Spin state crossover in Co3BO5

N. V. Kazak, M. S. Platunov, Yu. V. Knyazev, M. S. Molokeev, M. V. Gorev, S. G. Ovchinnikov, Z. V. Pchelkina, V. V. Gapontsev, S. V. Streltsov, J. Bartolomé, A. Arauzo, V. V. Yumashev, S. Yu. Gavrilkin, F. Wilhelm, and A. Rogalev. PHYSICAL REVIEW B. https://doi.org/10.1103/PhysRevB.103.094445

We have investigated the spin and oxidation states of Co in Co3BO5 using x-ray magnetic circular dichroism (XMCD) and dc magnetic susceptibility measurements. At low temperatures, XMCD experiments have been performed at the Co K-edge in Co3BO5 and Co2FeBO5 single crystals in the fully ferrimagnetically ordered phase. The Co (K-edge) XMCD signal is found to be related to the Co2+ magnetic sublattices in both compounds, providing strong experimental support for the low-spin (LS) Co3+ scenario. The paramagnetic susceptibility is highly anisotropic. An estimation of the effective magnetic moment in the temperature range 100–250 K correlates well with two Co2+ ions in the high-spin (HS) state and some orbital contribution, while Co3+ remains in the LS state. The crystal structure of the Co3BO5 single crystal has been solved in detail at the T range 296–703 K. The unit cell parameters and volume show anomalies at 500 and 700 K. The octahedral environment of the Co4 site strongly changes with heating. The generalized gradient approximation with Hubbard U correction calculations have revealed that, at low-temperatures, the system is insulating with a band gap of 1.4 eV, and the Co2+ ions are in the HS state, while Co3+ are in the LS state. At high temperatures (T > 700 K), the charge ordering disappears, and the system becomes metallic with all Co ions in 3d7 electronic configuration and HS state.