Hysteresis properties of magnetite and their dependence on particle size: A test of pseudo‐single‐domain remanence models

Abstract

This paper reports room temperature measurements of initial susceptibility χ 0, second Rayleigh coefficient B, coercive force H c, remanent coercive force H R, and the ratio J rs/J s of near‐saturation remanent and induced magnetizations for four samples containing single‐crystal magnetite cubes with mean sizes of 0.037, 0.076, 0.10, and 0.22 μm. While χ o and H c have weak dependences on particle size d, H c and J rs vary strongly with size, as d−0.55 and d−0.75, respectively, over the 0.076–0.22 μm range. These hysteresis properties and their size dependences are incompatible with data for >l‐μm crushed and annealed magnetite grains. Nearly perfect unstrained crystals tend to have H c and J rs values 2–5 times smaller than those of pulverized grains of nominally the same size. Magnitudes and size dependences of H c and J rs for >0.06‐μm magnetites are approximately, although not perfectly, accounted for by (1) metastable single‐domain (SD) particles, (2) rotatable SD‐like domain wall (DW) moments, or (3) DW displacements impeded by crystal defects and limited by self‐demagnetization. Only reversible DW displacements (or possibly domain rotations) opposed by self−demagnetization can explain the initial susceptibility data, however.