Low-temperature exchange coupling between Fe2O3 and FeTiO3:: Insight into the mechanism of giant exchange bias in a natural nanoscale intergrowth

Exchange bias (> 1 T at 10 K) has been observed in natural sample of Fe2O3 containing abundant nanoscale exsolution lamellae of FeTiO3. Exchange bias is first observed below the Neel temperature of FeTiO3 (55 K). Possible interface magnetic structures are explored within the framework of a classical Heisenberg model using Monte Carlo simulations. The simulations predict a threshold value of the Fe2O3 anisotropy constant, below which Fe3+ spins become tilted out of the basal plane in the vicinity of the interfaces. This tilting creates a c-axis component of magnetization in the Fe2O3 host that couples to the c-axis magnetization of the FeTiO3 lamellae. Exchange interactions across the interfaces are frustrated when the FeTiO3 lamellae contain an even number of Fe2+ layers, resulting in zero net exchange bias. Lamellae containing an odd number of Fe2+ layers, however, are negatively exchange coupled to the Fe2O3 host across both (001) bounding surfaces, and are the dominant source of exchange bias. Exchange bias is observed whenever there is a significant c-axis component to both the Fe2O3 magnetization and the applied field. An exchange bias of 0.9 T was obtained with an anisotropy constant of 0.1 K.