Strain control in SrRuO 3 thin films by using a lattice constant tunable buffer
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Strain control in SrRuO3 thin films by using a lattice constant tunable buffer K. Terai, T. Ohnishi, M. Lippmaa, H. Koinuma1 and M. Kawasaki2 Institute for Solid State Physics, University of Tokyo, Kashiwa, 277-8581, Japan. 1Frontier Collaborative Research Center, Tokyo Institute of Technology, Yokohama, 226-8503, Japan. 2 Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan. ABSTRACT Heteroepitaxial oxide thin films are usually grown on single crystal substrate which offer a similar lattice constant as the target material. In general, there are no substrates that are suitable for film fabrication and have a good lattice matching. In our previous report, we succeeded in fabricating a lattice constant tunable buffer by using a Ba1-xSrxTiO3 / BaTiO3 bilayer structure on SrTiO3. The in-plane lattice constant of the buffer layer can be tuned from 3.91 to 3.99 Å and the lattice constant is determined only by the Ba/Sr ratio in the Ba1-xSrxTiO3 layer. The buffer is suitable for growing both strain-free films and strained films. In this report we demonstrate the use of the lattice constant tunable buffer for strain control in SrRuO3 magnetic thin films. SrRuO3 has a magnetic anisotropy which changes under lattice strain. We show that the easy axis of magnetization is parallel to the film surface under tensile strain on a Ba0.5Sr0.5TiO3 buffer. The easy axis is perpendicular to the surface in compressively strained films. The tensile strain in a film also results in an increase of the ferromagnetic ordering temperature from a bulk value of 160 K to 164 K. INTRODUCTION Epitaxial strain can cause large changes in the transport properties and magnetic ordering in perovskite-type transition metal oxides. Various groups have reported on strain control in thin films by using lattice mismatch strain between a film and a single crystal substrate to alter the critical temperature of superconductors [1] or to modify the type of magnetic ordering [2] or the direction of the magnetic easy axis [3]. These studies require a substrate which has a lattice constant close to that of the target material. In general, there are only very few choices for suitable substrate materials. Exactly lattice-matched substrates are almost never available and heteroepitaxial films therefore inevitably suffer from compressive or tensile strain. To avoid strain, free-standing thin films have been prepared by substrate etching [4], but this is only works in some special cases In a previous report, we showed how to fabricate a buffer layer with a tunable in-palne lattice constant, based on Ba1-xSrxTiO3 (BSTO) and BaTiO3 (BTO) layers. [5] Figure 1 shows a schematic image of the buffer and a lattice constant diagram for the whole concentration range of BSTO, obtained by concurrent X-ray diffraction. [6] The buffer is constructed by covering the BTO layer grown on a SrTiO3 (STO) substrate surface with a thick BSTO layer. The BTO and the BSTO buffer thicknesses were 120 Å and 1800 Å, respectively. The buffer layers are grown
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