tion-element salts exhibit another
form of magnetism called antiferromagnetism. This occurs below a certain temperature, called the Néel
temperature, when an ordered array
of atomic magnetic moments spontaneously forms in which alternate moments have opposite directions.
There is therefore no net resultant
magnetic moment in the absence of
an applied Üeld. In manganese
Ûuoride, for example, this antiparallel arrangement occurs below a Néel
temperature of 72 K. Below this temperature the spontaneous ordering
opposes the normal tendency of the
magnetic moments to align with the
applied Üeld. Above the Néel temperature the substance is paramagnetic.
A special form of antiferromagnetism is ferrimagnetism, a type of magnetism exhibited by the *ferrites. In
these materials the magnetic moments of adjacent ions are antiparallel 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 lattices it is possible to design ferrimagnetic substances with speciÜc
magnetizations for use in electronic
components.
magnetite A 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 lodestone, are natural magnets; these
were used as compasses in the ancient world. Magnetite is widely distributed and occurs as an accessory
mineral in almost all igneous and
metamorphic rocks. The largest deposits of the mineral occur in N Sweden.
magnetochemistry The branch of
physical chemistry concerned with
measuring and investigating the
magnetic properties of compounds. It
is used particularly for studying transition-metal complexes, many of
which are paramagnetic because
they have unpaired electrons. Measurement of the magnetic susceptibility allows the magnetic moment of
the metal atom to be calculated, and
this gives information about the
bonding in the complex.
magnetomechanical ratio See
gyromagnetic ratio.
magneton A unit for measuring
magnetic moments of nuclear,
atomic, or molecular magnets. The
Bohr magneton µ B has the value of
the classical magnetic moment of an
electron, given by
µ B = eh/4πm e = 9.274 × 10
–24 A m
2
,
where e and m e are the charge and
mass of the electron and h is the
Planck constant. The nuclear magneton, µ N is obtained by replacing the
mass of the electron by the mass of
the proton and is therefore given by
µ N = µ B .m e /m p = 5.05 × 10
–27 A m
2 .
Main-Smith–Stoner rule An empirical 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 subshells 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 spectroscopic evidence by Edmund Stoner in
1924. The Main-Smith–Stoner rule is
a consequence of the *Pauli exclusion principle. The rule was one of
the key developments that led to the
enunciation of the Pauli exclusion
principle in 1925.
malachite A secondary mineral
form of copper carbonate–hydroxide,
339
malachite
m
www.AzShimi.ir www.AzShimi.com
form of magnetism called antiferromagnetism. This occurs below a certain temperature, called the Néel
temperature, when an ordered array
of atomic magnetic moments spontaneously forms in which alternate moments have opposite directions.
There is therefore no net resultant
magnetic moment in the absence of
an applied Üeld. In manganese
Ûuoride, for example, this antiparallel arrangement occurs below a Néel
temperature of 72 K. Below this temperature the spontaneous ordering
opposes the normal tendency of the
magnetic moments to align with the
applied Üeld. Above the Néel temperature the substance is paramagnetic.
A special form of antiferromagnetism is ferrimagnetism, a type of magnetism exhibited by the *ferrites. In
these materials the magnetic moments of adjacent ions are antiparallel 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 lattices it is possible to design ferrimagnetic substances with speciÜc
magnetizations for use in electronic
components.
magnetite A 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 lodestone, are natural magnets; these
were used as compasses in the ancient world. Magnetite is widely distributed and occurs as an accessory
mineral in almost all igneous and
metamorphic rocks. The largest deposits of the mineral occur in N Sweden.
magnetochemistry The branch of
physical chemistry concerned with
measuring and investigating the
magnetic properties of compounds. It
is used particularly for studying transition-metal complexes, many of
which are paramagnetic because
they have unpaired electrons. Measurement of the magnetic susceptibility allows the magnetic moment of
the metal atom to be calculated, and
this gives information about the
bonding in the complex.
magnetomechanical ratio See
gyromagnetic ratio.
magneton A unit for measuring
magnetic moments of nuclear,
atomic, or molecular magnets. The
Bohr magneton µ B has the value of
the classical magnetic moment of an
electron, given by
µ B = eh/4πm e = 9.274 × 10
–24 A m
2
,
where e and m e are the charge and
mass of the electron and h is the
Planck constant. The nuclear magneton, µ N is obtained by replacing the
mass of the electron by the mass of
the proton and is therefore given by
µ N = µ B .m e /m p = 5.05 × 10
–27 A m
2 .
Main-Smith–Stoner rule An empirical 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 subshells 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 spectroscopic evidence by Edmund Stoner in
1924. The Main-Smith–Stoner rule is
a consequence of the *Pauli exclusion principle. The rule was one of
the key developments that led to the
enunciation of the Pauli exclusion
principle in 1925.
malachite A secondary mineral
form of copper carbonate–hydroxide,
339
malachite
m
www.AzShimi.ir www.AzShimi.com