Magnetic hysteresis of magnetite at high temperature: grain size variation

Abstract

SUMMARY Thermoremanent magnetization (TRM), the primary magnetic memory of igneous rocks, depends for its stability through geologic time on mineral carriers with high coercivities and high unblocking temperatures. The palaeomagnetic record of past magnetic field directions and intensities is the key to unraveling Earth's tectonic history. Yet we still do not fully understand how the familiar mineral magnetite, in the micrometer grain size range typically responsible for stable TRM, acquires and holds its signal. Direct indicators of magnetite remanence-carrying capacity and coercivity at high temperature T are saturation remanence relative to saturation magnetization Mrs/Ms and coercive force Hc. This study is the first to measure the variation of these hysteresis properties for magnetite, from room temperature to the Curie point, across the entire size range from 25 nm to 135 µm, covering superparamagnetic, single-domain, vortex, pseudo-single-domain and multidomain magnetic behaviour. The paper focuses on: (1) Hc(T) and Mrs(T) observations and their reproducibility; (2) mathematical relationships of Hc(T) and Mrs(T) to Ms(T), used in modelling TRM and for unbiased comparisons of thermal variations; (3) the shapes of magnetite grains and the number of domains they contain, revealed by demagnetizing factors N = Hc/Mrs and (4) the grain size dependences of Hc and Mrs at ordinary and elevated T, delineating domain structure changes and mechanisms of coercivity.