  Recently synthesised Ba4NbMn3O12 belong to cluster magnets - systems with
tightly bound groups of magnetism ions, in this case Mn3 trimers. Often such
magnetic clusters can be described by molecular orbitals (MO), however strong
electron correlations may invalidate this description. To understand the
electronic and magnetic state of Ba4NbMn3O12 we carried out ab initio
calculations and show that this system is better described not in MO picture,
but as a system with electrons localized on the Mn ions, with strong
intra-cluster and weaker inter-cluster exchange. The calculated spin of the Mn3
trimer is S=2, in agreement with the experiment. The predicted magnetic
structure of Ba4NbMn3O12 is that of ferromagnetic layers of Mn3 trimers,
stacked antiferromagnetically.
