First principles calculations on the magnetic state of the mixed valence cobalt ludwigite

Density functional non-collinear spin calculations are carried out on the mixed valence ludwigite Co3O2BO3 to determine the low temperature magnetic order of this material. Low spin is found in tri-valent cobalt, whose consequence is to prevent the formation of a ferromagnetic short-bonded cobalt dimer, a mechanism which favors dimerization in the similar compound Fe3O2BO3. Thus, magnetism blocks the structural dimerization transition in Co3O2BO3. This is the most significant difference observed between the electronic and magnetic structure of the two homo-metallic ludwigites. Magnetic arrangement of the set of di-valent metal cations is the same as in Fe3O2BO3. Bulk magnetization is due to ferromagnetic alignment in a sub-group of Co2+ which in this compound is not compensated by spin canting in demagnetized Co3+, as observed in the anti-ferromagnetic state of Fe3O2BO3. Calculations predict a bulk magnetization of ~ 30 emu/g, with negligible contribution of the orbital magnetic moments of cobalt.