INTRODUCTION

Up till now we have talked about magnetic fields and how they can be produced by electrical currents. We have also talked about anomalous magnetic fields associated with magnetite ore deposits and other geological bodies. Why do these bodies influence the magnetic field in their vicinity? What’s meant by "magnetization" of materials? What’s the connection between magnetic properties and electric currents? Where are the currents? Why do they exist?

MAGNETIZATION

Magnetization simply means the (net) dipole moment per unit volume of a material. Thus it is measured in Amperes per meter. The magnetization may require an external magnetic field to sustain it (induced magnetization) or it may simply sustain itself (remnant magnetization, permanent magnetization).

INDUCED MAGNETIZATION AND SUSCEPTIBILITY

This kind of magnetization is dependent on the presence of an external magnetic field. For weak external fields like that of the earth, the induced magnetization is proportional to the external field and is reversible. If the external field vanishes so does the induced magnetization. Many magnetic anomalies, especially on the continents, are caused by induced magnetization.

Let’s do a simple experiment to clarify the relationship between the external field and the magnetization. First we’ll obtain a long helix of wire (a "solenoid") and send a steady current through it. Inside the solenoid we now have a uniform magnetic field (Bo). Next we insert a long rock core into the solenoid and note a slightly different field (B) which is the sum of Bo and a field from the sample (Bm).

If we use a weak magnetic field (Bo), we’ll discover than Bm is proportional to Bo. The slope of the graph of Bm versus Bo is just "k", the magnetic susceptibility. By its definition as a ratio, susceptibility is a dimensionless number. For almost all earth materials k is much, much less than one. In the old cgs system of units k is also dimensionless but equals k in the modern SI system divided by 4 pi. Be very careful using tables of susceptibilities as the units are often messed up.

For earth materials we can often treat k as a scalar quantity. In advanced treatments (ES934) we’ll see that many earth materials are magnetically anisotropic from various textural causes and that k should be represented by a tensor.

The induced magnetization (M) of the sample is defined as Bm divided by the permeability of free space (vacuum). The definition in terms of dipole moment per unit volume suggests a picture of so many little aligned unit dipoles in a given volume of material. What are these little dipoles?

DIAMAGNETIC MATERIALS

For these materials the susceptibility and Bm are both negative. In these materials the little dipoles are simply electrons orbiting around atomic nuclei; orbiting electrons are equivalent to a circular electrical current flowing in the opposite sense. In the absence of an external field (Bo) these dipoles are randomly aligned and the magnetization is zero. When an external field is applied the orbiting electrons speed up or slow down depending on their orientation (an expression of Lenz‘s Law). In either case the fields of the electrons, as found using the Biot Savart Law, are opposite to Bo. In other words, the susceptibility is negative.

Although these effects on the orbiting electrons occur in all materials, in most rocks and minerals other effects predominate. Common minerals such as quartz, calcite, halite, galena, sphalerite and graphite are diamagnetic. K is of the order of about 10^-5 in the SI system.

PARAMAGNETIC MATERIALS

In these substances the magnetization is caused by alignment of unpaired electron spins and, as "spin" implies, can only be understood in terms of quantum physics. It turns out that electrons mostly exist in pairs with equal and opposite spins that have no net magnetic effect. However some elements such as Fe, Ni and Co have unpaired electrons. These unpaired spinning electrons act like tiny dipoles and, just like a current loop, can line up with Bo and thus augment it so Bm and k are positive. As Bo is increased the alignment becomes better and thus Bm increases. At values of the external field much larger than the field of the earth, the alignment becomes essentially perfect and Bm saturates. In contrast, random oscillations of the dipoles at high temperatures reduce Bm so that at depth in the earth minerals cannot be significantly paramagnetic.

The ferromagnesian silicates such as olivine, pyroxene, amphibole, biotite and garnet are common paramagnetic minerals. K is of the order of 10^-3.

"FERROMAGNETIC MINERALS"

In paramagnetic minerals the spinning electrons act more or less independently of their neighbors. In ferromagnetic minerals, in contrast, it is possible for spins of nearby electrons to line up ("coupled spins"). In these cases k is positive and can be very large, even bigger than one! As with paramagnetic minerals, magnetization is prevented or destroyed by thermal oscillations at high temperatures.

The ferromagnetic minerals (using the term broadly) contain elements such as Fe, Ni and Co. Common examples are magnetite, hematite, ilmenite and pyrrhotite. These minerals are often present in only trace quantities in rocks but, because of the huge values of k, may completely dominate the magnetic susceptibility of the rock.