ADVENTURES AT THE NANOSCALE: LAMELLAR MAGNETISM AND EXCHANGE BIAS IN BILLION-YEAR-OLD METAMORPHIC TITANOHEMATITE

Metamorphic rocks from Modum, Norway exhibit extreme magnetic exchange bias when cooled in zero-field below the ordering temperature of ilmenite. Exchange bias refers to a horizontal shift of a magnetic hysteresis loop caused by coupling between two magnetic materials across an interface. It is a phenomenon that lies at the heart of the sensor inside every magnetic hard drive. The discovery that not only did natural samples of titanohematite containing nanoscale ilmenite exsolution lamellae display exchange bias, but that the shift observed was one of the largest ever seen in either natural or synthetic materials, lead Peter Robinson on a 20-year adventure to the nanoscale and beyond in search of the origin of this effect. His journey began with the discovery of lamellar magnetism in the early 2000s, as an explanation for the unusually strong and stable interface remanence carried by nanoscale exsolution lamellae of ilmenite in a titanohematite host – and vice versa – and continued with the more recent discovery of the importance of interface structure, magnetic spin orientation, exchange coupling, frustration, lamellar geometry, lattice-preferred orientation and the orientation of the external magnetic field. An unfinished symphony at the time of Peter’s death, we present here the culmination of Peter’s theory of the atomic-magnetic basis of exchange bias in titanohematite system, including: a) the effects of lamellar shapes on magnetic coupling, b) the high-T acquisition of lamellar magnetism and low-T acquisition of magnetization of ilmenite lamellae, c) the intensity of lamellar magnetism and the consequent ilmenite magnetism in populations of randomly oriented crystals, d) lattice-preferred orientation of the titanohematite host crystal populations, and e) the effects of magnetic domain walls in the host on hysteresis properties. The models and observations presented not only provide a comprehensive framework for understanding the highly unusual magnetic properties of exsolved titanohematite, but an elegant proof of the original theory of lamellar magnetism that sparked one of Peter’s most profound contributions to the subject.