Biomedical Engineering Reference
In-Depth Information
ferromagnetism, while for T < T
c
it is ferromagnetic. As the temperature
increases, thermal motion or entropy competes with the ferromagnetic
tendency for dipoles to align. When the temperature rises beyond a certain
point, called the Curie temperature, there is a second-order phase tran-
sition and the system can no longer maintain a spontaneous magnetiza-
tion, although it still responds paramagnetically to an external i eld. Below
that temperature, there is a spontaneous symmetry breaking and random
domains form. h e Curie temperature itself is a critical point, where the
magnetic susceptibility is theoretically ini nite and there is no net magne-
tization, while domain-like spin correlations l uctuate at all length scales.
13.1.2.2.3 Applications of Ferromagnetic Materials
From the survey of literature it is found that the properties of ferromag-
netic materials mainly depend on the purity of the materials, grain size
of the materials, and the strength and applied frequency of the magnetic
i elds. In some cases the crystalline and nanosized materials as well as
nanocomposite magnetic materials are potential candidates for a variety
of new applications. h ey i nd their applications in the i elds of image sen-
sors, catalysis, biotechnology, drug delivery systems, medical diagnostics,
magnetic refrigeration, and magnetically assisted chemical separation of
radioactive and hazardous wastes. h e ferromagnetic materials are used
in audio tape, video tape and magnetic disk storage for computers. h e
recording head of a tape recorder or the writing head of a disk drive applies
a i eld that magnetizes a small portion of the tape or disk. h e magnetism
in each portion remains until another magnetic i eld changes it. When each
magnetized section is moved under the playback head of a tape player or
the reading head of a disk drive, the moving magnetic i eld induces small
currents, which are amplii ed and turned into either music or data bits. If
the domains were unable to remember the i eld that had been applied to
them, none of this would be possible.
Radio frequency (RF) electronic applications have had iron powder-
based ferromagnetic materials for many years due to their high saturation
resistance. Some of these are used quite extensively in consumer lighting
ballasts, computers, consumer electronics and related application.
13.1.2.3 Ferroelastic
Ferroelastic crystals are, in some respects, like the more familiar ferro-
magnetic and ferroelectric materials. Instead of spontaneous magnetiza-
tion or polarization, these materials develop a spontaneous strain below a
phase transition from paraelastic to ferroelastic. h e transition is accom-
panied by the critical behavior of the elastic compliance (the ef ective
