Non-monotonous variation of structural instability in self-doped Ni-Mn-Sn based shape memory alloys

A series of Ni-doped Ni50+xMn35−xSn15 samples for x = −1, 0, 1, 2, 3 and 4 have been investigated by transport and dc magnetization measurements. The parent sample (x = 0) undergoes martensitic phase transition around 180 K during cooling along with ferromagnetic Curie point (TC) to be 340 K. Interestingly, Ni doping at the Mn site leads to rather unusual non-monotonous variation of martensitic transition temperature (TMS) which is precisely corroborated by the similar non-monotonous variation of lattice volume (V300K). Between −1 ≤ x ≤ 2, the variation of various physical parameters in the studies alloys is conventional and can be explained on the basis of e/a ratio of the samples. However, for x> 2, parameters such as TC, TMS, V300K etc. show sudden change in the nature of their variation which is anomalous and cannot be accounted by the e/a ratio formalism. Present study reveals that possible reason behind this non-monotonous variation is closely connected with a self-sustained mechanism originating from the change in the strength of hybridization between Ni 3d eg state and the excess Mn at the Sn site on Ni substitution. The samples also show large negative magnetoresistance near TMS with its magnitude showing similar non-monotonous variation with Ni doping.