Composite multiferroic with giant magnetoelectric coupling: The effect of Fe-Co alloying studied from first principles. M. Fechner, S. Ostanin, A. Ernst, J. Henk, ...
Selected Results
Composite multiferroic with giant magnetoelectric coupling: The effect of Fe-Co alloying studied from first principles M. Fechner, S. Ostanin, A. Ernst, J. Henk, K. Mohseni, H. L. Meyerheim, and I. Mertig in cooperation with I. V. Maznichenko Martin-Luther-Universit¨at Halle-Wittenberg, Halle, Germany and J. B. Staunton University of Warwick, Warwick, United Kingdom The coexistence of magnetism and ferroelectricity in the same crystalline phase of a multiferroic material could in principle involve magnetoelectric (ME) coupling. Magnetoelectric coupling, that is switching of the magnetization by an electric field or switching of the electric polarization by a magnetic field, is of prime interest with regard to information storage in nanometer-sized memories with four logic states. In single-phase multiferroics (MFs), however, the electric polarization and the magnetization interact weakly; and further, ferromagnetism shows up far below room temperature. A more robust magnetoelectricity may occur in artificial multiferroics which are composed of ferromagnetic thin films grown epitaxially on a ferroelectric substrate. Early ab initio studies of composite MFs have revealed a promising direction for the future [1]. In a previous study of ultrathin magnetic films deposited on the TiO2 terminated (001) surface of BaTiO3 , we found a significant magnetoelectric coupling across the Fe/BaTiO3 interface [2]. These films exhibit an unexpected change in their magnetic structure with increasing Fe film thickness. More precisely, the magnetic order changes from strongly ferromagnetic for the singlemonolayer Fe system to ferrimagnetic with almost vanishing magnetization upon deposition of a second Fe layer. Ferromagnetic order is restored for thicker Fe films. This effect is attributed to the magnetovolume instability of Fe and can be understood in terms of hybridiza46
Fig. 1: Geometry of Me L /BaTiO3 (Me=Fe, Co) for thicknesses L = 1 (a), L = 2 (b), and L = 3 (c). Spheres represent the adsorbed metal (Me, black), Ba (red), Ti (green), and O (blue).
tion of electronic states and of structural relaxation. However, a spin-reorientation transition was not found. Both magnetic order and magnetic anisotropy of Fe films are very sensitive to alloying with other 3d elements. This suggests that alloying could also change considerably the magnetoelectricity, a conjecture which calls for first-principles simulations of chemical order in composite multiferroics. Here, we report on such simulations of Fe1−x Co x films on BaTiO3 (001). We choose Co due to its weak magneto-elasticity and because it is expected to stabilize magnetically soft Fe films. The structural relaxation of (Fe1−x Co x )L /BaTiO3 interfaces (Fig. 1) were
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Fig. 3: Topography of submonolayer Fe islands on bulk BaTiO3 (001). Fe islands appear as bright dots 2 Fig. 2: Magnetoelectric coupling in 2-monolayer in the 50 × 50 nm constant-current STM image. thick Fe1−x Co x on BaTiO3 . Bottom: Energy differ- The inset shows the height profile along the white ence ∆E = E AFM − EFM versus Co concentration x. line. U = 3.1 V, I = 700 pA. Top: Magnetic moment M per atom in the adlayer versus x. Data for electric polarization P ↑ (P↓ ) in (Fig. 2, top). In addition, the magneto-electric the BaTiO3 substrate are given by filled triangles coefficient shows a giant maximum at this con(open squares). Lines serve as guide to the eye. centration. xCo
Further investigations on these systems will be conducted in close cooperation with the Experimental Dept. I. The geometric structure of the Fe/BaTiO3 interface will be investigated by surface X-ray diffraction and scanning tunneling spectroscopy. As a first result, we show a scanning tunneling microscopy (STM) image of a submonolayer of Fe deposited on BaTiO3 (001) in Fig. 3. Besides a 400 pm high step which corresponds to a complete BaTiO3 unit cell, islands of approximately 170 − 190 pm height are found (inset in Fig. 3), in agreement with theoretical predictions.
obtained by a pseudopotential code [3], with film thickness L ≤ 3. The precise atomic positions were subsequently used for electronicand magnetic-structure calculations within multiple-scattering theory (cf. Ref. [2]). Alloying of Fe and Co was modeled within the coherent-potential approximation [4]. For pure Co films, a strong ferromagnetic order is predicted for all studied film thicknesses. As expected, alloying of Co and Fe stabilizes ferromagnetism in these systems. The most spectacular case is the bilayer (Fe1−x Co x )2 /BaTiO3 since for L = 2 Fe-Co alloys possess an electrically switchable magnetization. Note that the pure Fe film (x = 0) References exhibits a ferrimagnetic structure with nearly [1] M. Fechner, I. V. Maznichenko, S. Ostanin, A. zero magnetization [2]. Ernst, J. Henk, and I. Mertig, phys. stat. sol. (b) For Co concentrations x less than 25 %, the 247, 1600 (2010) antiferromagnetic (AFM) configuration is pre- [2] M. Fechner, I. V. Maznichenko, S. Ostanin, A. ferred (Fig. 2, bottom). For x > 25 %, ferErnst, J. Henk, P. Bruno, and I. Mertig, Phys. Rev. B 78, 212406 (2008) romagnetic (FM) order becomes stable. This phase transition depends slightly on the elec- [3] G. Kresse and J. Furthm¨uller, Phys. Rev. B 54, 11169 (1996) tric polarization in substrate: The critical concentration for P↑ is slightly less than for P↓ [4] B. L. Gyorffy, Phys. Rev. B 5, 2382 (1972) 47
