AAWSAP DIRD, Metallic Spintronics, March 2010

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similar to conventional HDDs; but the racetrack memory would have much higher
read/write performance than HDD.
4 . Antiferromagnetic Metal Spintronics
Recently, MacDonald and coworkers (Reference 84) predicted that effects similar to
GMR and STT observed with ferromagnets ought to occur in multilayer systems where
the ferromagnetic (F) components are replaced by antiferromagnets (AFM). First, they
predicted that the resistance of an AFM spin valve- where two AFM layers are
separated by an N spacer- could depend on the relative orientations of the magnetic
moments in the two AFM layers (antiferromagnetic GMR = AGMR). Second, they
predicted that injecting a sufficiently strong current density into an AFM should affect
its magnetic state via current-induced spin torque. These new AFM effects could lead to
new all-AFM spintronics where AFMs are used in place of Fs.
Replacing F metals with AFM metals in a spintronic device has several advantages.
While AGMR of an AFM spin valve was predicted (Reference 84) to be similar in
magnitude to GMR in standard F spin valves, the critical current needed to alter the
magnetic order in AFMs can be smaller than for Fs, partly because spin torques can act
through the entire AFM volume. The estimate of the necessary current density j
109
N
A/m 2 (Reference 84) was less than the typical j "' 1011 A/m 2 needed to reverse the
magnetic order in F/N/F multilayers (Reference 5-9, 40-43, 48-51). Finally, using AFM
metals in spintronic devices in place of F metals would eliminate unwanted effects of
shape anisotropy on the magnetic stability of small elements, thus potentially offering
better control of the magnetic state in nanoscale systems and easing fabrication
requirements.
Following the original predictions of MacDonald and coworkers (Reference 84), Xu et al.
(Reference 85) calculated the AGMR for a simple AFIM/N/AFM/N = FeMn/Cu/FeMn/Cu
multilayer, and Gomonay and Loktev (Reference 86) provided additional theoretical
evidence that polarized current can destabilize the equilibrium state of an AFM. Note,
however, that all calculations to date are for perfect samples and depend on quantum
coherence. It is known that disorder can reduce the predicted effects. Experiments are
thus crucial to see if any such effects are visible in real samples.
4.1 ANTIFERROMAGNETIC GMR
To the author's knowledge, the only experimental study searching for AGMR was
performed by Wei et al. (Reference 87). They have measured current-in-plane {CIP)
and current-perpendicular-to-plane (CPP) magnetoresistances (MR) of magnet1ic
multilayers containing two antiferromagnetic layers separated by a nonmagnetic layer.
Such an antiferromagnetic spin valve, AFM/N/AFM, was predicted (Reference 84) to
exhibit AGMR similar to GMR seen in ferromagnetic spin valves, F/N/F, containiing two
ferromagnetic layers separated by a nonmagnetic layer. Note, however, that
calculations (Reference 84) assumed ballistic transport in samples with perfect layers
and interfaces where AGMR is a consequence of quantum interference effects. Thus,
any disorder that produces diffusive scattering and weakens quantum interference will
weaken any such AGMR .
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