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

Abstract

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 15K 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<40K).