Magnetic anisotropy is the directional dependence of a material's magnetic properties. In the absence of an applied magnetic field, a magnetically isotropic material has no preferential direction for its magnetic moment, while a magnetically anisotropic material will align its moment with one of the easy axes. An easy axis is an energetically favorable direction of spontaneous magnetization that is determined by the sources of magnetic anisotropy listed below. The two opposite directions along an easy axis are usually equivalent, and the actual direction of magnetization can be along either of them (see spontaneous symmetry breaking).
Magnetic anisotropy is a prerequisite for hysteresis in ferromagnets: without it, a ferromagnet is superparamagnetic.
Read more about Magnetic Anisotropy: Sources of Magnetic Anisotropy, Anisotropy Energy of A Single-domain Magnet
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—Henry David Thoreau (1817–1862)