spin of the particles lines up at an
angle to the direction of theÜeld,
precessing around this direction. See
also nuclear magnetic resonance.
spinel A group of oxide minerals
with the general formula F2+R 2 3+O 4 ,
where F2+= Mg, Fe, Zn, Mn, or Ni and
R3+= 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 face-
centred cubic arrangement of O2–
ions. The A cations occupy one
eighth of the tetrahedral holes and
the B cations occupy the octahedral
holes. Example of the spinel struc-
ture 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 non-
magnetic 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 labelA molecule or group
that contains an unpaired electron
and can be attached to another mol-
ecule. The spin of the unpaired elec-
tron can be detected by electron
paramagnetic resonance. The tech-
nique of spin labelling is used to in-
vestigate 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 per-
turbation, such as magnetic reso-
nance, 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 curveA curve that sepa-
rates a metastable region from an un-
stable region in the coexistence
region of a binaryÛuid. 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 atÜrst
(see spinodal decomposition). The
spinodal curve is not a sharp bound-
ary in real systems as a result ofÛuc-
tuations.spinodal decompositionThe
process of moving towards equilib-
rium in a part of a phase diagram in
which the order parameter is con-
served. Spinodal decomposition is ob-
served in the quenching of binary
mixtures. See also spinodal curve.spin–orbit couplingAn interac-
tion between the orbital angular mo-
mentum 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 asso-
ciated with spin–orbit coupling are
comparable to the sizes of energies
associated with electrostatic repul-
sion between the electrons, the mul-
tiplets in this case being described as
having intermediate coupling.499 spin–orbit coupling
s