Lamellar Magnetism: Ferrimagnetic Effect of Cation Monolayers at Interfaces of Nanoscale Exsolution Lamellae

Naturally exsolved members of the hematite-ilmenite series (Fe2O3-FeTiO3), with exsolution lamellae down to 1-2 nm thick, have very strong stable remanent magnetization. Monte Carlo simulations of electrostatic/magnetic interactions indicate the magnetic moment is controlled by Fe2± Fe3+ contact layers ~0.23 nm thick on (0001) between canted antiferromagnetic (CAF) hematite and paramagnetic (PM) ilmenite. The layers reduce charge imbalance on interfaces; reduced even more by Fe2±Fe3+ ordering in the layers. They are spin coupled with hematite so that lamellar magnetism (LM) has properties of thermal stability and coercivity similar to hematite. Magnetic moments of two contact layers are combined with one uncompensated spin of a hematite layer to produce a net moment of ~4 Bohr magnetons per lamella. The nature of LM depends on whether the host is hematite or ilmenite, and whether moments of individual lamellae are in or out of phase. In-phase lamellae are favored during exsolution in grains with (0001) parallel to the magnetizing field. Intensity depends on in-phase lamellae and high lamellar yield, achieved by precipitation under supersaturated conditions, giving a high interface/volume ratio. LM is a CRM developed in the field CAF hematite + PM ilmenite below the eutectoid at ~520°C at 1 Atm, but probably at higher T at the 5-10 kbar of origin of studied LM materials. Demagnetization T's of 580-650°C during short-term experiments reproduce those of Ti-contaminated hematite, much higher than precipitation T's. Higher coercivity in hematite-hosted than ilmenite-hosted phases is explained by different layer configurations and growth models. With contact layers only ~0.23 nm thick, LM is hard to prove. Besser et al. 1967 showed that sublattice magnetizations in hematite are parallel to a crystallographic a axis; spin-canted moment 90° to a. LM requires principal moments to be parallel to sublattice magnetizations of hematite, not the spin-canted direction, therefore LM would be supported by moments of crystals subparallel to a. This occurs in a majority of 17 hemo-ilmenite single crystals, with a and c axes oriented by electron backscatter diffraction, then isolated and measured for NRM and AMS.