Magnetization

[1] It is represented by a pseudovector M. Magnetization can be compared to electric polarization, which is the measure of the corresponding response of a material to an electric field in electrostatics.

The origin of the magnetic moments responsible for magnetization can be either microscopic electric currents resulting from the motion of electrons in atoms, or the spin of the electrons or the nuclei.

Magnetization is not necessarily uniform within a material, but may vary between different points.

The magnetization field or M-field can be defined according to the following equation:

is the volume element; in other words, the M-field is the distribution of magnetic moments in the region or manifold concerned.

This makes the M-field completely analogous to the electric polarization field, or P-field, used to determine the electric dipole moment p generated by a similar region or manifold with such a polarization:

Those definitions of P and M as a "moments per unit volume" are widely adopted, though in some cases they can lead to ambiguities and paradoxes.

[1] The M-field is measured in amperes per meter (A/m) in SI units.

[2] The behavior of magnetic fields (B, H), electric fields (E, D), charge density (ρ), and current density (J) is described by Maxwell's equations.

In diamagnets and paramagnets, the relation is usually linear: where χ is called the volume magnetic susceptibility, and μ is called the magnetic permeability of the material.

[4] and for the bound surface current: so that the total current density that enters Maxwell's equations is given by where Jf is the electric current density of free charges (also called the free current), the second term is the contribution from the magnetization, and the last term is related to the electric polarization P. In the absence of free electric currents and time-dependent effects, Maxwell's equations describing the magnetic quantities reduce to These equations can be solved in analogy with electrostatic problems where In this sense −∇⋅M plays the role of a fictitious "magnetic charge density" analogous to the electric charge density ρ; (see also demagnetizing field).

Rather than simply aligning with an applied field, the individual magnetic moments in a material begin to precess around the applied field and come into alignment through relaxation as energy is transferred into the lattice.

Magnetization reversal, also known as switching, refers to the process that leads to a 180° (arc) re-orientation of the magnetization vector with respect to its initial direction, from one stable orientation to the opposite one.

Technologically, this is one of the most important processes in magnetism that is linked to the magnetic data storage process such as used in modern hard disk drives.

[7] One way to do this is to heat the object above its Curie temperature, where thermal fluctuations have enough energy to overcome exchange interactions, the source of ferromagnetic order, and destroy that order.

Another way is to pull it out of an electric coil with alternating current running through it, giving rise to fields that oppose the magnetization.

[8] One application of demagnetization is to eliminate unwanted magnetic fields.

For example, magnetic fields can interfere with electronic devices such as cell phones or computers, and with machining by making cuttings cling to their parent.