Magnetocrystalline Anisotropy

Magnetocrystalline anisotropy is the dependence of the internal energy of a ferromagnetic material on the direction of its magnetization. In a crystal lattice, certain crystallographic directions are preferred directions, or easy axes, for the magnetization; the crystal is easier to magnetize in these directions and requires more energy to magnetize in other directions. It is a special case of magnetic anisotropy. The spin-orbit interaction is the primary source of magnetocrystalline anisotropy.

Magnetocrystalline anisotropy has a great influence on industrial uses of ferromagnetic materials. Materials with high magnetic anisotropy usually have high coercivity; that is they are hard to demagnetize. These are called "hard" ferromagnetic materials, and are used to make permanent magnets. For example, the high anisotropy of rare earth metals is mainly responsible for the strength of rare earth magnets. During manufacture of magnets, a powerful magnetic field aligns the microcrystalline grains of the metal so their "easy" axes of magnetization all point in the same direction, freezing a strong magnetic field into the material.

On the other hand, materials with low magnetic anisotropy usually have low coercivity, their magnetization is easy to change. These are called "soft" ferromagnets, and are used to make magnetic cores for transformers and inductors. The small energy required to turn the direction of magnetization minimizes core losses, energy dissipated in the transformer core when the alternating current changes direction.