SOME MAGNETIC-PROPERTIES OF SYNTHETIC AND NATURAL HEMATITE OF DIFFERENT GRAIN-SIZE

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.