Effect of excess Ni on martensitic transition, exchange bias and inverse magnetocaloric effect in Ni2+xMn1.4−xSn0.6 alloy

The martensitic transition, exchange bias (EB) and inverse magnetocaloric effect (IMCE) of bulk Ni2+xMn1.4−xSn0.6 (x = 0, 0.06, 0.12, 0.18) Heusler alloy is investigated in this paper. Substitution of Mn by Ni causes an increase in the martensite transition temperature (TM), decrease in Curie temperature of austenite phase (TCA) and also a decrease in the saturation magnetic moment (Msat). While the decrease in TCA and Msat is explained by the dilution of the magnetic subsystems and on the other hand the increase in TM is due to the increase of valence electron concentration per atom (e/a). All the alloys shows EB effect below a certain temperature (T∗) and EB field (HEB) value is almost thrice in magnitude for x = 0.18 sample compared to x = 0 sample at 5 K. In these alloys, Ni/Mn atoms at regular site couples antiferromagnetically (AFM) with the excess Ni atoms at Mn or Sn sites and this AFM coupling plays the key role in the observation of EB. For the IMCE, the change in magnetic entropy (ΔSM) initially increased with excess Ni concentration upto x = 0.12 but then a drastic fall in ΔSM value is observed for the sample x = 0.18 but the relative cooling power (RCP) value is increased continuously with the excess Ni concentration.