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Strictly
speaking, there is no such thing as a “nonmagnetic” material. Every material
consists of atoms; atoms consist of electrons spinning around them, similar to
a current carrying loop that generates a magnetic field. Thus, every material
responds to a magnetic field. The manner in which this response of electrons
and atoms in a material is scaled determines whether a material will be
strongly or weakly magnetic. Examples of ferromagnetic materials are
materials such as Fe, Ni, Co, and some of their alloys. Examples of ferrimagnetic
materials include many ceramic materials such as nickel zinc ferrite and
manganese zinc ferrite. The term “nonmagnetic,” usually means that the materia
is neither ferromagnetic nor ferrimagnetic. These “nonmagnetic” materials are
further classified as diamagnetic (e.g., superconductors) or paramagnetic.
In some cases, we also encounter materials that are antiferromagnetic or
superparamagnetic.
Ferromagnetic
and ferrimagnetic materials are usually further classified as either soft or
hard magnetic materials. High-purity iron or plain carbon steels are examples
of a magnetically soft material as they can become magnetized, but when the
magnetizing source is removed, these materials lose their magnet-like behavior.
Permanent magnets or hard magnetic materials retain their
magnetization. These are permanent “magnets.” Many ceramic ferrites are used to
make inexpensive refrigerator magnets. A hard magnetic material does not lose
its magnetic behavior easily.
The
magnetic behavior of materials can be traced to the structure of atoms. The
orbital motion of the electron around the nucleus and the spin of the electron
about its own axis cause separate
magnetic moments. These two motions (i.e., spin and orbital) contribute to the
magnetic behavior of materials. When the electron spins, there is a magnetic
moment
associated with that motion. The magnetic moment of an electron due to
its spin is known as the Bohr magneton (µB). This is a
fundamental constant and is defined as:
Figure taken from :The Science and Engineering of Materials by Donald R. Askeland, Pradeep P. Fulay, Wendelin J. Wright (Very good Book)
Figure shows origin of
magnetic dipoles: (a) The spin of the electron produces a magnetic field with a
direction dependent on the quantum number ms. (b) Electrons orbiting around the
nucleus create a magnetic field around the atom.
The
magnetization M represents the increase in the inductance due to the core material,
so we can rewrite the equation for inductance as
The first part of this equation is simply the effect of the
applied magnetic field. The second part is the effect of the magnetic material that is present. In materials, stress causes strain, electric field (E ) induces
dielectric polarization (P), and a magnetic field (H) causes magnetization (µ0M)
that contributes to the total flux density B.
The magnetic susceptibility (χm) , which is the ratio
between magnetization and the applied field, gives the amplification produced by the material:
Both µr and χm refer to the degree to which
the material enhances the magnetic field and are therefore related by
As noted before, µr and χm, therefore, the χmvalues for ferromagnetic and
ferromagnetic
materials depend on the applied field (H). For ferromagnetic and ferrimagnetic materials, the term µ0M>>µ0H . Thus, for these
materials,
We sometimes interchangeably refer to either inductance or
magnetization. Normally, we are interested in producing a high inductance B or
magnetization M. This is accomplished by selecting materials that have a high
relative permeability or magnetic susceptibility.
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