2.6 Spin-transfer Effect and Current-induced Magnetization Switching
It has been recently shown, both theoretically and experimentally,
that a spin-polarized current when passing through a small
magnetic conductor can affect its magnetization state. The
interaction between spin polarized current and magnetization in
small ferromagnetic bodies can produce steady state precessional
magnetization dynamics, that is self-oscillating behavior, or even
the switching of magnetization
direction [44]-[46]. Both types
of dynamical behavior have potential applications in magnetic
storage technology and spintronics. In this respect, it was
predicted, and later confirmed, that spin-polarized current can
lead to current-controlled hysteretic switching in magnetic
nanostructures. This kind of behavior may become very important
for applications such as current-controlled switching of magnetic
random access memory elements and stabilization of magnetic
hard-disk read heads. Steady precessional oscillations of
magnetization due to spin polarized currents have also interesting
potential applications for the realization of current-controlled
microwave oscillators integrable with semiconductor electronics.
This kind of oscillators could be used to realize a very new
design of clocks for synchronization of electronic devices.
Here spin-polarized current induced dynamics is studied in the
case of a uniformly magnetized ferromagnetic thin
film [47]. Magnetization dynamics is described
by the Landau-Lifshitz-Gilbert equation and the effect of
spin-polarized currents is taken into account through the
additional torque term derived by Slonczewski in
Ref. [44]. This model can be applied to
describe the magnetization dynamics in the free layer of trilayers
structures constituted by two ferromagnetic layers separated by
nonmagnetic metal layer (typically the system is a Co-Cu-Co
trilayers as sketched in Fig. 2.20). One of the magnetic
layer is ``fixed'', namely has a given and constant value of
magnetization (indicated with
in Fig. 2.20) while
the second magnetic layer is a thin film where the magnetization
is ``free'' to change and where dynamics takes place. This kind of
structure is traversed by an electric current whose direction is
normal to the plane of the layer (generally this configuration in
called ``current perpendicular to plane (CPP) geometry''). The
fixed layer is instrumental to provide a controlled polarization
(on the average parallel to the fixed magnetization direction) of
the electron spins which travel across the trilayers, from the
fixed to the free layer.
Figure 2.20:
Sketch of Trilayers Co-Cu-Co structure.
It important to underline that the effect of spin induced torque
is predominant on the effect of the magnetic field generated by
the current itself for structures which have small enough
transversal dimensions. By using reasonably estimate it has been
predicted and then verified experimentally that the effect of the
current generated magnetic field can be considered negligible for
transversal dimension as small as nm.
Subsections
in Fig. 2.20) while
the second magnetic layer is a thin film where the magnetization
is ``free'' to change and where dynamics takes place. This kind of
structure is traversed by an electric current whose direction is
normal to the plane of the layer (generally this configuration in
called ``current perpendicular to plane (CPP) geometry''). The
fixed layer is instrumental to provide a controlled polarization
(on the average parallel to the fixed magnetization direction) of
the electron spins which travel across the trilayers, from the
fixed to the free layer.
It important to underline that the effect of spin induced torque
is predominant on the effect of the magnetic field generated by
the current itself for structures which have small enough
transversal dimensions. By using reasonably estimate it has been
predicted and then verified experimentally that the effect of the
current generated magnetic field can be considered negligible for
transversal dimension as small as 
nm.
