Page 310 - Electrical Properties of Materials

292 Magnetic materials

Permanent magnet
or solenoid

Incident Reflected
circularly circularly
polarized polarized
wave wave

Fig. 11.31
Schematic representation of an Ferrite rod Circular
isolator. waveguide

magnetic dipole. The effect of this torque is insignificant, except when the ex-
y
tra magnetic field rotates with the speed of precession—and, of course, in the
right direction. But this is exactly what happens for one of the circularly po-
μ larized waves when its frequency is equal to the frequency of precession. The
interaction is then strong, and energy is taken out of the electromagnetic wave
B B in order to increase the angle of precession. Hence, for one given frequency
x
z (the resonance frequency) and one sense of rotation (that of the reflected wave)
the electromagnetic wave is absorbed.
x The quantum explanation is based on the resonance phenomena discussed
in the last section. The electromagnetic wave is absorbed because its energy is
Fig. 11.32
used to sponsor transitions between the respective energy levels. Unfortunately,
The magnetic dipole moment
quantum mechanics provides no intuitive description of the effect of circularly
precesses around the constant
polarized waves. You either believe that the result comes out of the mathem-
magnetic field, B. An additional
atical description of the problem or, alternatively, you stick to the classical
magnetic field, B x , gives an extra
torque, trying to increase the angle of picture. This is what many quantum physicists do, but to ease their conscience,
precession. they put the offending noun between inverted commas. They do not claim that
anything is really precessing, nonetheless, they talk of ‘precession’.
11.12.2 Sensors

Magnets can be used to sense position, force, torque, speed, rotation, acceler-
ation, and of course current and magnetic field. Since the advent of light and
powerful neo-magnets, their use has been rapidly expanding, as for example in
anti-lock brakes and in activating airbags.

11.12.3 Magnetic read-heads
Practical devices for recording sound appeared in the middle of the 1930s and
their video counterparts, which needed a much wider bandwidth, came about
20 years later. The information had to be read by so-called read-heads, which
were also suitable for writing the information upon the tape. The revolution
in read-heads came with the discovery of the giant magnetoresistance. It was
so much better than those existing at the time that they reached commercial
application in less than a decade after the discovery, earning billions of dollars
for IBM. Interestingly enough their reign was short-lived. Within a few years