Fyzikální ústav Akademie věd ČR

The exchange interaction through interface of dissimilarly ordered magnetic materials (e.g. an antiferromagnet (AF) in contact with a ferro- or ferrimagnet (F)) gives rise to a characteristic exchange bias (EB) effect [1] (i.e. shifted magnetic hysteresis loop with an enhanced coercivity), which is of fundamental importance for novel hard magnets, recording media, spin valve and tunneling devices. Having in mind the strong spin-orbit coupling and enormous magnetic anisotropy displayed by some actinide compounds, we employed uranium-based antiferromagnets to study the exchange anisotropy effects in thin magnetic films.

A series of UO₂-magnetite films with a variable layers thickness was produced by reactive sputtering. A very strong effect, reaching 0.26 T in a bilayer with the thinnest Fe₃O₄ layer (90 Å), was found [2]. Magnetization measurements demonstrate that a remarkably large EB 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). 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 magnetic moments remain pinned up to 120 K that correlates with the change in the easy magnetization axis of Fe₃O₄ across the Verwey transition. The effects are observed in the samples of quality controlled by different methods such as ex-situ x-ray resonant scattering, conventional x-ray diffraction and transmission electron microscopy.

Fig. 1

References:
[1] W.H. Meiklejohn, C.P. Bean, Phys. Rev. B 102 (1956) 1413.
[2] E.A. Tereshina, Z. Bao, L. Havela, S. Danis, C. Kuebel, T. Gouder, R. Caciuffo, Exchange bias in UO₂/Fe₃O₄ thin films above the Néel temperature of UO₂ Appl. Phys. Lett. 105 (2014) 122405.