Spin state crossover in Co3BO5
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. © 2021 American Physical Society. ; We acknowledge many inspirational discussions on the topic of this paper with the late Natalia Ivanova and Leonard Bezmaternykh. The authors acknowledge A. Ney for allowing us to use the XANES spectrum of film. We are grateful to the Russian Foundation for Basic Research (Project No. 20-02-00559) and President Council on Grants (Project No. МК-2339.2020.2) for supporting this paper. This paper was carried out within the state assignment of the Russian Ministry of Science and High Education via program "Quantum"' (No. AAAA-A18-118020190095-4). We also acknowledge support by Russian Ministry of Education and Science via Contract No. 02.A03.21.0006. We acknowledge financial support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2017-83468-R) and from the regional Government of Aragon (E12-20R RASMIA project).