Gagan Sharma 1 , Ajay Gupta 2 , Parasmani Rajput 3 , Andrew Wildes 4 , Valeria Lauter 5
1 UGC-DAE Consortium for Scientific Research, Indore-452001, India,
2 Center for Spintronic Materials, Amity University, Sector 125, Noida , India
3 Atomic & Molecular Physics Division, Bhaba Atomic Reserach Centre, Mumbai-85, India
4
ILL, BP 156, 38042 Grenoble Cedex, France,
5
ORNL, Chestnut Ridge Bldg 8920, Oak Ridge, TN 37830, USA
Email: gags.phy@gmail.com
Abstract: Interface of ferromagnetic thin films (CoFeB) on MgO (001) has been studied using two complementary techniques i.e. Polarized Neutron Reflectivity (PNR) and X-ray reflectivity (XRR). For this purpose ultrathin CoFeB films of different thicknesses were deposited on treated MgO (001). Using the thicknesses as obtained from XRR, PNR data has been fitted. The magnetic moment as obtained from preliminary fitting was observed to be increases linearly with increasing thicknesses. By extrapolating the linear behavior, a 2.4 nm thick magnetic dead layer was observed.
1. INTRODUCTION
Magnetic tunnel junction (MTJ), consists of two ferromagnetic electrodes sandwiching an insulating barrier between them. In MgO based MTJ’s, the experimentally achieved values of TMR are more than an order of magnitude less than theoretically predicted one. One of the factor, which significantly reduces the tunneling properties of magnetic tunnel junction is the presence of an imperfect interface between ferromagnetic thin film and barrier layer [1]. linearly with increasing thicknesses By extrapolating the linear behavior we deduced the thickness of magnetic dead layer around 2.4 nm. The data will be further analyzed in order to extract information about
B diffusion in MgO.
Sample
S1
S2
Thickness
(nm)
Au CoFeB
2.6±0.1
8.3±0.1
2.6
12.3
Roughness
(nm)
σ
Au/CoFeB
σ
CoFeB/MgO
0.5±0.05
0.3±0.05
0.5 0.3
For example, formation of a thin magnetic dead layer
(i.e. FeO) at the interface of M/MgO (M = Fe,
CoFeB) is detrimental for TMR applications.
Ambiguity exists in the literature about the presence of magnetic dead layer at M/MgO interface [2-5]. The
S3 2.4
15.2
(a)
S3
0.7 0.4
R+ Experimental
R- Experimental
R+ Fitted
R- Fitted
S3
0.1
0.2
0.3
objective of the present work is to deduce the magnetic dead layer (if any) at CoFeB/MgO interface. S2
0.1
0.2
0.3
S2
0.4
2. EXPERIMENTAL
The as-received polished MgO(001) substrates were given cleaning treatments in order to improve the surface quality. Deposition of Co
23
Fe
65
B
17 films of three different thicknesses were carried out at room temperature using ion beam sputtering in a UHV chamber with a base pressure of 2
×
10
−
7 mbar. X-ray reflectivity measurements were done using Bruker D8
S1
0.1
0.2
0.3
0.4
q (A
-1
) 0.4
0.8
1.2
1.6
-1 q (nm )
Figure: Fitted a)XRR b) PNR data for CoFeB films of different thickness
(a)
22
20
18
S1
Advance diffractometer equipped with a multilayer mirror on the incident beam side to give a parallel monochromatic beam of Cu K α radiation. Polarized
Neutron reflectivity measurements were performed at
Oak Ridge National Laboratory (ORNL), USA. The measurements were done in presence of high magnetic field of 1T. MOKE measurements were done in longitudinal geometry in presence of magnetic field of
16
14
12
10
MDL ~ 2.4 nm
8
6
4
2
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
(b)
1500 Oe.
3. RESULTS
From XRR one can get thickness and interface roughness for each layer with 1 Å resolution. The parameters as obtained from XRR fitting were used to fit PNR data. The magnetic moment as obtained from preliminary fitting was observed to be increases
Thickness (nm)
REFERENCES
[1] W. H. Butler et al., Phys. Rev. B (2001) 63 054416
[2] Y. Y. Huang et al., Phys. Rev. B (1993) 47 183
[3] Laloë et al., Appl. Phys. Letters
93 (2008) 012505
[4] C. Tusche, et al., Phys. Rev. B 74 (2006) 195422
[5] Sven Döring et al., Phys. Rev. B
83 (2011) 165444