Las Capas y Paquetes

If a grain contains complete electron shells, an induced magnetisation will cause electron spins to precess in an antiparallel direction to that of the applied magnetic field, resulting in a weak, negative magnetic moment (Fig. 2.1) (Tarling and Hrouda, 1993). This behaviour

is termed diamagnetism (O’Reilly, 1984; Tarling and Hrouda, 1993; Dunlop and Özdemir, 1997). As magnetisation is directly linked to magnetic susceptibility, diamagnetic minerals (e.g. quartz and calcite) have a weak, negative magnetic susceptibility (<-10^-6 SI; Dunlop and Özdemir, 1997).

All other grains, those containing atoms with incomplete electron shells, are paramagnetic (O’Reilly, 1984; Tarling and Hrouda, 1993; Dunlop and Özdemir, 1997).

The partial alignment of magnetic moments parallel to the applied magnetic field (Fig. 2.1) produces a relatively weak (~10–100 × 10^-6 SI), positive magnetic susceptibility (Tarling and Hrouda, 1993). Paramagnetism, the alignment of isolated and non-interacting first transition series elements (Dunlop and Özdemir, 1997), is therefore predominantly observed in

ferromagnesian silicates (e.g. biotite, pyroxene and amphibole) as well as some Fe-Ti oxides (e.g. ilmenite) and iron sulphides (e.g. pyrite) (Tarling and Hrouda, 1993).

The difference in susceptibility values for diamagnetic and paramagnetic minerals implies that paramagnetism is dominant. However, temperature fluctuations can alter the alignment of electron spins within paramagnetic minerals to an extent that susceptibility is

Diamagnetic

Paramagnetic

Ferromagnetic (s.s.)

Antiferromagnetic

Ferrimagnetic

Canted antiferromagnetic Direction of applied magnetic field

Direction of induced magnetisation (size reflects strength) Direction of susceptibility (size reflects strength)

Fig. 2.1: The magnetic susceptibility to an applied field of materials with various magnetic behaviours (after Tarling and Hrouda, 1993; Dunlop and Özdemir, 1997).

inversely proportional to increasing temperature; the Curie-Weiss law (Dunlop and Özdemir, 1997). In contrast, the magnetic susceptibility of diamagnetic minerals is temperature

independent (Tarling and Hrouda, 1993), implying diamagnetic behaviour may be important at high temperature. Characteristically, both diamagnetic and paramagnetic grains do not hold a magnetisation after the applied external field is removed (Butler, 1992).

2.1.2:2 Ferromagnetism

Ferromagnetic (sensu lato) grains generally produce a strong, positive susceptibility (up to three orders of magnitude greater than paramagnetic grains) in response to an applied external field due to a significant abundance of first transition series elements (Tarling and Hrouda, 1993). As such, only a small quantity of ferromagnetic (s.l.) grains is required to dominate the magnetic signature of a sample. Removal of the external field leaves ferromagnetic (s.l.) grains with a remanent magnetisation for a variable period of time dependent on grainsize (Dunlop and Özdemir, 1997). Although rare in nature, true ferromagnetic (sensu stricto) substances (e.g. pure iron) adopt a strong magnetisation consistently parallel to the applied magnetic field through direct exchange coupling of electrons between adjacent cations, which induces a common spin direction (Fig. 2.1) (Borradaile and Jackson, 2004). However, most minerals are complex structures comprising of cations and anions (Borradaile and Jackson, 2004). Magnetisation is instead controlled by the crystal lattice structure through the formation of magnetic sub-lattices, which instigates superexchange (indirect) coupling of electrons between cations across an intervening anion (Dunlop and Özdemir, 1997).

The magnetic moments of the two cations within the magnetic sub-lattice are directionally opposite, with one parallel to the external field and the other antiparallel (Tarling and Hrouda, 1993). If the net magnetic moments of each direction in the sub-lattice are equal, the grain behaves antiferromagnetically and produces a weak, positive susceptibility (Fig. 2.1) (Tarling and Hrouda, 1993). Occasionally some grains display canted antiferromagnetism, whereby the induced magnetic moments, although still equal, deviate from exact (anti-)parallelism with the external field and result in a slight, positive susceptibility orthogonal to the applied

field (Fig. 2.1) (Tarling and Hrouda, 1993; Borradaile and Jackson, 2004). A significant difference in the amount of first transition series cations in adjacent magnetic sub-lattices results in a strong, positive susceptibility parallel to the external magnetic field and is termed ferrimagnetic behaviour (Fig. 2.1) (Tarling and Hrouda, 1993). Ferromagnetism (s.l.) is superimposed onto paramagnetic behaviour and may be removed by increasing the applied field strength or heating above a grains Curie (ferrimagnetic) or Néel (antiferromagnetic) Temperature (T_C and T_N respectively).

The magnetic response of ferromagnetic (s.l.) grains to an applied field is strongly dependent on grainsize (Butler, 1992). Grains ~>10 µm are divided into discrete volumes of atoms, with a specific net magnetic moment parallel to one of several crystallographic ‘easy’

axes, by Bloch walls and are termed magnetic domains (Fig. 2.2) (Butler, 1992; Dunlop and Özdemir, 1997). Under zero field conditions the magnetic domains and their associated magnetic moment will orient themselves to minimize the internal magnetostatic energy (Fig.

2.2). If a weak magnetic field is applied to a true multidomain (MD) grain (e.g. >100 µm Single-domain

Multidomain

Multidomain

Multidomain

magnetic domain

Bloch wall

Fig. 2.2: Potential imposed configurations of magnetic moments, to reduce the magnetostatic energy, in ferromagnetic (s.l.) grains of variable sizes in the absence of an applied magnetic field (Dunlop and Özdemir, 1997).

for magnetite), the domains with magnetic moments sub-parallel to the external field will grow, by Bloch wall translation, at the expense of differently oriented domains and thus produce a net magnetic moment aligned parallel to the magnetic field (Tarling and Hrouda, 1993). A greater amount of domains oriented parallel to a specifically oriented external field will therefore result in a stronger magnetic susceptibility. The number of magnetic domains decreases with grainsize until only one domain can be accommodated; single-domain (SD) grains for magnetite are < 1 µm (Butler, 1992). As SD grains only contain one domain, the orientation of its magnetic moment reflects its remanent magnetisation (Fig. 2.2). Application of an external magnetic field to an SD grain (anti-)parallel to the remanent magnetisation will generate no magnetic response (Tarling and Hrouda, 1993). Instead, if the applied magnetic field is oriented orthogonally to the original remanent magnetisation, a slight deviation will be induced (Fig. 2.2), i.e. SD grains display a stronger susceptibility along a plane orthogonal to its actual magnetisation (Tarling and Hrouda, 1993). Grains ranging from 1–100 µm are termed pseudo-single-domain (PSD) as crystal lattice imperfections act to pin domain walls, effectively reducing their capability to translate under an applied magnetic field (Dunlop and Özdemir, 1997).