In condensed matter physics, magnetic anisotropy describes how an object's magnetic properties can be different depending on direction. In the simplest case, there is no preferential direction for an object's magnetic moment. It will respond to an applied magnetic field in the same way, regardless of which direction the field is applied. This is known as magnetic isotropy. In contrast, magnetically anisotropic materials will be easier or harder to magnetize depending on which way the object is rotated.

For most magnetically anisotropic materials, there are two easiest directions to magnetize the material, which are a 180° rotation apart. The line parallel to these directions is called the easy axis. In other words, the easy axis is an energetically favorable direction of spontaneous magnetization. Because the two opposite directions along an easy axis are usually equivalently easy to magnetize along, and the actual direction of magnetization can just as easily settle into either direction, which is an example of spontaneous symmetry breaking.

Magnetic anisotropy is a prerequisite for hysteresis in ferromagnets: without it, a ferromagnet is superparamagnetic.

Sources

The observed magnetic anisotropy in an object can happen for several different reasons. Rather than having a single cause, the overall magnetic anisotropy of a given object is often explained by a combination of these different factors:

Magnetocrystalline anisotropy
The atomic structure of a crystal introduces preferential directions for the magnetization.
Shape anisotropy
When a particle is not perfectly spherical, the demagnetizing field will not be equal for all directions, creating one or more easy axes.
Magnetoelastic anisotropy
Tension may alter magnetic behaviour, leading to magnetic anisotropy.
Exchange anisotropy
Occurs when antiferromagnetic and ferromagnetic materials interact.