Geology Reference
In-Depth Information
3.5
Magnetic Properties
of Rocks
It is not exaggerated to say that the ferromagnetic
properties of some crystalline solids are at the
base of the plate tectonics revolution during the
1960s. In ferromagnetic materials, the permanent
magnetic moments of neighbor atoms are not
independent as a consequence of electrostatic
interactions and quantum phenomena, which de-
termine their alignment and a magnetization that
can be several orders of magnitude greater than
the paramagnetic response induced by the same
external field. Ultimately, such a large magne-
tization is a consequence of the tendency of
unpaired electrons of neighbor atoms to avoid
sharing of their orbits, in which case they would
acquire opposing spins, and to align their intrinsic
magnetic moments.
This interaction exists independently from the
application of an external field, and determines
a spontaneous alignment of the spins through
distinct regions of each crystal called
magnetic
domains
(Fig.
3.11
). Therefore, a single magnetic
domain has a net non-zero
spontaneous magne-
tization
even in absence of external field. The
reason for which a ferromagnetic substance does
not reveal, in normal conditions, any apparent
magnetization is illustrated in Fig.
3.11
. Each
crystal grain has a preferred direction of magneti-
zation, and is divided into a series of magnetic do-
mains whose spontaneous magnetization is alter-
nate and parallel to this direction. At macroscopic
scale, these preferred directions are randomly
distributed. Therefore, the net magnetization is
zero. When we apply an external field
H
,the
domain walls start moving to favor the growth of
domains with a direction of magnetization close
to that of the applied field and the simultaneous
reduction of size for the other domains. For small
values of
H
, this process is reversible, so that if
we remove the external field the magnetization
returns to zero. If the applied field increases
sufficiently, the domain walls are progressively
destroyed, as illustrated in Fig.
3.12
, until the
total magnetization reaches a saturation value,
which corresponds to a complete alignment of
Fig. 3.11
Arrangement of mineral grains (in
gray
)and
magnetic domains (regions separated by
thin lines
)in
an unmagnetized polycrystalline solid.
Arrows
are spin
directions
Fig. 3.12
Arrangement of mineral grains and magnetic
domains in a magnetized polycrystalline solid close to
saturation
the spins. Now let us imagine to reduce pro-
gressively the intensity of the external field. In
this instance, the domains start reforming, but
the progression is not exactly the reverse of the
previous one. In fact, a key feature of the process
described above is represented by the relevant
loss of internal energy in so far as the external
field increases and the mineral grains rearrange
their magnetic domains. This is a consequence of
the presence of crystal defects, which prevents a
continuous adaptation of the domain geometry as
the magnitude of the external field increases. The
jerky rearrangement of the domains wall geome-
try determines the formation of eddy currents that
