Low lying magnetic states of the mixed valence cobalt ludwigite

There are two interpretations offered for the different structural and magnetic properties of the mixed valence homo-metallic ludwigites, Co3O2BO3 and Fe3O2BO3. One of them associates the physical behavior to charge ordering processes among the cations, as is well known in simpler oxides. The other attributes the effects to local pairwise magnetic interactions. Recently first principles calculations in the iron ludwigite have shown that the structural cation dimerization is due to the formation of strong magnetic dyads supporting the second model. Here we confirm the dominance of magnetic interactions to explain the absence of dimerization in the cobalt compound. Density functional non-collinear spin calculations are carried out on Co3O2BO3 to determine its low temperature magnetic order. Low spin is found on tri-valent cobalt sites, thus preventing the formation of the ferromagnetic dyad, the mechanism which favors dimerization in Fe3O2BO3. We conclude that the difference between high spin Fe3+ and low spin Co3+ pairwise interactions is responsible for the observed differences between the two compounds. The pairwise magnetic interactions also explain the difference between the existence of low temperature bulk AF state in the Fe ludwigite and its absence in the Co material.
Comment: 17 pages, 2 Tables, 3 Figures