magnétisation

Magnetization, or magnétisation, is a property of magnetic materials describing how their internal magnetic moments align in response to external stimuli. It is represented by a vector field M, called the magnetization, defined per unit volume. In SI units, M is measured in ampere per metre (A/m). The magnetic flux density B and the magnetic field strength H are related by B = μ0(H + M) in linear, isotropic media; equivalently, M expresses how much the material contributes to the total B beyond the applied field.

At the microscopic level, magnetization arises from the alignment of electron spins and, to a lesser extent, orbital motions. In many materials, moments align due to exchange interactions, forming domains—regions where moments point in the same direction. The macroscopic magnetization is the net moment per unit volume. In paramagnetic and diamagnetic materials, M is proportional to H with a small susceptibility χ: M = χH. In ferromagnets, M can have a spontaneous component below the Curie temperature, and M versus H exhibits hysteresis.

In linear, isotropic, non-hysteretic materials, χ is a material constant, and μ = μ0(1 + χ) is the permeability. In anisotropic or nonlinear media, χ is a tensor and the magnetization depends on the direction of H and its magnitude. Hysteresis leads to remanence and coercivity, while saturation magnetization M_s is reached at strong fields.

Temperature strongly affects magnetization: thermal agitation reduces order, vanishing above the Curie temperature in ferromagnets. Measurement techniques include vibrating-sample magnetometry and SQUID magnetometry; magnetization is a fundamental parameter in devices such as transformers, motors, magnetic recording, and magnetic resonance imaging.