Magnetic moment distribution in non-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys

In this work we present a model for the net magnetic moment per formula unit measured in Ni-Mn-Ga Ferromagnetic Shape Memory Alloys, analyzing the different types of magnetic interactions appearing in such compounds. We have studied four compositions ranging from a 44% to a 52% Ni content. We found that excess Mn atoms occupy empty positions at Ni and Ga sites. We consider that the Mn atoms at Ni sites (Mn/Ni) can determine that Mn/Ga atoms couple either ferromagnetic (FM) or antiferromagnetically (AF) to Mn/Mn atoms. If Mn/Ga are nearest neighbors to properly-sited Mn/Mn atoms they present AF coupling. However, when Mn/Ni atoms are at closer distances than Mn/Ga ones, so that they become nearest neighbors to Mn/Mn, the AF coupling between Mn/Ni and Mn/Mn atoms is stronger and overcomes that between Mn/Ga and Mn/Mn ones. Therefore Mn/Ga and Mn/Mn will couple ferromagnetically. Polarized neutron diffraction experiments will help to elucidate the validity of our model.