Magnetisation

Magnetisation is the vector field that describes the density of magnetic dipole moments within a material. It is measured in amperes per metre (A/m) and signifies how strongly a material responds to a magnetic field. In SI units, the magnetic induction B and the magnetic field H relate to magnetisation M by the equation B = μ0(H + M). For linear, isotropic materials, magnetisation is often written as M = χH, where χ is the volume magnetic susceptibility and μr = 1 + χ is the relative permeability.

Materials exhibit several magnetic behaviours. Diamagnetic materials acquire a weak magnetisation opposite to an applied field, with small, negative susceptibility. Paramagnetic materials have unpaired electrons that align with the field, giving a small positive susceptibility that typically decreases with temperature (Curie or Curie–Weiss law). Ferromagnetic materials show spontaneous magnetisation below the Curie temperature and form domains; their response to fields is nonlinear and may exhibit hysteresis, remanence (residual magnetisation) and coercivity. Ferrimagnetic and antiferromagnetic materials involve more complex arrangements of magnetic moments, leading to net magnetisation that is reduced or compensated.

Microscopically, magnetisation in ferromagnets arises from exchange interactions that align electron spins. Domains form to minimise magnetostatic energy, and magnetisation can change by domain-wall motion or by rotation of moments in response to external fields. At high temperatures, ferromagnets lose spontaneous order and become paramagnetic.

Magnetisation is measured with magnetometers such as vibrating sample magnetometers or SQUIDs. It underpins many technologies, including data storage, electric motors, transformers and sensors, and is a key parameter in characterising magnetic materials.