AAWSAP DIRD, Metallic Spintronics, March 2010

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UNCLASSIFIED// FOR OFFIElIAk WSE OPtkY
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/Fl/N/F2/AFM/N = Cu(50 or 100nm)/CoFe(3 or 10nm)/Cu(10nm)/CoFe(3 or
l0nm)/FeMn (3 or 8nm)/Au(5nm) multilayer (or inv,erted 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 dipolar
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
Fl/N/F2/AFM EBSV.
J
F1
-
N
F2
-
AFM - ---­
N
"°
20
e
,
,,,,
,
·60
I
I
0
·20
0
30
0
.30
10 l
0
-10 -
20
0
·20
B(mTJ
B(mT)
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 solid 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), SO 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 (8-nm) F layers. (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 RAPin
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
UNCLASSIFIED/,' FOA OfifilCI0Lt !P&i Alf! Y
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