spin of the particles lines up at an
angle to the direction of the Üeld,
precessing around this direction. See
also nuclear magnetic resonance.
spinel A group of oxide minerals
with the general formula F
2+ R 2
3+ O 4 ,
where F
2+ = Mg, Fe, Zn, Mn, or Ni and
R
3+ = Al, Fe, or Cr, crystallizing in the
cubic system. The spinels are divided
into three series: spinel (MgAl 2 O 4 ),
*magnetite, and *chromite. They
occur in high-temperature igneous or
metamorphic rocks.
spinel structure An ionic crystal
structure shown by compounds of
the type AB 2 O 4 . There is a facecentred cubic arrangement of O
2–
ions. The A cations occupy one
eighth of the tetrahedral holes and
the B cations occupy the octahedral
holes. Example of the spinel structure are MgAl 2 O 4 , Fe 3 O 4 , and Mn 3 O 4 .
A
• An interactive version of the structure
spin glass An alloy of a small
amount of a magnetic metal
(0.1–10%) with a nonmagnetic metal,
in which the atoms of the magnetic
element are randomly distributed
through the crystal lattice of the nonmagnetic element. Examples are
AuFe and CuMn. Theories of the
magnetic and other properties of
spin glasses are complicated by the
random distribution of the magnetic
atoms.
spin label A molecule or group
that contains an unpaired electron
and can be attached to another molecule. The spin of the unpaired electron can be detected by electron
paramagnetic resonance. The technique of spin labelling is used to investigate proteins and biological
systems.
spin–lattice relaxation A process
in which electrons in a crystal return
to the distribution for equilibrium
statistical mechanics after some perturbation, such as magnetic resonance, has caused more electron
spins to be in high-energy states. The
excess energy of these high-energy
states is taken up by vibrations of the
lattice of the solid.
spinodal curve A curve that separates a metastable region from an unstable region in the coexistence
region of a binary Ûuid. Above the
spinodal curve the process of moving
towards equilibrium occurs by
droplet nucleation, while below the
spinodal curve there are periodic
modulations of the order parameter,
which have a small amplitude at Ürst
(see spinodal decomposition). The
spinodal curve is not a sharp boundary in real systems as a result of Ûuctuations.
spinodal decomposition The
process of moving towards equilibrium in a part of a phase diagram in
which the order parameter is conserved. Spinodal decomposition is observed in the quenching of binary
mixtures. See also spinodal curve.
spin–orbit coupling An interaction between the orbital angular momentum and the spin angular
momentum of an individual particle,
such as an electron. For light atoms,
spin–orbit coupling is small so that
*multiplets of many-electron atoms
are described by *Russell–Saunders
coupling. For heavy atoms, spin–orbit
coupling is large so that multiplets of
many-electron atoms are described
by *j-j coupling. For medium-sized
atoms the sizes of the energies associated with spin–orbit coupling are
comparable to the sizes of energies
associated with electrostatic repulsion between the electrons, the multiplets in this case being described as
having intermediate coupling.
499
spin–orbit coupling
s
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angle to the direction of the Üeld,
precessing around this direction. See
also nuclear magnetic resonance.
spinel A group of oxide minerals
with the general formula F
2+ R 2
3+ O 4 ,
where F
2+ = Mg, Fe, Zn, Mn, or Ni and
R
3+ = Al, Fe, or Cr, crystallizing in the
cubic system. The spinels are divided
into three series: spinel (MgAl 2 O 4 ),
*magnetite, and *chromite. They
occur in high-temperature igneous or
metamorphic rocks.
spinel structure An ionic crystal
structure shown by compounds of
the type AB 2 O 4 . There is a facecentred cubic arrangement of O
2–
ions. The A cations occupy one
eighth of the tetrahedral holes and
the B cations occupy the octahedral
holes. Example of the spinel structure are MgAl 2 O 4 , Fe 3 O 4 , and Mn 3 O 4 .
A
• An interactive version of the structure
spin glass An alloy of a small
amount of a magnetic metal
(0.1–10%) with a nonmagnetic metal,
in which the atoms of the magnetic
element are randomly distributed
through the crystal lattice of the nonmagnetic element. Examples are
AuFe and CuMn. Theories of the
magnetic and other properties of
spin glasses are complicated by the
random distribution of the magnetic
atoms.
spin label A molecule or group
that contains an unpaired electron
and can be attached to another molecule. The spin of the unpaired electron can be detected by electron
paramagnetic resonance. The technique of spin labelling is used to investigate proteins and biological
systems.
spin–lattice relaxation A process
in which electrons in a crystal return
to the distribution for equilibrium
statistical mechanics after some perturbation, such as magnetic resonance, has caused more electron
spins to be in high-energy states. The
excess energy of these high-energy
states is taken up by vibrations of the
lattice of the solid.
spinodal curve A curve that separates a metastable region from an unstable region in the coexistence
region of a binary Ûuid. Above the
spinodal curve the process of moving
towards equilibrium occurs by
droplet nucleation, while below the
spinodal curve there are periodic
modulations of the order parameter,
which have a small amplitude at Ürst
(see spinodal decomposition). The
spinodal curve is not a sharp boundary in real systems as a result of Ûuctuations.
spinodal decomposition The
process of moving towards equilibrium in a part of a phase diagram in
which the order parameter is conserved. Spinodal decomposition is observed in the quenching of binary
mixtures. See also spinodal curve.
spin–orbit coupling An interaction between the orbital angular momentum and the spin angular
momentum of an individual particle,
such as an electron. For light atoms,
spin–orbit coupling is small so that
*multiplets of many-electron atoms
are described by *Russell–Saunders
coupling. For heavy atoms, spin–orbit
coupling is large so that multiplets of
many-electron atoms are described
by *j-j coupling. For medium-sized
atoms the sizes of the energies associated with spin–orbit coupling are
comparable to the sizes of energies
associated with electrostatic repulsion between the electrons, the multiplets in this case being described as
having intermediate coupling.
499
spin–orbit coupling
s
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