tion-element salts exhibit another
form of magnetism called antiferro-
magnetism. This occurs below a cer-
tain temperature, called the Néel
temperature, when an ordered array
of atomic magnetic moments sponta-
neously forms in which alternate mo-
ments have opposite directions.
There is therefore no net resultant
magnetic moment in the absence of
an appliedÜeld. In manganese
Ûuoride, for example, this antiparal-
lel arrangement occurs below a Néel
temperature of 72 K. Below this tem-
perature the spontaneous ordering
opposes the normal tendency of the
magnetic moments to align with the
appliedÜeld. Above the Néel temper-
ature the substance is paramagnetic.
A special form of antiferromagnet-
ism is ferrimagnetism, a type of mag-
netism exhibited by the *ferrites. In
these materials the magnetic mo-
ments of adjacent ions are antiparal-
lel and of unequal strength, or the
number of magnetic moments in one
direction is greater than those in the
opposite direction. By suitable choice
of rare-earth ions in the ferrite lat-
tices it is possible to design ferri-
magnetic substances with speciÜc
magnetizations for use in electronic
components.
magnetiteA black mineral form of
iron oxide crystallizing in the cubic
system. It is a mixed iron(II)-iron(III)
oxide, Fe 3 O 4 , and is one of the major
ores of iron. It is strongly magnetic
and some varieties, known as lode-
stone, are natural magnets; these
were used as compasses in the an-
cient world. Magnetite is widely dis-
tributed and occurs as an accessory
mineral in almost all igneous and
metamorphic rocks. The largest de-
posits of the mineral occur in N Swe-
den.
magnetochemistryThe branch of
physical chemistry concerned with
measuring and investigating the
magnetic properties of compounds. It
is used particularly for studying tran-
sition-metal complexes, many of
which are paramagnetic because
they have unpaired electrons. Meas-
urement of the magnetic susceptibil-
ity allows the magnetic moment of
the metal atom to be calculated, and
this gives information about the
bonding in the complex.magnetomechanical ratioSee
gyromagnetic ratio.magnetonA unit for measuring
magnetic moments of nuclear,
atomic, or molecular magnets. The
Bohr magneton μBhas the value of
the classical magnetic moment of an
electron, given by
μB= eh/4πme= 9.274 × 10 –24Am^2 ,
where e and meare the charge and
mass of the electron and h is the
Planck constant. The nuclear magne-
ton, μNis obtained by replacing the
mass of the electron by the mass of
the proton and is therefore given by
μN= μB.me/mp= 5.05 × 10 –27Am^2.Main-Smith–Stoner ruleAn em-
pirical rule in the theory of atomic
structure stating that for a principal
quantum number n the number of
electronic quantum states that can
have the orbital quantum number l is
2(2l+ 1). This rule describes the sub-
shells of atoms. It was put forward
on the basis of chemical evidence by
J. D. Main-Smith and independently
on the basis of magnetic and spectro-
scopic evidence by Edmund Stoner in- The Main-Smith–Stoner rule is
a consequence of the *Pauli exclu-
sion principle. The rule was one of
the key developments that led to the
enunciation of the Pauli exclusion
principle in 1925.
malachiteA secondary mineral
form of copper carbonate–hydroxide,
339 malachite
m