Tunning the magnetic transition temperature and spin structure in Cr-doped Co₃BO₅Ludwigite

Ludwigites have formula unit M₂M'BO₅, where M and M’ are transition metals. These compounds may exhibit different physical properties ranging from structural ordering, charge ordering, coexistence of magnetic order and paramagnetism, metamagnetic transition, magnetocaloric effect, dimensional crossover, spin crossover and spin glass. This variety of physical behavior has been attributed to a combination of strong correlation and low-dimensional effects. So, these materials offer a unique opportunity to study the correlation between magnetism and dimensionality. The homometallic Co₃BO₅ ludwigite, with high-spin (HS) Co²⁺ and low-spin (LS) Co³⁺, shows an AFM ordering at 42 K. When the compound is doped with non-magnetic ions, in most cases, and contrary to expected, the magnetic interactions are strengthened and the magnetic transition temperature increases. On the other hand, when doped with magnetic ions, such as Fe³⁺, Mn³⁺, magnetic disorder leads the system to a spin glass state, except for Cr, which occupies a unique site in the structure. Particularly, when Co₃BO₅ is doped with Cr³, forming the Co_2.5Cr_0.5BO₅, the magnetic interactions are strengthened, and T_N rises to 76 K. Despite the presence of three different magnetic ions, Co²⁺, Co³⁺ and Cr³⁺, in the HS states, frustrated magnetic interactions are established. The magnetic dimensionality increases from 2D in Co₃BO₅ to 3D in Co_2.5Cr_0.5BO₅. Here we extend our study of Cr-doped ludwigite Co_3-xCr_xBO₅, comprising a broader range of Cr concentrations to understand the role of Cr, which yield changes of the magnetic structure and dimensionality in these compounds. X-ray diffraction results show that Cr³⁺ occupy a unique site and gradually expand the unit cell as the Cr content increases, thus favoring the appearance of HS Co³⁺. The magnetic transition temperature increases gradually with the increase of Cr content in the compound, reaching 115 K for x=1.0. We show that depending on the Cr concentration, the compound adopts different spin structures. It goes from AFM for low Cr content, through ferrimagnetic (FIM) for intermediate concentrations, and back to AFM for high Cr concentrations.