Half-metallic Ferromagnetism of perovskite oxide Sn-doped SrRuO3

Abstract

In this thesis, we investigate the electronic structure and properties of SrRuO3 as a candidate of half-metallic ferromagnets. We will suggest two methods modulating the band structure of SrRuO3 to become half-metallic ground states. The first method is the doping of Sn atoms as substitute for Ru atoms. Due to the excellent band positioning between Ru 4d t2g bands and Sn 5s bands, only small decrease of Ru 4d t2g majority bandwidth can induce the half-metallic band structure of SrRuO3. We carried out first-principles electronic structure calculations for the series of solid solutions Sr(Ru1-xSnx)O3 of orthorhombic perovskite adopting LSDA+U scheme with 1 eV of the effective value of on-site Coulomb interaction Ueff for Ru d-orbitals. We present the electronic structure and analysis of projected density-of-states of Sr(Ru1-xSnx)O3 for x = 1/8, 1/4, 1/2, 5/8, 3/4, 7/8. From the result, we can observe that the band width of Ru t2g majority band decreased with Sn-doping. Since Sn 5s band has no overlap with Ru 4d band, the decreased hopping by B-site substituted Sn atoms narrows the bandwidth of Ru 4d t2g band. And we also analyzed for the disorder in B-site atoms at fixed Sn-doping ratio. In the super-cell of 2×2×2 times cubic perovskite unit cell, we calculated all the possible B-site configurations of Ru atoms and Sn atoms. The Ru t2g band in clustering types of defects are more dispersive than that in homogeneous types of defects. The averaged density-of-states over all possible B-site configurations at fixed Sn-doping ratio can suggest more reliable prediction for the phase transition to the half-metallic ground state. The calculated density-of-states shows half-metallic ferromagnetic ground states in the range of 0.6 < x < 0.75 of Sn-doping. And the metal-insulator transition occurs at x ~ 0.75. The calculation including the spin-orbit coupling showed that the checker-boarded type of B-site defects has non-collinear spin structures but the total energy of the checker-boarded type is larger by 0.6 eV than the plane type. The second method is an epitaxial strain on SrRuO3. The change of lattice parameter can modulate the bandwidth of the material and we found that the expansion of volume by 9% can induce the half-metallic ground state of SrRuO3. In experiments, the wide range of epitaxial strain can be given by the selection of substrate materials having different size of lattice. For better accuracy of calculated lattice parameters with experimental value, we used the PBEsol functional which can give improved bulk properties than PBE and LSDA. The band structure for 2.5% tensile epitaxial strain showed the maximally decreased Ru 4d t2g bandwidth in LSDA+U and PBEsol+U calculations. Accidentally the best fitting substrate to the lattice size of 2.5% tensile strained SrRuO3 was SrSnO3 that is previously used by the doping material. Our study is a new revenue in search of half-metallic materials in the perovskite family. Considering the potential application of half-metal in spintronics applications, we expect that the half-metallic Sn-doped SrRuO3, when realized, can contribute to the development of devices and technology in spintronics.