Chapter 34
Magnetic Materials
Generally every material responds to a magnetic field in one way or another: materials may be weakly
repelled by a magnet (diamagnetism), weakly attracted to a magnet (paramagnetism), or strongly attracted to
a magnet (ferromagnetism). Each of these phenomena is described below.
34.1 Diamagnetism
In adiamagneticmaterial, the atoms of the material have no magnetic dipole moment. The external magnetic
field alters the speed of electrons in their orbits around the atomic nucleus, which induces an internal magnetic
field that repels the external field.
34.2 Paramagnetism.
In aparamagneticmaterial, the atoms of the material do have a magnetic dipole moment. The dipole mo-
ments of the atoms align themselves with the external magnetic field to create an internal field that is weakly
attracted to the external magnetic field.
34.3 Ferromagnetism
Ferromagnetic materials are strongly attracted to magnets, and can be made into permanent magnets. The
ferromagnetic elements are iron, cobalt, and nickel, along with the rare earth elements gadolinium and dys-
prosium.
In ferromagnetic materials, the material is divided into a number ofmagnetic domains, each of which has
dimensions on the order of 1 mm or so. Within each domain, the atomic dipole moments are aligned in the
same direction. In an unmagnetized ferromagnetic material, each domain has its magnetic moment oriented
in a different (random) direction, so that the dipole moments of the material as a whole tend to cancel out. But
if a ferromagnetic material is exposed to an external magnetic field, the domains will tend to align themselves
with the external field, so that the material as a whole takes on a net dipole moment.
Unlike diamagnetic and paramagnetic materials, ferromagnetic materials respond nonlinearly when placed
in an external magnetic field. When a ferromagnetic material is placed within an external magnetic field, and
the external field is then removed, the field in the ferromagnetic material willnotdisappear; a remanent
magnetic field will remain, turning the material into a permanent magnet. The nonlinear response of a ferro-
magnetic material to an external magnetic field is calledhysteresis(hiss-tuh-REE-sus) (Fig. 34.1).