Material with high permeability.

The magnetization induced in a ferromagnetic substance is many orders of magnitude larger than in a paramagnetic or diamagnetic substance.

A ferromagnet is made up of domains within which the electron spins are aligned. When the magnetization of the ferromagnet is zero, the domain spins are randomly aligned with respect to each other. The application of a magnetic field of strength H causes the domains with spin aligned along H to become more dominant. The domain boundaries grow so increasing the magnetization M of the material.

The magnetic flux density B within the ferromagnetic material is given by

The response of B to the applied field H is characterized by an S-shaped curve known as a hysteresis curve. After being magnetized a ferromagnetic material will retain some magnetization (i.e. It becomes a magnet). Placing a ferromagnet in an alternating current will demagnetize the material.

Iron is a ferromagnetic material (hence the name). The magnetic flux densty of iron saturates at about 2 teslas.