Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 92

a)

Canted
antiferromagnetism Ferromagnetism Antiferromagnetism Ferrimagnetism

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b)

Figure 3.7 Schematic illustration of the alignment of magnetic
dipoles in materials with different types of magnetism. Redrawn,
with permission, from Harrison and Freiberg ( 2009 ).

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A material s magnetism, or in the case of a rock the

magnetism of an individual mineral grain, can be either
single-domain or multidomain. The greater the number of
domains in alignment, the stronger the magnetism, the
limit being when all the domains are aligned ( Fig. 3.6c ).
In this case the maximum magnetism possible for the
material is reached and it is effectively a single domain.
When the external field is removed the domains revert to
their original state, but if the external field is suf ciently
strong it may cause irreversible changes to the domains.
The material will then have a remanent magnetism.
The most important ferromagnetic material is iron, but
materials with this kind of magnetism rarely occur in the
natural environment. Figure 3.7 shows the various types of
ferromagnetism. In a ferromagnet the intra-domain mag-
netic dipoles are parallel and the material has a strong
intrinsic magnetism and high susceptibility. Materials
where the magnetic dipoles, of equal strength, are antipar-
allel with equal numbers of dipoles in each direction are
known as antiferromagnetic. Imperfect antiparallelism of
the dipoles, i.e. canted antiferromagnetism, may cause a
small intrinsic magnetism, but the material does not
acquire remanence. An example is haematite.
Another form of ferromagnetism occurs either when the
antiparallel sub-domains of the lattice have unequal mag-
netisation or when there is more of one sub-domain type
than the other (when the crystal lattice has two types of
ions with different electron spins). This is known as ferri-
magnetism; ferrimagnetic materials have high positive sus-
ceptibilities and can acquire remanent magnetism. Nearly
all magnetic minerals are ferromagnetic, including mono-
clinic pyrrhotite, maghaemite, ilmenite and magnetite.
For simplicity we will follow Clark ( 1997 ) in referring to
all strongly magnetic minerals as ferromagnetic (sensu
lato). Ferromagnetic materials can have high susceptibility
and can produce very strong magnetic responses in geo-
physical surveys. As the temperature increases, thermal

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c)

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Figure 3.6 Schematic illustration of magnetic domains. In the
unmagnetised piece of iron (a), the domains are randomly oriented.
Imparting an approximate alignment to the domains creates a weak
magnet (b). Magnetic saturation occurs when all the domains are
aligned (c). Note that the sizes of the individual domains are
greatly exaggerated.

that their magnetic dipoles align with an external eld
exhibit a weak positive susceptibility and are paramagnetic.
They can produce very weak magnetic responses visible in
high-resolution geophysical surveys. In both cases only
induced magnetism is possible.

Ferromagnetism
Materials in which the atomic dipoles are magnetically
coupled are known as ferromagnetic; the nature of the
coupling determines the material

s magnetic properties.
Ferromagnetism can be understood using the concept of
magnetic domains. Domains are volumes in the lattice
within which the magnetic vectors are parallel ( Fig. 3.6 ) .
A series of domains with different magnetism represent a
lower energy state than a single uniform direction of mag-
netism; their magnetic fields interact and they align to
minimise the magnetic forces between adjacent domains.
When domains are randomly oriented they cancel each
other and the material is non-magnetic ( Fig. 3.6a ) . In the
presence of an external magnetic field, normally the Earth

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s
magnetic field in the geological environment, the domains
may grow by aligning with the external field leading to a
net magnetism of the object as a whole ( Fig. 3.6b ), i.e. the
material becomes magnetic with an induced magnetism.