- Dr. Evgeniya Tereshina: Exchange Bias in Heterostructures Based on UO2
- 24. 2. 2016, 14:50
- lecture room F2, first floor Ke Karlovu 5
- more information
Abstract:
Interfacial exchange interaction in bilayers consisting of two dissimilarly ordered magnetic materials (e.g. an antiferromagnet (AF) and a ferro- or ferrimagnet (F)) may give rise to a phenomenon called magnetic exchange bias (EB) effect [1]. The EB manifests itself as a shift of a magnetic hysteresis loop along the field direction when the bilayer is field-cooled below the Néel temperature (TN) of the AF. This property is of great value for magnetic recording applications. Despite the conceptual simplicity, a generally accepted theory that predicts the EB behavior for an apt pair of materials is still missing. This is due to the materials-related effects (e.g. domains and spin structures, field effects etc.) and the difficulty to meet the conditions necessary for the presence of a well-defined interface.
Critical dependence of EB on magnetic anisotropy brings us the possibility to use actinides with strong spin-orbit interaction as the key ingredient. Here we report exchange bias studies in magnetic bilayers consisting of a stoichiometric UO2 film grown epitaxially on different substrates and covered with polycrystalline metallic (Ni80Fe20 and Fe) and highly textured oxide (Fe3O4) layers of variable thickness. A large longitudinal exchange bias ~ 2.6 kOe is found in UO2/Fe3O4 bilayers [3] while UO2 combined with metallic ferromagnets displays perpendicular exchange coupling with an order of magnitude smaller EB. Interestingly, the magnetization measurements on UO2/ Fe3O4 show that a remarkably large EB is present at temperatures higher than the bulk TN = 30.8 K of UO2. 0ur 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. The former can be caused by proximity effects of Fe3O4 on TN combined with magnetic anisotropy of UO2, persisting above a continuous magnetic-ordering transformation. The effects are observed for the samples of quality controlled by different methods such as X-ray Photoelectron Spectroscopy, conventional X-ray Diffraction, Transmission Electron Microscopy and Rutherford Backscattering Spectroscopy.
[1] W. H. Meiklejohn and C. P. Bean, "New magnetic anisotropy", Phys. Rev. B 102, 1413 (1956).
[2] V. Sechovsky, L. Havela, in: Magnetic Materials, K.H.J. Buschow (Ed.), Elsevier, Amsterdam, 1998, Vol. 11, p. 1.
[3] E. A. Tereshina et al., Appl. Phys. Lett. 105, 122405 (2014).