We have investigated the spin and oxidation states of Co in ${\mathrm{Co}}_3{\mathrm{BO}}_5$ 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 ${\mathrm{Co}}_3{\mathrm{BO}}_5$ and ${\mathrm{Co}}_2{\mathrm{FeBO}}_5$ single crystals in the fully ferrimagnetically ordered phase. The Co ($K$-edge) XMCD signal is found to be related to the ${\mathrm{Co}}^{2+}$ magnetic sublattices in both compounds, providing strong experimental support for the low-spin (LS) ${\mathrm{Co}}^{3+}$ 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 ${\mathrm{Co}}^{2+}$ ions in the high-spin (HS) state and some orbital contribution, while ${\mathrm{Co}}^{3+}$ remains in the LS state. The crystal structure of the ${\mathrm{Co}}_3{\mathrm{BO}}_5$ 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 ${\mathrm{Co}}^{2+}$ ions are in the HS state, while ${\mathrm{Co}}^{3+}$ 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 $3d^7$ electronic configuration and HS state.

• Received 15 September 2020
• Accepted 8 March 2021

DOI:https://doi.org/10.1103/PhysRevB.103.094445