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

In room-temperature hysteresis, 14 submicron hematites (0.12-0.45 µm) had large coercive forces Hc (150-350 mT), while 22 natural 1-5.5 mm hematite crystals had Hc = 0.8-23 mT (basal-plane measurements). Single-domain (SD) and multidomain (MD) hematites owe their high Hc mainly to magnetoelastic anisotropy, caused in fine particles by internal strains and in large crystals by defects like dislocations, with a smaller contribution by triaxial magnetocrystalline anisotropy. A strong correlation between Hc and the defect moment Md measured below hematite's Morin transition also favors magnetoelastic control. Saturation remanence/saturation magnetization ratios Mrs/Ms and coercivity ratios Hcr/Hc (Hcr is remanent coercive force) are distinctive: Mrs/Ms = 0.5-0.9, Hcr/Hc = 1.02-1.17 for MD hematites; Mrs/Ms = 0.5-0.7, Hcr/Hc = 1.45-1.62 for SD hematites. In high-temperature (20-690°C) hysteresis, Hc(T) ~ Ms(T) to a power 1.8-2.4 above 385°C. Magnetoelastic wall pinning by crystal defects is thus more likely than control by domain nucleation which depends on magnetocrystalline anisotropy. Our results compare well with existing Hc vs. crystal size d data. A suggested peak in Hc around 15 µm and a proposed slope change around 100 µm are both questionable. Using only near-saturation data, Hc varies continuously as d−0.61 from ≈0.1 µm to 2 mm. The SD threshold size d0 may be >15 µm but there is no strong evidence that d0 ≈100 µm. Direct domain observations are needed to settle the question. Augmented data sets for Hc and Mrs vs. d show that SD hematite is increasingly affected by thermal fluctuations below ≈0.3 µm and generally confirm a superparamagnetic threshold size ds of 0.025-0.03 µm.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2013JB010739

Hysteresis and coercivity of hematite

We report the successful synthesis of ferrimagnetic magnetite powders made using an arc-discharge method in a partial oxygen atmosphere. X-ray and electron diffraction measurements show that the powders also contain some antiferromagnetic hematite and a small amount of FeO and Fe that has not oxidized. The Raman data show that there is a small fraction of ferrimagnetic maghemite that cannot be seen in the x-ray diffraction data. There is a wide particle size distribution where there are nanoparticles as small as 7 nm, larger faceted nanoparticles, and particles that are up to 25 μm in diameter. The saturation magnetization at high magnetic fields is ∼74% of that found in the bulk magnetite, where the lower value is due to the presence of some antiferromagnetic hematite. The temperature dependence of the saturation magnetization changes at the Verwey transition temperature, and it has a power low dependence with an exponent of 3/2 at low temperatures and 2.23 at high temperatures above the Verwey transitio...

High spin-dependent tunneling magnetoresistance in magnetite powders made by arc-discharge

Domain wall-stress interaction causes pronounced increases in attenuation and decreases in the velocities of ultrasonic (30 MHz) waves in hematite at 298°K relative to the values in a saturating magnetic field. For compressional modes, these effects are about 10 db/cm in attenuation and less than 1% in velocity and may be eliminated by the application of a magnetic field of 2 kilogauss in the basal (c) plane. At zero applied field, it was not possible to propagate shear waves in the single crystal; as the field is varied from 2 to 4 kG, the attenuation decreases by about 30 db/cm, and the velocities increase by as much as 10%. These data are consistent with the fact that hematite exhibits weak ferromagnetism at room temperature, with the spins largely contained in the c plane. The large fields needed to saturate the crystal indicate the presence of a hard component of magnetization. These magnetoelastic interactions can also be eliminated by cooling through the Morin temperature (TM=261°K), below which point the spins are aligned parallel to the trigonal c axis and the weak ferromagnetism vanishes. At low pressures, TM increases by 3.6°K/kb and occurs at 298°K near 30 kb. The results of this study have important consequences for ultrasonic studies of the elastic properties of third-transition metal oxides, for static compression experiments on these materials, and for studies of phase transitions in solids.

Magnetoelastic interactions in hematite: Implications for geophysics

SUMMARY

Grain-size dependence of some remanent magnetic properties of a natural low-temperature haematite is reported. The haematite was obtained from a gossan in Montarnu (France) and upgraded by magnetic separation. Seven grain-size fractions were prepared ranging from 10 down to <0.25 μm by means of microprecision sieving and centrifuging. The haematite has an average crystallite size of 60 nm and contains minor contents of Si, Al and H2O. Its unit cell dimensions are very close to those of ideal haematite.



The magnetic properties of the haematite are strongly dependent on its thermal history. For the original unheated haematite fractions, the isothermal saturation remanence (Jrs) decreases with grain size. The remanent coercive force (Hcr), remanent acquisition coercive force (Hcr,) and median destructive field of the saturation remanence (H1/2I) are maximal in the 1.0-0.5 μm fraction. This grain-size range can be understood as the SD threshold size in analogy with ferrimagnetic minerals. Jrs/(Hcr x Xin) is a useful grain-size indicator. Lowtemperature cycling of Jrs shows identical behaviour for all fractions; the remanence decreases slightly and shows no sharp Morin transition.



After annealing the haematite fractions at 700 °C, Jrs is considerably reduced relative to the original haematite predominantly due to removal of the defect moment. Both Hcr and Hcr, have increased and their maximal values are shifted to the 2.1-1.0 μm fraction. All fractions show a much sharper Morin transition during low-temperature cycling of Jrs due to crystallite enlargement. TRM decreases with grain size.



The different rock-magnetic properties before and after annealing might provide a basis for the discrimination in sediments between a haematite DRM with a high-temperature origin and a haematite CRM which has a low-temperature origin. Two natural red beds, one with a CRM and one with a DRM associated with high-temperature haematite, broadly show the expected differences in coercivity and in annealing behaviour, although a full account of all aspects of the observed behaviour can not yet be given.

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Rockmagnetic properties of fine-grained natural low-temperature haematite with reference to remanence acquisition mechanisms in red beds

Powder patterns of magnetic domains on various crystallographic surfaces of pure and doped hematite single crystals have been studied at room temperature and in the vicinity of the Morin transition. The observations below room temperature were made with an apparatus employing thermoelectric cooling modules. It was concluded that the domains were either slabs or cylinders of irregular transverse cross section with axes lying parallel to the basal plane. The domain walls were determined to be 180° ones. At the Morin transition, a light diffuse colloidal line is formed at the boundary between two phases, one weakly ferromagnetic and the other antiferromagnetic. On warming or cooling, the boundary sweeps across the crystal in a characteristic fashion.

J. A. Eaton

Papers

Magnetic Domains in Hematite At and Above the Morin Transition