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NEOCLASSICAL THEORY OF INTERACTION 79

2.6.2 Basic Description of Ferro-Materials

Let us start from some classification:
1. Ferromagnetism is the basic mechanism by which certain materials (such as iron, nickel,
cobalt, most of their alloys, some rare elements as gadolinium and neodymium, etc.) form
the structure with strong and close to saturation magnetic moment domain and are intensely
attracted to magnets. Many of them are metals with high thermal and electric conductivity.
Ferromagnetics are extensively used as permanent magnets in loudspeaker and
microphones, magnetic particles in medical and biology research, in power motor,
generators and transformers, electromagnet cores and relays, cassette magnetic tape, stripe
on the back of credit card, medical equipment, as a low frequency (up to 100 kHz) magnetic
field shielding, and wide-ranging variety of different devices.
2. Ferrimagnetism essentially is the same form of magnetism as ferromagnetism. However,
the ferrimagnetics are chiefly isolators or semiconductors, i.e. they might be transparent
well into the radio and optical spectrum. It is explained by the fact that they consist of the
different type of neighboring ions of unequal magnetic moments, which are polarized in
opposite directions. As a result, the ferrimagnets are weakly attracted by the magnetic field
as compared to ferromagnets. Magnetite Fe O , ferrites like ZnO, MgFe O and ZnFe O ,
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4
3 4
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yttrium iron garnet Y Fe O , gallium gadolinium garnet Gd Ga O , optically transparent
3 5 12 3 5 12
oxyphosphate PbFe O(PO )3 are examples. A wide variety of ferrimagnetic material
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4
applications include a low frequency (up to 100 kHz) magnetic field shielding, magnetic
analog and digital data storages like Magnetoresistive Random-Access Memory (MRAM)
and computer hard drives with Giant MagnetoResistive (GMR) reading head, microwave
nonreciprocal devices like circulators, switches, isolators, magneto-optic devices based on
Faraday and Kerr effect, etc. This list always grows following the progress in new material
development.
3. In antiferromagnets, the moments of the aligned and anti-aligned ions entirely balance
thereby nullifying the net magnetization, despite the magnetic ordering. These alignment
effects only occur at temperatures below a certain critical temperature, called the Curie
temperature (for ferromagnets and ferrimagnets) or the Néel temperature (for
antiferromagnets).
4. In ferroelectrics, the domain structure is formed by the electric dipoles. The strong external
E-field (10 – 100 kV/m) is required to reorient most of the domains in the direction of the
applied field or close to it. This process, called polling, induces the piezoelectric (world
“piezo” is Greek for “push”) properties forcing the ferroelectric element to expand or
contract its dimensions (under the influence of the permanent external field) or vibrate
(under the influence of the external time-varying field). Dielectric constants of
ferroelectrics can be enormous—in the range of 1000–5000 for pure BaTiO and up to
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50,000 if the titanium, Ti, is replaced by
zirconium, Zr. Since the piezoelectric
poling effect can be reversible, the
piezoelectric materials are used to convert
electrical energy into mechanical energy
and vice-versa. There is a broad range of
applications for ferroelectrics materials.
Piezo drives and motors are vital in today's
ultra-precision motion control systems,
Figure 2.6.1 Relative dielectric or sonars, miniaturized Micro Electro
magnetic constant over temperature Mechanical Systems (MEMS) switches, in