Dimensional crossover in Cr-doped Co3BO5

Abstract

The crystal structure and the magnetic properties of the ${\mathrm{Co}}_{2.5}{\mathrm{Cr}}_{0.5}{\mathrm{BO}}_{5}$ ludwigite have been investigated by x-ray diffraction, magnetization, and specific heat experiments. Cr ions mainly occupy sites 4 of the ludwigite lattice, substituting approximately half of the ${\mathrm{Co}}^{3+}$. It changes the interatomic distances and favors a high spin state for the remaining ${\mathrm{Co}}^{3+}$ ions. Doping the homometallic ${\mathrm{Co}}_{3}{\mathrm{BO}}_{5}$ with magnetic Cr drastically increases the magnetic transition temperature from 42 K, for the undoped compound, to 76 K for ${\mathrm{Co}}_{2.5}{\mathrm{Cr}}_{0.5}{\mathrm{BO}}_{5}$. The latter contains three different magnetic ions, ${\mathrm{Co}}^{2+}, {\mathrm{Co}}^{3+}$ and ${\mathrm{Cr}}^{3+}$ in high spin states, as evidenced by magnetization measurements. The specific position of Cr in the structure (sites 4) gives rise, according to calculations of the exchange interactions and the Goodenough-Kanamori rules, to a few frustrated magnetic interactions. This is responsible for the high ferrimagnetic ordering temperature of 76 K, when compared to that of the pure compound. The experimental results imply a magnetic structure for the system that we show is consistent with the known interactions and the Goodenough-Kanamori rules. Low temperature specific heat and magnetization measurements reveal that the two-dimensional nature of the magnetism ${\mathrm{Co}}_{3}{\mathrm{BO}}_{5}$ when doped with Cr acquires a three-dimensional character with a high temperature ferrimagnetic order.