AAWSAP DIRD, Metallic Spintronics, March 2010

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In F/N/F trilayers, the relative orientation of the magnetizations of the two Fs is
controlled by an externally applied magnetic field B. To achieve well-defined antiparallel
and parallel states of the two Flayers, the moment of one Fis often "pinned," via
exchange coupling (exchange bias) to an adjacent AFM layer (Reference 88, 89),
leaving the moment of the other free to reverse in much smaller B. In a simple
AFM/N/AFM sample, just applying a field Bis not expected to be efficient because of the
weak effect of external fields on magnetic moments in AFMs. To achieve better control
of the AFMs, AFM/N/AFM spin valves can be sandwiched between two F layers to give
F/AFM/N/AFM/F, with the two AFM layers differently exchange-coupled to their
respective F neighbors. Applying a magnetic field to change the magnetic order of the F
layers should then also affect the order of the AFM layers. In addition to AFM/N/AFM
and F/AFM/N/AFM/F multilayers, Wei et al. (Reference 87) have studied a variety of
structures-F/AFM/N/AFM, AFM/F/N/AFM, F/AFM, and single F and AFM layers-to
isolate the MR observations of interest from potential spurious effects.
For small applied currents, neither standard current-in-plane (CIP) MR measurements
on extended multilayer films nor CPP MR measurements with point contacts showed
any MRs for samples of all types. For larger applied currents, small positive CPP MRs
(resistance is highest at saturation) were sometimes observed in samples with at least
one F layer, while no MR was seen in samples with no Fs. These observations suggest
sputtered AFM/N/AFM multilayers do not show AGMR, possibly owing to mostly diffusive
transport in such imperfect films. Small MRs observed at higher currents in films with F
layers may be associated with the suppression at high currents of spin accumulation
induced within and around Fs (Reference 90). Further studies on high-quality films are
still needed to verify any possible existence of AGMR in such structures.
4.2 ANTIFERROMAGNETIC STT
Stimulated by the theoretical studies in (Reference 84-86), four experimental searches
for effects of STT on AFMs have been published so far (Reference 91-94 ), all working
with exchange-biased spin-valves (EBSVs) of the form AFM/Fl/N/F2. Here the AFM lies
outside the "active" GMR region of the two F layers and serves mainly to "pin" the
magnetization of the adjacent F2 layer to a higher reversing (switching) field than that
of the "free" Fl layer, leaving the Fl layer free to rotate at a lower field. The pinning is
produced either by heating the sample to above the blocking temperature of the AFM,
applying a magnetic field, and then cooling to room temperature with the field on or by
applying a magnetic field during sample growth. Wei et al. (Reference 91) used a point
contact to inject a high de current density j "' 1012 A/m 2 approximately CPP into an
EBSV film. Urazhdin and Anthony (Reference 92) sent a de CPP current density j ~ 5 x
1011 A/m 2 into electron-beam-lithography-fabricated nanopillar EBSVs. Tang et al.
(Reference 93) sent a de current-in-plane (CIP) current density ~ 1010 A/m 2 into an
EBSV film with a metallic AFM. Dai et al. (Reference 94) sent an ac CIP current density j
"' 109 A/m 2 into an EBSV film with an insulating AFM. In all of the studies (Reference
91-94), it was found that a sufficiently high bias current can influence the magnetic
reversal of the "pinned" Fl layer. As the pinning (ex,change bias) is known to be
associated with interfacial AFM moments (Reference 88, 89), this observation can be
taken as evidence of effects of the current on the AFM predicted in (Reference 84-86),
given that other spurious effects (for example, Joule heating) can be ruled out.
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