Low-temperature exchange coupling between Fe2 O3 and FeTi O3: 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 Fe2 O3 containing abundant nanoscale exsolution lamellae of FeTi O3. Exchange bias is first observed below the Néel temperature of FeTi O3 (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 Fe2 O3 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 Fe2 O3 host that couples to the c -axis magnetization of the FeTi O3 lamellae. Exchange interactions across the interfaces are frustrated when the FeTi O3 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 Fe2 O3 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 Fe2 O3 magnetization and the applied field. An exchange bias of 0.9 T was obtained with an anisotropy constant of 0.1 K.