The simplest form of crystal anistropy is uniaxial anisotropy. For cubic crystals the anisotropy energy can be expressed in terms of the direction cosines (, , ) of the internal magnetisation with respect to the three cube edges. Due to the high symmetry of the cubic crystal this can be expressed in a simple manner as a polynomial series in the direction cosines. This can be simplified further to
When the first term of Equation 5.1 becomes a minimum for directions, whilst for it is a minimum for directions.