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 Mₛ. 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, Ni₅₀Mn₄₀Sb₁₀ and Ni₅₀Mn₃₇Sn₁₃. We show that in the vicinity of Mₛ, the magnetic correlations at temperatures T<Mₛ are antiferromagnetic, whereas they are ferromagnetic above Mₛ and well beyond the Curie temperature of the austenitic state.