ning there is also a spin magnetic
moment (see spin); atomic nuclei also
have magnetic moments.magnetic quantum numberSee
atom.magnetismA group of phenomena
associated with magneticÜelds.
Whenever an electric currentÛows a
magneticÜeld is produced; as the or-
bital motion and the *spin of atomic
electrons are equivalent to tiny cur-
rent loops, individual atoms create
magneticÜelds around them, when
their orbital electrons have a net
*magnetic moment as a result of
their angular momentum. The mag-
netic moment of an atom is the vec-
tor sum of the magnetic moments of
the orbital motions and the spins of
all the electrons in the atom. The
macroscopic magnetic properties of a
substance arise from the magnetic
moments of its component atoms
and molecules. Different materials
have different characteristics in an
applied magneticÜeld; there are four
main types of magnetic behaviour:
(a) In diamagnetismthe magnetiza-
tion is in the opposite direction to
that of the appliedÜeld, i.e. the sus-
ceptibility is negative. Although all
substances are diamagnetic, it is a
weak form of magnetism and may be
masked by other, stronger, forms. It
results from changes induced in the
orbits of electrons in the atoms of a
substance by the appliedÜeld, the di-
rection of the change opposing the
appliedÛux. There is thus a weak
negative susceptibility (of the order
of –10–8m^3 mol–1) and a relative per-
meability of slightly less than one.
(b) In paramagnetismthe atoms or
molecules of the substance have net
orbital or spin magnetic moments
that are capable of being aligned in
the direction of the appliedÜeld.
They therefore have a positive (but
small) susceptibility and a relativepermeability slightly in excess of
one. Paramagnetism occurs in all
atoms and molecules with unpaired
electrons; e.g. free atoms, free radi-
cals, and compounds of transition
metals containing ions with unÜlled
electron shells. It also occurs in met-
als as a result of the magnetic mo-
ments associated with the spins of
the conducting electrons.
(c) In ferromagnetic substances,
within a certain temperature range,
there are net atomic magnetic mo-
ments, which line up in such a way
that magnetization persists after the
removal of the appliedÜeld. Below a
certain temperature, called the Curie
point(or Curie temperature) an in-
creasing magneticÜeld applied to a
ferromagnetic substance will cause
increasing magnetization to a high
value, called the saturation magneti-
zation. This is because a ferromag-
netic substance consists of small
(1–0.1 mm across) magnetized re-
gions called domains. The total mag-
netic moment of a sample of the
substance is the vector sum of the
magnetic moments of the compo-
nent domains. Within each domain
the individual atomic magnetic mo-
ments are spontaneously aligned by
exchange forces, related to whether
or not the atomic electron spins are
parallel or antiparallel. However, in
an unmagnetized piece of ferromag-
netic material the magnetic mo-
ments of the domains themselves are
not aligned; when an externalÜeld is
applied those domains that are
aligned with theÜeld increase in size
at the expense of the others. In a
very strongÜeld all the domains are
lined up in the direction of theÜeld
and provide the high observed mag-
netization. Iron, nickel, cobalt, and
their alloys are ferromagnetic. Above
the Curie point, ferromagnetic ma-
terials become paramagnetic.
(d) Some metals, alloys, and transi-magnetic quantum number 338m