ð Microscopic examination shows that magnets are composed of tiny regions or domains about 1 mm length or width.

        Each is like a tiny magnet.

ð Nickel, cobalt and some of the rare earths (gadolinium, dysprosium) exhibit a unique magnetic behavior which is called

         ferromagnetism

ð The name comes from the fact that iron (ferrum in Latin) is the most common and most dramatic example.

ð Samarium and neodymium in alloys with cobalt have been used to fabricate very strong rare-earth magnets.

ð Ferromagnetic materials exhibit a long-range ordering phenomenon at the atomic level which causes the unpaired electron spins

        to line up parallel with each other in a region called a domain.

ð Within the domain, the magnetic field is intense, but in a bulk sample the material will usually be unmagnetized because the many

        domains will themselves be randomly oriented with respect to one another.

ð Ferromagnetism manifests itself in the fact that a small externally imposed magnetic field, say from a solenoid, can cause the

        magnetic domains to line up with each other and the material is said to be magnetized.

ð The driving magnetic field will then be increased by a large factor which is usually expressed as a relative permeability for the

         material.

ð There are many practical applications of ferromagnetic materials, such as the electromagnet (previous section)