AAWSAP DIRD, Metallic Spintronics, March 2010

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currents larger than the critical current, STT exceeds the damping torque and causes
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to spiral away from B eff, with a steadily increasing precession angle. The ultimate
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result can be either stable steady-state precession of S around B eff (black trajectory in
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Figure 6) or magnetic reversal of S into a state anti parallel to B eff, depending on the
angular dependence of STT and damping torques.
Almost all experimental observations of STT rely on the GMR phenomenon to detect the
current-induced reorientation of magnetic moments in nanodevices. Typical
measurements include (1) measuring static device resistance R=V/I as a function of
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applied de bias current I in an applied magnetic field B and (2) measuring R versus B
at a constant I. Figure 3 shows how the differential resistance dV/dl of a Cu point
contact to Co/Cu magnetic multilayer varies with the bias voltage V (equivalent to I)
applied across the contact. Here the multilayer magnetic moments are saturated out of
the plane of the layers by a sufficiently large externa l magnetic field (B ;:: 2T). The
onset of STT-driven magnetic precession is revealed by a peak in differential resistance
of the contact. The peak in dV/dl indicates the transition into precession is a reversible
process, and in a small range of currents, one can continuously increase or decrease
the angle of precession. However, other scenarios are also possible- for example, fast
transitions between static and steady-state precession states with current-dependent
dwell time.
If the applied magnetic field is small, the
magnetic system can have more than one
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low-energy state. In the simple case of a 6 a.2
magnetic element with uniaxial
anisotropy, STT can trigger a transition ­
=a 8.1
between two static states that are
energetically favorable. An example of
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such behavior is shown in Figure 7. Here
u 8.0
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the current is driven across a trilayer
Py20nm/Cu12nm/Py4.Snm spin-valve
-0.4 -0.2 0.0 0.2 0.4
structure patterned by electron beam
I (mA.)
lithography into a nanopillar with a 40 x
Figure 7. Spin-torque-driven magnetic switching
120 nm 2 cross-sectional area (Reference
for a Py20nm/Cu12nm/Py4.5nm spin valve with a
40x120-nm 2 cross-sectional area, as the
48). The differential resistance of the
magnetization of the thin (free) magnetic layer is
nanopillar exhibits a hysteresis as a
aligned parallel and antiparallel to the thicker
function of an applied bias current as the
magnetic layer by an applied current. (Reference
magnetization of the thin (free) permalloy
48)
(Py) layer is aligned parallel and
antiparallel to the thick (hard) Py layer by the current.
The two examples presented above (Figures 3 and 7) demonstrate how simple de
resistance measurements can be used for STT observation. Here the measured
resistance of a device provides indirect information about the relative orientation of
magnetic elements in the device. However, measured critical currents highlighted by
sharp variations in the resistance remain the only experimental information that can be
used to quantitatively compare theory and experiment. Moreover, the de measurements
in Figure 3 provide no information about fast evolution of magnetization in the device
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