ning there is also a spin magnetic
moment (see spin); atomic nuclei also
have magnetic moments.
magnetic quantum number See
atom.
magnetism A group of phenomena
associated with magnetic Üelds.
Whenever an electric current Ûows a
magnetic Üeld is produced; as the orbital motion and the *spin of atomic
electrons are equivalent to tiny current loops, individual atoms create
magnetic Üelds around them, when
their orbital electrons have a net
*magnetic moment as a result of
their angular momentum. The magnetic moment of an atom is the vector 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 magnetic Üeld; there are four
main types of magnetic behaviour:
(a) In diamagnetism the magnetization is in the opposite direction to
that of the applied Üeld, i.e. the susceptibility 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 applied Üeld, the direction of the change opposing the
applied Ûux. There is thus a weak
negative susceptibility (of the order
of –10
–8 m
3 mol
–1
) and a relative permeability of slightly less than one.
(b) In paramagnetism the atoms or
molecules of the substance have net
orbital or spin magnetic moments
that are capable of being aligned in
the direction of the applied Üeld.
They therefore have a positive (but
small) susceptibility and a relative
permeability slightly in excess of
one. Paramagnetism occurs in all
atoms and molecules with unpaired
electrons; e.g. free atoms, free radicals, and compounds of transition
metals containing ions with unÜlled
electron shells. It also occurs in metals as a result of the magnetic moments associated with the spins of
the conducting electrons.
(c) In ferromagnetic substances,
within a certain temperature range,
there are net atomic magnetic moments, which line up in such a way
that magnetization persists after the
removal of the applied Üeld. Below a
certain temperature, called the Curie
point (or Curie temperature) an increasing magnetic Üeld applied to a
ferromagnetic substance will cause
increasing magnetization to a high
value, called the saturation magnetization. This is because a ferromagnetic substance consists of small
(1–0.1 mm across) magnetized 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. Within each domain
the individual atomic magnetic moments 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 ferromagnetic material the magnetic moments of the domains themselves are
not aligned; when an external Üeld is
applied those domains that are
aligned with the Üeld increase in size
at the expense of the others. In a
very strong Üeld all the domains are
lined up in the direction of the Üeld
and provide the high observed magnetization. Iron, nickel, cobalt, and
their alloys are ferromagnetic. Above
the Curie point, ferromagnetic materials become paramagnetic.
(d) Some metals, alloys, and transimagnetic quantum number
338
m
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moment (see spin); atomic nuclei also
have magnetic moments.
magnetic quantum number See
atom.
magnetism A group of phenomena
associated with magnetic Üelds.
Whenever an electric current Ûows a
magnetic Üeld is produced; as the orbital motion and the *spin of atomic
electrons are equivalent to tiny current loops, individual atoms create
magnetic Üelds around them, when
their orbital electrons have a net
*magnetic moment as a result of
their angular momentum. The magnetic moment of an atom is the vector 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 magnetic Üeld; there are four
main types of magnetic behaviour:
(a) In diamagnetism the magnetization is in the opposite direction to
that of the applied Üeld, i.e. the susceptibility 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 applied Üeld, the direction of the change opposing the
applied Ûux. There is thus a weak
negative susceptibility (of the order
of –10
–8 m
3 mol
–1
) and a relative permeability of slightly less than one.
(b) In paramagnetism the atoms or
molecules of the substance have net
orbital or spin magnetic moments
that are capable of being aligned in
the direction of the applied Üeld.
They therefore have a positive (but
small) susceptibility and a relative
permeability slightly in excess of
one. Paramagnetism occurs in all
atoms and molecules with unpaired
electrons; e.g. free atoms, free radicals, and compounds of transition
metals containing ions with unÜlled
electron shells. It also occurs in metals as a result of the magnetic moments associated with the spins of
the conducting electrons.
(c) In ferromagnetic substances,
within a certain temperature range,
there are net atomic magnetic moments, which line up in such a way
that magnetization persists after the
removal of the applied Üeld. Below a
certain temperature, called the Curie
point (or Curie temperature) an increasing magnetic Üeld applied to a
ferromagnetic substance will cause
increasing magnetization to a high
value, called the saturation magnetization. This is because a ferromagnetic substance consists of small
(1–0.1 mm across) magnetized 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. Within each domain
the individual atomic magnetic moments 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 ferromagnetic material the magnetic moments of the domains themselves are
not aligned; when an external Üeld is
applied those domains that are
aligned with the Üeld increase in size
at the expense of the others. In a
very strong Üeld all the domains are
lined up in the direction of the Üeld
and provide the high observed magnetization. Iron, nickel, cobalt, and
their alloys are ferromagnetic. Above
the Curie point, ferromagnetic materials become paramagnetic.
(d) Some metals, alloys, and transimagnetic quantum number
338
m
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