Magnetocrystalline anisotropy

In physics, a ferromagnetic material is said to have magnetocrystalline anisotropy if it takes more energy to magnetize it in certain directions than in others. These directions are usually related to the principal axes of its crystal lattice. It is a special case of magnetic anisotropy.The hexagonal lattice cell.The tetragonal lattice cell.The rhombohedral lattice cell. In physics, a ferromagnetic material is said to have magnetocrystalline anisotropy if it takes more energy to magnetize it in certain directions than in others. These directions are usually related to the principal axes of its crystal lattice. It is a special case of magnetic anisotropy. The spin-orbit interaction is the primary source of magnetocrystalline anisotropy. It is basically the orbital motion of the electrons which couples with crystal electric field giving rise to the first order contribution to magnetocrystalline anisotropy. The second order arises due to the mutual interaction of the magnetic dipoles. 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 such that 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. Magnetocrystalline anisotropy arises mostly from spin–orbit coupling. This effect is weak compared to the exchange interaction and is difficult to compute from first principles, although some successful computations have been made. The magnetocrystalline anisotropy energy is generally represented as an expansion in powers of the direction cosines of the magnetization. The magnetization vector can be written M = Ms(α,β,γ), where Ms is the saturation magnetization. Because of time reversal symmetry, only even powers of the cosines are allowed. The nonzero terms in the expansion depend on the crystal system (e.g., cubic or hexagonal). The order of a term in the expansion is the sum of all the exponents of magnetization components, i.e., α β is second order. More than one kind of crystal system has a single axis of high symmetry (threefold, fourfold or sixfold). The anisotropy of such crystals is called uniaxial anisotropy. If the z axis is taken to be the main symmetry axis of the crystal, the lowest order term in the energy is The ratio E/V is an energy density (energy per unit volume). This can also be represented in spherical polar coordinates with α = cos φ sin θ, β = sin φ sin θ, and γ = cos θ: The parameter K1, often represented as Ku, has units of energy density and depends on composition and temperature.