Weiss domain

Discovered by the French physicist Pierre-Ernest Weiss (1865–1940).

When you create an external magnetic field, the walls of the domains shift. The domains magnetized in the direction of the external field become larger. This is at the expense of domains magnetized in other directions.

As the field strength increases, this process continues until all the domains are magnetized in the direction of the external field. The material is then magnetically saturated.