Defense Intelligence Reference Document Metallic Spintronics

UNCLASSIFIED/ (FOR OFFICIAL u5& &rtlY
The original observation of the effect was reported by Wei et al. (Reference 91), who
measured magnetoresistance of a point contact to EBSV film at room temperature ( ~
295K) with negative current flowing from the contact tip into the film. The sample
geometry is shown in Figure 12a. A point contact is used to inject a de current into a
sputtered N/F1/N/F2/AFM/N = Cu(S0 or 100nm)/CoFe(3 or 10nm)/Cu(10nm)/CoFe(3 or
l0nm)/FeMn (3 or 8nm)/Au(Snm) multilayer (or inverted versions thereof-that is,
Cu/FeMn/CoFe/Cu/CoFe/Au). The sample is heated to ~450K (above the blocking
temperature of FeMn) and then cooled in a magnetic field of 180 Oersteds to exchange-
bias the "pinned" layer F2 to a higher magnetic field than needed to reverse the "free"
layer F2. The top layer is covered by a 5-nm-thick layer of Au to protect it from
atmospheric contamination. The magnetic field H is applied in the plane of the layers
and along the direction of exchange bias. Magnetic coupling between the two F layers
should be negligible, because the N layer is thick enough (10 nm) to eliminate
exchange coupling, and the two Flayers are wide enough ( ~ mm) to minimize di polar
coupling. The bottom N layer is Cu, made thick enough (50 or 100 nm) to approximate
an equipotential, thereby generating an approximately CPP current flow through the
F1/N/F2/AFM EBSV.
F1
N
F2
IIFM
N
J
-
---- .... _. .__. -- e
; '✓
B(mTj
Figure 12. (a} Schematic of point contact to sample geometry. Omitted is a 5-nm-thick protective Au
capping layer between the point contact and the multilayer. The bottom N layer is also much thicker
than shown to help produce a nearly CPP current. (b) R (vertical scale) versus applied magnetic field B
for a series of currents I, The solld black curves are hysteresis curves starting from large positive Band
finishing at large negative B. The grey curves start at large negative B and finish at positive B. In the
dark curves, the "free" layer, Fl, switches at~ ·5 mT and the pinned layer, F2, switches at fields
ranging from below -40 mT (large positive I) to about - 60 mT (large negative I). (c-e) Grey-scale plots
of R versus B for different values of I. White is maximum R (antiparallel state) and black is minimum R
(parallel state). Lines are linear fits to the data at 30 percent (dashed white), 50 percent {solid white)
and 70 percent (dashed black) of maximum R. Sample (c) is the sample of (b), with the AFM layer on
the bottom (furthest from the point contact}. Sample {d) is similar to (c), except inverted, so that the
AFM layer is on top (closest to the point contact). Sample (e} differs from {b) only in that it has two
equally thick (B•nm) Flayers. (Reference 91}
Figure 12b shows magnetoresistance curves for a series of currents I applied to a point
contact with resistance R = 0.92 n. The dark curves show sweeps from positive to
negative field, and the lighter curves show sweeps back down from negative to positive
field. For this contact, I = 30 mA corresponds to j ~ 2 x 108 A/cm 2. For dark curves, a
large positive field +B along the pinning direction causes the moments of both the
"free" and "pinned" F layers to point along +B, producing the minimum resistance RP in
this parallel orientation of the two Fs. Reducing the magnitude of B, the "free" F2 layer
reverses at a small-magnitude negative B, giving the maximum resistance RAP in
antiparallel configuration. Finally, a larger magnitude negative B breaks the exchange-
bias pinning of the "pinned" Fl layer, and its moment rotates to along -B, returning the
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UNCLASSIFIED/ (FOR OFFICIO I 1166 Qlllalf