Magnetic and M\"ossbauer spectroscopy (MS) measurements have been performed on a single crystal of Co2.25Fe0.75O2BO3 with ludwigite structure. Two magnetic transitions (TN=115$\hskip0.3em${}K and TC=70$\hskip0.3em${}K) were traced from the ac susceptibility temperature dependence. The MS spectra as a function of temperature clearly show the onset of magnetic ordering below 115 K. Magnetization measurements on the parent Co3O2BO3 and Fe3O2BO3 compounds have been done for comparison. In Fe3O2BO3 the anisotropy of the different phases has been determined, showing that the anisotropy axis changes from the a to the b axis in the low-temperature antiferromagnetic phase. High magnetic uniaxial anisotropy has been detected for both Co3O2BO3 and Co2.25Fe0.75O2BO3. From the angle-dependent magnetization measurements it is found that in both compounds the easy axis of magnetization is the b [010] axis, where an antiferromagnetic component is superimposed on the main ferromagnetic component. In the c direction the behavior is purely antiferromagnetic. In Co2.25Fe0.75O2BO3 a strong reduction of the remanent magnetization and a very strong increase in coercive field along the b axis with respect to those found in Co3O2BO3 were observed from magnetic hysteresis cycles measured below TC. The increase of coercive field is caused by the increase of defects upon Co substitution by Fe.

Co₃O₂BO₃ and Co₂FeO₂BO₃ single crystals with a ludwigite structure are fabricated, and their crystal structure and magnetic properties are studied in detail. Substituted ludwigite Co₂FeO₂BO₃ undergoes two-stage magnetic ordering at the temperatures characteristic of Fe₃O₂BO₃ (T _N1 ≈ 110 K, T _N2 ≈ 70 K) rather than Co₃O₂BO₃ (T _N = 42 K). This effect is explained in terms of preferred occupation of nonequivalent crystallographic positions by iron, which was detected by X-ray diffraction. Both materials exhibit a pronounced uniaxial magnetic anisotropy. Crystallographic direction b is an easy magnetization axis. Upon iron substitution, the cobalt ludwigite acquires a very high magnetic hardness.

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.