Anomalous exchange bias induced by spin-flop transition in epitaxial hematite/permalloy heterostructures

Exchange bias (EB) between an antiferromagnet (AFM) and a ferromagnet (FM) leads to a preferential orientation of the ferromagnet’s moments and has a significant impact on magnetic memory technologies. In addition, it offers crucial insights into the AFM order which is usually difficult to measure. Here, we report anomalous EB induced by field cooling (FC) across the spin-flop (Morin) transition (T_M~270K) of an epitaxial hematite (0001)/permalloy heterostructure. The exchange field (H_Ex) reaches maximum (~40 Oe) near T_M and decreases away from it. This result shows that FC across the Morin transition imprints information from the permalloy on the hematite without cooling through its Néel temperature. Surprisingly, field heating (FH) and FC yield almost identical temperature dependence of H_Ex. Interestingly, below 50 K, a significantly larger H_Ex~150 Oe emerges at 10 K. We propose that a nonzero H_Ex close to T_M arises from the competition between magnetocrystalline and dipolar anisotropies in hematite. The two anisotropies balance each other out near T_M producing a relatively small unidirectional anisotropy created through FC/FH process. This study demonstrates that the spin orientation of the hematite can be altered by either FC or FH across the Morin transition when interfaced with a ferromagnet. Future studies will focus on understanding the microscopic origin of this unusual thermal behavior of exchange bias in spin-flop/ferromagnetic heterostructures.