Magnetic Materials

Please Do subscribe this blog by clicking "Joint this Site" Button.

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 (µ_B). 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 (χ_m) , which is the ratio between magnetization and the applied field, gives the amplification produced by the material:

Both µ_r and χ_m refer to the degree to which the material enhances the magnetic field and are therefore related by

As noted before, µ_r and χ_m, therefore, the  χ_mvalues 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.