Spin and valence variation in cobalt doped barium strontium titanate ceramics

Abstract

In the present decade, owing to half-metallic ferromagnetism, controlled 3d transition metal-doping based defect engineering in oxide perovskites attracts considerable attention in the pursuit of spintronics. We aim to investigate the electronic structure of Co-doped barium strontium titanate (Ba_0.8Sr_0.2Co_xTi_1−xO₃ where x = 0, 0.1, 0.2) solid solution. Structural, vibrational and microscopic properties indicate the cationic substitution of Co at the octahedral Ti position along with a displacive kind of tetragonal-to-cubic phase transformation. X-ray photoelectron spectroscopy evidences the reduction in the valence state from Co³⁺ to Co²⁺ and Ti K edge X-ray absorption spectroscopy endorses the higher lattice symmetry with increasing Co doping. Orbital hybridization triggered electron hopping between O 2p and Co e_g orbitals results in a spin fluctuation from the occupation t⁶_2ge⁰_g for x = 0.1 to the occupation t⁶_2ge¹_gL for x = 0.20 (L designates a hole in the O 2p shell) aligned state observed from density functional theory calculations. The dominating crystal field energy as compared to intra-atomic exchange (Hund) energy decides the spin-orbital degeneracy for the Co 3d orbital to induce spin fluctuations.