Magnetic moment and chemical order in off-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys

Recent studies have shown that the total magnetic moment in off-stoichiometric Ni-Mn-Ga alloys depends not only on electronic concentration but also on the degree of chemical order in the alloy. We have performed neutron diffraction experiments and magnetization measurements for determining the preferential atomic order and saturation moment in off-stoichiometric compounds (44-52 at.% Ni), having excess Mn and deficient in Ga. These alloys include isoelectronic alloys with different magnetic moments and were chosen in an effort to study the impact of chemical order on the magnetic moment distribution. In this work, we present an improved model of magnetic interaction between Mn atoms, which carry most of the localized magnetic moment of the alloys. The Mn atoms at Ga sites, which are nearest neighbors to properly sited Mn, couple antiferromagnetically to the dominant moment. In contrast, Mn atoms at Ga sites, which are nearest neighbors to Mn at Ni sites, couple ferromagnetically. Mn at Ni sites is always antiferromagnetic (AF). The new model is supported by the exchange variation with the Mn-Mn distance and demonstrates excellent agreement between experimental and calculated magnetic moments. The proposed model is shown to better explain the observed experimental results as compared to the rigid band model and previous localized moment models that assumed AF coupling for all off-site Mn atoms.