A natural hematite of hydrothermal origin shows evidence of time‐dependent behavior related to the sample preparation procedure (involving significant crushing and milling). A strongly magnetic phase with a Curie temperature (∼630°C) corresponding to that of maghemite is present after storage for one‐and‐a‐half‐years, while being absent in the original sample material. The magnetic properties of the hematite‐maghemite in relation to mechanical treatment are compared and possible mechanisms of the hematite transformation are discussed. The hematite‐maghemite transformation is suggested to have resulted primarily from recrystallization induced by shear stress resulting from crushing and milling.

A remanence-based technique, developed recently in magnetic recording media research with the aim of identifying the nature and intensity of interparticle magnetic-interaction fields, is reported. The technique is based on Wohlfarth's linear relation linking isothermal remanent magnetization (IRM) acquisition and saturation IRM (SIRM) d.c. demagnetization with the field as a parameter, deviation from the linearity being attributed to many-body effects. Physical interpretation of an interaction-dependent parameter is discussed. The method is demonstrated on artificial samples of synthetic fine-grained haematite with various grain sizes and shapes and different initial magnetic configurations (i.e. the degree of spin-spin ordering). The observed negative character of interactions, yielding a net demagnetizing effect, is attributed to the specific arrangement of magnetic particles in the specimens and/or to a significant portion of superparamagnetic particles in some specimens.

The effect of uniaxial pressure on the remanent magnetization of artificial samples containing synthetic or natural magnetite of various grain sizes was examined. Changes in intensity, declination and inclination of initial remanent magnetization were determined while the sample was being compressed. It was shown that the applied uniaxial pressure caused a decrease in inclination except when the initial inclination was either 0° or 90° with respect to the stress axis. The remanent magnetization rotated towards the plane perpendicular to the stress direction. This effect is most pronounced when the initial inclination is 30° with respect to the uniaxial pressure direction.

The behaviour of some magnetic properties of natural and synthetic haematite of different grain size is examined. The natural haematite was obtained from the hydrothermal deposit Kadan (Czech and Slovak Federal Rep.). Six grain-size fractions ranging from 120 to 40-mu-m were prepared by means of sieving and two further fractions down to 5-mu-m by wet ultrasonic sieving. Since the behaviour of the fractions is similar, that of only four representative samples is reported. In addition, the behaviour of one submicron synthetic haematite fraction (0.5-mu-m) prepared by oxidation of ferrous sulphate (uniform in size and shape) was investigated.The initial remanence value (J(r)) seems to increase with decreasing grain size. During alternating field (AF) demagnetization, all fractions behaved similarly, except for the submicron fraction which is considerably softer than the others. Normalized (isothermal remanent magnetization) IRM acquisition curves were similar for all fractions.Parameters of the anisotropy of magnetic susceptibility (AMS) display significant changes, mainly during IRM acquisition. During AF demagnetization, the anisotropy degree P exhibits a slight increase (some %), while the behaviour of the shape factor T is complicated. The anisotropy ellipsoid exhibits a tendency to rotate. Significant changes in the AMS parameters occur during IRM acquisition. Curves of P and T vs. IRM acquisition field, for various grain-size fractions, show no coherent pattern. For all the samples studied, the T vs. H curve exhibits a threshold value at which change in the type of arrangement of easy axis of magnetization occurs. For the IRM acquisition fields higher than some 320 kA m-1, the minimum susceptibility axis parallels the direction of the IRM acquisition field.Hysteresis curves of the fractions are similar to each other. The Preisach distribution function was determined and it indicates that the reversible part of the magnetization process plays an important role comparatively. Based on the coercivity data presented no unambiguous conclusion could be drawn from the single-domain (SD)-multidomain (MD) transition, associated with a coercivity maximum.

We present the results of a palaeomagnetism and magnetic fabric study in six sampling sites of the lower Ordovician redbeds of the Cap de la Chevre formation, Crozon Peninsula, France. Measurement of anisotropy of magnetic susceptibility (AMS) shows that these beds are affected by a weak but significant tectonic strain during the Hercynian folding of the Armorican Palaeozoic cover. Samples containing small elliptical reduction spots allowed us to check the control of strain on AMS. Site mean AMS data were then used to provide strain estimates at each site. Analysis of remanent magnetization through thermal and chemical stepwise demagnetization procedures allowed us to isolate three magnetic components. Component A, defined in the lower unblocking temperature range, is a recent overprinting which clusters around the present-day dipole field direction. Components B and C, defined respectively in the intermediate and higher unblocking temperature ranges, appear to be pretectonic in age. The fold test, however, is inconclusive for both, because of the significant dispersion still remaining after the simple tilt correction. Assuming that at least a part of this dispersion is due to strain, we have applied the strain removal technique using AMS-derived strains at each site. This results in a clustering of component C from four sites, allowing recovery of the primary Ordovician paleomagnetic direction (D = 261-degrees, I = 71-degrees, alpha-95 = 11-degrees). In contrast, unstrained component B shows a small circle distribution, containing the formation mean of unstrained component C. The hypothesis that component B and C distributions arise from beds being tilted during a long-term magnetization acquisition in the Ordovician field is discussed, taking into account the extensional geodynamic context during deposition of lower Palaeozoic redbeds in Brittany.

Stress effect on remanent magnetization and anisotropy of magnetic susceptibility of synthetic samples of magnetite (Fe3O4) and haematite (Fe2O3) of submicron grain size is reported. Both the parameters were investigated under uniaxid pressure within the range of elastic deformation. All the measured parameters display significant changes of different pattern.

We have investigated natural Ni- and Co-bearing minerals with respect to their basic magnetic properties. Samples of cobaltite CoAsS monocrystals from Tunaberg locality (SW of Stockholm, Sweden) showed some anomalous magnetic properties, especially high remanent magnetic polarization, high Q-coefficients, distinctive blocking temperatures and high stability of remanent magnetic polarization to alternating fields. Experiments indicated that at least two types of cobaltite exist with different magnetic properties.

The results of studying the magnetic properties of rocks with a vibrating-sample magnetometer are presented. The limiting parameters of the measuring system are given and demonstrated on a concrete example of measurements. The possibilities of identifying stochastic models of the distribution of Preisach-type microcoercitive forces are discussed and the result of the identification is presented for a sample of pure haematite.

The values of the initial magnetic susceptibility κ were measured with different instruments. The values of saturated magnetic polarization J s , the coercive force H c , the alternating current demagnetization curves J rs J rso = F( H ) and the hysteresis curves J = J ( H ) were determined for synthetic samples containing synthetic magnetite with grain sizes of < 40 μm, 40–80 μm, 80–120 μm and 120–160 μm, and concentrations of 0.1–5.0% by volume. Analogous parameters were also determined for synthetic samples containing natural and synthetic haematite with a grain size of 40–80 μm and concentration of 1.0–10.0% by volume. The results indicate a significant effect of the magnetic interactions between the grains of the magnetic minerals in the samples and the ways of determining the grain size and concentration of magnetite or haematite in rock samples.

The theory of multivariate statistical processing of the anisotropy of magnetic susceptibility, measured on a group of specimens, originating from a single geological body (outcrop, locality, etc.), is described. The result of the processing is an estimate of the mean normalized tensor and the estimates of the principal susceptibilities, derived from it, together with the respective intervals of confidence, and the estimates of the principal directions with the respective regions of confidence. An anisotropy test for a group of specimens is proposed. The function of the ANS21 computer program employed is briefly described and an example of its output plot is presented.

In the first part of this paper, the main geological and geophysical applications of magnetic susceptibility anisotropy are briefly described. In the second part, the data on the magnetic anisotropy of various rock types are summarized and some conclusions concerning its significance are deduced. For this purpose, all accessible data were used.