Page 308 - Electrical Properties of Materials

290 Magnetic materials

H
M

Fig. 11.29
A propagating spin wave.

before that polarization persists when moving from a ferromagnetic into a non-
magnetic material. The same situation occurs in tunnelling; the polarization
persists. A spin-up electron will still be a spin-up electron after it has tunnelled
through the insulator. But whether it is cordially welcome in F 2 or grudgingly
accepted depends on the polarization of F 2 whether its electrons are polarized
in the same or the opposite direction. If in the same direction, the resistance is
low, if in the opposite direction the resistance is high, the same phenomenon
that occurred in the conducting device. The difference is that the change in
resistance is higher at tunnelling than at conduction by about a factor of 2.

11.11.3 Spin waves and magnons
Spin
up Although spin waves were discovered long before anybody thought of spin-
J c J c tronics, I think this is still the best place to briefly describe it. The wave may be
characterized by the magnetization vector precessing about an applied mag-
Spin
down netic field, B, as shown in Fig. 11.29. The wave can also be regarded as a
particle (the analogue of the electromagnetic wave–photon duality), in which
Fig. 11.30 case it is called a magnon.
In the presence of a charge current
consisting of spin-up and spin-down 11.11.4 Spin Hall effect and its inverse
electrons, it is observed that the
different polarizations are deflected in The configuration in which the spin Hall effect is observed is even simpler
opposite directions. than that for the ordinary Hall effect. There is no need for an applied mag-
netic field nor is there a need for a magnetic material. It can be observed in a
non-magnetic metal or semiconductor; the only condition is to have spin–orbit
∗
∗ interaction and of course a charge current needs to flow. The result is that
The coupling between an element’s
spin and its orbital magnetism. spin-up and spin-down electrons are deflected in opposite directions as shown
in Fig. 11.30. Hence at the top of the material, perpendicular to the charge cur-
rent, there is an excess of spin-up electrons and at the bottom of the material
there is an excess of spin-down electrons. Note that there is no charge current
in the perpendicular direction but there is a spin current, another example of
a pure spin current. This is again something hard to get used to. There is no
movement of charge but a spin current is nonetheless present. The inverse ef-
fect can also be observed. If a spin current flows, that will induce a charge
†
† This follows from the Onsager rela- current perpendicular to it. The effect is useful for detecting spin current.
tions which establish reciprocal relation-
ships between various physical quantit-
ies. 11.11.5 Spin and light
We know that both electric and magnetic vectors rotate in circularly polarized
light. Hence comes the reasonable hypothesis that spin polarized carriers can