Theory Meets Experiment in Low-Dimensional Structures with Correlated Electrons
Prague, Czech Republic, July 1 – 4, 2019
Magnetic anisotropy of UO2 studied on UO2/Fe3O4 thin films
Large magnetic anisotropy (MA) of the material is a keystone to the high-density magnetic storage. It defines the easy magnetization direction and ensures information storage by preventing the magnetization from switching. In bilayers, MA can be controlled via pinning of a ferromagnetic (F) layer by an antiferromagnet (AF) in the exchange bias (EB)  systems . In this work we aim at evaluating the anisotropy constant in the antiferromagnetic uranium dioxide by making use of exchange bias effect study in the UO2/Fe3O4 bilayers deposited on various substrates. The shift of the magnetic hysteresis curve along the field direction of the magnetic bilayer field-cooled below the Néel temperature (TN) of the antiferromagnet is related to its anisotropy constant KAF [3,4] via the critical thickness of the AF layer, above which the antiferromagnet is strong enough to pin the magnetization of a ferromagnet. Using reactive sputtering from metallic U and Fe targets, we prepared sets of the UO2/Fe3O4 samples with the varied thickness of UO2 (50-300 Å) and with a constant thickness of magnetite (~270±20 Å). The stoichiometry of each deposited layer was controlled by x-ray photoelectron spectroscopy (XPS). The samples were further characterized by Rutherford back-scattering spectrometry (RBS) and x-ray diffraction (XRD). Magnetization study revealed a large EB in the samples . Using the approach of Ref. 4, we find the anisotropy constants of UO2 in the samples ~0.1-0.3 MJ/m3. We further compare the experimental results with those obtained theoretically using a first-principles density-functional+dynamical-mean-field-theory (DFT+DMFT) method  in conjunction with a quasi-atomic (Hubbard-I) treatment of U 5f states. We predict a strong easy-axis anisotropy to originate from the substrate-induced in-plain tetragonal compression of the UO2 layer.
The samples were prepared in the framework of the TALISMAN project of the European Commission Joint Research Centre, ITU Karlsruhe. RBS measurements have been carried out at the CANAM (Centre of Accelerators and Nuclear Analytical Methods) infrastructure LM 2015056 and supported by OP RDE, MEYS, Czech Republic under the project CANAM OP, CZ.02.1.01/0.0/0.0/16_013/0001812 and by the Czech Science Foundation (GACR 18-03346S and 18-02344S). Part of the work was supported by “Nano-materials Centre for Advanced Applications,” Project CZ.02.1.01/0.0/0.0/15_003/0000485, financed by ERDF.
 W. H. Meiklejohn and C. P. Bean, Phys. Rev. B 102 1413 (1956).
 S. van Dijken et al., J. Appl. Phys. 97 063907 (2005).
 D. Mauri et al., J. Appl. Phys. 57, 3047 (1987).
 Ch. Binek et al., J. Magn. Magn. Mater. 234, 353 (2001).
 E. A. Tereshina et al., Appl. Phys. Lett. 105 122405 (2014).
 M. Aichhorn et al., Comp. Phys. Comm. 204, 200 (2016).