Biomedical Engineering Reference
In-Depth Information
13.1 Introduction
13.1.1 Ferroics
When a crystal changes its symmetry as a function of temperature, pres-
sure, etc., it goes from one phase to another, which we refer to as the phe-
nomenon known as phase transition. Ferroic crystals are those crystals
which involve at least one phase transition which changes the directional
symmetry of the crystal. Ferroic materials possess symmetry and some-
times undergo a change of symmetry spontaneously when environmental
conditions are changed. h e term “ferroic materials” is a general term cov-
ering ferromagnetic materials, ferroelectric materials, ferroelastic materi-
als, ferrotoroidic materials, etc.
Ferroic materials are associated with multiscale phenomena from
atomic/nano scale (atomic/ionic displacement, spin, etc.), mesoscopic
scale (domain) to macroscopic scale (strain, electric ef ects, magnetic
ef ects). Accordingly, they have signii cant scientii c interest and techno-
logical importance.
13.1.2 Classii cation of Ferroics
Ferroics can be classii ed into three categories (Figure 13.1) which are based
on the dif erent terms of the density of stored free enthalpy. One of them
is primary ferroics, which includes ferromagnetic, ferroelectric, ferroelastic
and ferrotoroidic materials. A primary ferroic material exhibits a spontane-
ous magnetization, a spontaneous polarization or a spontaneous strain. h e
second category is secondary ferroics and the last is tertiary ferroics.
Here we are mainly concerned with the primary ferroics, which can be
further classii ed as:
•
Ferroelectric materials
possess a spontaneous polarization
that is stable and can be switched hysteretically by an applied
Classification of ferroics
Primary ferroics
Secondary ferroics
Tertiary ferroics
Ferroelectric
Ferromagnetic
Ferroelastic
Ferrotoroidic
Figure 13.1
Classii cation of ferroics.
