Influence of intermartensitic transitions on transport properties of Ni2.16Mn0.84Ga alloy

Magnetic, transport and x-ray diffraction measurements of the ferromagnetic shape memory alloy Ni2.16Mn0.84Ga have revealed that this alloy undergoes an intermartensitic transition upon cooling, whereas no such a transition is observed upon subsequent heating. The difference in the modulation of the martensite forming upon cooling from the high-temperature austenitic state (five-layered (5M) martensite), and the martensite forming upon the intermartensitic transition (seven-layered (7M) martensite) strongly affects the magnetic and transport properties of the alloy and results in a large thermal hysteresis of the resistivity p and magnetization M. The intermartensitic transition has an especially marked influence on the transport properties, as is evident from a large difference in the resistivity of the 5M and 7M martensites, (rho(5M) - rho(7M))/rho(5M) approximate to 15%, which is larger than the jump of resistivity at the martensitic transition from the cubic austenitic phase to the monoclinic 5M martensitic phase. We assume that this significant difference in p between the martensitic phases is accounted for by nesting features of the Fermi surface. It is also suggested that the nesting hypothesis can explain the uncommon behaviour of the resistivity at the martensitic transition, observed in stoichiometric and near-stoichiometric Ni-Mn-Ga alloys.