Page 100 - Maxwell House

80 Chapter 2

nonvolatile Random Access Memory (RAM) meaning the information is retained if the
power is turn off, car ignition and cigarette lighters, acceleration sensors, resonators,
micromirrors, inkjet printheads and actuators, transducers, electrooptic devices, tunable
microwave devices, ferroelectric field effect transistors, ferroelectric random access
memory (FeRAM), miniature capacitors for integrated circuits, intruder alarms, thermal
imaging devices, etc. The principal ferroelectric materials are lead (Pb)-based materials -
Pb(Zr, Ti)O or PZT, layered perovskites – SrBi Ta O or SBT and Bi Ta O or BiT and
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3 12
2 3
3
2
TiO -based materials as (Ba,Sr)TiO .
3
3
5. The characteristic feature of ferro-materials is that the increase in the material temperature
gradually destroys the domain structure. Then at some unique for each material critical
temperature called the Curie temperature the domain structure abruptly vanishes.

Above this temperature ferromagnetics and ferrimagnetics become paramagnetics while
ferroelectrics behaves as paradielectrics. The generalized temperature behavior of relative
dielectric / magnetic constant around the Curie point is shown in Figure 2.6.1. It means that
any devices containing such materials must be thermostabilized if we expect the wide
temperature deviation in a system.
2.6.3 Ferromagnetics
An unmagnetized piece of
the ferromagnetic material
consists of randomly
oriented and
spontaneously
magnetized to saturation
small domains, as shown
in Figure 2.6.2a. Each
domain acts as tiny bar
Figure 2.6.2 a) An unmagnetized piece of ferromagnetic magnet with magnetic
material, b) boundary shift between domains showing greater saturation moment ,
alignment, c) total alignment and in the absence of an

external magnetic field,
the total magnetic moment of bulk ferromagnetic is zero or close to zero (see Figure 2.6.2a).
Each domain is separated from its neighbors by a domain wall (Bloch wall). In the wall region,
the direction of magnetization smoothly turns from that of one domain to that of its neighbor,
as shown in Figure 2.6.2d . From quantum
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physics, we know that in materials there are two
main sources of magnetic moments: the
continuously moving electrons around the nucleus
(orbital moments) and electrons rotation around
their own axis (spin moments). The main role in
domain magnetization plays the spin magnetic
moments due to a restraining force of quantum
Figure 2.6.2d Bloch domain wall nature tending to keep electron near each other and
model

14 Public Domain Image, source: http://www.energeticforum.com/renewable-energy/20331-
enlightened-magnetism-full-proof-ken-wheelers-theories-6.html