Magnetic anisotropy in rocks

Summary

A method of deriving the remanent coercivity spectrum from the isothermal remanence vs. applied field characteristic is described and applied to a series of Triassic sandstones and late Cretaceous–early Tertiary clays and limestones from France. Low- (0-1 kOe), intermediate- (1-3 kOe), and high- (3-18 kOe) coercivity remanence fractions are identified with magnetite, specularite, and haematite pigment respectively. In the limestones, specularite is the only important magnetic phase; the sandstones contain in addition considerable pigment; and the clays contain all three phases. The relative pigment content indicated magnetically agrees with the colour of each rock type. When the sandstones are heated in air, enough magnetite is produced below 675°C to account for 20-25 per cent of the saturation remanence. Secondary magnetite production may therefore be serious when red beds are thermally demagnetized, even though the heating is done in air. The source of this secondary magnetite seems to be a non-magnetic mineral, rather than haematite. Secondary haematite production is insignificant below 675°C, but the pigment coercivity spectrum becomes harder with annealing, as observed previously with synthetic fine-particle haematites. Either annealing-out of defect ferromagnetism or impurities entering the haematite lattice could explain the results. Either coercivity spectra or rotational hysteresis curves can be used to estimate pigment/specularite ratios in red beds and to monitor mineralogical changes resulting from heating, but the coercivity spectrum method is much simpler to use.

Magnetic Mineralogy of Unheated and Heated Red Sediments by Coercivity Spectrum Analysis

A quantitative investigation of the current-induced torque in antiferromagnets represents a great challenge due to the lack of an independent method for controlling N\'eel vectors. By utilizing an antiferromagnetic insulator with the Dzyaloshinskii-Moriya interaction $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$, we show that the N\'eel vector can be controlled with a moderate external field, which is further utilized to calibrate the current-induced magnetic dynamics. We find that the current-induced magnetoresistance change in antiferromagnets can be complicated by resistive switching that does not have a magnetic origin. By excluding nonmagnetic switching and comparing the current-induced dynamics with the field-induced one, we determine the nature and magnitude of current-induced effects in $\mathrm{Pt}/\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ bilayer films.

Quantitative Study on Current-Induced Effect in an Antiferromagnet Insulator/Pt Bilayer Film.

SUMMARY Five goethite concentrates, separated from natural samples of different supergene geological origin, are sieved in 12 well-defined grain-size fractions ranging from 250 pm down to <5 pm. The goethites show differences in their isomorphous substitution and in their contents of dispersed non-Fe elements. The crystallite size ranges from 240 to 480& which is considerably smaller than the grain size of even the smallest fraction. Nevertheless grain-size dependent trends are present in most observed rockmagnetic parameters. The behaviour in a grain-size dependent framework is reported for the initial susceptibility (Xi"), the high field susceptibility (xhf), the saturation magnetization (J,), the saturation remanence (Jrs; acquired in fields of 16 X lo6 Am-' or 20 T), the coercive force (H,), ferromagnetic coercive force (H, remanent coercive force (Her) and the remanent acquisition coercive force (Her,). X,,, H, and H,, decrease with grain size for each sample. A tendency to decrease with grain size is observed for Jr,. Xi, shows a weak maximum in the intermediate grain-size range. H,,,,,. is approximately constant in the coarse grain-size fractions and falls in the same grain-size range as Xi, rises. No grain-size dependence of J, is observed. Her. does not show a consistent grain-size dependence: in three samples it is grain-size independent and the other two it decreases with grain sue. There is a great variety between the samples in the absolute figures of J, (30-450 X Am2 kg-I), J,, (15-115 X lop3 Am2 kg-I), H, (2-20 x lo4 Am-'), H,,,,,. (2-70 X lo4 Am-'), H,, (40-320 X lo4 Am-') and H,,, (100-550 X lo4 Am-'). Xi, shows smaller differences (0.50-1.50 X m3 kg-I) and Xh, is approximately equal for all samples (varying from 0.40 x lop6 m3 kg-' down to 0.20 x lop6 m3 kg-' with decreasing grain size). Sample differences and some ratios of these parameters are discussed in terms of different goethite chemical composition, crystallite size and the presence of intergrowths. The magnetic properties of mirocrystalline goethite are best understood by taking into account crystallite size and magnetic interaction amongst crystallites.

Magnetic properties of natural goethite. I: Grain-size dependence of some low- and high-field related rockmagnetic parameters measured at room temperature

A critical review of the heavy elastica

The stress induced anisotropy has been determined for several natural crystals of hematite for compressions of up to 300 Kg/cm2 along the trigonal axis and along different directions within the basal plane. The induced anisotropy is predominantly uniaxial, but substantial higher harmonics are present for compressions along the trigonal axis. It is larger for compressions within the basal plane in accordance with the magnetostriction experiments of Urquhart and Goldman. Stress induced anisotropy as a possible explanation of the high coercive forces of hematite powders is discussed, as shape and intrinsic crystalline anisotropy are negligible.

Hartmut Porath

Papers

Stress induced magnetic anisotropy in natural single crystals of hematite