Theoretical Study On Magnetic And Martensitic Transformation Of Ni-Mn-based Heusler Ferromagnetic Shape Memory Alloy

The effect of alloy composition on total energy, crystal structure and electronic structure of Ni-Mn-X (X=Ga, Sn, Sb) ferromagnetic shape memory alloys has been investigated by first-principles plane-wave pesedupotential method based on density functional theory. The mechanism of effect of alloy composition on martenstic transformation and magnetic properties has been clarified. These works provide theoretical guidance for the design of novel magnetic drive memory material.The results show that variations of Mn content have significant effect on the structure of Ni-Mn-Ga. When Mn content is 25 at.%, Ni-Mn-Ga exhibits martensitic transformation. This is because that the phase stability of martensite is higher than that of parent phase. The lattice constant of Ni-Mn-Ga parent phase increases with increasing Mn content, which is related with the structure stability. With increase of Mn content, Mn at Mn site is antiparallel with Mn at Ga site, resulting in the decrease of parent phase and martensite. The addition of Co changes the Mn-Mn exchange from antiferromagnetic to ferromagnetic in Ni-Mn-Ga parent phase, while the Mn-Mn exchange in martensite remains antiferromagnetic. Therefore, the saturation magnetization difference between parent phase and martensite increases.The calculation results indicate that the lattice constants of Ni-Mn-Ga parent phase decrease with Mn content. With increasing Mn content, the magnetic moment of Ni-Mn-Sn parent phase decreases. This is due to that Mn at Mn site is antiparallel with Mn at Sn site. Substitution of Co for Ni changes the Mn-Mn exchange from antiferromagnetic to ferromagnetic in Ni-Mn-Ga parent phase, leading to the increase of saturation magnetization difference between parent phase and martensite. In addition, it is noted that Co addition has little effect on majority-spin density of states (DOS), but changes minority-spin DOS significantly. The analysis of DOS indicates that the total DOS at the Fermi level can be used to evaluate the phase stability of parent phase.The theoretical studies show that the lattice constants of Ni-Mn-Sn parent phase decrease with Mn content. With increasing Mn content, the magnetic moment of Ni-Mn-Sn parent phase decreases. This is due to that Mn at Mn site is antiparallel with Mn at Sb site.