Exchange bias in UO_2/Fe_3O_4 thin films above the Neel temperature of UO_2

摘要

Magnetic exchange bias (EB) has been investigated on antiferromagnetic/ferrimagnetic heterostructures consisting of a stoichiometric UO_2 film grown epitaxially on a LaAlO_3 substrate and covered with highly textured Fe_3O_4 layers of variable thickness (t_(Fe_3O_4) = 90-700 A). EB values reaching ～2.6 kOe were observed at lowest temperatures in UO_2/Fe_3O_4 with the thinnest Fe_3O_4. Magnetization measurements demonstrate that a remarkably large exchange bias in UO_2/Fe_3O_4 is present at temperatures higher than the bulk Neel temperature of UO_2 (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 UO_2 and Fe_3O_4 and by exchange biasing within magnetite (up to 25% of the total effect at 5 K in the UO_2/Fe_3O_4 sample with the largest EB). The former can be caused by proximity effects of Fe_3O_4 on T_N combined with a magnetic anisotropy of UO_2, persisting above a continuous magnetic-ordering transformation. For magnetite, the results indicate the presence of pinned moments up to 120K that correlates with the change in the easy magnetization axis of Fe_3O_4 across the Verwey transition.