Anisotropy is a dependence of energy level on some direction. If the magnetic moments in a material have a bias towards one particular direction (the easy axis) then the material is said to have uniaxial anisotropy, like cobalt. If the bias is towards many particular directions, then the material has multiple easy axes and it possesses cubic anisotropy. Cubic crystals such as iron and nickel have this property. Uniaxial and cubic anisotropy are forms of magnetocrystalline anisotropy as their properties in this respect arise from the crystalline structure of the material.

Figure 1 Energy density due to uniaxial anisotropy as a function of the angle θ from a magnetic moment μ. The maximum energy has been normalised to zero for clarity.

Figure 2 : Normalised cubic anisotropy energy surfaces w_c (θ,φ) for (left) iron and (right) nickel. The different shapes of the surfaces are a reflection of the sign of K₁ -- iron has a positive K₁, nickel a negative K₁

The anisotropy energy in transition metal magnets arises from spin-orbit coupling.