Geology Reference
In-Depth Information
rationalizing the SI system is that SI susceptibility values
are a factor 4
p
greater than corresponding c.g.s. values.
In a vacuum the magnetic field strength
B
and magne-
tizing force
H
are related by
B
=
m
0
H
where
m
0
is the
permeability of vacuum (4
p
¥ 10
-7
Hm
-1
). Air and
water have very similar permeabilities to
m
0
and so this
relationship can be taken to represent the Earth's mag-
netic field when it is undisturbed by magnetic materials.
When a magnetic material is placed in this field, the
resulting magnetization gives rise to an additional mag-
netic field in the region occupied by the material, whose
strength is given by
m
0
J
i
. Within the body the total
magnetic field, or magnetic induction,
B
is given by
BH J
=
m
+
m
0
0
i
Ferrimagnetism
Ferromagnetism
Antiferromagnetism
Substituting equation (7.6)
Fig. 7.3
Schematic representation of the strength and orientation
of elementary dipoles within ferrimagnetic, ferromagnetic and
antiferromagnetic domains.
B
=
m
H
+
m
kH
=
(
1
+
k
)
m
H
=
m
m
H
0
0
0
R
0
where
m
R
is a dimensionless constant known as the
rela-
tive magnetic permeability
.The magnetic permeability
m
is
thus equal to the product of the relative permeability and
the permeability of vacuum, and has the same dimen-
sions as
m
0
. For air and water
m
R
is thus close to unity.
All substances are magnetic at an atomic scale. Each
atom acts as a dipole due to both the spin of its electrons
and the orbital path of the electrons around the nucleus.
Quantum theory allows two electrons to exist in the
same state (or electron shell) provided that their spins
are in opposite directions. Two such electrons are called
paired electrons and their spin magnetic moments can-
cel. In
diamagnetic
materials all electron shells are full and
no unpaired electrons exist. When placed in a magnetic
field the orbital paths of the electrons rotate so as to pro-
duce a magnetic field in opposition to the applied field.
Consequently, the susceptibility of diamagnetic sub-
stances is weak and negative. In
paramagnetic
substances
the electron shells are incomplete so that a magnetic field
results from the spin of their unpaired electrons. When
placed in an external magnetic field the dipoles cor-
responding to the unpaired electron spins rotate to
produce a field in the same sense as the applied field
so that the susceptibility is positive.This is still, however,
a relatively weak effect.
In small grains of certain paramagnetic substances
whose atoms contain several unpaired electrons, the
dipoles associated with the spins of the unpaired elec-
trons are magnetically coupled between adjacent atoms.
Such a grain is then said to constitute a single
magnetic do-
main
. Depending on the degree of overlap of the electron
orbits, this coupling may be either parallel or antiparallel.
In
ferromagnetic
materials the dipoles are parallel (Fig.
7.3), giving rise to a very strong spontaneous magnetiza-
tion which can exist even in the absence of an external
magnetic field, and a very high susceptibility. Ferromag-
netic substances include iron, cobalt and nickel, and
rarely occur naturally in the Earth's crust. In
antiferromag-
netic
materials such as haematite, the dipole coupling
is antiparallel with equal numbers of dipoles in each
direction. The magnetic fields of the dipoles are self-
cancelling so that there is no external magnetic effect.
However, defects in the crystal lattice structure of an
antiferromagnetic material may give rise to a small net
magnetization, called
parasitic antiferromagnetism
. In
ferri-
magnetic
materials such as magnetite, the dipole coupling
is similarly antiparallel, but the strength of dipoles in
each direction are unequal. Consequently ferrimagnetic
materials can exhibit a strong spontaneous magnetiza-
tion and a high susceptibility. Virtually all the minerals
responsible for the magnetic properties of common
rock types (Section 7.3) fall into this category.
The strength of the magnetization of ferromagnetic
and ferrimagnetic substances decreases with tempera-
ture and disappears at the
Curie temperature
. Above
this temperature interatomic distances are increased
to separations which preclude electron coupling, and
the material behaves as an ordinary paramagnetic
substance.
