Anomalous pressure effect on the magnetic properties of Ni-Mn based shape memory alloys

Abstract

The magnetic behavior of three Ni-Mn-(Sn,In) based shape memory alloys are investigated under hydrostatic pressure. Among them, Ni51.2Mn32.8In16 (NM-In16) and Ni51Mn35Sn14 (NM-Sn14) have their martensitic transition close to room temperature and undergo several magnetic transitions at lower temperatures. They order ferromagnetically at TCA, which is just above room temperature. However, the ferromagnetism is destroyed by the martensitic transition at TMS (<TCA). The ferromagnetic order is revoked further at a lower temperature TCM (<TMS) in the martensite phase. The third alloy, namely, Ni50Mn34Sn16 (NM-Sn16), has a relatively stable ferromagnetic state (Curie point TCA=347 K), and it survives below the martensitic transition temperature, TMS. Our magnetic study under hydrostatic pressure indicates that TMS and TCM (only for the first two alloys) increase systematically with pressure. However, the saturation magnetization at base temperature is found to be an increasing function of pressure for NM-Sn14 only, and a decreasing trend is observed for the other two alloys. Most interestingly, NM-Sn14 shows a large enhancement in magnetocaloric effect (MCE) with pressure, whereas the other two compositions show a drop in MCE with pressure. A subtle competition between the change in Ni-Mn hybridization strength and the Mn-Mn magnetic interaction with pressure can be responsible for the observed anomalous behavior.