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EXAFS Studies of Local Structure in MTJ
MRSEC (Johns Hopkins University) DMR05-20491
Background: CoFeB-MgO magnetic tunnel junctions (MTJs) are leading spintronic devices that relies on quantum
Mechanical tunneling of electrons from one magnetic metal (CoFeB) to another across an insulating barrier of MgO.
The electrical resistance of such MTJs is dictated by the orientation of the two CoFeB metals, parallel or antiparallel.
The performance of MTJ is measured by tunneling magnetoresistance (TMR), which is only20% initially, but greatly
increases to 200% after high temperature annealing (Fig. 1). We reveal the evolution of the local structure during annealing.
This work: Working with researchers at Brookhaven National Lab, we used extended x-ray absorption fine structure
(EXAFS) at special energies (K-edge of Co and Fe) to study subtle structural change within the MTJ after annealing
from 20 sec to 36 min. EXAFS, a special x-ray spectroscopy using intense synchrotron radiation, measures the local
atomic arrangements of Co and Fe atoms with the so-called first shell Debye-Waller factor 2 , which reveals how tightly
bound the atoms are within the materials. We found, as shown in Fig. 2, that as annealing commences the value
of 2 dramatically decreases with a concomitant increase of the change of the resistance (TMR).
250
0.014
200
36 minutes
0.013
200
0.012
150

2
 (Å )
TMR (%)
0.010
100
100
0.009
TMR(%)
0.011
150
0.008
50
0.007
50
as prepared
0.006
0
0.005
0
-150
-100
-50
0
50
100
150
Magnetic Field (Oe)
Fig. 1: TMR curve of an MgO-MTJ before and
after annealing at 36 min at 400 C. The inset
is the TEM cross section.
400
800
1200
1600
2000
0
2400
Annealing Time (seconds)
Fig. 2 First shell Debye-Waller factor 2 and
TMR as a function of annealing time.
A. K. Rumaiz, J. C. Woicik, W. G. Wang, J. Jordan-Sweet, G. H. Jaffari, C. Ni, J. Q. Xiao, and C. L. Chien,
“Effects of annealing on the local structure of Fe and Co in CoFeB-MgO-MTJs, an EXAFS study,” Appl. Phys. Lett. 96, 1125023 (2010).

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