Lamellar magnetism: effects of interface versus exchange interactions of nanoscale exsolutions in the ilmenite-hematite system

Abstract

We have examined finely exsolved oxides of the hematite-ilmenite solid-solution series found in slowly cooled middle Proterozoic igneous and metamorphic rocks. These oxides impart unusually strong and stable remanent magnetization. Transmission electron microscopy (TEM) analysis shows multiple generations of ilmenite and hematite exsolution lamellae, with lamellar thicknesses ranging from millimeters to 1-2 nanometers. Rock-magnetic experiments suggest that the remanence is thermally locked to the antiferromagnetism of the hematite component of the intergrowths, yet is stronger than expected for a canted antiferromagnetic hematite or coexisting paramagnetic Fe-Tiordered (R 3̄) ilmenite. In alternating field experiments a stable magnetization is observed in these samples to fields of 100 to 120 mT, indicating that the natural remanent magnetization (NRM) is stable over billions of years. This feature has implications for understanding magnetism of deep rocks on Earth, or on planets like Mars that no longer have a magnetic field. Atomic-scale simulations of an (R 3̄) ilmenite lamella in a hematite host, based on empirical cation-cation and spin-spin pair interaction parameters, show that boundary regions of the lamellae are occupied by ‘‘contact layers’’ with a hybrid composition of Fe ions, intermediate between Fe2+-rich layers in ilmenite and Fe3+-rich layers in hematite. In this paper we review current data and explore further the nature of the interface.