ferromagnetism - definition

Definition: Phenomenon in some magnetically ordered materials in which there is a bulk magnetic moment and the magnetisation is large. The electron spins of the atoms in the microscopic regions, domains, are aligned. In the presence of an external magnetic field the domains oriented favourably with respect to the field grow at the expense of the others and the magnetisation of the domains tends to align with the field. Above the Curie temperature, the thermal motion is sufficient to offset the aligning force and the material becomes paramagnetic. Certain elements (iron, nickel and cobalt), and alloys with other elements (titanium, aluminium) exhibit permeabilities up to 104 (ferromagnetic materials). Some show marked hysteresis and are used for permanent magnets, magnetic amplifiers etc. In ferromagnetic substances, within a certain temperature range, there are net atomic magnetic moments, which line up in such a way that magnetisation persists after the removal of the applied field. Below a certain temperature, called the Curie point (or Curie temperature) an increasing magnetic field applied to a ferromagnetic substance will cause increasing magnetisation to a high value called the saturation magnetisation. This is because a ferromagnetic substance consists of small magnetised regions called domains. The total magnetic moment of a sample of the substance is the vector sum of the magnetic moments of the component domains.