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Tunneling Magnetoresistance in
(Co1-x,Fex) Electrode
Magentic Tunnel Junctions
Computational Work: Amy Bengtson, Dane Morgan,
Materials Science Program
Experimental Work: J. Joshua Yang, C-X. Ji,
Austin Chang, Materials Science Program
Magnetic tunnel
junctions (MTJs) have important applications in
nonvolatile magnetic random access memory and
advanced magnetic recording heads. However,
there is still significant uncertainty about how
composition and structure in the magnetic alloy
can influence the overall MTJ performance. In
this work, we have done the first study
separating the role of crystal structure on
tunneling magnetoresistance (TMR) in the (Co,Fe)/AlOx/(Co,Fe)
MTJ system. Careful experiments (J-J. Yang,
C-X. Ji, A. Chang) were preformed to measure how
the TMR changes with a face-centered cubic (fcc)
electrode and a body-centered cubic (bcc)
electrode. The results show that MTJs with a
bcc electrode have superior TMR than those with
a fcc electrode.
The experimental
work is supported by computational ab initio
studies (A. Bengtson and D. Morgan).
Computational studies are ideal for separating
the TMR values of bcc and fcc (Co,Fe), however
the work was limited to TMR values in bulk
crystals since the exact structure at the
junction is unknown. Our computational results
support the experimental evidence that
electrodes with a bcc crystal structure have a
higher TMR value than those with fcc crystal
structure.
Figure 1: Changing the crystal structure
of the electrode in a MTJ from fcc to bcc
increases the TMR value.
We then extended the
above work to study the role of composition,
x, in (Co100-x,Fex) on
TMR and find an initial drastic increase in TMR
value and then a gradual decrease in TMR as x
increases up to 50%. The results suggest
that an fcc electrode doped with Fe will have a
higher TMR value than a pure Co electrode.
Figure 2: Spin polarization (hollow
squares (fcc) and circles (bcc)) and the
corresponding TMR (solid squares (fcc) and
circles (bcc)) values vs. concentration of Fe in
the Co100-xFex FM
electrode calculated with ab initio
methods. The solid line with stars is the TMR
trend with the Fe concentration estimated based
on the ab initio predicted spin
polarization values and the known stable crystal
structureat each composition. (Figure and
figure caption are from J. Yang, JAP 2008).
We gratefully acknowledge
financialsupport from the
Wisconsin Alumni Research
Foundation and
computing support the
National Science
Foundation (NSF) National Center for
Supercomputing Applications (NCSA), award number
DMR060007. Experimental work supported by the
Department of Energy (DOE), Office of
Basic Energy Science, Division of Materials
Science, award number DE-FG02-99ER45777, the
Wisconsin Alumni Research Foundation (WARF), and
the Wisconsin Distinguished Professorship.
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