Exchange bias in UO2/Fe3O4 thin films above the Neel temperature of UO2

Magnetic exchange bias (EB) has been investigated on antiferromagnetic/ferrimagnetic heterostructures consisting of a stoichiometric UO2 film grown epitaxially on a LaAlO3 substrate and covered with highly textured Fe3O4 layers of variable thickness (t(Fe3O4) = 90-700 angstrom). EB values reaching similar to 2.6 kOe were observed at lowest temperatures in UO2/Fe3O4 with the thinnest Fe3O4. Magnetization measurements demonstrate that a remarkably large exchange bias in UO2/Fe3O4 is present at temperatures higher than the bulk Neel temperature of UO2 (T-N = 30.8 K). Our study reveals a complex nature of the effect: we suggest that it is provided both by strong exchange coupling between UO2 and Fe3O4 and by exchange biasing within magnetite (up to 25% of the total effect at 5K in the UO2/Fe3O4 sample with the largest EB). The former can be caused by proximity effects of Fe3O4 on TN combined with a magnetic anisotropy of UO2, persisting above a continuous magnetic-ordering transformation. For magnetite, the results indicate the presence of pinned moments up to 120 K that correlates with the change in the easy magnetization axis of Fe3O4 across the Verwey transition. (C) 2014 AIP Publishing LLC.