Magnetization

Magnetization, M, is the magnetic moment per unit volume, V of a material, defined in terms of the magnetic moments of its constituents by:
 * $$\mathbf{M}=\frac{1}{V}\sum_{j=1}^N \mathbf{m_j}$$

where the magnetic moment mj of the j-th constituent is a vector property that determines the torque the object experiences in a magnetic field tending to align its moment with the field. Here, N is the number of magnetic moments in the volume V. The M-field is measured in amperes per meter (A/m) in SI units. Usually the magnetization is referred to a particular location r by imagining the volume V to be microscopic, and is anticipated to change with time t in the general case (perhaps because the moments are moving), defining a magnetization field, M(r, t).

At a microscopic level, the origin of the magnetic moments responsible for magnetization is traced to angular momentum, such as due to motion of electrons in atoms, or to spin, such as the intrinsic spin of electrons or atomic nuclei.