Page 309 - Electrical Properties of Materials

Some applications 291

be excited by circularly polarized light. Such interaction has indeed been re-
ported in GaMnAs. This turns out to be a very fast effect, in the femtosecond
range, suggesting direct transformation from the angular momentum of light
to magnetic moment.

11.11.6 Spin transfer torque
Spin polarized current is known to exert a torque that can rotate the local
magnetic moment when it is injected into a ferromagnetic material. The local
magnetic moments feel the torque and start to precess (i.e. spin waves are
excited) around the local effective magnetic field. This will go on until the
local magnetization becomes parallel with the injected spin polarization. Inter-
estingly, the effect is still there in the absence of charge current. Rotation of
magnetic moment may also be caused by pure spin current, as has been proved
using the configuration of Fig. 11.27(a). In terms of the F 1 –N–F 2 and F 1 –I–F 2
sandwiches this means that the polarization of the second ferromagnetic layer
could be changed by a spin current injected into it. This provides the basis
of a very promising memory element, discussed in A6.3.5. Another effect of
spin-transfer torque is to move domain walls by interacting with the magnetic
moments present there. The inverse effect may also occur. Spin waves may
generate spin currents. The phenomenon is known as spin pumping.

11.12 Some applications

Until the 1950s the only significant application was for electrical machines
and transformers. Modern technology brought some new applications: most
notable among these being the use of magnetism for storing information. In
fact, in 1985 for the first time, the sales of magnetic products for Information
Technology in the USA exceeded those for all other technologies. We shall dis-
cuss information storage in more detail in Appendix VI. In the present section
we shall briefly mention only four applications, a unique one, as isolators, a
general one, as sensors, an old one, magnetic read-heads, and an even older
one, electric motors.

11.12.1 Isolators
My next example is a device which lets an electromagnetic wave pass in one
direction but heavily attenuates it in the reverse direction. It is called an isol-
ator. The version I am going to discuss works at microwave frequencies and
uses a ferrite rod, which is placed into a waveguide and biased by the magnetic
field of a permanent magnet (Fig. 11.31). The input circularly polarized wave
may propagate unattenuated, but the reflected circularly polarized wave (which
is now rotating in the opposite direction) is absorbed. Thus, the operation of
the device is based on the different attenuations of circularly polarized waves
that rotate in opposite directions.
The usual explanation is given in classical terms. We have seen that a mag-
netic dipole will precess in a constant magnetic field. Now if in addition to
the constant magnetic field in the z-direction there appears a magnetic vector
in the x-direction (Fig. 11.32), then there is a further torque acting upon the