THE EFFECT OF COMPOSITION, ANNEALING TEMPERATURE AND THICKNESS ON MAGNETORESISTANCE OF FERROMAGNETIC MULTILAYER STRUCTURES NAZIMAH BINTI KHAMIS @ SUBARI A thesis submitted in fulfilment of the requirements for the award of the degree of Master of Science (Physics) Faculty of Science Universiti Teknologi Malaysia FEBRUARY 2010 iii Specially dedicated to my beloved husband, Norazam Ahmad, and to my daughters, Nuraishah Maisarah and Nurul Hidayah. iv ACKNOWLEDGEMENTS اﻠﺤﻤﺪﻠﻠﮫﺮﺐاﻟﻌﻠﻣﯿﻦ. Kupanjatkan syukur kepada-Mu Ya Allah, atas kurniaan kekuatan dan kesihatan untuk menyelesaikan kajian ini. I would like to express a special appreciation to my supervisor, Professor Yussof Wahab, for his beneficial guidance, assistance and advice throughout the completion of this study. And thousand thanks to my co-supervisor, Professor Samsudi Sakrani, for his fruitful suggestion and opinion during this M.Sc journey. To fellow postgraduates students at Physics Department especially those in IRPA SET group, your presence always inspires me to give the best I could to finish this project. Thank you very much to Encik Nazari Kamarudin and Puan Fazilah Lasim for their unforgettable support in the experimental work at Vacuum Lab, Physics Department. Not forgetting Puan Wani and Encik Faizal from Ibnu Sina Institute, thank you for the kind assistance in finishing this research. Thank you also to Encik Azmi Ibrahim from Telekom R&D for the XRD analysis and Encik Hishamudin Abdul Rahim for AFM analysis. A very deep gratitude to Ministry of Science, Technology and Innovation (MOSTI), for providing financial assistance through research funding IRPA 09-0206-0057-SR005/09-06, and RMC, UTM for short term research funding number 75120. To my husband, thanks for the never ending love and support, and finally to my daughters, I left this path to be followed someday, Amin. v ABSTRACT Giant Magnetoresistance (GMR) refers to a phenomenon of a considerable drop in electrical resistance in ultrathin ferromagnetic/non-magnetic layers structure, when a sufficiently high magnetic field is applied to the structure. Since the first time it was observed in late 1980’s, this area of research has attracted a very strong interest due to the deep fundamental physics and its promising technological potential especially in data storage industry. This dissertation discusses the magnetoresistance of ferromagnetic/non-magnetic multilayers fabricated using electron beam evaporation technique. The magnetoresistance value is studied towards the effect of compositional variance, a range of post deposited annealing temperature, and multilayer thickness in terms of number of repetition of the bilayer as well as the non-magnetic layer thicknesses. The results on compositional variance show as deposited Co/Cu/Ni gives the largest magnetoresistance value of 8.75% followed by Ni/Cu/Ni (5.85%), Co/Ag/Co (2.64%) and Ni/Ag/Ni (1.48%). These four ferromagnetic multilayers were given heat treatment with a range of elevating temperature. The temperature ranges between 200°C to 350°C, resulting the magnetoresistance of the above structures increases to 14.25%, 10.68%, 7.45% and 4.38% respectively. However, the thermal stability for each multilayer systems were degraded after further annealing at a temperature called the blocking temperature. From the investigation, it was found that the blocking temperatures (or the optimum annealing temperature) for Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni were 280°C, 290°C, 310°C and 280°C respectively. The study of thickness dependence GMR, focus only on Co/Cu/Co structures. It was shown that in [Co/Cu/Co]n structures, the magnetoresistance increase as n increases from 1 to 20. Meanwhile, for Cox/Cuy/Cox structures, the magnetoresistance increases as y decreases from 15 nm to 1 nm with x is fixed at 6 nm. The X-ray diffraction study for the optimum structures shows peaks of Co, Cu, Ni and Ag which describe the multilayers are in the crystalline form. The atomic force microscopy (AFM) image analysis for Co and Cu thin films with elevated annealing temperature reveals that the higher the annealing temperature, the larger the grain size of the multilayer. The EDX spectrum analysis confirmed the presence of Co, Cu, Ni, and Ag in the multilayers. vi ABSTRAK Kemagnetorintangan raksaksa (GMR) merujuk kepada satu penyusutan rintangan ketara dalam lapisan ferromagnet/bukan magnet ultra nipis apabila medan magnet yang cukup kuat dikenakan terhadap struktur tersebut. Semenjak pertama kali ditemui pada tahun 1988, bidang penyelidikan ini telah menarik minat yang kuat di kalangan penyelidik disebabkan oleh asas fiziknya yang mendalam dan potensi teknologinya terutama terhadap industri penyimpanan data. Tesis ini membincangkan magnetorintangan dari lapisan ferromagnet/bukan magnet yang di fabrikasi menggunakan kaedah penyejatan alur elektron. Nilai magnetorintangan dikaji terhadap tiga parameter utama iaitu kesan terhadap komposisi berlainan, kesan terhadap suhu sepuh-lindap selepas pendepositan, dan kesan ketebalan multilapisan tersebut dalam bentuk bilangan ulangan dwilapisan dan ketebalan lapisan bukan magnet. Keputusan dalam kajian variasi komposisi menunjukkan selepas pendepositan multilapisan Co/Cu/Ni memberikan nilai magnetorintangan tertinggi iaitu 8.75%, diikuti oleh Ni/Cu/Ni (5.85%), Co/Ag/Co (2.64%) dan Ni/Ag/Ni (1.48%). Keempat-empat struktur multilapisan ini kemudian diberikan rawatan suhu. Julat suhu adalah dari 250°C hingga 350°C. Hasilnya menunjukkan magnetorintangan struktur di atas masing-masing meningkat kepada 14.25%, 10.68%, 7.45% dan 4.38%. Walau bagaimanapun, kestabilan terma untuk setiap struktur multilapisan menurun pada suhu sepuh lindap yang lebih tinggi. Suhu ini dipanggil suhu sekatan. Daripada kajian, didapati suhu sekatan (ataupun suhu sepuh lindap optimum) untuk struktur Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co dan Ni/Ag/Ni masing-masing adalah 280°C, 290°C, 310°C dan 280°C. Kajian tentang kesan ketebalan terhadap GMR difokuskan kepada struktur Co/Cu/Co sahaja. Pada struktur [Co/Cu/Co]n, didapati magnetorintangan bertambah dengan meningkatya nilai n dari 1 hingga 20. Bagi struktur CoxCuyCox pula, magnetorintangan bertambah apabila y menyusut dari 15 nm kepada 1 nm. Kajian belauan sinar-X terhadap terhadap struktur multilapisan optimum menunjukkan kehadiran puncak-puncak Co, Cu, Ni dan Ag, yang mana menunjukkan multilapisan berstruktur hablur. Analisis imej mikroskop imbasan daya atom terhadap filem nipis Co dan Cu bagi suhu yang meningkat menunjukkan pada suhu sepuh lindap yang lebih tinggi, saiz butiran multilapisan adalah lebih besar. Analisis spektrum serakan tenaga sinar-X mengesahkan kehadiran unsur-unsur Co, Cu, Ni dan Ag dalam multilapisan yang dikaji. vii TABLE OF CONTENTS CHAPTER TITLE DECLARATION ii DEDICATION iii ACKNOLEDGEMENTS iv ABSTRACT v ABSTRAK vi TABLE OF CONTENTS vii LIST OF TABLES x LIST OF FIGURES xi LIST OF ABBREVIATIONS xiv LIST OF SYMBOLS xv LIST OF APPENDICES 1 2 PAGE xviii INTRODUCTION 1 1.1 Background of Research 1 1.2 Introduction to Giant Magnetoresistance 2 1.3 Research Objectives 5 1.4 Research Scopes 5 1.5 Research Problem Statement 6 1.6 Thesis Outline 8 THEORY AND LITERATURE REVIEW 9 2.1 Introduction to Ferromagnetism 9 2.1.1 Physical Origin of Ferromagnetism 10 2.1.2 Domain and Domain Walls 12 viii 2.1.3 Hysteresis 13 2.1.4 Curie Temperature and Annealing Temperature 13 2.2 Introduction to GMR Theory 2.2.1 Mott’s Two Current Model 17 2.2.2 Resistor Model 19 2.2.3 Spin-dependent Scattering 22 2.2.4 Spin-dependent Conduction in Ferromagnet 25 2.3 Band Structure 3 15 26 2.3.1 Spin-orbit Coupling 27 2.3.2 Exchange-split d Bands 28 2.3.3 The Conductivity of 3d Metals 31 2.3.4 The Present of Interfaces 32 2.3.5 Band Matching in Magnetic Multilayer 32 2.3.6 CASTEP 33 2.3.7 Density Functional Theory 34 2.4 Fabrication of Ferromagnetic Multilayer Structures 34 2.5 Development of GMR Research 35 2.6 Spintronics : GMR Future Technology Application 37 RESEARCH METHODOLOGY 39 3.1 Sample Fabrication 39 3.1.1 Substrate and Source Material 39 3.1.2 Substrate Pre-clean and Preparation 40 3.1.3 Electron Beam Evaporation Technique and Principles 40 3.1.4 Annealing Process 42 3.1.5 Sample Features and Structures 44 3.2 Thickness Control and Measurement 46 3.2.1 Film Thickness Monitor (FTM) 46 3.2.2 Surface Profiler 48 3.3 Magnetoresistance Measurement 3.3.1 Four Point Probe Sample 48 50 ix 3.3.2 Magnetoresistance Measurement Set-up 3.4 Structural Analysis of Ferromagnetic Thin Films 4 53 3.4.1 X-Ray Diffractometer Analysis 53 3.4.2 Energy Dispersive X-Ray Analysis 54 3.4.3 Atomic Force Microscope Analysis 55 3.5 Band Structure Analysis 56 RESULTS AND DISCUSSION 57 4.1 Introduction 57 4.2 Thickness of Films 58 4.3 Compositional Dependence Magnetoresistance 59 4.3.1 Band Structure Factor 62 4.3.2 Lattice Matching Factor 67 4.4 Annealing Temperature Dependence Magnetoresistance 69 4.5 Thickness Dependence Magnetoresistance 77 4.5.1 Non-magnetic Layer Thickness Dependence 78 4.5.2 Number of Multilayer Dependence 81 4.6 Spectroscopy and Surface Morphology Analysis 4.6.1 84 X-Ray Diffractometer/ Energy Dispersive X-Ray Analysis 4.7 Surface Morphology Analysis 5 52 85 93 SUMMARY AND CONCLUSIONS 100 5.1 Summary and Conclusion 100 5.2 Suggestions 103 REFERENCES 106 APPENDIX 117 x LIST OF TABLES TABLE NO TITLE PAGE 2.1 Curie temperature of ferromagnetic elements. 14 3.1 List of annealed samples. 43 3.2 List of compositional varying samples (C-series) and non-magnetic thickness varying samples (N-seies). 44 3.3 List of number of multilayer varying samples (R). 45 4.1 Difference in thickness measurement observed by Film Thickness Monitor and Surface Profiler. 58 4.2 Lattice parameter for ferromagnetic metals (Co, Ni, Fe, Cr) and non-magnetic metal (Cu,Ag). 67 4.3 The average lattice mismatch and the GMR for Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni multilayer. 69 4.4 Sample description for XRD scans and followed by EDX scans. 85 xi LIST OF FIGURES FIGURE NO TITLE PAGE 1.1 Schematic representation of the GMR effect, after Tsymbal et.al, 2001. 2.1 Schematic illustration of electron transport in a multilayer for parallel (a) and antiparallel (b) magnetization of the successive ferromagnetic layers. 24 The electronic band structure (left panels) and the density of states (right panels) of Cu (a) and fcc Co for the majority-spin (b) and the minority-spin (c) electrons. 30 2.2 Beam Evaporation 4 3.1 Edwards Electron Photograph. System 42 3.2 The FTM Display connected to quartz crystal inside the e-beam chamber 47 3.3 Four point probe measurement of sheet resistance. 49 3.4 Four point sample for magnetoresistance measurement. 51 3.5 Magnetoresistance measurement set-up. Dotted arrows, indicating the magnetic field direction, whether from A to B or from B to A. 52 4.1(a) Magnetoresistance response towards external magnetic field for Co(6.5)/Cu(4.0)/Ni(5.5) multilayers. 59 4.1(b) Magnetoresistance response towards external magnetic field for Ni(7.5)/Cu(4.0)/Ni(7..5) multilayers. 60 4.1 (c) Magnetoresistance response towards external magnetic field for Co(6.0)/Ag(5.0)/Co (6.0) multilayers. 60 4.1 (d) Magnetoresistance response towards external magnetic field for Ni(7.0)/Ag(4.0)/Ni(7.0) multilayers. 61 4.2 Electronic band structures (left panels) and density of states (right panels) of a) Co and b) Cu. 63 4.3 Electronic band structures (left panels) and density of 64 xii states (right panels) of a) Cu and b) Ni. 4.4 Electronic band structures (left panels) and density of states (right panels) of a) Co and b) Ag. 65 4.5 Electronic band structure (left panels) and density of states (right panels) of a) Ni and b)Ag. 66 4.6 (a) Magnetoresistance curve for Co(6.5nm)/Cu(4.0nm)/Ni(5.5nm), annealed at different temperatures. 70 4.6 (b) Magnetoresistance curve for Ni(7.5nm)/Cu(4.0nm)/Ni(7.5nm), annealed at different temperatures. 71 4.6 (c) Magnetoresistance curve for Co(6.0nm)/Ag(5.0nm)/Co (6.0nm), annealed at different temperatures. 72 4.6 (d) Magnetoresistance curve for Ni(7.0nm)/Ag(4.0nm)/Ni(7.0nm), annealed at different temperatures. 73 4.7 Annealing temperature versus magnetoresistance ratio for Co(6.5nm)/Cu(4.0nm)/Ni(5.5nm), Ni(7.5nm)/Cu(4.0nm)/Ni(7.5nm), Co(6.0nm)/Ag(5.0nm)/Co (6.0nm) and Ni(7.0nm)/Ag(4.0nm)/Ni(7.0nm). 77 4.8 The magnetoresistance effect on Cu thicknesses for Co/Cu/Co multilayers 79 4.9 Cu thicknesses versus magnetoresistance of Co/Cu/Co multilayers in experiment fitted with the expected result based on equation 4.1. 81 4.10 Number of multilayers repetition versus magnetoresistance for Co/Cu/Co structures. 82 4.11 Two theta XRD patterns for [Co/Cu]n , with n = 2,5,16 and 20. All samples were annealed at 270°C for two hours. 83 4.12 (a) The two-theta XRD pattern , (b) The EDX spectrum for Co(6.5)/Cu(4.0)/Ni(5.5). 86 4.13 (a) The two-theta XRD pattern, (b) The EDX spectrum for Ni(7.5)/Cu(4.0)/Ni(7.5) 88 4.14 (a) The two-theta XRD pattern, (b) The EDX spectrum for Co(7.0)/Ag(4.0)/Co (7.0) 90 4.15 (a) The Two-theta XRD pattern, (b) The EDX spectrum for Ni(6.0)/Ag(5.0)/Ni (6.0) 91 4.16 AFM topography images of as deposited Co film (a), and Co films annealed at 200°C (b), 250°C (c), 280°C (d) and 350°C (e). 95 4.17 RMS roughness and average grain diameter of Co thin 96 xiii films as a function of annealing temperature. 4.18 AFM topography images of as deposited Cu film (a), and Cu films annealed at 200°C (b), 250°C (c), 280°C (d) and 350°C (e). 98 4.19 RMS roughness and average grain diameter of Cu thin films as a function of annealing temperature. 99 xiv LIST OF ABBREVIATIONS OMR - Ordinary magnetoresistance AMR - Anisotropic magnetoresistance GMR - Giant magnetoresistance CMR - Colossal magnetoresistance TMR - Tunnelling magnetoresistance AFM - Atomic force microscope FESEM - Field emission scanning electron microscope XRD - X-Ray diffractometer EDX - Energy dispersive X-Ray HDD - Hard disk drive CIP - Current-in-plane MBE - Molecular beam epitaxy DC - Direct current RF - Radio Frequency MRAM - Magnetic random access memory MTJ - Magnetic tunnel junction FTM - Film thickness monitor DFT - Density functional theory FM - Ferromagnetic NM - Non-magnetic RMS - Root mean square xv LIST OF SYMBOLS Ag - Argentum B - Magnetic flux density C - Quartz constant Co - Cobalt Cu - Copper d NM - Non-magnetic layer thickness d FM - Ferromagnetic layer thickness d - Thickness, film d - Plane spacing in atomic lattice E - Electric field e - Charge of electron F - Correction factor f - Frequency ∆f - Frequency change H - External magnetic field Hs - Saturation magnetic field I - Current i , - Intrinsic resistivities for spin channels Jn - Exchange constant K - Breadth constant kF - Fermi’s momentum xvi L - Crystallite length lNM - Mean free path of conduction electron MR - Magnetoresistance MR% - Magnetoresistance ratio m - Mass of electron Ni - Nickel n - Avogadro number n - Integer, determine by order n (Ef) - Density of states N - R - Number of four layer unit cells withimultilayermultilayer multilayer Resistance R - Resistance, at certain magnetic field R AP - Resistance for anti-parallel alignment RP - Resistance for parallel alignment Rmin - Saturation magnetoresistance R , - Resistance for two spin channel Rs - Semiconductor sheet resistance Si - Silicon Si3 N 4 - Silicon nitride T - Temperature Tc - Curie temperature V - Voltage Zi - Height value of each point Zave - Average surface height - Spin asymmetry parameter β - Breadth of peak phase - Independent conductivity - Conductivities for up-spin - Conductivities for down-spin Drude - Drude’s conductivity xvii ρ↑ - Spin-up ρ↓ - Spin-down - Resistivity of non-magnetic layer ρq - Quartz density ρf - Thin film density F - Fermi’s velocity - Velocity in spin channel - X-ray wavelength - Mean free path - Pai - Relaxation time of electrons - Planck constsnt - Micro - Angle δRMS - Root mean square roughness 2θ - Diffraction angle (two-theta) Δ - Delta θ - Angle < - Less than NM ħ xviii LIST OF APPENDICES APPENDIX A TITLE Photographs of Characterization Instruments PAGE 117 CHAPTER 1 INTRODUCTION 1.1 Background of Research The phenomenon of magnetism, that is the mutual attraction of two pieces iron or iron ore had been known since 4000 years ago. The ancient Greeks have been reported to experiment with this mysterious force but the application were limited at that time, until scientific research was made by William Gilbert (1504-1603) 400 years ago. He wrote the book ‘De Magnet’ which explained his research findings on magnetic properties. Before 19th century, the only important application of magnet is Compass. Hans Christian Oersted (1775-1851) observed that magnetic field interact with electric current in 1820. Inspired with this, he creates the first electromagnet in 1825 based on the principle that electric current produce magnetic field. This was a scientific breakthrough in magnetism and since that, researches towards magnetic material were rapidly grown and its applications were widely increased. 2 Magnetoresistance is the properties of some materials to lose or gain electrical resistance when an external magnetic field is applied to them. The effect was first discovered by William Thomson in 1857, but he was unable to lower the electrical resistance by more than 5%. This effect was later called ordinary magnetoresistance (OMR). Ordinary magnetoresistance arises from the effect of Lorentz force on the electron trajectories due to the applied magnetic field. It does not saturate at the saturation magnetic field and usually small in metals which is less than 1% in fields of 1 Tesla (Tsymbal and Pettifor, 2001). More recent researches discovered materials showing anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), colossal magnetoresistance (CMR) and tunneling magnetoresistance (TMR). The discovery of giant magnetoresistance phenomena in 1988 was a breakthrough in magnetic material and thin film magnetism. Since then, researches in thin film magnetism are looking forward to obtain larger magnetoresistance ratio because in principal, larger magnetoresistance ratio ascribe bigger data storage volume. 1.2 Introduction to Giant Magnetoresistance Giant Magnetoresistance or GMR is a quantum mechanical effect observed in thin film structures composed of alternating ferromagnetic and non-magnetic layers. The effect is a significant decrease in electrical resistance in the presence of magnetic field. In the absence of an applied magnetic field, the direction of magnetization of adjacent ferromagnetic layers is antiparallel due to a weak anti-ferromagnetic coupling between layers, and it decreases to a lower level of resistance when the magnetization of 3 the adjacent layers align due to an applied external field. The spins of the electrons of the non-magnetic metal align parallel or antiparallel with an applied magnetic field in equal numbers, and therefore suffer less magnetic scattering when the magnetizations of the ferromagnetic layers are parallel. Figure 1.1 shows the schematic representation of GMR effect. The upper diagram (a), shows change in the resistance of the magnetic multilayer as a function of applied magnetic field. The middle diagram (b), is the magnetization configurations (indicated by the arrows) of the multilayer at various magnetic field. The diagram shows that the magnetizations are aligned antiparallel at zero field whereas the magnetizations are aligned parallel when the external field H is larger than the saturation field Hs. multilayer. The lower diagram (c), shows the magnetization curve of the magnetic 4 Figure 1.1: Schematic representation of the GMR effect, after Tsymbal and Pettifor, 2001. 5 1.3 Research Objectives This dissertation was carried out to reach the following objectives: i. To fabricate and design the GMR structures by electron beam evaporation method. ii. To study the effect of fabrication parameters and post-annealing parameters towards the GMR values. iii. To investigate the dependencies of composition, annealing temperature and thickness on the GMR value. 1.4 Research Scopes This dissertation emphasizes on deposition and characterization of different ferromagnetic multilayer structures. A ferromagnetic multilayer structure basically consists of ferromagnetic layer separated by a thin conducting non-magnetic layer. Ferromagnetic materials used in this study are Cobalt (Co) and Nickel (Ni). conducting materials used are Copper (Cu) and Argentum (Ag). The The multilayer structures studied are namely Co/Cu/Ni, Ni/Ag/Ni, Co/Ag/Co and Ni/Cu/Ni. These ferromagnetic multilayer structures were deposited using electron beam method, which were then followed by annealing. The Magnetoresistance Ratio (MR%) for each structures is measured, focusing on effect of different fabrication and post-annealing parameters. The following parameters are studied: Composition difference of 6 ferromagnetic materials and conducting materials; annealing temperature ranged from 200°C to 350°C; conducting layer thickness vary from 1 to 15 nm, and the number of the repetition of the bilayer from 1 to 20 bilayer. Structural characterizations using methods of Atomic Force Microscopy, Field Emission Scanning Electron Microscopy and X-Ray Diffractometry are also carried out as supportive evidences of the result obtained. Atomic Force Microscope (AFM) was used to probe the surface morphology of the magnetic and non-magnetic thin film layer. Field Emission Scanning Electron Microscope (FESEM) was used to see the surface structure of the magnetic and nonmagnetic thin film layer. X-Ray Diffractometer (XRD) was used to provide information about the crystallographic structure of the magnetic and non-magnetic layer. 1.5 Research Problem Statement In the early 1980s, thin film magnetism was applied to higher-density to nonvolatile random access memory by Honeywell in their development of anisotropic magnetoresistance (AMR) memory. Later, a new path leading to the integration of magnetic devices into computer technology began to emerge with the discovery of giant magnetoresistance (GMR). GMR heads are more sensitive compared to the previous anisotropic magnetoresistive (AMR) heads. 7 Different from GMR heads, an AMR heads employs a special conductive material that changes its resistance in the presence of magnetic field. As the head passed over the surface of the disk, this material changes its resistance as the magnetic fields changes corresponding to the stored patterns on disk. A sensor is used to detect these changes in resistance, which allows the bits on platter to be read. While the older AMR heads typically exhibit about 2% of resistance change when passing from one magnetic polarity to another, for GMR heads the value is between 5% to 8%. This means GMR heads can detect much weaker and smaller signals, which is the key to increasing areal density. Magnetic disk drive products have had their areal density increased by factor of 35 million since the introduction of the first disk drive, in 1957. Since 1991, the rate of increase has accelerated to 60% per year, and since 1997 it has accelerated further to an incredible 100% per year (Wolf et. al, 2006). The acceleration was the result of the introduction of AMR read heads in 1991 and GMR read heads in 1997. Today, nearly all disk drives in the industry incorporated the GMR read-head design, which is faster in speed and smaller in size. The achievements of switching from AMR read head technology to GMR read head technology would not be possible without a detailed understanding of the physics of GMR. To further understand GMR, a deeper insight into ferromagnetic multilayer structures is pertinent. The technique of fabrication, fabrication parameters such as material composition, number of bilayer, non-magnetic layer thickness and also the post annealing treatment, produces a variety of spin transport behavior in ferromagnetic multilayer structures. All these parameters significantly influence the GMR effect in the structures. 8 Study on magnetic and non-magnetic material composition can provide better understanding on magnetic and non-magnetic coupling in GMR phenomenon. Detailed information about band structure and lattice matching could be associated. From the study on post annealing treatment, the GMR response on a range of annealing temperatures could be observed. Study on the number of bilayer could enhance the understanding of spin dependent transport where supportive evident to resistor model could be obtained. The GMR response towards increasing non-magnetic layer thickness could be provided from the non-magnetic layer thickness study. 1.6 Thesis Outline A general background and brief introduction to GMR effect are discussed in Chapter 1. This is followed by objectives, scope of the study, significances of the study and the outline of the dissertation. In Chapter 2, an overview of GMR properties in ferromagnetic multilayer structures is presented. Chapter 3 discussed about the methodology which is focused on fabrication procedures, measurement technique and characterization technique of ferromagnetic multilayers. A general overview of the measurement and characterization technique is discussed. Results of all the experimental work such as effects on the GMR towards composition difference, various annealing temperature, various conducting layer thickness, number of the repetition of the bilayer and also structural characterization are presented in Chapter 4. Chapter 5 summarizes and concludes the findings obtain in the earlier chapter, and also provides suggestions for future work in this area of research. CHAPTER 2 THEORY AND LITERATURE REVIEW 2.1 Introduction to Ferromagnetism Ferromagnetic materials are generally used in the making of GMR sensors because of two advantages. First, ferromagnetic thin film has high resistance that is pertinent for obtaining a high magnetoresistance value. Second, the anisotropic characteristic or in other words the homogeneity in all direction in ferromagnetic thin film can be made uniaxial subjected to external magnetic field strength applied to the thin film (Chen, 2005). Ferromagnetism is the common and usual form of magnetism as we are familiar with, as exhibited in horseshoe magnets and refrigerator magnets. It contributes to most of the magnetic behavior encountered in everyday life. 10 Since the early time of magnetic study, the term ‘ferromagnet’ was used for any material that could exhibit spontaneous magnetization that is a net magnetic moment in the absence of external magnetic field. This general definition is still in common use nowadays. Recently, however, different classes of spontaneous magnetization have been identified when there is more than one magnetic ion per primitive cell of the material. This leads to a stricter definition of ferromagnetism. In particular, ferromagnetism can be ascribed to materials where all of its magnetic moments/ions add to a positive contribution to the net magnetization. If some of the magnetic ions subtract from the net magnetization (partially anti-aligned), then the material is ferrimagnetic. If the magnetic ions are completely anti-aligned so as to have zero net magnetization, despite the magnetic ordering, it is an antiferromagnet. 2.1.1 Physical Origin of Ferromagnetism The property of ferromagnetism is due to the direct influence of two effects from quantum mechanics that are the spin and the Pauli Exclusion Principle. Pauli Exclusion Principle is a quantum mechanical principle that states no two identical electrons may occupy the same quantum state simultaneously. The spin of an electron when combined with its orbital angular momentum results in a magnetic dipole moment and creates a magnetic field. In many materials 11 however, especially those with a completely filled electron shells, the total dipole moment of all the electrons is zero, or in other words the spins are in up/down pairs. Only atoms with partially filled shells (for example unpaired spins) can experience a net magnetic moment in the absence of the external magnetic field. A ferromagnetic material has many such electrons, and if they are aligned they create a measurable macroscopic field. For this reason Fe, Co, Ni are ferromagnet. These permanent dipoles (or spins) tend to align in parallel to an external magnetic field, an effect called paramagnetism. A related but much smaller effect is diamagnetism, due to the orbital motion induced by an external field, resulting in a dipole moment opposite to the applied field. Ferromagnetism involves an additional phenomenon; the dipoles tend to align spontaneously, without any applied field. This is a purely quantum-mechanical effect. According to classical electromagnetism, two nearby magnetic dipoles will tend to align in opposite direction (which would create an antiferromagnetic material). In a ferromagnet however, they tend to align in the same direction because of the Pauli Exclusion Principle. According to Pauli, two electrons with the same spin state cannot lie at the same position, and thus feel an effective additional repulsion that lowers their electrostatic energy. This difference in energy is called the exchange energy and induces nearby electron to align. 12 2.1.2 Domains and Domain Walls At long distances, after many thousands of moments/ions, the exchange energy is overtaken by the classical tendency of dipoles to anti-align. This is why, in an equilibrated (non-magnetized) ferromagnetic material, the dipoles in the whole material are not aligned. They are organizing into magnetic domains or also known as Weiss domain. The domains are aligned (magnetized) at short range, but at long range adjacent domains are anti-aligned. The transition between two domains, where the magnetizations flips, is called a Domain Wall. It is called whether a Bloch wall or Neel wall depending upon whether the magnetization rotates parallel or perpendicular to the domain interface. This transition is a gradual transition on the atomic scale which covers a distance of about 300 ions for iron (Tsymbal and Pettifor, 2001). Generally, an ordinary piece of iron has little or no net magnetic moment. However, if it is placed in a strong enough magnetic field, and the moments will remain re-oriented when the field is tuned off, therefore creating a ‘permanent’ magnet. This magnetization as a function of the external field is described by a hysteresis curve. Although this state of aligned domains is not a minimal-energy configuration, it is extremely stable and has been observed to persist for millions of years in seafloor magnetite aligned by the Earth’s magnetic field whose poles can thereby be seen to flip at long intervals. The net magnetization can be destroyed by heating and then cooling (annealing) the material without an external magnetic field. 13 2.1.3 Hysteresis Hysteresis is well known in ferromagnetic materials. When an external magnetic field is applied to a ferromagnet, the atomic dipoles align themselves with the external field. Even when the external field is removed, part of the alignment will be retained and the material has become magnetized. The relationship between magnetic field strength (H) and magnetic flux density (B) is not linear in such materials. If the relationship between the two is plotted for increasing levels of field strength, it will follow a curve up to a point where further increases in magnetic field strength will result in no further change in flux density. This condition is called magnetic saturation. If the magnetic field is then reduced linearly, the plotted relationship will follow a different curve back towards zero field strength at which point it will be offset from the original curve by an amount called the remanent flux density or remanence. If this relationship is plotted for all strengths of applied magnetic field the result is sort of Sshaped loop. The thickness of the middle bit of the S describes the amount of hysteresis, related to the coercivity of the material. 2.1.4 Curie Temperature and Annealing Temperature The Curie point in ferromagnetic material is the temperature above which it losses its characteristic ferromagnetic ability. At temperatures below the Curie point, the magnetic moments are partially aligned within magnetic domains in ferromagnetic materials. As the temperature is increased towards the Curie point, the alignment 14 (magnetization) within each domain decreases. Above the Curie point, the material is purely paramagnetic and there are no magnetized domains of aligned moments. While annealing process improved the microstructure of the magnetic multilayer thin film, it does have a threshold limit. Annealing for a temperature extending 400°C decreasing the magnetoresistance value of a magnetic multilayer ( Malkinski et.al,2000). Another group reveals that annealing at 350°C decrease the magnetoresistance value and further annealing up to 650°C diminishing the magnetoresistance signal (Hecker et. al, 2002). Some other researchers annealed their magnetic multilayers below the temperature of 400°C (Hall et.al, 1996; Suzuki et.al, 1997). It is expected that the threshold annealing temperature for magnetic multilayer thin film range between 350°C and 400°C. At temperatures above the Curie point, an applied magnetic field has a paramagnetic effect on the magnetization, but the combination of the paramagnetism with ferromagnetism leads to the magnetization following a hysteresis curve with the applied field strength. The destruction of magnetization at the Curie temperature is a second-order phase transition and critical point where the magnetic susceptibility is theoretically infinite. Table 2.1 below shows the Curie temperature for ferromagnetic elements. Table 2.1 : Curie temperature of ferromagnetic elements. Material Curie Temperature, Tc Iron (Fe) 768°C (1043K) Cobalt (Co) 1121°C (1388 K) Nickel (Ni) 354°C (629 K) 15 While the Curie point of bulk Iron, Cobalt and Nickel are high, the Curie point for thin layers of such elements had to be much lower due to lesser number of magnetic moments in the layers. Due to this, the range of annealing temperature for Iron, Cobalt and Nickel layers need to be decided prior to temperature dependence study for the layers. The GMR value of the magnetic layers after annealing is been made, can be used as indication of their magnetic ordering. The GMR value for magnetic layers or multilayers should increase with the increasing annealing temperature as the magnetic alignment decrease. When the Curie point is reached, the magnetization is completely disordered and this degrades the GMR value. 2.2 Introduction to GMR theory In understanding the transport properties in metallic superlattice, we should consider that the transport behaviours are affected by many inherent complexities of the material. Many possible complications arise in these types of material such as interfacial interdiffusion at various lateral length scales, bulk defects, and structural changes as a function of an individual layer and overall thickness, different length scales affecting the structures, magnetism and transport, and differences in the magnetotransport along the different directions in the superlattices (Warda et al, 2004). However, to make it simple to understand, GMR can be qualitatively understood using Mott model, which was introduced in 1936 to explain the sudden increase in resistivity of ferromagnetic metals as they are heated above Curie temperature (Mott, 1964). Mott proposed two main points. First, the electrical conductivity in metals can 16 be described in terms of two largely independent conducting channels that are the upspin and down-spin electrons. These electrons are distinguished according to the projection of their spins along the quantization axis. The probability of spin-flip scattering processes in metals is normally small as compared to the probability of scattering processes in which the spin is conserved. This means that the up-spin and down-spin electrons do not mix over long distances and therefore the electrical conduction occurs in parallel for the two spin channels. Second, in ferromagnetic metals the scattering rates of the up-spin and downspin electrons are quite different, whatever the nature of the scattering center is. According to Mott, the electric current is primarily carried by electrons from the valence sp bands due to their low effective mass and high mobility. The d bands play an important role in providing final states for the scattering of the sp electrons. In ferromagnets the d band are exchange-split, so that the density of states is not the same for the up-spin and down-spin electrons at the Fermi energy. The probability of scattering into these states is proportional to their density, so that the scattering rates are spin-dependent which is different for the two conduction channels. 17 2.2.1 Mott’s Two Current Model At low temperature, the current is carried in parallel by spin-up and spin-down electrons. The introduction of impurities generates a residual resistivity ρ↑(0) and ρ↓(0) for each spin direction. The overall resistivity is then: ρ (0) = (0) (0) (0) (0) (2.1) Two processes that are known to mix the momentum from one spin channel to the other one are the elastic scattering of electrons by spin waves and mutual spin-flip process between two interacting electrons. However the description of these mechanisms is beyond the scope of this section. If we denote the average velocity in each spin channel by and the electric field is E, then we can write - 1 e 1 E = + ( ) m 2 (2.2a) and - 1 e 1 E = + ( ) m 2 (2.2b) where is the relaxation time of the electrons, e is the charge of electron, m is the mass of electron. 18 Then for each spin channel, we define intrinsic resistivities as: i , = ne , (2.3a) or , = m ne , (2.3b) 2 where n is the Avogadro number. The resistivity is then calculated as : (T ) E i i (2.4) Equation (2.4) hence gives 4 ( ) 4 (2.5) and each spin resistivity can be written as : , (T ) , (0) i, (T ) (2.6) where i, is the intrinsic resistivity of each spin channel of the pure metal. 19 2.2.2 Resistor Model Current-in-plane GMR or CIP GMR can be qualitatively understood using the very simple resistor model (Edwards et al., 1992). According to the resistor model, each metallic layer (and each interface) is treated as an independent resistor. Within each spin conduction channel the resistors are added in parallel or in series depending on the relationship between the mean free path and the layer thickness. If the mean free path is short compared to the layer thickness, then each layer conducts the electric current independently and the resistor should be added in parallel. Under this circumstance, it is obvious that the resistance of the parallel and antiparallel are the same, making the GMR is zero. These observations indicate that to obtain non-zero GMR the mean free path should be sufficiently long. This is in agreement with the qualitative picture of GMR (Figure 2.1 a), which is based on the possibility for the electrons to propagate across the non-magnetic layer freely sensing the magnetizations of the two consecutive ferromagnetic layers. If the length of the mean free path is longer compared to the layer thickness, the probability of scattering within the multilayer is the sum of scattering probabilities within each layer and each interface. Thus, within a given spin channel the total resistance is the sum of resistances of each layer and each interfaces or in other words the resistors are connected in series. Magnetic multilayers exhibiting giant magnetoresistance is good example for this limiting case. In building up the resistor network for the multilayer, consider a unit cell which consist of four layers, two ferromagnetic and two non-magnetic as shown in Figure 2.1 (a) and 2.1 (b). Assume the global spin-quantization axis is the same with magnetization direction. In each ferromagnetic layer the electron spin can be either parallel or antiparallel to the magnetization direction. If the electron spin is parallel to the magnetization direction, the electron is locally a majority-spin electron. If the electron spin is antiparallel to the magnetization direction, the electron is a minority-spin electron. The majority and minority spin resistivities of the ferromagnetic layer are 20 different and denote as ρ↑ and ρ↓ respectively. The resistance of the bilayer which consists of the ferromagnetic layer and the non-magnetic layer for the two spin channel is equal to: R , = NM d NM , d FM (2.7) where NM is the resistivity of non-magnetic layer, d NM is the thickness of non-magnetic layer, d FM is the thickness of ferromagnetic layer. For simplicity, we assumed the interface resistance between the ferromagnetic and non-magnetic layer has been omitted. Using the resistances which are defined in Equation 2.7, the equivalent network of resistors for the parallel and antiparallel magnetizations are shown in Figure 2.1 (c) and 2.1 (d) respectively. The total resistance of the parallel-aligned multilayer is given by: RP = N R R R R (2.8) where N is the number of the four layer unit cells within the multilayer. The total resistance of the antiparallel-alignment multilayer R AP is then equal to: R AP = N R R 2 (2.9) Therefore, the magnetoresistance ratio can be determined by a simple expression which is: 21 ( R R ) 2 R R AP RP = R RP 4 R R (2.10) By using Equation 2.7 and 2.10 we can pinpoint the main factors which determine the GMR. By assuming that the resistance of the spacer layer is small compared to the resistance of the ferromagnetic layer, the expression for GMR is: 2 R ( ) ( 1) 2 = R 4 4 (2.11) where is the spin asymmetry parameter and is equal to / . From Equation 2.11 it is obvious that the magnitude of the GMR is strongly dependent on the asymmetry in the resistivity between the two spin conduction channels in ferromagnetic layers. Large asymmetry where >>1 or <<1 is an important requirement for obtaining high values of GMR. If there is no spin asymmetry in the resistivity of the two spin channels or in other words 1 , the GMR will be zero. The finite resistance of the spacer layer is given by: R ( 1) 2 R d d 4 NM 1 NM d FM d FM (2.12) 22 where is equal to NM / . It is clear that for any given value of , the GMR will decrease with decreasing d NM / d FM . Using the resistor model, few hypotheses can be made. Firstly, in stacking multilayers, the GMR are higher than in single multilayers because stacking the multilayers is equivalent to adding up more resistors in series to a circuit, thus give higher resistance. Secondly, in single multilayers (magnetic/non-magnetic/magnetic), the magnetization of the magnetic layers needs to be in anti-parallel state to obtain higher resistance and higher GMR. And thirdly, in single multilayers (magnetic/nonmagnetic/magnetic), in order to obtain higher GMR value, a low resistivity of a nonmagnetic spacer layer is needed. 2.2.3 Spin-dependent Scattering Using Mott’s argument and model it is straightforward to explain GMR in magnetic multilayers. Consider a collinear magnetic configuration as shown in Figure 2.1, and assume that the scattering is strong for the electron with the spin antiparallel to the magnetization direction, and is weak for electrons with spin parallel to the magnetization direction. This is supposed to reflect the asymmetry in the density of states at the Fermi level, in accordance with Mott’s second argument. For the parallelaligned magnetic layers (Figure 2.1 (a)), the up-spin electrons pass through the structure almost without scattering, because their spin is parallel to the magnetization of the layers. On the contrary, the down-spin electrons are scattered strongly within both magnetic layers, because their spin is antiparallel to the magnetization of the layers. 23 Since conduction occurs in parallel for the two spin channels, the total resistivity of the multilayer is determined mainly by the highly-conductive up-spin electrons and appears to be low. For the antiparallel-aligned multilayer which is depicted in Figure 2.1 (b), both the up-spin and down-spin electrons are scattered strongly within one of the magnetic layers, because within one of the layers the spin is antiparallel to the magnetization direction. Therefore, the total resistivity is high. Wang and Li, (1995), suggested when the ambient temperature is lower than the Curie temperature of magnetic materials, the direction of polarized spins of different magnetic atoms are parallelly ordered, however, to different magnetic clusters, they are disordered, therefore the interface layers can be described as superparamagnetic layers and the exchange coupling between the ferromagnetic films can be omitted. 24 Fig. 2.1 : Schematic illustration of electron transport in a multilayer for parallel (a) and antiparallel (b) magnetization of the successive ferromagnetic layers. In Figure 2.1, the magnetization directions are indicated by the arrows. The solid lines are individual electron trajectories within the two spin channels. It is assumed that the mean free path is much longer than the layer thicknesses and the net electric current flows in the plane of the layers. Bottom panels show the resistor network within the two-current series resistor model. For the parallel-aligned multilayer as in Figure 2.1 (a), the up-spin electrons pass through the structure almost without scattering, whereas the down-spin electrons are scattered strongly within both ferromagnetic layers. Since conduction occurs in parallel for the two spin channel, the total resistivity of the multilayer is low. 25 For the antiparallel-aligned multilayer as in Figure 2.1 (b), both the up-spin and downspin electrons are scattered strongly within one of the ferromagnetic layers, and the total resistivity is high. Figure 2.1 (c) and 2.1 (d) show the resistor network within two current series resistor model. For the parallel aligned multilayer, as in Figure 2.1 (c), resistance for the up-spin electron is low and resistance for the down-spin electron is high results in low total resistivity. For the antiparallel aligned multilayer, both up-spin and down-spin electrons experience high resistance, which resulting in high total resistivity. 2.2.4 Spin-dependent Conduction in Ferromagnet According to Mott’s first argument, the conductivity of a metal is the sum of the independent conductivities for the up-spin and down-spin electrons, which can be expressed by: (2.13) Within each conduction channel the conductivity is determined by number of factors which Drude expressed them as follows: 2 Drude e2 kF 6 (2.14) 26 where Drude is Drude conductivity per spin, e 2 / is approximately equal to 0.387 x 10-4 is the spin quantum conductance, k F is the Fermi momentum and is the mean free path. The mean free path can be expressed by: F (2.15) (2.15) where F is Fermi velocity and is the relaxation time. 2.3 Band Structure The electronic band structure of the multilayer is believed to be the most important property which determines the spin dependent conduction and the GMR. In most experimental works, 3d transition metal that are Co, Fe and Ni, and their alloys are used in combination with non-magnetic spacer layer such as Cr or the noble metals like Cu, Ag and Au. The electronic band structure of these metals is characterized by several similar features that are spin-orbit coupling, exchange-split d bands in 3d metals, the conductivity of d bands, the present of interfaces in a magnetic multilayer and band matching. 27 2.3.1 Spin-orbit Coupling Spin orbit coupling in 3d transition metals is very weak and the electronic structure for the up-spin and down-spin electron can be considered independently. 3d elements are characterized by the presence of the 4s, 4p and 3d valence states which are distinguished by their orbital momentum. A dispersive sp bands is similar to freeelectron band is created by 4s and 4p states (Tsymbal and Pettifor, 2001). The sp electrons have a high velocity, a low density of states and consequently a long mean free path. These features are believed to be mainly responsible for the conductivity in 3d metals. Contradict to these features, the d band is localized in a relatively narrow energy window and is characterized by a high density of states and a low velocity of electrons. In the interval of energy where the sp and d bands cross, electrons cannot be considered as independent because of the strong sp-d hybridization, which modify substantially the band structure. Hybridization changes dramatically the properties of sp electrons which is reflected in the band bending and results in a reduced velocity associated with the sp band. Jaya et al. (2002), evaluated the interlayer exchange coupling of two local moment ferromagnetic sublayers in a magnetic/non-magnetic/magnetic multilayer at different spacer (non-magnetic) thickness. The interlayer exchange coupling was found to have an oscillating behaviour with respect to the non-magnetic layer thickness and it oscillates between ferromagnetic and antiferromagnetic configurations. Significant influences of the electron correlation effects on the interlayer exchange coupling and the correlation effects were found to change the nature and magnitude of the exchange coupling. The oscillation period of the interlayer exchange coupling was also found to depend on the band occupation. 28 Gubbiotti et al. (2005), found the presence of bilinear and biquadratic exchange coupling in Co/Ru/Co trilayers. The trilayers effective uniaxial anisotropy on the top of Co film thickness has been estimated. This biquadratic coupling, usually referred to as ‘orange peel coupling’ or ‘Neel coupling’ depends on the roughness of the layers and favors the parallel alignment of the layers magnetic moments so it enters as a positive contribution to bilinear exchange coupling. 2.3.2 Exchange-split d Bands In ferromagnetic 3d metals, the d band is exchange-split. Due to the localized nature of the d electrons, two d electrons experience a strong Coulomb repulsion provided that they have antiparallel spins and occupy the same orbital. To reduce the energy it is advantageous for the d electrons to have parallel oriented spins because the Pauli’s Exclusion Principle does not permit two electrons with the same spin to approach each other closely that is occupy the same orbital. Hence, the Coulomb interaction is reduced. Therefore, the Coulomb repulsion in conjunction with the Pauli’s Exclusion Principle leads to the ferromagnetic exchange interaction and favors the formation of spontaneous magnetic moment. However putting all the electrons into states with the same spin direction increases the total kinetic energy. The increase is being larger in two cases. Firstly, when the d band is wide, and secondly, when the d band’s density of states is low. Therefore there are two competing tendencies that have to be balanced in order to find whether the ferromagnetic ordering is favored. The condition which has to be satisfied 29 for the appearance of ferromagnetism is the famous Stoner criterion Jn (Ef) > 1. J is the exchange constant which is approximately 1eV for transition metals, n (Ef) is the density of states for a given spin at the Fermi energy. The Stoner criterion is satisfied for bcc Fe, fcc Co and fcc Ni. Due to the exchange splitting of the d band, the number of the occupied states is different for the up-spin and down-spin electrons and giving rise to the non-zero magnetic moments of 2.2 μB, 1.7 μB and 0.6 μB for Fe, Co and Ni respectively. In order to distinguished between the high and low-occupied spin states, the terms ‘majority-spin electrons’ and ‘minority-spin electrons’ are usually used. The band structure of Co, as a representative of the ferromagnetic 3d metals is shown in Figure 2.2 (b) and 2.2 (c). 30 Figure 2.2 : The electronic band structure (left panels) and the density of states (right panels) of Cu (a) and fcc Co for the majority-spin (b) and minority-spin (c) electrons. After Tsymbal and Pettifor, 2001. 31 2.3.3 The Conductivity of 3d Metals The conductivity is determined by the position of Fermi energy with respect to the d bands. In the case of Cu, the d bands are fully occupied and the Fermi levels lies within the sp band as in Figure 2.2 (a). Electrons within the sp band have high velocity and low density of states which lower the probability of scattering. Therefore, the mean free path is long and Cu becomes a very good conductor. The same case goes for another noble metal like Ag and Au. On the other hand, for ferromagnetic metal like Co the majority d band is fully occupied whereas the d majority band is only partly occupied as depicted in Figure 2.2 (a). This is the result of exchange splitting. The Fermi level lies within the sp band for the majority spins but within the d band for the minority spin. The exchange splitting of the spin bands lead to crucial difference in the conductivity between the majority- and minority- spin electrons. For the majority spins the conductivity is governed by sp electrons and is high. This is the same case like Cu. However for the minority spins, the conductivity is not entirely determined by the sp electrons. Because of the strong sp-d hybridization which mixes the sp and d states the contribution of both the sp and d electrons become important. The minority bands represent hybridized spd bands which are not dispersive and have a high density of states. As a result, the mean free paths associated with these bands are relatively short and the minority-spin conductivity is low. These arguments, which are based on the spin polarized band structure, explain the strong spin asymmetry in the conductivity of bulk Co. 32 2.3.4 The Present of Interfaces The present of interfaces in magnetic multilayer adds a new important feature towards the spin dependent transport in bulk elemental ferromagnets. Two adjacent metals creating the interface have different band structures. This will lead to a potential step at the interface, and results in the transmission probability across the interface being less than 1. If the interface separates the ferromagnetic and non-magnetic metals, the transmission will be spin-dependent due to the spin dependence of the band structure of the ferromagnetic layer. 2.3.5 Band Matching in Magnetic Multilayer Another important aspect to be considered in magnetic multilayer is band matching. Band matching plays important role in spin-dependent interface scattering due to the intermixing of atoms near the interfaces. By ignoring the change in the chemical state of the atoms that is by assuming that their energy levels and magnetic moment are identical to those in the bulk of the adjacent layers, the intermixing at the interfaces produce a random potential which is strongly spin-dependent. This spin dependency is a direct consequence of the good band matching for the spins in Co/Cu, which implies a small scattering potential and the poor band matching for the minority spins in Co/Cu, which implies a large scattering potential. A similar behavior takes place in Fe/Cr multilayer. Thus the matching or mismatching of the bands between the ferromagnetic and nonmagnetic metals results in spin-dependent potentials at disordered interfaces which contribute to GMR. 33 2.3.6 CASTEP CASTEP is a commercial and academic software package which uses Density Functional Theory with a plane wave basis set to calculate electronic properties of solids. CASTEP programme is a first principle quantum mechanic (ab initio) code for performing electronic structure calculations. Within the density functional formalism, it can be use to simulate a wide range of material including crystalline solids, surfaces, molecules, liquids and amorphous materials. Those are the properties of any material that can be thought of as an assembly of nuclei and electrons can be calculated with the only limitation being the finite speed and the memory of the computers being used. In Physics Department, Faculty of Science UTM, CASTEP was bought for use on parallel computers by researchers and was located at Computer Instrumentation Laboratory. CASTEP is a fully featured first principles code and its capabilities are quite numerous. The basic quantity is the total energy from which many other quantities are derived. For example the derivative of total energy with respect to atomic positions results in the forces and the derivative with respect to cell parameters give stresses. These are then use to perform full geometry optimizations and possibly finite temperature of molecular dynamics. Furthermore, symmetry and constraints (both internal and external to the cell) can be imposed in the calculations, either as defined by user, or automatically using in-built symmetry detection. 34 2.3.7 Density Functional Theory (DFT) Density Functional Theory (DFT) is a quantum mechanical theory used in physics and chemistry to investigate the electronic structure (principally the ground state) of many body systems, in particular atoms, molecules, and the condensed phases. With this theory, the properties of many-electrons system can be determined by using functionals, that is functions of another function, which in this case the spatially dependent electron density. DFT is among the most popular and versatile methods available in condensed-matter physics, computational physics and computational chemistry. 2.4 Fabrication of Ferromagnetic Multilayer Structures The first multilayer that demonstrated GMR effect was using molecular beam epitaxy (MBE) technique (Baibich et.al,1988). This method is considered expensive and time consuming. Less expensive method compared to MBE are direct current (DC) magnetron sputtering method (Dieny et.al, 1990; Bernabe et.al,19991; Peterson et.al, 2003); radio frequency (RF) sputtering method (Spizzo et.al, 2003) and ion beam sputtering method (Colis et.al, 2000). Other technique reported including thermal evaporation method (Marszalek et.al, 2004). Among various types of thin film deposition techniques, electron beam evaporation method was chosen in this study because of several factors. Electron beam evaporation method is chosen in this study because of its suitability for depositing high melting point ferromagnetic metals (Zeltser and Smith,1996). This method also have 35 been reported to produce uniform GMR (Kenane et al., 2006). Karcher and Kolesnikov, 2005, reported of less intermixed interfaces and smaller grain sizes of films deposited using electron beam technique compared to sputtered films. This technique allows the production of thin film coatings from pure elements, including most metals, as well as numerous alloys and compounds. Electron beam evaporation offers several advantages over competing processes including precise control of low or high deposition rates, excellent material utilization, co-deposition and sequential deposition systems and a uniform low temperature deposition (Anklam et al., 1995). Electron beam offers higher evaporation rates, freedom from contamination, precise rate controls at very low deposition levels, precise film composition and cooler substrate temperatures. The materials used for evaporation are available in near limitless shapes and form, the most common being pellets, slugs and disks. 2.5 Development of GMR research GMR is the most fascinating discovery in thin film magnetism. The discovery of GMR in 1988 lead to the invention of IBM’s record breaking 16.8 gigabyte of hard disk drive for desktop computers using their special GMR structure over a decade later. First GMR based Magnetic Random Access Memory (MRAM) consist of mixture of nickel, iron and cobalt provided a nine fold improvement in read-access time. By assessing the microstructure of GMR materials, this yield was enhanced. Recent research include developing new technologies for making layers that are uniform and can withstand high temperature to prolonged the durability of material layers. 36 Since the invention of the hard-disk drive in 1956, the technology of the magnetic head sensors has never ceased to evolve. Today’s sensors are drastically different from those used in those early heads; they can detect and transmit information from recorded data at densities greater than 200 Gbit/in2 and data rates approaching 1 GHz according to Childress and Fontana Jr., (2005). Numerous advances in nanomagnetics, magnetic ultrathin films, magneto-electronics as well as device processing, have fueled the remarkable progress of the GMR technology. Besides revealing new vision in data storage industry, GMR sensors are used in traffic and army. The sensors reduce the size and power needs of traffic monitoring system and may help save lives and money in weapons-detection system. Other than that, GMR smart shock absorbers are used in high performance mountain bike. Other applications are as diverse as solid state compass and detection of landmines. 37 2.6 Spintronics : GMR Future Technology Application In semiconductor devices, taking advantage of the charge of electrons is most essential. On the other hand, magnetic materials are exploited on the basis of electron spin. To develop new electronics, it is interesting to combine both features, the charges and the spin of electrons, because in this case, the spin of electrons that carries the information can be used as an added degree of freedom in novel electronic devices. Thus the development of functional ferromagnetic semiconductors is a key to the development of spintronics, which will certainly be the devices of the future (Hong, 2006). Ten years after the GMR discovery, all hard disk drives included GMR-based read heads. This has led to a significant improvement in the storage density and has demonstrated the potential of applications based on spintronics. Nonvolatile random access magnetic memories, including magnetic tunnel junctions, (MTJs) as the storage element could be another great application of spintronics. Actually, their low power consumption, their high scalability and their nonvolatility place them in a good position on the random access memory market with respect to semiconductor-based memories (Schuhl and Lacour, 2005). Current efforts in designing and manufacturing spintronic devices involve two different approaches. First, perfecting the existing GMR based technology by either developing new materials with larger spin polarization of electrons or making improvement or variations in the existing device that allow for better spin filtering. Second, finding novel way of both generation and utilization of spin polarized current. These include investigations of spin transport in semiconductors and looking for ways in which semiconductors can function as spin polarizers and spin valves. 38 The importance of these efforts lies in the fact that the existing metal-based devices do not amplify signals (although they successful switches or valves), whereas semiconductor-based spintronics devices could in principle provide amplification and serve as multi functional devices. Perhaps even more importantly, it would be much easier for semiconductor-based spintronics devices to be integrated with traditional semiconductor technology. CHAPTER 3 RESEARCH METHODOLOGY 3.1 Sample Fabrication Sample fabrication processes include the selection of substrate and source material, substrate pre-clean and preparations, deposition using electron beam method and post deposit annealing process. 3.1.1 Substrate and Source Material The corning glass slides (CORNING Micro Slides model no: 2947) were used as substrate. Cobalt (Co), Nickel (Ni), Copper (Cu) and Silver (Ag) granules manufactured by FLUKA with purity 4N (99.99%) were used as target materials. 40 3.1.2 Substrate Pre-clean and Preparation The existence of contaminants such as particles, metals, dust and watermarks on the surface of the substrate can cause poor adhesion between the film and the substrate and this will affect the structure of the samples grown. Therefore substrates needed to be cleaned with degreasing solvent such as chromic acid before samples deposition. Substrates were submerged in chromic acid solution and then immersed in an ultrasound bath for 30 minutes. Then the substrates were taken out from the bath and rinsed with distilled water for two minutes. The substrates were then blow-dried using a dryer. 3.1.3 Electron Beam Evaporation Technique and Principles Electron beam evaporation is a form of physical vapor deposition in which a target anode is bombarded with an electron beam given off by a charged tungsten filament under high vacuum condition. The electron beam causes atoms from the target to transform into gaseous phase. These atoms then precipitated into solid forms, coating the substrate in the vacuum chamber with a thin layer of the anode (target) material. Edwards Auto 306 Evaporation System (Auto 306), which is shown in Figure 3.1 is used to fabricate ferromagnetic multilayer structures. The system consists of a tungsten filament, a shuttle, graphite crucibles, a voltage and current sources, a rotary pump, a turbo pump, film thickness monitor (FTM) and a control panel. The deposition 41 chamber is evacuated to 10-6 Torr using both the rotary and the turbo pumps. First the rotary pump was used to pump down the pressure of the chamber to 10-2 Torr. Then the turbo pump assisted the rotary pump to pump down the pressure down to 10-6 Torr. The following steps were taken to fabricate ferromagnetic multilayer structures. i. The chamber was pumped down to 10-2 Torr using the rotary pump. ii. The vacuum chamber was opened and the corning glass substrate was placed on top of the graphite crucibles. The target source was then put into one of the crucible. The vacuum chamber was then closed. iii. The chamber was pumped down to 10-6 Torr by using the turbo pump. This procedure took 2 hours. iv. When the vacuum system reached the pressure of 10-6 Torr, degassing step was taken by supplying approximately 70% of current for a period of 15 minutes. This step was to ensure the residual gas that may exist in the chamber from the previous deposition process was being removed out. v. A 3 kV voltage was set. The current was then raised slowly until the voltage reading started to dropped. Usually in the case of cobalt, nickel, copper and silver deposition the current reading ranges from 1.5 to 2.0 mA. vi. The voltage source, current source and the height of the crucible were adjusted to obtain the suitable deposition rate that is 0.01 nm/s. The deposition rate reading was shown on the film thickness monitor (FTM). 42 vii. When the appropriate thickness reached, the shutter was then used to block the evaporation from reaching the substrate. The current and voltage source was then stopped. Figure 3.1 : Edwards Electron Beam Evaporation System Photograph. 3.1.4 Annealing Process Annealing is a heat treatment in which a sample is exposed to an elevated temperature for an extended time and then slowly cooled. Annealing heat treatment is largely characterized by induced structural changes which are ultimately responsible for altering the sample properties. In magnetoresistance studies, annealing process may alter the magnetic properties of the thin film which may affect the magnetoresistance value (Carbucicchio et al., 1999). 43 Table 3.1 below summarizes the annealed samples structures where the annealing temperatures range from 280°C to 380°C. Table 3.1 : List of annealed samples Sample structure sample label annealing annealing temperature time Co(6.5)/Cu(4.0)/Ni(5.5) T1(a) as deposited 2 hour Co(6.5)/Cu(4.0)/Ni(5.5) T1(b) 280°C 2 hour Co(6.5)/Cu(4.0)/Ni(5.5) T1(c) 290°C 2 hour Co(6.5)/Cu(4.0)/Ni(5.5) T1(d) 300°C 2 hour Co(6.5)/Cu(4.0)/Ni(5.5) T1(e) 310°C 2 hour Co(6.5)/Cu(4.0)/Ni(5.5) T1(f) 350°C 2 hour Co(6.5)/Cu(4.0)/Ni(5.5) T1(g) 380°C 2 hour Ni(7.5)/Cu(4.0)/Ni(7.5) T2(a) as deposited 2 hour Ni(7.5)/Cu(4.0)/Ni(7.5) T2(b) 280°C 2 hour Ni(7.5)/Cu(4.0)/Ni(7.5) T2(c) 290°C 2 hour Ni(7.5)/Cu(4.0)/Ni(7.5) T2(d) 300°C 2 hour Ni(7.5)/Cu(4.0)/Ni(7.5) T2(e) 310°C 2 hour Ni(7.5)/Cu(4.0)/Ni(7.5) T2(f) 350°C 2 hour Ni(7.5)/Cu(4.0)/Ni(7.5) T2(g) 380°C 2 hour Co(6.0)/Ag(5.0)/Co (6.0) T3(a) as deposited 2 hour Co(6.0)/Ag(5.0)/Co (6.0) T3(b) 280°C 2 hour Co(6.0)/Ag(5.0)/Co (6.0) T3(c) 290°C 2 hour Co(6.0)/Ag(5.0)/Co (6.0) T3(d) 300°C 2 hour Co(6.0)/Ag(5.0)/Co (6.0) T3(e) 310°C 2 hour Co(6.0)/Ag(5.0)/Co (6.0) T3(f) 350°C 2 hour Co(6.0)/Ag(5.0)/Co (6.0) T3(g) 380°C 2 hour 44 Ni(7.0)/Ag(4.0)/Ni(7.0) T4(a) as deposited 2 hour Ni(7.0)/Ag(4.0)/Ni(7.0) T4(b) 280°C 2 hour Ni(7.0)/Ag(4.0)/Ni(7.0) T4(c) 290°C 2 hour Ni(7.0)/Ag(4.0)/Ni(7.0) T4(d) 300°C 2 hour Ni(7.0)/Ag(4.0)/Ni(7.0) T4(e) 310°C 2 hour Ni(7.0)/Ag(4.0)/Ni(7.0) T4(f) 350°C 2 hour Ni(7.0)/Ag(4.0)/Ni(7.0) T4(g) 380°C 2 hour 3.1.5 Samples Features and Structures Tables 3.2 and 3.3 summarize the other sample properties and sample structures deposited using electron beam evaporation system. Table 3.2 : List of compositional varying samples (C-series) and non-magnetic thickness varying samples (N-series). Sample type Sample label Sample features/ structures C1 Co(6.5)/Cu(4.0)/Ni(5.5) C2 Ni(7.5)/Cu(4.0)/Ni(7..5) C3 Co(6.0)/Ag(5.0)/Co (6.0) C4 Ni(7.0)/Ag(4.0)/Ni(7.0) N1(a) Co(6)/Cu(1)/Co (6) thickness ranging from N1(b) Co(6)/Cu(3)/Co (6) 1 nm to 15 nm N1(c) Co(6)/Cu(5)/Co (6) N1(d) Co(6)/Cu(9)/Co (6) Compositional vary Non-magnetic layer 45 N1(e) Co(6)/Cu(13)/Co (6) N1(f) Co(6)/Cu(15)/Co (6) N2(a) Ni(7)/Ag(1)/Ni(7) N2(b) Ni(7)/Ag(3)/Ni(7) N2(c) Ni(7)/Ag(5)/Ni(7) N2(d) Ni(7)/Ag(9)/Ni(7) N2(e) Ni(7)/Ag(13)/Ni(7) N2(f) Ni(7)/Ag(15)/Ni(7) Table 3.3 : List of number of multilayer varying samples (R). Sample type Sample label Sample features/ structures Number of repetition of the R1 [Co(6)/Cu(3)/Co (6)]1 multilayer ranging from 1 R2 [Co(6)/Cu(3)/Co (6)]2 to 20 multilayer. R3 [Co(6)/Cu(3)/Co (6)]3 R4 [Co(6)/Cu(3)/Co (6)]4 R5 [Co(6)/Cu(3)/Co (6)]5 R6 [Co(6)/Cu(3)/Co (6)]6 R7 [Co(6)/Cu(3)/Co (6)]7 R8 [Co(6)/Cu(3)/Co (6)]8 R9 [Co(6)/Cu(3)/Co (6)]9 R10 [Co(6)/Cu(3)/Co (6)]10 R11 [Co(6)/Cu(3)/Co (6)]11 R12 [Co(6)/Cu(3)/Co (6)]12 R13 [Co(6)/Cu(3)/Co (6)]13 R14 [Co(6)/Cu(3)/Co (6)]14 R15 [Co(6)/Cu(3)/Co (6)]15 R16 [Co(6)/Cu(3)/Co (6)]16 R17 [Co(6)/Cu(3)/Co (6)]17 R18 [Co(6)/Cu(3)/Co (6)]18 46 3.2 R19 [Co(6)/Cu(3)/Co (6)]19 R20 [Co(6)/Cu(3)/Co (6)]20 Thickness Control and Measurement Thickness measurements in fabricating ferromagnetic multilayer structures in this study were done by using two complementary methods. During the deposition process, the thickness of the magnetic or non-magnetic layer was controlled using Film Thickness Monitor (FTM) which attached to the electron beam evaporation system. The same layer thickness was then measured using Surface Profiler to confirm the thickness measured by the FTM. 3.2.1 Film Thickness Monitor (FTM) The Film Thickness Monitor (FTM) is used to monitor the thickness of film deposited on the substrate. When the expected thickness is reached, the shutter is then used to block the evaporated materials from reaching the substrate. The Film Thickness Monitor (FTM) is a microprocessor based frequency counter capable of converting frequency changes into deposition rates and thickness information for a range of deposition materials. It is used in conjunction with a quartz crystal, which is placed in the deposition field whose output frequency is controlled by crystal. 47 Figure 3.2 : The FTM Display which connected to quartz crystal inside the e-beam chamber Quartz crystal is capable of measuring film thickness and deposition rates during the deposition process. It is based on the frequency change in the quartz crystal upon the mass of the material being deposited. Thickness d and frequency change ∆f can be express as : d where f .C a f 2 f (3.1) f is the resonans frequency (5 or 6 Mcs-1), ∆f is the frequency change in quartz crystal, C is constant (1670 Kcs-1), ρq is the density of quartz and ρf is the density of thin film. The negative sign is because frequency reduces from larger value to smaller value. When f = 5 Mcs-1, ∆f = 1 cs-1 which is equal to the increase of mass of the material being deposited, m = 1.8 x 10-8 gcm-2. ∆f can be calibrated to give the exact and correct d value. Linearity of d and ∆f is restricted with the mass of the material being 48 deposited, because m which can occupy a maximum value. After the maximum value, the quartz crystal needs to change with a new one for further measurement. 3.2.2 Surface Profiler Another method of measuring film thickness is by using surface profiler. To confirm the thickness measured by FTM, a single layer of cobalt, copper, silver and nickel layer was measured using Dektak 3 Surface Profiler and the result was as shown in Table 4.1. The surface profiler consists of a stylus with a 25 microns tip diameter. It probes the surface of the samples and measure step heights from as low as 1 nm to maximum 50 microns. Thickness measurements are made electromechanically by moving the sample beneath the diamond-tipped stylus, according to a user programmed scan length and speed. Prior to measurement, a step was created on the sample to provide surface variations on the sample surface. 3.3 Magnetoresistance Measurement Magnetoresistance measurement in this study was made using four point probe method. According to Chen et. al, (unpublished), in a sheet resistance measurement, several resistances need to be considered as shown in Figure 3.3 (a). The probe has a probe resistance Rp. It can be determined by shorting two probes and measuring their 49 resistances. At the interface between the probe tip and the film, there is a probe contact resistance, Rcp. When the current flows from the small tip into the semiconductor and spreads out in the semiconductor, there will be a spreading resistance, Rsp. Finally the film itself has a sheet resistance Rs. The equivalent circuit for the measurement of sheet resistance by using the fourpoint probe is shown in Figure 3.3 (c). Two probes carry the current and the other two probes sense the voltage. Each probe has a probe resistance Rp, a probe contact resistance, Rcp and a spreading resistance Rsp associated with it. However these parasitic resistances can be neglected for the two voltage probes because the voltage is measured with a high impedance voltmeter, which draws very little current. Thus the voltage drops across these parasitic resistances are insignificantly small. The voltage reading from the voltmeter is approximately equal to the voltage drop across the sheet resistance. Figure 3.3: Four point probe measurement of sheet resistance. 50 By using the four-point probe method, the sheet resistance can be calculated: Rs F V , I (3.2) where V is the voltage reading from the voltmeter, I is the current carried by the two current-carrying probes, and F is a correction factor. For collinear or in-line probes with equal probe spacing, the correction factor F can be written as a product of three separate correction factors: F= F1 F2 F3 (3.3) F1 corrects for finite sample thickness, F2 corrects for the finite lateral sample dimension, and F3 corrects for the placement of the probes with finite distances from the sample edges. For very thin samples with the probes being far from the sample edge, Chen et. al, (unpublished), estimated F2 and F3 are approximately equal to one, and the expression of the semiconductor sheet resistance becomes: Rs 3.3.1 V ln 2 I (3.4) Four Point Probe Sample The deposited multilayer was silver pasted with a copper wire at four point contact to make it a four point probe sample. The schematic diagram of four point probe sample is as shown in Figure 3.4. By using the four point probe resistance measurement method, the thin film resistance can be calculated with the common formula of resistance that is: 51 R V I (3.5) where V is the voltage reading from the voltmeter and I is the constant current supplied to the sample. The four point probe resistance measurement method can eliminate the effect by the probe resistance, probe contact resistance and spreading resistance (Schroeder,1998). non-magnetic layer voltmeter A B copper wire constant current source silver paste copper wire silver paste magnetic layer Figure 3.4 : Four point sample for magnetoresistance measurement. 52 3.3.2 Magnetoresistance Measurement Set-up The four point probe sample which was connected to a voltmeter and a constant current source (DPS 175) was then fitted into the magnetoresistance measurement setup. This set-up consists of two magnetic poles (EMU 75 magnetic field generator), a digital gauss meter (DGM-102) and a magnetic field indicator. These components were manufactured by Roorkee Scientific Equipment. The schematic diagram that represents the magnetoresistance measurement set-up is as shown in Figure 3.5. The direction of the applied magnetic field was changed by changing the current direction using a switch. ≈ 5cm magnetic pole A B magnetic pole sample digital gauss meter magnetic field indicator Figure 3.5 : Magnetoresistance measurement set-up. Dotted arrows, indicating the magnetic field direction, whether from A to B or from B to A. 53 The magnetoresistance (MR) is calculated using the formula below: MR │ R Rmin │ x 100% Rmin (3.6) where R is the resistance at certain applied magnetic field strength and Rmin is the resistance at where saturation magnetization takes place, at 2000 Gauss. 3.4 Structural Analysis of Ferromagnetic Thin Films Structural studies of ferromagnetic thin film were done by X-Ray Diffraction analysis (XRD), Atomic Force Microscope analysis (AFM), and Field Emission Scanning Electron Microscope analysis (FESEM). 3.4.1 X-Ray Diffractometer Analysis Diffraction refers to various phenomena associated with the bending of waves when they interact with obstacles in their path. It occurs with any type of wave including X-rays. When X-rays hit an atom, they make the electronic cloud move. The movement of these charges re-radiates waves with the same frequency which is known as Rayleigh scattering. The scattered waves can themselves be scattered but this secondary scattering is assumed to be negligible. A similar process occurs upon 54 scattering neutron waves from the nuclei or by a coherent spin interaction with an unpaired electron. These re-emitted wave fields interfere with each other either constructively or destructively producing a different pattern on a detector film. The resulting wave interference pattern is the basis of diffraction analysis. X-rays wavelengths are comparable with inter-atomic distances and thus are an excellent probe for this length scale. 3.4.2 Energy Dispersive X-Ray Analysis The scanning electron microscope (SEM) is a type of electron microscope that creates various images by focusing a high energy beam of electrons onto the surface of a sample and detecting signals from the interaction of the incident electrons with the sample’s surface. In a typical SEM electrons are thermoionically emitted from a tungsten or lanthanum hexaboride (LaB6) cathode and are accelerated towards an anode. Alternatively, electrons can be emitted via field emission (FE). Field emission is the emission from the surface of a condensed phase into another phase due to the presence of high electric fields. In this phenomenon, electrons with energies below the Fermi level tunnel through the potential barrier at the surface, with the high electric field sufficiently narrows for the electrons to have a non-negligible tunneling probability. 55 3.4.3 Atomic Force Microscope Analysis The Atomic Force Microscopy enables not only to image the surface in atomic resolution but also to measure the force at nano-newton scale. In Atomic Force Microscope (AFM), an atomically sharp tip is scanned over a surface with feedback mechanisms that enable the piezo-electric scanners to maintain the tip at a constant force (to obtain height information), or height (to obtain force information) above the sample surface. Tips are typically made from Si3 N 4 or Si , and extended down from the end of a cantilever. AFM head employs an optical detection system in which the tip is attached to the underside of a reflective cantilever. A diode laser is focused onto the back of a reflective cantilever. As the tip scans the surface of the sample, moving up and down with the contour of the surface, the laser beam is deflected off the attached cantilever into a dual element photodiode. The photodetector measures the difference in light intensities between the upper and lower photodetectors, and then converts to voltage. Feedback from the photodiode difference signal, through software control from the computer, enables the tip to maintain either a constant force or constant height above the sample. In the constant force mode the piezo-electric transducer monitors real time height deviation. In the constant height mode the deflection force on the sample is recorded. 56 3.5 Band Structure Analysis Band structures of ferromagnetic and nonmagnetic metal were obtained by simple modeling using CASTEP modeling software. CASTEP incorporates the Density Functional Theory (DFT) in its calculations. CASTEP exploits DFT mechanical code to simulate the properties of solids, interfaces and surfaces for a wide range of material classes including ceramics, semiconductors and metals. In this study CASTEP were used to simulate the electronic band structure and density of states for metals involved that are Copper (Cu), Nickel (Ni), Cobalt (Co) and Argentum (Ag). CHAPTER 4 RESULTS AND DISCUSSION 4.1 Introduction This chapter describes the experimental results to the magnetoresistance of the samples which were fabricated using electron beam evaporation technique. The effect of several fabrication parameters such as composition, annealing temperature and thickness on magnetoresistance was studied. The parameters are material composition, temperature treatment and sample thickness. The structural characterization of the samples using characterization method discussed in chapter three are also presented. 58 4.2 Thickness of Films The thicknesses of the deposited film were measured using Film Thickness Monitor (FTM) during the deposition process. The thickness is then reconfirmed by using DEKTAK 3 Surface Profiler. Table 4.1 below shows the thickness measurement obtained by Film Thickness Monitor and using Surface Profiler. Table 4.1 : Difference in thickness measurement observed by Film Thickness Monitor and Surface Profiler. Thickness measured using Surface Profiler (nm) 15.2 Deviation R1, [Co(6)/Cu(3)/Co (6)]1 Thickness observed using FTM (nm) 15 R2, [Co(6)/Cu(3)/Co (6)]2 30 30.5 1.7% R3, [Co(6)/Cu(3)/Co (6)]3 45 45.7 1.6% R4, [Co(6)/Cu(3)/Co (6)]4 60 60.8 1.3% R5, [Co(6)/Cu(3)/Co (6)]5 75 76.2 1.6% R6, [Co(6)/Cu(3)/Co (6)]6 90 91.2 1.3% R7, [Co(6)/Cu(3)/Co (6)]7 105 106.6 1.5% R8, [Co(6)/Cu(3)/Co (6)]8 120 121.9 1.6% Sample structure 1.3% Comparing the thicknesses from FTM measurements and Dektak 3 Surface Profiler measurements for eight samples (R1-R8), gives the average variance of 1.5%. This results show that the thickness being measured by FTM during the deposition process only slightly differs from counter measurement made using Surface Profiler. This confirms that the thickness measurements are precise and acceptable. 59 4.3 Compositional Dependence Magnetoresistance The magnetoresistance as has been discussed in Chapter 3, is ascribed to a considerable drop in the percentage of resistance of a sandwiched magnetic/nonmagnetic layers when a sufficiently high magnetic field is applied to the sample (Sakrani et al., 2006). Figure 4.1 (a), (b), (c) and (d) below show the magnetoresistance curve of Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni respectively. The curves show the changes in the resistance of the samples in the vicinity of applied magnetic field. The resistance change is described as; when there is no magnetic field applied, magnetization of the ferromagnetic layers ( Co-Ni, Ni-Ni, and Co-Co) are anti-parallel with each other. Based on spin-dependent scattering principle, in this condition, the resistivity is high thus the magnetoresistance is also high. When the applied magnetic field is increased, it will align the magnetization of the ferromagnetic layers gradually until they become completely parallel with each other. In this condition, the resistivity is low and as a result, the magnetoresistance is low. 10 9 Magnetoresistance (%) 8 7 6 5 4 3 2 1 0 -2500 -2000 -1500 -1000 -500 -1 0 500 1000 1500 2000 2500 Magnetic Field, Gauss Figure 4.1 (a) : The external magnetic field versus the magnetoresistance response for Co(6.5)/Cu(4.0)/Ni(5.5) multilayers. 60 6 Magnetoresistance (%) 5 4 3 2 1 0 -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500 Magnetic Field, Gauss Figure 4.1 (b) : The external magnetic field versus the magnetoresistance response for Ni(7.5)/Cu(4.0)/Ni(7..5) multilayers. 2.5 Magnetoresistance (%) 2 1.5 1 0.5 0 -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500 Magnetic Field, Gauss Figure 4.1 (c) : The external magnetic field versus the magnetoresistance response for Co(6.0)/Ag(5.0)/Co (6.0) multilayers. 61 1.6 Magnetoresistance (%) 1.4 1.2 1 0.8 0.6 0.4 0.2 0 -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500 Magnetic Field, Gauss Figure 4.1 (d) : The external magnetic field versus the magnetoresistance response for Ni(7.0)/Ag(4.0)/Ni(7.0) multilayers. From Figure 4.1, it was found that as deposited Co/Cu/Ni multilayer shows magnetoresistance value of 8.77%, as deposited Ni/Cu/Ni multilayer shows magnetoresistance value of 5.28%, as deposited Co/Ag/Co shows magnetoresistance value of 2.27% and as deposited Ni/Ag/Ni show magnetoresistance value of 1.48% respectively. All the measurements were made at room temperature with magnetic field of -2000 to 2000 gauss applied in plane with the sample. Since GMR was discovered in 1988, a large number of magnetic multilayer structures which display GMR effect were discovered. The highest published values of magnetoresistance to date are 220% in Fe/Cr multilayers (Schad et al., 1995) and 120% in Co/Cu multilayers (Parkin et.al, 1991). In comparison with the results obtained in Figure 4.1 (a), 4.1 (b), 4.1 (c) and 4.1 (d), fairly large values of GMR were obtained which 62 are 22% of Co/Ag at room temperature (Araki, 1993) 28% of Ni/Ag at 4.2K (Rodmacq et al., 1993) and 9% of Ni/Cu at 4.2K (Sato, et al., 1994) Ferromagnetic 3d metals have a pronounced spin asymmetry in their conductivity due to the presence of exchange split d bands and therefore the magnetoresistance should be high. But however, many findings reported that some multilayers are highly magnetoresistive whereas the others are not, although they contain ferromagnetic 3d metals. It looks like that spin asymmetry in the band structure is a necessary but not sufficient condition for high GMR values. Parkin, 1994, after tested some 30,000 different multilayer combinations of different element and dimensions at IBM proposed a hypothesis that GMR to a great extent is determined by ferromagnetic metal/non-magnetic metal pair, rather by an individual material considered separately. To support this explanation GMR was found to be much lower in Co/Cr and Fe/Cr. A 3% GMR in Co/Cr has been reported (Parkin et. al, 1990), and 5.5% GMR in Fe/Cu (Monchesky et.al., 1999) as compared to Fe/Cr and Co/Cu multilayers. 4.3.1 Band Structure Factor There are two factors which are crucial for obtaining high values of GMR in terms of composition dependence. These are the band matching and lattice matching between the ferromagnetic and nonmagnetic metals (Tsymbal and Pettifor, 2001). Rao and Freeman, 1998, and Gebele et al., 2000, explained that a good band matching for one spin orientation between a ferromagnetic metal (FM) and a nonmagnetic (NM) 63 metal implies high transmission for this spin across the FM/NM interface. On the other hand, a large band mismatch for the other spin orientation implies that the transmission is poor. In Co/Cu/Ni multilayer the band structure of Co/Cu and Cu/Ni collaborate to a good GMR value compare to Ni/Cu in Ni/Cu/Ni multilayer, Co/Ag in Co/Ag/Co multilayer and Ni/Ag in Ni/Ag/Ni multilayer. Figure 4.2 : Electronic band structures (left panels) and density of states (right panels) of a) Co and b) Cu. From Figure 4.2, the band structure for non-magnetic Cu shows fully occupied d bands and the presence of a dispersive sp band at the Fermi energy, which results in high conductivity of Cu. The band structure of ferromagnetic Co shows the exchange-split d bands and the Fermi energy lies within the sp bands which also lead to high spin conductivity. Both spin conductivity contribution from Co and Cu are believed to be responsible for high GMR value in Co/Cu/Ni multilayers as shown in Figure 4.1 (a). 64 Figure 4.3 : Electronic band structures (left panels) and density of states (right panels) of a) Cu and b) Ni. Figure 4.3 shows that Ni band structures are quite the same as Cu. Fermi energy for both Cu and Ni lies within the sp bands. Due to the high velocity of electrons within the sp bands and the low density of states with resultant low probability of scattering, the mean free path is long and thus making Cu and Ni a good conductor. Spin conduction contribution from Cu and Ni, results in high GMR value as in Figure 4.1 (b). 65 Figure 4.4 : Electronic band structures (left panels) and density of states (right panels) of a) Co and b) Ag. The electronic and atomic structure for Ag is similar to Cu. Nevertheless, Ag is not as good band matching for 3d ferromagnets compared to Cu (Tsymbal and Pettifor, 2001). Although Ag is a very good conductor, which is characterized by the Fermi energy that lies within the sp bands as depicted in Figure 4.4 (b) , due to a relatively poor band matching with Co, provide Co/Ag multilayer with a relatively poor spin transmission across the Co/Ag interface. This resulted to a relatively low GMR value for Co/Ag multilayer, as shown in Figure 4.1 (c) when compared to Co/Cu and Ni/Cu (Figure 4.1 (a)). 66 Figure 4.5 : Electronic band structure (left panels) and density of states (right panels) of a) Ni and b)Ag. Ni is strong ferromagnet with entirely filled majority-spin d bands. However poor band matching with Ag leads to poor spin transmission across the Ni/Ag interface. The band mismatch for Ni and Ag is shown in Figure 4.5. Comparing the density of states for both Ni and Ag, the occupied energy level for sp bands shows a significant different, which resulting in low GMR value in Ni/Ag multilayers. Figure 4.1 (d) shows a lowest GMR value for Ni/Ag/Ni multilayer compared to Co/Cu/Ni, Ni/Cu/Ni and Co/Ag/Co as depicted in Figure 4.1 (a), (b), (c) respectively. 67 4.3.2 Lattice Matching Factor Another factor that influence the GMR value is lattice matching. According to Tsymbal and Pettifor, 2001, lattice matching of the subsequent layers is very important factor for GMR as lattice mismatched leads to the formation of misfit dislocation and other structural defects at the interfaces. Scattering by this defect in the nonmagnetic spacer layer is spin-independent, resulting in a reduction of GMR. Although the scattering by defects in a ferromagnet layer could be spin dependent, the spin asymmetry in the scattering potentials will vary depending on structural details. The presence of various types of defects will make the average of the scattering potential only weaklydependent on the spin, which can lead to reduce values of GMR. Table 4.2 below summarizes the lattice parameter a, for Co, Cu, Ni , Ag, Fe and Cr which explain how lattice matching and lattice mismatch contributes to GMR value in Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co, Ni/Ag/Ni, Co/Cu/Co and Fe/Cr/Fe. Table 4.2 : Lattice parameter for ferromagnetic metals (Co, Ni, Fe, Cr) and nonmagnetic metal (Cu,Ag). Thin film Lattice structure material parameter, a (Å) Co 3.54 Face centered cubic, fcc Cu 3.61 Face centered cubic, fcc Ni 3.52 Face centered cubic, fcc Ag 4.09 Face centered cubic, fcc Fe 2.87 Face centered cubic, fcc Cr 2.88 Face centered cubic, fcc 68 Thin films of Co, Cu, Ni and Ag grow in fcc structure with lattice parameter 3.54Å, 3.61Å and 3.52Å and 4.09Å respectively. In Co/Cu/Ni multilayer, the Co/Cu lattice mismatch is 1.98% and the Cu/Ni lattice mismatch is 2.56% and give the average lattice mismatch is 2.27%. The GMR value for Co/Cu/Ni is 8.77% as shown in Figure 4.1 (a). In Ni/Cu/Ni multilayer the lattice mismatch is 2.56% and the GMR value of 5.28%, as shown in Figure 4.1(b). With the same principle applies to Co/Ag/Co and Ni/Ag/Ni, lattice mismatch are 15.54% and 16.20% thus gives the GMR value of 2.27% and 1.48% as the results are shown in Figure 4.1 (c) and 4.1 (d) respectively. From the overall results obtain in Figure 4.1, it appears that small lattice mismatch gives higher magnetoresistance value. In order to make better comparison for each multilayer, the associated interface, lattice mismatch, and GMR are summarized in Table 4.3. 69 Table 4.3 : The average lattice mismatch and the GMR for Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni multilayer. Multilayer Interface Co/Cu/Ni Co/Cu Lattice Mismatch 3.54 3.61 3.54 Cu/Ni 3.61 3.52 3.52 Ni/Cu/Ni Ni/Cu 3.52 3.61 3.52 Cu/Ni 3.61 3.52 3.521 Co/Ag/Co Co/Ag 3.54 4.09 3.54 Ag/Co 4.09 3.54 3.54 Ni/Ag/Ni Ni/Ag 3.52 4.09 3.52 Ag/Ni 100 1.98% Average Lattice Mismatch GMR 2.27% 8.77% 2.56% 5.28% 15.54% 2.27% 16.20% 1.48% 100 2.56% 100 2.56% 100 2.56% 100 15.54% 100 15.54% 100 16.20% 4.09 3.52 100 16.20% 3.52 4.4 Annealing Temperature Dependence Magnetoresistance Thermal treatment or annealing is a common procedure for producing structural changes. Structural changes would change the magnetic properties of the multilayers. Chen, 2003, indicated that the optimum time for annealing Co/Cu multilayer is about 2 hour. By assuming Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni have almost similar 70 features like Co/Cu, the annealing time chosen was 2 hours. All the samples had undergone annealing temperature between 200°C to 350°C. Magnetoresistance measurement was then carried out and the results were shown in Figure 4.6 (a), 4.6 (b), 4.6 (c) and 4.6 (d). 16 as deposited 14 200 deg Magnetoresistance (%) 12 250 deg 10 280 deg 8 290 deg 6 310 deg 350 deg 4 2 0 -2500 -2000 -1500 -1000 -500 -2 0 500 1000 1500 2000 2500 Magnetic Field, Gauss Figure 4.6 (a) : Magnetoresistance curve for Co(6.5nm)/Cu(4.0nm)/Ni(5.5nm), annealed at different temperatures for 2 hours. As deposited Co(6.5nm)/Cu(4.0nm)/Ni(5.5nm) multilayer shows GMR of 8.77% (as dotted line in Figure 4.6 (a)). Annealing at 200°C, 250°C and 280°C increased the GMR to 10.68%, 11.36% and 14.25% respectively. Further annealing at 290°C, 310°C and 350°C however, lower the GMR to 12.75%, 6.37% and 3.05% respectively. The blocking temperature for this system which was indicated by the highest GMR percentage curve is approximately 280°C. 71 12 as deposited Magnetoresistance (%) 10 230 deg 260 deg 8 290 deg 300 deg 6 310 deg 4 330 deg 2 0 -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500 -2 Magnetic Field, Gauss Figure 4.6 (b) : Magnetoresistance curve for Ni(7.5nm)/Cu(4.0nm)/Ni(7.5nm), annealed at different temperatures for 2 hours. Figure 4.6 (b) shows that for Ni(7.5nm)/Cu(4.0nm)/Ni(7.5nm) multilayer, the as deposited GMR was 5.86%. Annealing temperature of 230°C, 260°C and 290°C increased the GMR to 7.97%, 8.98% and 10.68% respectively. Further annealing at elevated temperatures of 300°C, 310°C and 330°C cause the decreased of the GMR to 6.25%, 4.10% and 1.93 respectively. It seems that the degradation of GMR starts at around 290°C. 72 8 as deposited Magnetoresistance (%) 7 230 deg 6 260 deg 5 290 deg 4 310 deg 3 320 deg 350 deg 2 1 0 -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500 Magnetic Field, Gauss Figure 4.6 (c) : Magnetoresistance curve for Co(6.0nm)/Ag(5.0nm)/Co (6.0nm), annealed at different temperatures for 2 hours. Figure 4.6 (c) shows the as deposited Co(6.0nm)/Ag(5.0nm)/Co (6.0nm) was 2.63%. Then the GMR were enhanced to 3.74%, 4.37%, 5.87% and 7.45% when the multilayer was annealed at 230°C, 260°C, 290°C and 310°C respectively. Higher temperature of 320°C and 350°C reduced the GMR to 6.98% and 1.83% respectively. The blocking temperature for Co(6.0nm)/Ag(5.0nm)/Co (6.0nm) was observed at around 310°C. 73 5 4.5 as deposited Magnetoresistance (%) 4 200 deg 3.5 230 deg 3 260 deg 2.5 280 deg 2 290 deg 1.5 320 deg 1 0.5 -2500 -2000 -1500 -1000 0 -500 -0.5 0 500 1000 1500 2000 2500 Magnetic Field, Gauss Figure 4.6 (d) : Magnetoresistance curve for Ni(7.0nm)/Ag(4.0nm)/Ni(7.0nm), annealed at different temperatures for 2 hours. The as deposited GMR for Ni(7.0nm)/Ag(4.0nm)/Ni(7.0nm) shows a value of 1.48% as shown in figure 4.6 (d). Annealing at 200°C, 230°C, 260°C and 280°C increased the GMR to 1.88%, 2.13%, 3.97% and 4.38% respectively. Further annealing at 290°C and 320°C decreased the GMR to 3.21% and 2.76% respectively. The blocking temperature for this structure occurs at around 280°C. Generally, for Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni multilayers, the as deposited GMR was enhanced by annealing up to a temperature called the blocking temperature where beyond this temperature limit, the GMR were dramatically decreased. The increasing GMR in Co/Cu/Ni multilayer were caused by the diffusion of Cu along 74 the Co and Ni grain boundaries (May et al.,1998 and Wilhelm et al., 1999). Kuch et al. (1999) observed diffusion behaviour in Ni/Cu/Co trilayers under photoelectron emission spectroscopy. In the same agreement, Anna et al. (1999) also suggested the same diffusion behaviour to Co/Ag/Co multilayer, while Loloee et al., (1995), observed the diffusion behaviour to Ag/Co and Cu/Co interfaces. Therefore, the same argument is associated with the increasing GMR when annealing the Ni/Cu/Ni and Ni/Ag/Ni . As diffusion of Cu into Ni matrix, Ag into Co matrix and Ag into Ni matrix were strongly believed to increase the GMR in multilayers mentioned earlier, the term ‘diffusion’ need to be clarified. In magnetic multilayer, diffusion refers to a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is gradual mixing of material. Siritaratiwat et al., (2000), explained that the interlayer (non-magnetic) diffusion into ferromagnetic grain boundaries or matrix creating intra-layer magnetic discontinuities in ferromagnetic layers which elevate interlayer antiferromagnetic coupling and consequently, causes the increment of GMR. The degradation of GMR when annealing temperatures exceed the blocking temperature is associated with the intermixing of nonmagnetic atoms (Cu and Ag) with the ferromagnetic atoms (Co and Ni). Hecker et al., (2003), suggested this intermixing resulted in the decrease of magnetic homogeneity for ferromagnetic/non-magnetic interfaces. If diffusion is the key factor of enhancing GMR value, intermixing is the key factor for the degradation of GMR value for temperature treated ferromagnetic multilayer structures. 75 According to Dinia et al., (2002), intermixing refers to the mixing of two magnetic and non-magnetic atoms from two neighboring boundaries after diffusion takes place. The intermixing resulted in a new type of combined magnetic and nonmagnetic atoms. Hecker et al. (2002) & Kim et al. (2002) suggested the temperature range between diffusion and intermixing to take place is called the ‘thermal stability’ for a certain multilayers. Intermixing of atoms promotes significant decay of the transport properties and magnetic properties of the multilayers. In other words, the alloying tendency of ferromagnetic and nonmagnetic atoms which occurs above the blocking temperature cause the decay of GMR and the changes in the magnetic properties of the multilayers investigated. Belozrov et al., (2003), explained that the increasing magnetoresistance is due to a small atomic diffusion at the origin of smothered interfaces. In Co/Cu, Cu/Ni, Co/Ag and Ni/Ag interfaces the recrystallization process of Co, Cu, Ni and Ag during the early stage (0°C -300°C) of annealing increased the GMR for each samples. However, when the annealing temperature exceed to 350°C, GMR for each samples dropped to around 20%. This is because at higher temperature, Co, Cu, Ni and Ag species are weakly soluble to each other. While the solubility increases with the increasing annealing temperature, however, at temperature of about 350°C and above, the solubility become weaker. As the annealing temperature is increased, Co and Ni atoms are gradually precipitated from the Cu and Ag matrix and form Co and Ni clusters (Yu et al., 1995). Their bodies and interfaces with Cu and Ag matrix become the conduction electron spindependent scattering centers. Very small Co and Ni particles or atoms may loose their magnetic moment due to interaction with the nonmagnetic Cu or Ag matrix, which 76 adversely affects the GMR. On the other hand, when the annealing temperature is increased to a large value i.e >350°C, Co and Ni particle become large, thus directly cause the appearance of a ferromagnetic interaction between larger Co and Ni particles or in other words, antiparallel alignment for better GMR does not occur in portions of the samples. It was interesting to note that the systems which exhibit high values of GMR such as Fe/Cr, Co/Cu, Co/Ag are all immiscible (Gijs & Bauer, 1997). This fact indicates that intermixing at the interfaces is not favorable to GMR. One of the reasons for this might be a reduction in the magnetic moments in the intermixed region which negatively affects GMR. In addition, intermixing may result in misaligned spins, which are weakly coupled with the ferromagnetic layer, or magnetically ‘dead’ layers. Figure 4.7 shows the magnetoresistance effect on annealing temperature for multilayers discussed previously. It appears that beyond the blocking temperature limits the sudden and significant decreases of magnetoresistance occurs to all the studied multilayers. 77 Figure 4.7 : Annealing temperature versus magnetoresistance for Co(6.5nm)/Cu(4.0nm)/Ni(5.5nm), Ni(7.5nm)/Cu(4.0nm)/Ni(7.5nm), Co(6.0nm)/Ag(5.0nm)/Co (6.0nm) and Ni(7.0nm)/Ag(4.0nm)/Ni(7.0nm). 4.5 Thickness Dependence Magnetoresistance When considering the dependence of GMR on thickness of the multilayer, there are three thickness parameters need to be taken into account. Firstly, the magnetic layer thickness dependence, secondly, the non-magnetic layer thickness dependence and thirdly, the number of the repetition of the multilayer. Results in this section focus only on the thickness effect on GMR exhibit by Co/Cu/Co multilayer because the high value of GMR reported by other researchers (Parkin et al., 1994; Marrows et al., 1994; Vavassori et al., 2003; and Turilli et al., 1999) and the simplicity of producing it rather than Co/Cu/Ni multilayer. 78 This section would not further discussing the effect of magnetic layer thickness towards the GMR of Co/Cu/Co multilayer. This is because of the previous work by Chen, 2005, reported that the optimum thickness of Co magnetic layer is 6 nm. This was supported by work done by Vavassori, 2003, and Turilli, 1999. The results obtained in Section 4.5.1 and 4.5.2 used the assumption that the optimum magnetic layer thickness dependence to produce a good GMR value was 6 nm. 4.5.1 Non-magnetic Layer Thickness Dependence A number of Co(6nm)/Cu(x nm)/Co(6nm) multilayers with x = 1 to 15 nm were prepared using electron beam evaporation method, same as the method used in the previous section. As discussed in Section 4.4 which shows better GMR value obtained upon annealing, all of the samples were annealed at 270°C for 2 hours. The GMR versus Cu thicknesses for each sample is then plotted as in Figure 4.8. 79 Figure 4.8 : The magnetoresistance effect on Cu thicknesses for Co/Cu/Co multilayers. Figure 4.8 shows the value of GMR decreases monotonically with the increasing of the non-magnetic (Cu) layer thicknesses. According to Dieny et al., (1991), this decrease can be qualitatively associated with two contributing factors that are scattering probability and the shunting current effect. Shunting current refers to current that pass around another direction in the multilayers. In other words, it is defined as unwanted short circuit between non-magnetic layer and magnetic layer and this is caused by Cu layer itself. According to Childress and Fontana Jr. (2005), the non-magnetic Cu layer become a major source of shunting current effect unless it is kept as thin as possible because of its high conductivity. However, as the non-magnetic layer is made thinner, coupling between the two magnetic layers must be well controlled for proper magnetoresistance operation. Thus the interfaces between the non-magnetic and 80 magnetic layers, as well as the non-magnetic layer itself need to carefully engineered to minimize this problem. With the increasing Cu thicknesses, the probability of scattering increases as the conduction electron traverses in the Cu layer, which reduces the flow of electrons between the ferromagnetic layers and consequently reduces the GMR value. The increasing thickness of the non-magnetic layer enhances the shunting current within the non-magnetic layer, which also reduces the GMR. Dieny et al. (1994) suggested these two contributions to GMR can be described by the following expression: R R R R o d NM exp l NM , d 1 NM d0 (4.1) where dNM is the non-magnetic layer thickness, lNM is related to the mean free path of the conduction electron in the spacer layer, do is the effective thickness which depends on the conductance of the system in the absence of the non-magnetic layer and (∆R/R)o is the normalization coefficient. The exponential factor represents the probability that the electrons is not scattered within the non-magnetic layer. The factor in the denominator describes the shunting effect due to the non-magnetic layer. The actual result obtained from the experiment, which is Figure 4.8 was fitted with the result based on Equation 4.1, 81 as shown in Figure 4.9. Both the actual results (dotted line) and the expected result (straight line) shows an exponential behavior, thus it proved that the result is in good agreement with work done by Dieny et al. (1994). 20 Magnetoresistance (%) 18 16 14 12 Experiment 10 Equation 4.1 8 6 4 2 0 0 2 4 6 8 10 12 14 16 Cu thicknesses (nm) Figure 4.9: Cu thicknesses versus magnetoresistance of Co/Cu/Co multilayers in experiment fitted with the expected result based on Equation 4.1. 4.5.2 Number of Multilayer Dependence A number of [Co(6nm)/Cu(3nm)]n were prepared with n ranging from 1to 20 multilayer(s). After being annealed at 270°C for two hours, the GMR for each sample was measured and the result is as shown in Figure 4.10. 82 Figure 4.10 : Number of multilayers repetition versus magnetoresistance for Co/Cu/Co structures. From Figure 4.10, it shows increasing trend of GMR as the number of multilayer repetition increases from 1 to 20. Generally, the improvement in the structural quality for the thicker multilayers is believed to contribute to the increasing GMR. This is clearly shown in Figure 4.11 where X-Ray diffraction (XRD) pattern for [Co/Cu]n multilayers for n = 2, 5, 16 and 20 were compared. 83 Figure 4.11: Two theta XRD patterns for [Co/Cu]n , with n = 2,5,16 and 20. All samples were annealed at 270°C for two hours. When the number of the multilayer is increased, the Co (111), Cu (200) and Cu (220) peaks were found to be narrower. This means that the increase of n leads to the increase of grain size (X-H, Xu et. al. 2004). The grain size was estimated by the Scherrer formula from the Co (111) peak width of XRD patterns. The grain size increases from 7.71 to 16.56 nm as n increases from 2 to 20 as shown in Figure 4.11. Thus, the increase of n cause the grain size (Co grain) to be larger where larger grain size leads to the better structural properties of Co/Cu multilayer which contributes to higher GMR value. Apart from the improvement of structural properties of Co/Cu multilayer, the major factor which is responsible for the behavior of GMR versus number of multilayers is the presence of diffuse scattering at the outer boundaries of the multilayer. The outer boundary scattering is a very important characteristic that increasing GMR for Co/Cu/Co 84 multilayers because their thickness is comparable to the mean free path of the conduction electrons in the conduction channel (Plaskett and McGuire,1993). However, when the multilayer is thick enough (n reaches 100) the GMR seem to saturate and then decrease. Plaskett and McGuire found at this thickness and above, diffuse outer boundary scattering reduces the conductivity of the good conduction channel and hence negatively effect the GMR. When the number of n increased, the number of interface is consequently increased. Since interface scattering is more significant than bulk scattering (Dong, 1999) it is responsible for the total resistivity of Co/Cu multilayer. The interface scattering at each Co/Cu interface sum up to the total resistivity of samples which according to Smadar and Nathan, (2001), increases with the increasing number of Co/Cu bilayers. When the resistivity of a particular multilayer is high, this resulted to a higher GMR value because resistivity is directly proportional to GMR value as expressed in GMR formula. 4.6 Spectroscopy and Surface Morphology Analysis This section presents the spectroscopy analyses that were done using XRD and EDX method while surface morphology analysis were done using AFM method. These analyses were made to compliment the previous results in which only comparing the magnetoresistance value towards composition, temperature and thickness parameters. 85 4.6.1 X-Ray Diffractometer/ Energy Dispersive X-Ray Analysis Samples of highest GMR value in temperature dependence analysis were scanned using XRD technique. After XRD measurement, the samples were undergone EDX analysis. The samples involved are as listed in Table 4.4 below: Table 4.4 : Sample description for XRD scans and followed by EDX scans. Sample no. Sample structure Annealing Annealing temperature time T1(b) Co(6.5)/Cu(4.0)/Ni(5.5) 280°C 2 hour T2(c) Ni(7.5)/Cu(4.0)/Ni(7..5) 290°C 2 hour T3(e) Co(6.0)/Ag(5.0)/Co (6.0) 310°C 2 hour T4(b) Ni(7.0)/Ag(4.0)/Ni(7.0) 280°C 2 hour The XRD technique is the primary tool for investigating the structure of the crystalline material while EDX is a technique use for identifying the elemental composition of the specimen. Figure 4.12, 4.13, 4.14 and 4.15 show the XRD pattern and the EDX spectrum for sample T1 (b), T2 (c), T3 (e) and T4 (b) respectively. 86 Figure 4.12 : (a) The two-theta XRD pattern , (b) The EDX spectrum for Co(6.5)/Cu(4.0)/Ni(5.5). From the X-ray diffraction (XRD) analysis of Co(6.5)/Cu(4.0)/Ni(5.5), the XRD pattern as in Figure 4.12 (a) shows significant peaks of Co (111) at 44.22°, Cu (200) at 50.43° and Ni (220) at 51.85°. Those peaks indicates that the Co(6.5)/Cu(4.0)/Ni(5.5) 87 multilayer was of a crystalline structure. As Copper is one of the materials that highly prone to oxidization, the doubt of CuO being formed in the multilayer disappeared as the pattern shows no CuO peak which usually appear at 38.72°. The EDX spectrum for Co(6.5)/Cu(4.0)/Ni(5.5) as in Figure 4.12 (b) shows Co, Ni and Cu peaks at 6.92 keV, 7.48 keV and 8.08 keV respectively. The percentage of Co, Ni and Cu are 4.43%, 2.77% and 3.72% respectively. The other elements found are Mg, Al, Na came from the substrate. 88 Figure 4.13 : (a) The two-theta XRD pattern, (b) The EDX spectrum for Ni(7.5)/Cu(4.0)/Ni(7.5) The XRD pattern of Ni(7.5)/Cu(4.0)/Ni(7..5) shows peaks of Ni (111) at 44.5°, Cu (200) at 50.43° and Ni (220) at 76.37° as can be observed in Figure 4.13 (a). No copper oxide peak was detected. The multilayer shows a crystalline structure. 89 The EDX spectrum for Ni(7.5)/Cu(4.0)/Ni(7..5) as in Figure 4.13 (b) shows Cu peak observed at 7.48 keV and Ni peak at 8.08 keV. Strong peak of silicon and oxygen observed at 1.71 keV and 0.41 keV come from the corning glass which Silicon Oxide is the major component, together with Na, Mg, Al, K and Ca. Figure 4.14 (a) shows the XRD pattern for Co(7.0)/Ag(4.0)/Co (7.0) . Peaks of Co (111) and Ag (220) appear at 44.2° and 64.43° respectively. The sample show a crystalline structure and no oxidized layer were formed because neither Cobalt Oxide nor Argentum Oxide observed in the pattern. From the EDX spectrum for Co(7.0)/Ag(4.0)/Co (7.0) as depicted in Figure 4.14 (b), Ag peak observed at 2.98 keV and Co peak observed at 6.92 keV. The percentage of Ag was 2.71% and Co was 9.47%. A relatively higher percentage of Co observed compared to Ag because the total thickness of Co in the sample was 14 nm while Ag thickness was only 4 nm. 90 Figure 4.14 : (a) The two-theta XRD pattern, (b) The EDX spectrum for Co(7.0)/Ag(4.0)/Co (7.0) 91 Figure 4.15 : (a) The Two-theta XRD pattern, (b) The EDX spectrum for Ni(6.0)/Ag(5.0)/Ni (6.0) The EDX spectrum for Ni(6.0)/Ag(5.0)/Ni (6.0) as in figure 4.15 (b) shows Ni peaks observed at 0.93 keV and 7.48 keV while Ag peak observed at 2.98 keV. percentage of Ni and Ag are 9.73% and 3.57% respectively. The 92 Generally, samples that exhibit GMR shows quite a crystalline structure. This is indicated by significantly low and broad peaks on the two-theta XRD pattern in Figure 4.12 (a), 4,13 (a), 4.14 (a) and 4.15 (a). These patterns of spectra are common for thin films. When the number of the multilayer increased, the total thickness of the thin film also increased and the GMR should be higher according to the resistor model. In this condition, the thin film properties would eventually change into bulk material properties and the peaks should be eventually higher and narrower. As a supporting evidence to this statement, in Figure 4.11, the two-theta XRD pattern for [Co(6nm)/Cu(3nm)]20 shows higher and narrower Cu and Co peaks compared to the two-theta XRD pattern for [Co(6nm)/Cu(3nm)]2. Thus, sample with higher number of multilayer shows better crystalinity and higher value of GMR. The EDX spectra as demonstrated in Figure 4.12 (b), 4.13 (b), 4.14 (b) and 4.15 (b), show relatively small percentage of Co, Cu, Ni and Ag compared to Silicon and Oxygen. This is due to the electron beam’s deep penetration properties. Electron penetration distance into corning glass substrate, which is greater than the multilayer thickness give strong signals or peaks of corning glass materials which are Silicon Oxide, Na, Mg, Al, K and Ca. If the number of the multilayer increased, according to resistor model, the GMR should increase. The increasing number of multilayer would increase the Co, Cu, Ni and Ag percentage in the spectra. 93 4.7 Surface Morphology Analysis The surface morphology analysis was done using Atomic Force Microscope (AFM). The purpose of the analysis was to study the surface morphology of ferromagnetic material and nonmagnetic material on increasing annealing temperature. Cobalt (Co) represented as ferromagnetic material while Copper (Cu) represented the non magnetic material in surface morphology studies of ferromagnetic multilayer structure. Since AFM probe onto the sample surface, the analysis was carried out by probing only on single layer of Co and Cu. The roughness of the film in terms of root mean square (RMS), indicating the mean of the root for deviation of a surface with respect to a perfect surface. The RMS roughness, RMS, RMS , is defined as: N RMS (Z i 1 i Z ave ) 2 N (4.1) where Z i is the height value of each point, Z ave is the average surface height and N is the number of points on the indicated surface (Sans et. al., 2004). The mean diameter of grain was obtained by assuming that the grain takes a circular shape. Figure 4.16 shows the three dimensional AFM topographic images of Co thin film. It appears that the surface roughness of Co thin film increase from the as deposited film stage as the annealing temperature increased up to 350°C. The surface roughness was 0.568 nm at as deposited stage. The value increases to 1.973 nm, 2.151 nm, 2.525 nm and 4.317 nm when the Co films were annealed to 200°C, 250°C, 280°C and 350°C respectively. The mean grain diameter at the as deposited stage was 5 nm. The mean 94 grain diameter increases to 29 nm, 34 nm, 37 nm, and 42 nm as the annealing temperature increased to 200°C, 250°C, 280°C and 350°C respectively. The average grain diameter and the surface roughness were depicted in Figure 4.17. As multilayers containing Co as ferromagnetic layer show high GMR value when the annealing temperature exceeded 280°C, as shown in Figure 4.6 (a) and Figure 4.6 (c), this suggested that the optimum surface roughness and the optimum grain diameter for systems containing Co were 2.225 nm and 37 nm respectively. However, it has been shown by Joyce et al. (1998) that RMS roughness is not a satisfactory indication of the interfacial structure. Joyce suggested that the interfacial structure must be examined more closely by taking diffuse reflectivity into account. In a different method, Rozenberg et al. (1999) in the X-Ray investigation of sputtered Co/Cu specimens found that a simple geometrical factor, namely, the difference in mean distance between the magnetrons and the surface of the samples, leads to noticeable changes in their structural parameters that are bilayer period, individual thickness of Co and Cu layers, and thickness fluctuations. The increase interfacial roughness may induce local contacts of neighboring Co layers, which decrease drastically the absolute value of magnetoresistance. 95 a) c) b) d) e) Figure 4.16 : AFM topography images of as deposited Co film (a), and Co films annealed at 200°C (b), 250°C (c), 280°C (d) and 350°C (e). 96 Figure 4.17: RMS roughness and average grain diameter of Co thin films as a function of annealing temperature. Figure 4.17 shows the surface roughness and the average grain diameter of Co thin films when the annealing temperature increases. The surface roughness increased with increasing annealing temperature. However, referring to previous results in Figure 4.6 (a) and (c), when the GMR dropped from the maximum values which occur at around 280°C-310°C there is no significant relation between the surface roughness and the degradation of GMR. Figure 4.18 shows the three dimensional AFM topographic images of Cu thin film. The surface roughness of Cu thin film increases from the as deposited stage as the annealing temperature increase up to 350°C. Generally the increasing roughness and grain diameter follow the same trend as Co thin films. The surface roughness was 0.676 nm at as deposited stage. The value increases to 2.132 nm, 2.435 nm, 2.861 nm and 97 4.617 nm when the Cu films were annealed to 200°C, 250°C, 280°C and 350°C respectively, depicted as in Figure 4.19. The mean grain diameter at the as deposited stage was 6 nm. The mean diameter increases to 27nm, 36 nm, 44 nm, and 40 nm as the annealing temperature increases to 200°C, 250°C, 280°C and 350°C respectively. Figure 4.18 shows the AFM topography images of Cu films and Figure 4.19 shows the mean grain diameter and surface roughness for Cu thin films. These results are associated with multilayers containing Cu as the nonmagnetic layer which are Co/Cu/Ni and Ni/Cu/Ni. Co/Cu/Ni multilayers show high GMR value when the annealing temperature reached 280°C while Ni/Cu/Ni multilayers show highest GMR value at 290°C. This suggests for multilayers containing Cu as non-magnetic layer, approximately 280°C to 290°C were the optimum annealing temperature. From Figure 4.19, it suggested the optimum surface roughness and average grain diameter for Co/Cu/Ni and Ni/Cu/Ni multilayer are 3.04nm and 44nm respectively. As the surface roughness and grain diameter increases with increasing annealing temperature, however, there is no significant relation between the degradation of GMR at temperature above 290°C with surface roughness and grain diameter. e) a) a) 98 a) b) c) d) e) Figure 4.18 : AFM topography images of as deposited Cu film (a), and Cu films annealed at 200°C (b), 250°C (c), 280°C (d) and 350°C (e). 99 Figure 4.19: RMS roughness and average grain diameter of Cu thin films as a function of annealing temperature. CHAPTER 5 SUMMARY AND CONCLUSION 5.1 Summary and Conclusion Ferromagnetic multilayer structures were successfully fabricated using the electron beam evaporation method. The magnetoresistance of the multilayer structures with different ferromagnetic and nonmagnetic composition, different layer thicknesses and different annealing temperatures were also successfully being carried out. The compositional study of ferromagnetic multilayer structures reveals that band matching and lattice matching are two important factors need to be taken into account prior to match a ferromagnetic material with a non-magnetic conducting material. Further insight into material band structure and density of states are important to determine whether or not the establishment of a good band matching between a ferromagnetic material and non-magnetic material. Generally a good band matching 101 between ferromagnetic and non-magnetic metal lead to high spin transmission across the ferromagnetic/nonmagnetic interface and a large band mismatch between ferromagnetic and non-magnetic metal implies a poor spin transmission. A good band matching between two materials (in this case between ferromagnetic material and non-magnetic material interface) is determined when this two materials have almost the same electronic band structure and density of states properties. Lattice matching between two adjacent ferromagnetic and non-magnetic materials plays important role in obtaining high GMR value. A good lattice matching favors high spin transmission at the interface while a poor lattice matching (lattice mismatch) leads to the structural defects due to interface misfit dislocation. The formations of structural defects at the interface causes a poor spin transmission and thus lower the GMR value. The key parameter need to be taken into account for the ferromagnetic and non-magnetic materials is the lattice parameter, a. A perfect lattice matching is when the lattice parameter of two adjacent ferromagnetic and non-magnetic layers is almost or the same value. Temperature dependence study proves that heat treatment or annealing to ferromagnetic multilayer structures provides better GMR value if compared to as deposit multilayers. The GMR increases with increasing annealing temperature until the temperature reaches a point called the ‘blocking temperature’. When the annealing temperature exceeds the blocking temperature, the GMR shows a dramatic decrease. Multilayers involved in these studies which are Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni reached their blocking temperature at 280°C, 290°C, 310°C and 280°C respectively. Diffusion and intermixing at the ferromagnetic/non-magnetic interface are two main reasons of the dramatic degradation of GMR. 102 Thickness dependence study in ferromagnetic multilayer structures is divided into two thickness parameters which are non-magnetic layer thickness dependence and number of multilayer dependence. In non-magnetic layer thickness dependence study, the results obtain shows GMR decreases in an exponential behavior as non-magnetic layer thickness increases. The decreases in GMR were associated with the scattering probability and the shunting current effects. When the number of multilayer was increased, the GMR value was also increased. In stacking multilayers, according to the resistor model, the GMR are higher than in single multilayers because stacking the multilayers is equivalent to adding up more resistors in series to a circuit, thus give higher resistance. In addition, the improvements in structural quality in thicker multilayers contributed in increasing GMR value. Apart from structural improvement and the resistor model, another important factor that contributes to enhance the GMR was the presence of diffuse scattering at the outer boundaries of the multilayer. Confirmation analyses were made using EDX, XRD and AFM technique. The EDX study for Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni multilayer proves the presence of Co, Cu, Ni and Ag elements in the EDX spectra. The XRD analyses also show the presence of Co, Cu, Ni and Ag peaks in the XRD spectra. The surface morphology analysis using AFM shows the surface roughness and grain diameter of Co and Cu increased as the annealing temperature increased. In conclusion, the objectives of this study were successfully achieved. It was shown that ferromagnetic multilayer structures fabricated using electron beam evaporation method exhibit considerably high GMR values after taking into 103 consideration the composition effects, the annealing temperature effects and the thickness effects prior to measuring the GMR. 5.2 Suggestions This thesis has initiated the effects of composition, annealing temperature, nonmagnetic layer thickness as well as the overall multilayer thickness on magnetoresistance of ferromagnetic multilayer structures namely Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni. Magnetic materials involved are Co and Ni while Cu and Ag are the non-magnetic materials. However, a more comprehensive study remains to be carried out. As a suggestion for future studies, instead of using single ferromagnetic material like Co and Ni as the magnetic layer, using alloy such as permalloy (Ni80Fe20) as magnetic layer could be carried out. A promising increment in annealed permalloy multilayers had been carried out by Kitada Masahiro and Yamamoto Kazuhiro, (1995). Permalloy has a high magnetic permeability, low coercivity, near zero magnetostriction and significant anisotropic magnetoresistance. The low magnetostriction is critical for industrial applications, where variable stresses in thin films would otherwise cause a ruinously large variation in magnetic properties. 104 GMR measurement in this study involved the application of external magnetic field of maximum 2000 gauss only. For further investigation, a stronger magnetic field in the order of Tesla could be applied to the sample. Large magnitude of external magnetic field probably could enhance the reversible or irreversible magnetization of magnetic layers and would probably enhance the magnetoresistance or otherwise. Since the magnitude of GMR is related to the asymmetry in the scattering rates within the two conduction channels (within magnetic/non-magnetic interfaces), it was expected that modifying the spin-dependent scattering by introducing appropriate impurities either at the interfaces or in the bulk of the ferromagnetic layers would enhance GMR. Further investigation on introducing appropriate impurities into the multilayers should be carried out in order to study the impurity dependence towards GMR behavior. Significant interactive effect between impurities and the GMR of CoFe/Cu multilayers were revealed by Masakiyo Tsunoda et.al, (2002). As the thickness of non-magnetic layers (non-magnetic with high conductivity metals, referring to Cu and Ag in this research) play important role in obtaining high GMR value, careful and precise control to produce a thin layer with minimum roughness need to introduced. For future work, controlling the non-magnetic layer properties can be done through oxygen plasma exposure on the top of the reference layer and/or lowpressure oxygen exposure during the growth of the non-magnetic layer. Peterson et.al, (2003), demonstrated a better non-magnetic layer quality with oxygen assisted growth in magnetic multilayers. 105 It was interesting to observe ‘ageing magnetoresistance’ in iron-bismuth thin films as reported by Hsu, (2005). According to Hsu, due to precipitation process, the magnetoresistance characteristics change with time. Ordinary magnetoresistance was observed initially but the magnetoresistance evolves into a negative and isotropic magnetoresistance, like GMR. 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PANanalytica X’pert PRO Materials Research X-ray Diffractometer 3. Seiko SPI 3800N Atomic Force Micrope