Chapter 6. Non-linear media
The best known examples of non-linear media are the ferromagnetic materials.
Ferromagnetic materials remain magnetized even when the external magnetic field is removed (they thus exhibit a non-linear dependence on the external field).
In ferromagnetic materials the individual dipoles remain aligned when the external magnetic field is removed.
The alignment can be destroyed by random thermal motion when the material is heated up (the transition temperature is called the Curie temperature).