magnetizationmagnetic

Magnetization is the vector field that describes the density of magnetic moments in a material. It is usually denoted M and has units of amperes per meter (A/m). Magnetization arises from the alignment of atomic and ionic magnetic moments, primarily those of unpaired electrons in atoms, and can persist even in the absence of an external field in some materials.

In classical electromagnetism, the response of a material to an applied magnetic field is described by the auxiliary field H and the magnetic flux density B. In SI units, B = μ0(H + M), where μ0 is the vacuum permeability. For many materials, M is proportional to H at low fields, M = χ_m H, defining the magnetic susceptibility χ_m. The proportionality factor yields the relative permeability μ = μ0(1 + χ_m).

Materials show different types of magnetism. Diamagnetic materials develop a small opposing magnetization (χ_m < 0). Paramagnetic materials have a small parallel magnetization (χ_m > 0 but small). Ferromagnetic materials can exhibit spontaneous magnetization from exchange interactions between electrons, forming magnetic domains. In zero field these domains are oriented randomly, but an external field or internal anisotropy can align them, producing strong magnetization. Ferromagnets often show hysteresis, with a saturation magnetization, a remanent magnetization, and a coercive field. Above the Curie temperature, ferromagnets lose their spontaneous magnetization and become paramagnetic.

At the microscopic level magnetization reflects the collective alignment of electron spins and orbital motions. Domain formations, anisotropy, temperature, and frequency of excitation influence the magnitude and dynamics of M. In practice, magnetization is measured with instruments such as vibrating sample magnetometers or SQUID magnetometers and is essential for devices ranging from transformers and motors to data storage and magnetic sensing.