'Magnetic anisotropy' means that magnetic properties depend on the direction in which they are measured. The design of some ferromagnetic materials of commercial importance depends on their magnetic anisotropy.
Microscopically, when the magnetization of a magnetic domain is aligned with a preferred crystallographic direction (e.g. <100> in iron), the magnetocrystalline energy is a minimum. These preferred directions are called 'axes of easy magnetization'. In a monocrystalline material or a polycrystalline material with a strong crystallographic texture, the microscopic easy axes of magnetization are found again on a macroscopic scale through the existence of a bulk direction of easy magnetization. This is usually termed magnetocrystalline anisotropy, because it is solely related to crystal symmetry. All other forms of magnetic anisotropy are induced. For example, magnetic anisotropy has been observed in some ferromagnetic alloys after cold rolling. It has been attributed to the rolling process, and has been termed 'roll magnetic anistropy'.
In our studies, magnetic anistropy has been recorded by detecting magnetic Barkhausen noise at a particular angle with respect to the specimen's axis. A parameter termed 'MBNenergy' was computed and plotted vs. angle. The elongation of the polar plot in a specific direction indicates a magnetic easy axis. The figure below shows an isotropic signal (0 % longitudinal surface strain) and a stress-induced magnetic easy axis (0.07 % and 0.20 % longitudinal surface strain).
