Magnetoresistance behavior of a ferromagnetic shape memory alloy: Ni_1.75Mn_1.25Ga

TL;DR

This study investigates the complex magnetoresistance behavior of Ni_1.75Mn_1.25Ga, revealing temperature-dependent switching linked to magnetic transitions and phase changes, supported by experimental data and ab initio calculations.

Contribution

It uncovers the temperature-dependent sign change in magnetoresistance and explains it through a ferrimagnetic ground state using density functional theory.

Findings

MR switches from negative to positive below 120K and back to negative at lower temperatures

Ab initio calculations show a ferrimagnetic ground state with anti-ferromagnetic Mn alignment

Hysteresis in MR is observed due to martensitic phase transition

Abstract

A negative-positive-negative switching behavior of magnetoresistance (MR) with temperature is observed in a ferromagnetic shape memory alloy Ni_1.75Mn_1.25Ga. In the austenitic phase between 300 and 120 K, MR is negative due to s-d scattering. Curiously, below 120K MR is positive, while at still lower temperatures in the martensitic phase, MR is negative again. The positive MR cannot be explained by Lorentz contribution and is related to a magnetic transition. Evidence for this is obtained from ab initio density functional theory, a decrease in magnetization and resistivity upturn at 120 K. Theory shows that a ferrimagnetic state with anti-ferromagnetic alignment between the local magnetic moments of the Mn atoms is the energetically favoured ground state. In the martensitic phase, there are two competing factors that govern the MR behavior: a dominant negative trend up to the saturation field due to the decrease of electron scattering at twin and domain boundaries; and a weaker positive trend due to the ferrimagnetic nature of the magnetic state. MR exhibits a hysteresis between heating and cooling that is related to the first order nature of the martensitic phase transition.