A ferromagnetic material is composed of mosaic of small regions within each of which all the atomic magnetic moments spontaneously line up parallel to each other by quantum mechanical exchange interaction within a certain temperature range. Each such spontaneously magnetized region is called a domain and the common direction of magnetic moment is called the domain axis. A domain is an extremely small region (e.g., a size of about 10-6 m -10-4 m) containing a large number (1010 – 1017) of atoms. The boundary between adjacent domains with a different orientation of magnetic moment is called a domain wall.
