magnetisationens

Magnetisationen, in physics often called magnetisation, describes the density of magnetic moments per unit volume within a material. It is represented by the magnetisation vector M and denotes how strongly a material responds to magnetic fields. The magnetic flux density B is related to the applied magnetic field H and the magnetisation by the relation B = μ0(H + M) in free space, where μ0 is the vacuum permeability. In linear, homogeneous materials, M = χm H, with χm the magnetic susceptibility.

Materials can exhibit different forms of magnetisation. Spontaneous magnetisation occurs in ferromagnets, ferrimagnets, and some antiferromagnets below characteristic temperatures (the Curie temperature for ferromagnets; the Néel temperature in some antiferromagnets). In these materials, magnetic moments tend to align even without an external field, forming magnetic domains. The overall magnetisation depends on the orientation of these domains and can be changed by external magnetic fields, producing hysteresis and, at high fields, saturation magnetisation Ms.

Paramagnetism and diamagnetism describe weaker, field-induced responses. Paramagnets show a net magnetisation that follows the applied field and disappears when the field is removed; diamagnets generate a magnetisation opposite to the applied field and are typically small and linear with the field.

Measurement of magnetisation is carried out with magnetometry and is often reported as M (in A/m) or via magnetic susceptibility. Temperature, impurities, and crystal structure influence magnetisation through changes in exchange interactions and domain structure.

Applications rely on controlled magnetisation, including data storage, electric motors and transformers, magnetic sensors, and medical imaging. See also magnetisation dynamics, hysteresis, and Curie temperature.