Types of Magnetic Materials

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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.

Superconductors are perfect diamagnets (χ_m=-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 (χ_m) 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, T_c 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.