119 Sn Mössbauer spectroscopy in the study of metamagnetic shape memory alloys

The effect of combined mechanical and thermal treatments in the magnetostructural properties is studied in a Ni-Mn-Sn metamagnetic shape memory alloy, by X-ray diffraction, powder neutron diffraction, calorimetry, magnetometry and 119Sn Mossbauer spectroscopy. A Ni50Mn35Sn15 alloy has been mechanically milled and then annealed at different temperatures in order to produce different microstructural states. The milling and the post-annealing processes do not produce any variation of transition temperatures and atomic order. However, the induced microstructural distortions affect greatly the martensitic transformation. Milling promotes antiferromagnetic coupling between Mn atoms, resulting in a significant decrease in the saturation magnetization that recovers with annealing. Moreover, the temperature width of the transformation is considerably affected by the mechanically induced local strain. Last but not least, the recovery of the microstructural distortions induced by milling is directly related to the intensity of the non-magnetic component revealed by 119Sn Mossbauer spectroscopy. This result opens the possibility of quantifying the whole contribution of defects and local strain on the martensitic transformation in Ni-Mn-Sn alloys.