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Magnetic Susceptibility / Permeability

The basics

Magnetic susceptibility is the ability of a material to become magnetised by an external magnetic field. It is (in the SI system) unitless & can be anisotropic although this is rarely considered in practical studies. An oscillating magnetic field will produce slightly different values of susceptibility if the frequency of oscillation is changed, due to physical factors like magnetic grain size. The value of susceptibility is usually expressed as a volumetric measure but after purification it is possible to measure mass-specific susceptibility for individual materials.

Magnetic permeability is a similar quantity & is the ability of a material to become magnetised by an electromagnetic field. It is of greatest relevance when considering the passage of electromagnetic radiation through a medium, e.g., radar.

Type of magnetism	Character	Example material
Diamagnetism	Susceptibility usually weak & opposing the ambient field direction	Peat
Paramagnetism	Very weakly susceptible	Lepidocrocite gamma-Fe₂O₃.H₂O & geothite FeOOH
Ferrimagnetism	Strongly susceptible & supports magnetic remanence	Cubic & inverse spinel oxides of iron magnetite Fe₃O₄ & maghaemite gamma-Fe₂O₃
Antiferromagnetism	Weakly susceptible & supports magnetic remanence	Rhombohedral hydroxide haematite alpha-Fe₂O₃ Soils over Devonian & Triassic rocks

Mechanisms of enhancement

In soils the principal mechanism is the Le Bourgne effect in which antiferromagnetic haematite is heated in a reducing atmosphere to form magnetite & then allowed to cool & re-oxidise to maghaemite. This can be summarised as:

alpha-Fe₂O₃ --(reduction)--> Fe₃O₄ --(oxidation)--> gamma-Fe₂O₃
For this to happen organic material must be present during heating to create the reducing atmosphere & temperatures should reach about 500^oC.

In addition, topsoil is almost universally two or three times as susceptible as the parent material below. Two possible mechanisms have been proposed, both using the above chemical pathway. One mechanism may simply be the slow accumulation within the soil of maghaemite particles from various thermal events including scrub fires, lightning strikes etc. The other mechanism seems to relate to bacterial activity although this is not well understood.

Some relatively non-magnetic rocks like chalk & some limestones produce magnetic soil on weathering. This is due to the liberation into the soil of individual crystals of highly magnetic magnetite that was originally deposited within the organisms from which the rock was formed.