Magnetisaation

Magnetisaation is the process by which a material develops a net magnetic moment due to the alignment of its internal magnetic moments in response to external influences or internal order. The term covers both induced magnetisation, produced when moments align with an applied magnetic field, and spontaneous magnetisation, which occurs in certain materials that exhibit magnetic order even without an external field, such as ferromagnets and ferrimagnets.

In macroscopic descriptions, magnetisation M is the magnetic moment per unit volume. It is related to the magnetic field B and the auxiliary field H by B = μ0(H + M) in SI units, where μ0 is the vacuum permeability. Important characteristics include the saturation magnetisation Ms, the remanent magnetisation Mr (the residual moment after removing the field), and the coercive field Hc (the field required to reduce magnetisation to zero). The relationship between M and H often forms a hysteresis loop, especially in ferromagnetic and ferrimagnetic materials, reflecting energy losses during cycling.

Temperature plays a key role: many magnets lose their order above the Curie temperature, at which spontaneous magnetisation vanishes. Materials are often classified as soft magnets (low Hc, easy magnetisation and demagnetisation) or hard magnets (high Hc, stable remanence).

Common materials include iron, nickel, cobalt and their alloys, ferrites, and rare-earth magnets such as NdFeB. Measurement techniques range from vibrating-sample magnetometry (VSM) and superconducting quantum interference devices (SQUID) to magneto-optical methods.

Applications span data storage, permanent magnets in motors and generators, transformers, and various sensing technologies.