AAWSAP DIRD, Metallic Spintronics, March 2010

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(Fa) is "hard" and used to polarize the current, while the spacer (N) is thin enough for
the polarized current to get through and excite the second "free" magnet (Fb), This
Fa/N/Fb trllayer structure is similar to a GMR spin valve. The GMR effect can thus be
used to monitor the orientation of Fb relative to Fa - GMR varies linearly with cose,
where e is angle between magnetic moments of Fa and Fb, and a phenomenological
description (Reference 46) gives the trilayer resistance R(0) = RF+ (RAF- RF) (1 ­
cos0)/2. When current flows across Fa/N/Fb, the current-induced torques act on both Fa
and Fb layers (Reference 3, 47). This is schematically illustrated in Figure 5. This
qualitative picture of STT .assumes both Fa and Fb layers are perfect spin filters, so that
electron spins aligned with the magnetic moment of, for example, Fa layer are
completely transmitted through the layer, while spins aligned antiparallel to the layer
moment are completely reflected. When electron current crosses the Fb/N/Fatrilayer
from right to left (Figure Sa), electrons transmitted t hrough Fa will be polarized along
Fa. If spin-diffusion length in N is long enough, this spin-polarized current will reach Fb
and exert a torque on Fb in a direction so as to align Fb with Fa. Repeating the argument
for Fb, we find that electrons reflected from Fb will be polarized antiparallel to Fb and,
hence, in turn exert a torque on Fa trying to align Fa antiparallel with Fb. The net result
is a pinwheel-type motion with both Fa and Fb rotating in the same direction (clockwise
in Figure Sa), as described previously by Slonczewski (Reference 3). When the current
crosses the trilayer from left to right, the directions of the torques are reversed (Figure
Sb)- the torque on Fa is trying to align Fa parallel with Fb, while the torque on Fb is
trying to align Fb antiparallel with Fa .
Fb N
Fa
F N
r
-¢ V
¢-
¢
(b)
Electron Current
Electron Current
Figure 5. Qualitative Picture of STT. (a) For left-going electrons magnetic moments (thin arrows) of both Fa and
Fb are rotated clockwise. (b) For right-going electrons the directions of the torques (thick arrows) on Fa and Fb are
reversed. (Reference 47)
The above discussion implies the asymmetry of STT with respect to current direction as
follows. Let's fix the orientation of the polarizer Fa; in experiments this is usually
accomplished by making Fa very thick (compared wit h Fb) or by pinning its orientation
with an adjacent antiferromagnetic layer via the phenomenon of exchange bias. If
initially Fb is almost parallel with Fa, the left-going electrons will stabilize this parallel
alignment, and no STT excitation is present. When current bias is reversed , the torque
on Fb will try to rotate Fb away from Fa and will result in STT excitation of the system.
This asymmetry with respect to current polarity is one of the main features of STT in
experiments; see, for instance, Figure 5, where STT excitations are present only at
negative bias.
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