Role of dilution on the electronic structure and magnetic ordering of spinel cobaltites

By means of the first-principles density functional theory (DFT + U) calculations and experiments, we investigate the role of dilution on the structural, magnetic, electronic, and optical properties of the antiferromagnetic (AFM) spinel Co3O4 having Ned temperature (T-N) similar to 30 K. As the octahedral cobalt site of spinel lattice is diluted with Ge, Al, Ti, Ru, and Sn cations, we observe a substantial increase in the size of the unit cell as well as destruction of the long-range magnetic ordering with a spin-orbit compensation effect. The ferrimagnetic ordering in diluted inverse spinels such as Co-2 Sigma O-4 (Sigma = Ti and Sn) emerges due to the difference in the magnetic moments of two sublattices A (3.87 mu(B)) and B (4.16 mu(B) for Co2SnO4 and 5.19 mu(B) for Co2TiO4). Experiments and DFT calculations indicate antiferromagnetic configuration for Co3O4 , Co2AlO4 (T-N similar to 4.8 K) spinels with an equal and opposite moment of similar to 2.60 mu B in tetrahedral sites of divalent Co ions and negligible contribution from trivalent B-site Co due to the complete filling of t(2g) levels having a giant crystal field of similar to 2.5 and 1.8 eV, respectively. However, in Co2GeO4 (T-N similar to 20.4 K) case AFM behavior originates due to the opposite spins at octahedral sites of divalent Co ions. The remaining spinels Co2TiO4 (T-N similar to 47.8 K), Co2RuO4 (T-N similar to 16 K), and Co2SnO4 (T-N similar to 41 K) are more favorable to ferrimagnetic structure as evident from our magnetization measurements with a different temperature dependence of magnetic moments A(T) and B(T) at tetrahedral A and octahedral B sites, respectively. The variation in the energy band gap (E-g = 1.68 -> 3.28 eV for Co2RuO4 -> Co2GeO4) obtained from DFT U calculations are in good agreement with our experimental results (E-g = 1.52 -> 3.16 eV) obtained from the diffusive reflectance spectroscopy. The extent of exchange splitting Delta(eg)(EX) of tetrahedral Co2+ varies between 1.8 and 1.3 eV for Co3O4 and Co2AlO4 , respectively. However, Delta(t2g)(Ex) exhibits a decreasing trend (5.2 -> 3.6 eV for Co3O4 -> Co2SnO4) with increasing the lattice parameter, except for cobalt-orthogermanate Co2GeO4.