AAWSAP DIRD, Metallic Spintronics, March 2010

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3. Spin-Transfer-Torque
This section focuses on the spin-transfer-torque (STT) phenomenon, which refers to a
novel method to control and manipulate magnetic moments in nanostructures by spin
currents-one of the forefront and most exciting areas in magnetism research today.
3.1 STT BASICS
The previous section showed that the magnetic state of a ferromagnet can affect its
electrical transport properties; for instance, the relative orientation of the magnetic
moments in magnetic multilayers underlies the phenomenon of GMR (Reference 1, 2).
The inverse effect, in which a large electrical current density can perturb the magnetic
state of a multilayer, has also been predicted (Reference 3, 4). Here the current
transfers vector spin between the magnetic layers and induces precession and/or
reversal of the layer magnetizations. Altering the magnetic state with spin currents is
based on quantum mechanical exchange interaction and represents a novel method of
magnetization control on the nanometer length scale and the picosecond time scale.
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The first observation of such a spin­
transfer phenomenon in magnetic
multilayers was recorded by Tsoi et al.
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(Reference 5) (see Figure 3). In this
experiment, the spin-transfer-induced
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excitations were produced by injecting
a 42
high-density electrical currents into a ­
Co/Cu magnetic multilayer through a
~
mechanical point contact. Point contacts
~ 41
smaller than 10 nanometers (nm) are
formed when a sharpened Cu metal
40
wire (tip) is carefully brought into
contact with the multilayer. The
39
extremely small cross-sectional area of
38~~~~~-~~~~~~-~
such a contact makes it possible to
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achieve current densities in excess of
1012 A/m 2. Because of its extremely
VM
sma ll size ( <10 nm), point contact is a
Figure 3. Differential Resistance dV / di of a
very efficient probe of electrical
Mechanical Point Contact as a Function of Current
transport properties in ext remely small
for a Series of Magnetic Fields. The peak In dV/dl
indicates the onset of SST excitations. The inset shows
sample volumes inaccessible with other
that the threshold current at the peak In dV/dl Increases
techniques (for example, electron­
linearly with the applied field. (Reference 5)
beam-lithography patterning). The
latter qualifies point contact as the smallest probe of STT today.
The STT phenomenon currently attracts considerable attention because it combines
poorly understood fundamental science questions with the promise of applications in a
broad range of technologies. In high-speed, high-density magnetic recording
technology, for instance, SST could replace the Oersted field currently used for writing
magnetic bits in storage media (for example, in magnetic random access memory
[MRAM]). This may lead to a sma ller and faster magnetic memory. Another possible
application is based on the spin-transfer-induced precession of magnetization, which
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