Magnetic anisotropy is the coupling of the magnetization of a material to particular directions, either at a local or a macroscopic level. It means that the measured magnetic properties depend on the direction in which the magnetic field is applied, and that in zero applied field, the magnetization will lie along preferred direction(s). Without magnetic anisotropy, there can be no coercivity and no remanent magnetic moment and hence no permanent magnets or magnetic recording media. For these technologies, high anisotropy is generally advantageous, as it causes the moment to remain fixed in some desired direction. For magneto-optic recording and proposed perpendicular recording media, perpendicular anisotropy is essential. For present-day longitudinal magnetic recording media, an anisotropy axis along the track direction allows greater thermal stability of magnetic bits, and hence smaller bit sizes. Minimizing anisotropy is equally important for technologies that require high susceptibility, such as transformers. Understanding what causes anisotropy and how to control it is thus crucial to the technological use of nearly all magnetic materials including films.

There are many sources of magnetic anisotropy in solids; for a description (see Magnetism in Solids: General Introduction; Localized 4f and 5f Moments: Magnetism; Transition Metal Oxides: Magnetism; and Density Functional Theory: Magnetism). All sources of anisotropy are fundamentally linked to the structure of the material, and therefore must have the symmetry of that structure. However, the symmetry as measured, for instance, by x rays is not necessarily the same as the local symmetry, and some substantial...