Study of electronic and magnetic properties and related x-ray absorption spectroscopy of ultrathin Co films on BaTiO 3 M. Hoffmann, 1, 2 St. Borek, 3 I. V. Maznichenko, 1 S. Ostanin, 2 G. Fischer, 4 M. Geilhufe, 2 W. Hergert, 1 I. Mertig, 1, 2 A. Ernst, 2 and A. Chass´ e 1 1 Institut f¨ ur Physik, Martin-Luther-Universit¨ at Halle-Wittenberg, D-06099 Halle, Germany 2 Max-Planck Institut f¨ ur Mikrostrukturphysik, Weinberg 2, D-06120 Halle 3 Department Chemie, Ludwig-Maximilians-Universit¨ at M¨ unchen, Butenandtstraße 11, D-81377 M¨ unchen, Germany 4 Institut National des Sciences Appliqu´ ees Toulouse, LPCNO, 135 Avenue de Rangueil, 31077 Toulouse, France (Dated: June 30, 2018) We present a first-principles study of electronic and magnetic properties of thin Co films on a BaTiO3(001) single crystal. The crystalline structure of 1, 2, and 3 monolayer thick Co films was determined and served as input for calculations of the electronic and magnetic properties of the films. The estimation of exchange constants indicates that the Co films are ferromagnetic with a high critical temperature, which depends on the film thickness and the interface geometry. In addition, we calculated x-ray absorption spectra, related magnetic circular dichroism (XMCD) and linear dichroism (XLD) of the Co L2,3 edges as a function of Co film thickness and ferroelectric polarization of BaTiO3. We found characteristic features, which depend strongly on the magnetic properties and the structure of the film. While there is only a weak dependence of XMCD spectra on the ferroelectric polarization, the XLD of the films is much more sensitive to the polarization switching, which could possibly be observed experimentally. PACS numbers: 75.50.Cc, 71.20.Lp, 71.15.Rf The interface between a magnetic thin film and a fer- roelectric material is the subject of several recent in- vestigations. 1–19 The major part of these studies is de- voted to Fe/ferroelectric interfaces, 2,4–7,10,12,14–18 since ferromagnetic iron is supposed to be a good candidate as a ferromagnetic electrode in two-component multi- ferroics. Although it was shown, that a ferroelectric film can be grown on an iron substrate and a func- tional heterojunction can be fabricated, 14,15 the multi- ferroic effects by polarization switching were found to be not strong enough, since the main changes of the func- tional properties occur only in the vicinity of the inter- face. 2,6,15 Another impediment is non-ideal compatibility between an iron and an oxide surface. Until now it was not shown that junctions with symmetric interfaces, a highly desirable condition for coherent electronic trans- port, 20 can be grown. Accordingly, ferromagnetic oxides La 1−x Sr x MnO 3 and SrRuO 3 were used as electrodes in multiferroic junctions. 3,8,9,11,19 Such interfaces can be al- most ideally grown, but the Curie temperature of these oxides is too low for functional devices. Therefore, search for an appropriate ferromagnetic/ferroelectric interface is still in progress. Among the above cited studies only few investigations deal with ultrathin ferromagnetic films on a ferroelec- tric substrate. 6,7,12,16 In particular, it was shown that two monolayer thick Fe films on the BaTiO 3 (001) surface might not be ferromagnetic because of the film geometry and magneto-elastic properties of iron. 6 Additionally, a substantial charge and spin moment transfer was found at the interface by altering the polarization direction. 6 Thus, ultrathin films of Fe on a BaTiO 3 (001) sin- gle crystal are magnetically unstable, 6 but cobalt ex- hibits usually stable ferromagnetic characteristics in many nanostructures. Therefore, we continue our work on ultrathin metallic films on ferroelectric single crystals and suggest in this paper to use Co as the ferromagnetic material on BaTiO 3 (001). Despite the comprehensive review about the progress in this field given by Vaz et al. 21 there are only few information about the interface of Co and perovskites like BaTiO 3 (BTO). In the framework of spin-polarized DFT calculations, as implemented in the Vienna ab ini- tio Simulation Package (VASP), multiferroic tunnel junc- tions of Co/BTO/Co were investigated by Cao et al. 13 They showed that a critical thickness of BTO unit cells is necessary for the appearance of ferroelectricity, which is inhibited by a depolarizing electrostatic field, caused by dipoles at the ferroelectric-metal interfaces. 22 In the work of Oleinik et al. 23 first-principles calculations were applied on Co/STO/Co(001) magnetic tunnel junctions, where a strong covalent bond between Co and O and an induced magnetic moment of |m s | =0.25 μ B at the Ti atom was observed. This is similar to the case of Fe/BTO/Fe tunnel junctions. In another work, Polisetty et al. 24 applied piezoelectrically controlled strain for elec- tric tuning of exchange-bias fields of BTO/Co/CoO het- erostructures. In our first-principles study, we investigated systemat- ically the crystalline structure of ultrathin Co films on BTO and their electronic and magnetic properties in de- pendence on the film thickness and the polarization of BTO. The calculations were performed using a so-called multi-code approach, in which atomic positions were ob- arXiv:1505.07306v2 [cond-mat.mtrl-sci] 29 May 2015