APPLIED PHYSICS LETTERS 88, 242505 共2006兲
Magnetic/nonmagnetic/magnetic tunnel junction based on hybrid organic
Langmuir-Blodgett-films
T. X. Wang, H. X. Wei, Z. M. Zeng, and X. F. Hana兲
State Key Laboratory of Magnetism, Beijing National Laboratory for Condensed Matter Physics,
Institute of Physics, Chinese Academy of Science, Beijing 100080, China
Z. M. Hong and G. Q. Shi
Department of Chemistry, Tsinghua University, Beijing 100084, China
共Received 17 March 2006; accepted 28 April 2006; published online 14 June 2006兲
The magnetic/organic/magnetic spin valve structure has been fabricated with ␲-conjugated
molecular pyrrole derivative 3-hexadecyl pyrrole as the spacer layer by Langmuir-Blodgett 共LB兲
technique. It shows giant magnetoresistance 共MR兲 as large as 20% at room temperature,
demonstrating that low-energy electrons can traverse the molecular barrier while remaining spin
polarized. The spin transport across organic structures is diffusive from its current-voltage 共I-V兲
curve under bias voltage up to 1 V. The MR value decreased when the LB-film layer increased. The
telegraph noise and the layer dependent MR value suggest that the spin-polarized transport signals
can be degraded by localized states in the molecular barriers and barrier quality. © 2006 American
Institute of Physics. 关DOI: 10.1063/1.2213177兴
Organic conductors and semiconductors are promising
electronic materials due to their lightweight, easy application
over larger areas and on a variety of substrates, and the potential for low cost fabrication. Various applications of these
materials include organic light emission diode 共OLED兲, transistors, solar cells, sensors, etc.1,2 The electronic properties
of these materials can be modified easily using chemical tuning of their structure. The materials can be processed using
self-assembly, monolayer deposition, etc., to achieve devices
in nanodimensions.3 Recent reports have demonstrated that
spin valves can be fabricated.4–6
Thin organic films of a thickness of a few nanometers
共a monolayer兲 are the source of high expectations for
being useful components in many practical and commercial
applications such as sensors, detectors, displays, and
electronic circuit components.7–9 The Langmuir-Blodgett
共LB兲-technique is one of the most promising techniques for
preparing such thin films as it enables 共i兲 the precise control
of the monolayer thickness, 共ii兲 homogeneous deposition of
the monolayer over large areas, and 共iii兲 the possibility to
make multilayer structures with varying layer composition.
An additional advantage of the LB technique is that monolayers can be deposited on almost any kind of solid
substrate.10–12
Pyrrole is one of the most used monomers for preparation of electroconducting polymeric materials. Indeed it offers several advantages such as the easy accessibility, the
polymerizability, both by chemical or electrochemical oxidation, the good specific conductivity, and chemical stability.13
Moreover, its variety of derivatives are building blocks of
microstructured polypyrrole 共PPy兲 for fabricating microdevices including reactors, actuators, and sensors.14–16 Here,
the insulating ␲-conjugated molecular pyrrole derivative
3-hexadecyl pyrrole 共3HDP兲 关Fig. 1共a兲 shows its molecular
structure兴 will be used as the spacer layer. By sputtering, LB
technique, ultraviolet 共UV兲 lithography, etc., we will sandwich the organic film between two ferromagnetic layers
a兲
Electronic mail: xfhan@aphy.iphy.ac.cn
forming a spin valve structure junction to obtain superior
electronic transport.
The spin valve structure we used is Ta共5 nm兲 /
Cu共20 nm兲 / Py共5 nm兲 / IrMn共10 nm兲 / Co60Fe20B20共4 nm兲/
organic spacer layer/Co60Fe20B20共4 nm兲/Py共20 nm兲/
Cu共20 nm兲 / Ta共5 nm兲. It has fixed bottom ferromagnetic
electrode and top free electrode. The metalic multilayer is
deposited with three-chamber high vacuum magnetron
sputtering system on the Si/ SiO2 substrate 共base vacuum
better than 10−7 Pa兲. First, the bottom electrode is deposited. Then, the samples are taken out from the chamber
with vacuum break for the cover of spacer layer by LB
technique. Figure 1共b兲 is the high magnification scanning
electronic microscope 共SEM兲 image of one-layer 3HDP
film extended by LB technique on ferromagnetic
Co60Fe20B20 film. It shows uniform surface on nanometer
scale with cluster structure at the air-molecular interface.
After immediately drying the film by dry N2, the samples
were transferred to the main chamber again for the top
electrode deposition. After that, the films are patterned to
junctions of different sizes by UV optical lithography, ion
etching, etc., for measurement.
The magnetoresistance of the fabricated devices was
measured by sending a current through the two electrodes by
four-probe method. Figure 2 showed the typical magnetore-
FIG. 1. 共a兲 The molecular structure of 3-hexadecyl pyrrole. 共b兲 The high
magnification SEM image of one-layer molecular LB film on Co60Fe20B20
film. It shows uniform surface on nanometer scale with cluster structure at
the air-molecular interface.
0003-6951/2006/88共24兲/242505/3/$23.00
88, 242505-1
© 2006 American Institute of Physics
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242505-2
Wang et al.
Appl. Phys. Lett. 88, 242505 共2006兲
FIG. 3. The hysteresis loop of multilayer films with different LB-film spacer
thicknesses by layers at room temperature 关共쎲兲 for one layer, 共〫兲 for two
layers, and 共䊊兲 for three layers兴.
FIG. 2. The magnetoresistance of organic tunnel junction with different
spacer thicknesses and junction sizes. 共a兲 one-layer spacer with junction size
of 5 ⫻ 10 ␮m2, 共b兲 two-layer spacer with junction size of 100⫻ 100 ␮m2 by
metal mask, and 共c兲 three-layer spacer with junction size of 15⫻ 30 ␮m2.
sistance 共MR兲 loop obtained at room temperature marked by
共a兲 for one-layer spacer junction with size of 5 ⫻ 10 ␮m2 and
共c兲 for three-layer spacer junction of 15⫻ 30 ␮m2 in size.
They showed obvious spin valve switching behavior with
outside magnetic field changing. But the resistance area
product 共RA兲 and the MR values are not uniform and the
curves showed degradation possibly due to the not ideal
spacer layer. Small coecivity force less than 10 Oe can be
observed during the resistance change for one-layer and
three-layer LB-film spacers, similar to magnetic tunnel junctions 共MTJs兲 of similar stucture with Al2O3 barrier we fabricated and the work of Kauffer.17 Namely, the resistance
changing corresponds to the switching of the free ferromagnetic layer. There is a consistent spin transportation in the
molecular layer.
The junction with Al2O3 barrier we fabricated has barrier thickness of about 1.2 nm; about 35% MR value is obtained under the similar bias voltage of about 1 mV and MR
values of 60% at room temperature, 80% at low temperature
observed after annealing.18 So the polarity 共P兲 of the
Co60Fe20B20 we used can reach 0.53 from Julliere’s
formulas.19 The largest MR value we have measured is 20%
in one-layer spacer junction under the similar bias voltage of
about 1 mV 关Fig. 2共a兲兴. This is 4 / 7 of that of as deposited
Al2O3 berrier junction, from which we conclude that the
electron spin is capable of maintaining a high degree of polarization during the process passing though the spacer layers. Other junctions with smaller MR have nice switching
behavior and better curve shape with more or less telegraph
noise. For three-layer junctions, only the highest MR value
of 6% is observed and still with degradation of curves. Figure 2共c兲 gave a curve with smaller MR value and smaller
degradation. It shows obvious time-dependent telegraph
noise signals suggesting the existence of localized states in
the barriers as that in self-assembled molecular 共SAM兲
layer.20 The origin of these states could be formed in the
physical process of molecular and top-layer deposition, by
chemical reactions between the monolayer and the metals,21
or by stress.22
For two-layer samples, no working junctions were obtained by optical lithography method. Only about 0.2% MR
value is detected at room temperature from a 100
⫻ 100 ␮m2 junction by metal mask method 关Fig. 2共b兲兴. And
it shows large coecivity. We guess that there must be a bad
spacer layer with too many defects in it. Only the anisotropy
magnetoresistance 共AMR兲 signal is detected. For identification, the M-H hysteresis loops are measured with vibrating
sample magnetometer 共VSM兲 at room temperature 共Fig. 3兲.
The samples with one-layer and three-layer spacers showed
good character in free layer and pinned layer though there is
little difference on magnetic moment coming from the intermixing layer at the interface of organic and free layer formed
by sputtering ferromagnetic particles into the gap of molecular clusters. The two-layer spacer sample shows rather large
coecivity value just like single layer high coecivity magnetic
material. It means that the top and bottom ferromagnetic layers coupled together strongly through the bad quality evenlayer LB film which is formed under the solution and easily
destroyed when the sample is taken out of the solution.
The character and quality of the LB-technique spacer
layer are further characterized by measurement of the I-V
curve for special size junction of 5 ⫻ 10 ␮m2. For one-layer
samples, linear behavior is observed under a rather big voltage to 1 V though they are slightly different in resistance
共Fig. 4兲. Slightly tunneling behavior is observed in junctions
with three-layer spacer 共inset of Fig. 4兲. So the diffusive
transport behavior should dominate for spin transportation in
such single layer organic material instead of tunneling which
can be due to the potential barrier at the interfaces for
multilayer film spacer. With the layer increasing, the junction
resistance increased, and the observed MR value decreased.
It is possibly from the scattering of the spin by polarons and
bipolarons13,23 in the organic molecular layer and also at the
interface between the layers for three-layers spacer.
In conclusion, the magnetic/organic/magnetic spin valve
structure has been fabricated with small ␲-conjugated molecular pyrrole derivative 3-hexadecyl pyrrole as the spacer
layer by LB technique. The junctions exhibit MR value up to
20% at room temperature. This demonstrates that spin-flip
scattering and spin-orbit processes can be weak enough in
molecular layers that these structures may prove useful in
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242505-3
Appl. Phys. Lett. 88, 242505 共2006兲
Wang et al.
Chinese National Natural Science Foundation 共10274103
and 50271081兲.
G. G. Malliaras and R. H. Friend, Phys. Today 58, 53 共2005兲.
D. K. Schwartz, J. Garnas, R. Viswanathan, and J. A. N. Zasadzinski,
Science 257, 508 共1992兲.
3
S. R. Forrest, Nature 共London兲 428, 911 共2004兲.
4
Z. H. Xiong, D. Wu, Z. V. Vardeny, and J. Shi, Nature 共London兲 427, 821
共2004兲.
5
M. Reufer, M. J. Walter, P. G. Lagoudakis, A. B. Hummel, J. S. Kolb, H.
G. Roskos, U. Scherf, and J. M. Lupton, Nat. Mater. 4, 340 共2005兲.
6
A. R. Rocha, V. M. García-Suárez, S. W. Bailey, C. J. Lambert, J. Ferrer,
and S. Sanvito, Nat. Mater. 4, 335 共2005兲.
7
Langmuir-Blodgett Films, edited by G. Roberts 共Plenum, New York,
1990兲.
8
J. D. Swalen, D. L. Allara, J. D. Andrade, E. A. Chandross, S. Garoff, J.
Israelachvilli, T. J. McCarthy, R. F. Pease, J. F. Rabolt, K. J. Wynne, and
H. Yu, Langmuir 3, 932 共1987兲.
9
M. Breton and J. Macromol, J. Macromol. Sci. Rev. Macromol. Chem.
C21, 61 共1981兲.
10
G. L. Gaines, Jr., Insoluble Monolayers at Liquid-Gas Interfaces 共WileyInterscience, New York, 1966兲.
11
A. Ulman, An Introduction to Ultrathin Organic Films 共Academic, New
York, 1991兲.
12
V. M. Kaganer, H. Möhwald, and P. Dutta, Rev. Mod. Phys. 71, 779
共1999兲.
13
G. Ruggeri, M. Bianchi, G. Puncioni, and F. Ciardelli, Pure Appl. Chem.
69, 143 共1997兲.
14
R. V. Parthasarathy and C. R. Martin, Nature 共London兲 369, 298 共1994兲.
15
E. W. H. Jager, E. Smela, and O. Inganäs, Science 290, 1540 共2000兲.
16
E. W. H. Jager, O. Inganäs, and I. Lundström, Science 288, 2335 共2000兲.
17
A. Käufler, Y. Luo, K. Samwer, G. Gieres, M. Vieth, and J. Wecker, J.
Appl. Phys. 91, 1701 共2002兲.
18
F. F. Li, X. F. Han, L. X. Jiang, J. Zhao, L. Wang, and Rehana Sharif, J.
Mater. Sci. Technol. 21, 289 共2005兲.
19
M. Julliere, Phys. Lett. 54A, 225 共1975兲.
20
J. R. Petta, S. K. Slater, and D. C. Ralph, Phys. Rev. Lett. 93, 136601
共2004兲.
21
A. V. Walker, T. B. Tighe, J. Stapleton, B. C. Haynie, S. Upilli, D. L.
Allara, and N. Winograd, Appl. Phys. Lett. 84, 4008 共2004兲.
22
K.-A. Son, H. I. kim, and J. E. Houston, Phys. Rev. Lett. 86, 5357 共2001兲.
23
R. R. Chance, D. S. Boudreaux, J. L. Bredas, and R. Silbey, in Handbook
of Conducting Polymers, edited by T. A. Skothein 共Marcel Dekker, New
York, 1986兲, Vol. 2, p. 825.
24
E. Y. Tsymbal, A. Sokolov, I. F. Sabirianov, and B. Doudin, Phys. Rev.
Lett. 90, 186602 共2003兲.
1
2
FIG. 4. The I-V curves of junctions with special size of 5 ⫻ 10 ␮m2 but
different spacer thicknesses of one-layer and three-layer LB-film 共inset兲.
They are slightly different in resistance and it is linear for one-layer spacer
junction, tunneling for three-layer spacer junction.
applications involving electron-spin manipulation. However,
we also find a strong layer dependence of the MR value. It
means that the spin polarization decreases by scattering from
the polarons, bipolarons, and the interface between the organic layers when the current diffused into the organic spacer
layer. The presence of telegraph noise suggests the existence
of localized states within the not ideal organic spacer layer.24
An improved procedures for extending the thin organic film
on magnetic surface and depositing molecular top contacts
are likely increase the MR. A better understanding of the spin
transport behavior in small molecular organic films is still
needed.
The project was supported by the State Key Project of
Fundamental Research of Ministry of Science and Technology Grant Nos. 2001CB613500 and 2002CB613500, China.
One of the authors 共X.F.H.兲 gratefully thanks the partial support of Distinct Researcher Foundation 共50325104兲 and
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