In an atom, the orbital and spin magnetic moments of its electrons may or may not add up to zero, depending on the electronic configuration. In some materials, their atoms have a net magnetic moment. When such a material is placed in an external magnetic field of induction \(\vec {B_0}\), the field exerts a torque on each atomic magnet. These torques tend to align the magnetic moments with the applied field. Due to this, the material as a whole acquires a net magnetic moment \(\vec {M_{net}}\) along \(\vec {B_0}\) and the material is said to be magnetized.
The net magnetic moment per unit volume is called the magnetization \(\vec{M_z}\) of the material.
\(\vec{M_z}\) = \(\vec {M_{net}}\)/V
where V is the volume of the material.
Even when the atomic magnetic moment is zero, application of magnetic field induces magnetism in the material. In this case, the magnetization has direction opposite to that of the applied field.
