Anisotropy of magnetic susceptibility and hysteresis parameters for natural magnetite particle assemblages: Micromagnetic analysis of focused-ion-beam nanotomography data with MERRILL

Three-dimensional shapes of 68 magnetite grains in pyroxene and 234 magnetite grains in plagioclase, were obtained by “slice-and-view” focused-ion-beam nanotomography (FIB-nt) on mineral separates from the Bushveld Intrusive Complex, South Africa. Electron backscatter diffraction (EBSD) determined the orientation of the magnetite inclusions relative to the crystallographic directions of their silicate hosts. For each particle, hysteresis loops in 20 equidistributed field directions were calculated by the finite-element micromagnetic code MERRILL. For each direction, the averages over the particle ensemble were compared to corresponding hysteresis loops measured with a vibrating sample magnetometer (VSM) on silicate mineral separates from the same samples. FIB-nt combined with micromagnetic modelling allows to explore the mechanisms controlling the magnetic anisotropy for each individual particle and to analyze the combined effect for bulk magnetic properties. This combination is of interest for anyone who interprets magnetic anisotropy because it helps understanding how domain states and crystal alignment in natural samples influence the measured anisotropy. Our results demonstrate that natural particle shapes, their orientations and domain states control the anisotropy of magnetic remanence, coercivity and susceptibility in natural particles. We can show for our specific examples, how the connection between mineral texture and magnetic anisotropy depends on specific domain states. Our data explain why natural magnetite particles at the transition between single-domain and single or multiple vortex states do not always follow the relation between axis orientation and  magnetic anisotropy which is theoretically expected for simple particle shapes.