Geology Reference
In-Depth Information
Fig. 3.18
Antiferromagnetic arrangement of Nickel
II
ox-
ide (NiO), which has the fcc crystal structure of NaCl,
with octahedral Ni
II
(not displayed). The
A
and
B
sublattices are shown in
green
and
yellow. Arrows
show the spin directions of Ni
atoms
(
black
and
red circles
)andO
2
sites
from the combined effect of electrostatic interac-
tion and a quantum-mechanical phenomenon, the
latter being the Pauli exclusion principle.
Although this description of ferromagnetism
explains the magnetic behavior of many metals,
the two series of magnetic minerals that are of
interest for plate tectonics, titanomagnetites and
titanohematites, belong to two special classes of
ferromagnetic materials, characterized by an “un-
usual” alignment of the atomic spins. Hematite
is an example of
antiferromagnetic
mineral. In
antiferromagnetism, the crystal lattice can be ide-
ally divided in two disjoint subsystems (or
sub-
lattices
),
A
and
B
, in such a way that neighboring
spins are coupled only within each subsystem,
where they have parallel alignment. Furthermore,
the magnetic moments belonging to adjacent sub-
lattices have antiparallel orientation, as shown
in the example of Fig.
3.18
. Therefore, if the
two subsystems have equal magnetic moment, the
resulting net magnetization will be zero.
In general, antiferromagnetic minerals have no
spontaneous magnetization and display a weak
magnetism, with a susceptibility ranging from
10
5
to 10
2
, similar to that of paramagnetic
materials. However, differently from the latter
substances, antiferromagnetic materials present
an ordered structure. In some cases the magnetic
moments of the
A
and
B
layers are not perfectly
antiparallel, so that a non-zero net magnetization
results from the vector difference between
the two spins. This kind of ferromagnetism
is referred to as
canted antiferromagnetism
.
Figure
3.19
shows the canted antiferromagnetic
arrangement of hematite, in which the small
net magnetization resulting from the imperfect
alignment of Fe
3C
spins is nearly perpendicular
to the crystal sublattices. If we place an
antiferromagnetic solid in an external magnetic
field parallel to the spin axes, the torque exerted
on the elementary current loops is nearly zero,
so that the ordered spin arrangement is not
disturbed. Therefore, the magnetic susceptibility
is smaller than that of a normal paramagnetic
substance. However, in so far as we increase
the temperature, the ordered arrangement tends
to be destroyed, so that the resulting canted
structure determines an
increase
of susceptibility,
contrarily to the case of paramagnetic materials.
However, above a critical value of temperature
the spin ordering disappears completely
and the solid behaves like a paramagnetic
substance. This temperature, which is called
Néel temperature
,
T
n
, is more appropriate than
the Curie temperature to describe the transition
point from antiferromagnetic arrangement to
paramagnetic disorder. The Néel temperature
is associated with a maximum of magnetic
susceptibility, as shown in Fig.
3.20
. In the case
of hematite, we have
T
n
Š
673
ı
C.
The other special class of ferromagnetic ma-
terials that are characterized by “unusual” align-
ment of the atomic spins is represented by the
ferrimagnetic
solids. Magnetite is an important
example of this class of magnetic minerals. In fer-
rimagnetic substances, the
A
and
B
sublattices of
an antiferromagnetic spin arrangement are occu-
pied by different magnetic atoms and sometimes
