Origins of superparamagnetism in self-accommodated and trained Ni50Mn37.5Sn12.5 single crystal

Abstract

The low temperature magnetic state of martensite in the shape memory Ni50Mn37.5Sn12.5 single crystal was confirmed from DC and AC magnetization measurements. At temperatures below the martensite finish and above the Curie temperature of martensite the martensite phase shows superparamagnetic behavior. On cooling growing superparamagnetic clusters manifest a strong interaction and develop, below the freezing temperature (~200 K), into a nonconventional spin glass state. The amount of retained austenite evaluated by high-energy synchrotron radiation was estimated at the level of 1.5 – 2%. Subsequent training procedure cleansed martensite from any retained austenite fraction. The evaluated cluster magnetic moment decreased following training (~9.5%). It is demonstrated that the retained austenite effectively subsidizes but not warrants the superparamagnetic exchange in martensite, which descends from alterations in interatomic spacing inflicted by martensitic transformation.