Exchange bias in UO₂/Fe₃O₄ thin films above the Néel 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 Å). EB values reaching ∼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 Néel 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 5 K in the UO₂/Fe₃O₄ sample with the largest EB). The former can be caused by proximity effects of Fe₃O₄ on T_N 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.