Tuning structure and magnetic order through Cr substitution in Co3O2BO3 Ludwigite

Mariano, D.L., Sanchez, D.R., Freitas, D.C., (...), Cohen, L.F., Ghivelder, L.// Journal of Alloys and Compounds//

https://doi.org/10.1016/j.jallcom.2025.181497

In the Co₃O₂BO₃ Ludwigite, there are four crystallographic sites for the metal ion Co. Three sites are occupied by Co $^{2 +}$ in a high-spin state, and one site by Co $^{3 +}$ in low-spin state, resulting in a compound where the number of Co $^{2 +}$ ions is twice that of Co $^{3 +}$ ions. Given the particular distribution of magnetic and non-magnetic ions, the compound can be visualized as consisting of magnetic planes containing Co $^{2 +}$ ions, separated by non-magnetic planes formed by Co $^{3 +}$ ions. In this work, were studied the structural changes, especially magnetic ones, caused by the gradual substitution of low-spin Co $^{3 +}$ (S=0) by Cr^3＋ ions (S = 3/2). We synthesized the Co $_{3 - x}$ Cr $_{x}$ O₂BO₃ series, and X-ray experiments showed a gradual increase in unit cell volume with increasing Cr concentration. Magnetization measurements show that this type of substitution drastically increases the magnetic transition temperature, from 42 K for the undoped compound to 115 K for the compound with the highest Cr concentration. The latter is the highest long-range magnetic ordering temperature recorded so far for cobalt Ludwigites. The results from specific heat and magnetization measurements show that this substitution changes the ground-state magnetic structure of the compound, transitioning from a ferrimagnetic state (at low Cr concentrations) to an antiferromagnetic state (at high Cr concentrations), in agreement with the results of exchange integral calculations.