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Diamagnetic Behavior: A magnetic field acting on any atom induces a magnetic dipole for the entire atom by influencing the magnetic moment caused by the orbiting electrons. These dipoles oppose the magnetic field, causing the magnetization to be less than zero. This behavior, called diamagnetism, gives a relative permeability of about 0.99995 (or a negative susceptibility approximately -10-6, note the negative sign). Materials such as copper, silver, silicon, gold, and alumina are diamagnetic at room temperature.
Diamagnetic Behavior: A magnetic field acting on any atom induces a magnetic dipole for the entire atom by influencing the magnetic moment caused by the orbiting electrons. These dipoles oppose the magnetic field, causing the magnetization to be less than zero. This behavior, called diamagnetism, gives a relative permeability of about 0.99995 (or a negative susceptibility approximately -10-6, note the negative sign). Materials such as copper, silver, silicon, gold, and alumina are diamagnetic at room temperature.
Superconductors are perfect diamagnets (χm=-1) ; they
lose their superconductivity at higher temperatures or in the presence of a
magnetic field. In a diamagnetic material, the magnetization (M) direction is
opposite to the direction of applied field (H).
Paramagnetism: When materials have unpaired electrons, a net
magnetic moment due to electron spin is associated with each atom. When a
magnetic field is applied, the dipoles
align with the field, causing a positive magnetization. Because the dipoles do
not interact, extremely large magnetic fields are required to align all of the
dipoles. In addition, the effect is lost as soon as the magnetic field is
removed. This effect, called paramagnetism, is found in metals such as
aluminum, titanium, and alloys of copper. The magnetic susceptibility (χm)
of paramagnetic materials is positive and lies between 10-4 and 10-5.
Ferromagnetism: Ferromagnetic behavior is caused by the
unfilled energy levels in the 3d level of iron, nickel, and cobalt. Similar
behavior is found in a few other materials, including gadolinium (Gd). In
ferromagnetic materials, the permanent unpaired dipoles easily line up with the
imposed magnetic field due to the exchange interaction, or mutual reinforcement
of the dipoles. Large magnetizations are obtained even for small magnetic
fields, giving large susceptibilities approaching 106. Similar to
ferroelectrics, the susceptibility of ferromagnetic materials depends upon the
intensity of the applied magnetic field. This is similar to the mechanical
behavior of elastomers with the modulus of elasticity depending upon the level
of strain. Above the Curie temperature, ferromagnetic materials behave as
paramagnetic materials and their susceptibility is given by the following equation,
known as the Curie-Weiss law:
In this equation, C is a constant that depends upon the material,
Tc is the Curie temperature, and T is the temperature above Tc. Essentially, the same equation
also describes the change in dielectric permittivity above the Curie temperature of
ferroelectrics. Similar to ferroelectrics, ferromagnetic materials show the formation of
hystereis loop domains and magnetic domains.
Antiferromagnetism: In materials such as manganese, chromium, MnO,
and NiO, the magnetic moments produced in neighboring dipoles line up in:
Figure taken from :The Science and Engineering of Materials by Donald R. Askeland, Pradeep P. Fulay, Wendelin J. Wright (Very good Book)
opposition to one another in the magnetic field, even though the
strength of each dipole is very high. This effect is illustrated for MnO in above Figure. These materials are antiferromagnetic and have zero magnetization.
The magnetic susceptibility is positive and small. In addition, CoO and MnCl2
are examples of antiferromagnetic materials.
Ferrimagnetism: In ceramic materials, different ions have
different magnetic
moments. In a magnetic field, the dipoles of cation A may line up
with the field,
while dipoles of cation B oppose the field. Because the strength
or number of dipoles is
not equal, a net magnetization results. The ferrimagnetic
materials can provide good
amplification of the imposed field. We will look at a group of
ceramics called ferrites that
display this behavior in a later section. These materials show a
large, magnetic-field
dependent magnetic susceptibility similar to ferromagnetic
materials. They also show
Curie-Weiss behavior (similar to ferromagnetic materials) at
temperatures above the Curie
temperature. Most ferrimagnetic materials are ceramics and are
good insulators of electricity.
Thus, in these materials, electrical losses (known as eddy current
losses) are much
smaller compared to those in metallic ferromagnetic materials.
Therefore, ferrites are used
in many high-frequency applications.
Superparamagnetism: When the grain size of ferromagnetic and
ferrimagnetic
materials falls below a certain critical size, these materials
behave as if they
are paramagnetic. The magnetic dipole energy of each particle
becomes comparable to
the thermal energy. This small magnetic moment changes its
direction randomly (as a
result of the thermal energy). Thus, the material behaves as if it
has no net magnetic
moment. This is known as superparamagnetism. Thus, if we produce
iron oxide
(Fe3O4) particles in a 3 to 5 nm size, they
behave as superparamagnetic materials. Such
iron-oxide superparamagnetic particles are used to form
dispersions in aqueous or
organic carrier phases or to form “liquid magnets” or ferrofluids.
The particles in the
fluid move in response to a gradient in the magnetic field. Since
the particles form a stable
sol, the entire dispersion moves and, hence, the material behaves
as a liquid magnet.
Such materials are used as seals in computer hard drives and in
loudspeakers as
heat transfer (cooling) media. The permanent magnet used in the
loudspeaker holds the
liquid magnets in place. Superparamagnetic particles of iron oxide
(Fe3O4) also can be
coated with different chemicals and used to separate DNA
molecules, proteins, and
cells from other molecules.
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