Half-metallicity induced by spin–orbit coupling in the superstructured perovskite Ca2TiRuO6

Abstract

The synthesis process and the characterisation of structural, morphological, compositional, optical, electrical and magnetic properties of the Ca₂TiRuO₆ material are described and analysed. Rietveld refinement of X-ray diffraction data showed the crystallisation of the material into a perovskite-like structure, given by the P2₁/n space group. Scanning electron microscopy reveals images of granular, porous surface morphology with submicrometre-sized grains. Semi-quantitative analysis of energy dispersive spectroscopy does not reveal the presence of compositional impurities. Diffuse reflectance spectroscopy analyses show a direct gap semiconductor-type optical bandgap E_g = 0.89 eV. Electrical measurements corroborate the semiconducting nature of the material with evidence of two types of transport mechanisms, variable range hopping at low temperatures and small polaron hopping at high temperatures. The I–V curves conform to varistor-type behaviour. The magnetic susceptibility as a function of temperature evidences irreversible behaviour characteristic of magnetically disordered systems and the magnetisation suggests antiferromagnetic behaviour with weak ferromagnetic hysteresis due to the canting of the spin moments of Ru⁴⁺ cations because of octahedral distortions in the crystal cell. Band structure and density of electronic states calculations exhibit half-metallic behaviour, caused by a shift of the 4d-Ru⁴⁺ orbitals from the valence band into the conduction band, crossing the Fermi level for the spin-down configuration, while the spin-up polarisation remains semiconducting. The half-metallic character is attributed to the strong spin–orbit coupling effects caused by the dequenching of the orbital angular momentum of 4d-Ru⁴⁺.