First principles calculation of magnetic order in a low-temperature phase of the iron ludwigite

The magnetic order of a low-temperature dimerized phase of Fe3O2BO3 is investigated through a density functional approach which considers full non-collinear spin-spin interactions, focusing on the 15 K crystalline structure. It is found that Fe spins in the (Fe-Fe)5+ dimer, formed during the room temperature structural change of Fe3O2BO3, are parallel and have little freedom to rotate under interaction with neighbor Fe atoms. While the Fe dimer behaves as a heavy single magnetic unit the spin magnetic moment of the third Fe3+ atom of the Fe triad has, on the contrary, much more freedom to rotate. This is responsible for a canted spin ordering, revealed by a rotation of ~80° of the trivalent Fe spin relative to the spin orientation of the dimer, due to spin-spin interaction with divalent Fe atoms outside the triad. Canting is thus seen to be responsible for the very low net magnetization, experimentally observed in this compound (T<40 K).