Magnetic correlations in martensitic Ni-Mn-based Heusler shape-memory alloys: Neutron polarization analysis

In ferromagnetic Ni-Mn-based Heusler alloys undergoing martensitic transformations, a sudden drop in the magnetization is observed as the temperature decreases through the martensitic-transformation temperature $M_s$. To understand the cause of the drop and thereby the nature of the magnetic coupling in this temperature region, we carry out neutron-polarization-analysis experiments on two prototypes, ${\text{Ni}}_{50}{\text{Mn}}_{40}{\text{Sb}}_{10}$ and ${\text{Ni}}_{50}{\text{Mn}}_{37}{\text{Sn}}_{13}$. We show that in the vicinity of $M_s$, the magnetic correlations at temperatures $T<M_s$ are antiferromagnetic, whereas they are ferromagnetic above $M_s$ and well beyond the Curie temperature of the austenitic state.