Magnetoresistance behavior of ferromagnetic shape memory alloyNi1.75Mn1.25Ga

Abstract

A negative-positive-negative switching behavior of magnetoresistance ͑MR͒ with temperature is observed in a ferromagnetic shape memory alloy Ni 1.75 Mn 1.25 Ga. In the austenitic phase between 300 and 120 K, MR is negative due to s-d scattering. Curiously, below 120 K 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 antiferromagnetic alignment between the local magnetic moments of the Mn atoms is the energetically favored 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 in 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.

Theoretical prediction and experimental study of a ferromagnetic shape memory alloy:Ga2MnNi

“…The reasons for this disagreement could be that the actual sample has Mn excess, which might cause Mn clustering leading to antiferromagnetic coupling between Mn atom pairs, as has been observed for other Mn excess systems. 22,32,33 Moreover, note that the theory does not consider the actual monoclinic structure (discussed below) which might favor a different magnetic ground state with anti-parallel coupling between Mn atoms.…”

Coexistence of charge-density wave and ferromagnetism in Ni2MnGa

“…Among the different FSMA's discovered so far, Ni 2 MnGa is at the center of attention because of large (10%) magnetic field induced strain and interesting magnetic behavior. [15][16][17] It is an ideal model system to explore the co-existence of ferromagnetism and CDW because martensite transition (which is a diffusionless first order transition from high symmetry austenite phase to a lower symmetry martensite phase) is known to be associated with phonon anomalies.…”

Twin-variant reorientation-induced large magnetoresistance effect in Ni50Mn29Ga21 single crystal

“…It is seen that the positive saturation MR of ϳ26.6% is developed in the T-range from 260 to 315 K, while the negative saturation MR of ϳ25.2% is formed from 230 to 315 K. For comparison, Ni-Mn-Ga alloys usually show the largest MRs during the narrow martensitic-austenitic phase transformation. [10][11][12] Figure 5 shows the saturation MR as a function of spatial angle ͑ ͒ between H and I for different magnitudes of H at room temperature ͑298 K͒ based on the preset initial nearly single-variant state S. It is noted that the direction of H was maintained along the z-axis, while that of I was kept along the l-direction of the single crystal so that was obtained by rotating the single crystal ͑also I͒ with respect to H. It is found that the MR value varies periodically with . The saturation MR has a periodic change between 0 and ϳ−25% for every 180°or half cycle of .…”

“…It is argued that the underlying mechanisms are mainly governed by the s-d scattering for the bulks and the spin transport for the thin films. 10,11 Among the Heusler alloys, Ni-Mn-Ga single crystals exhibit giant magnetic field-induced strains ͑MFISs͒ of ϳ6% ͑load-free͒ in the tetragonal martensitic phase and of ϳ10% in the orthorhombic martensitic phase as a result of the martensitic twin-variant reorientation induced by magnetic field, mechanical stress, and/or temperature. [13][14][15] This reorientation may also lead to a characteristic change in electrical resistivity due to crystallographic anisotropy.…”

“…5,6 More recently, the MR effect has been observed in some half-metallic Heusler alloys with the general formula Ni 2 Mn 1+x X 1−x ͑where X is a sp-element such as Sn, Sb, In, etc.͒ and in some Heusler-type ferromagnetic shape memory alloys ͑such as Ni-Mn-Ga alloys͒. [7][8][9][10][11][12] In particular, the reported MR effect in these Heusler alloys is basically in polycrystalline bulks and thin films with either positive or negative MR value, depending on applied magnetic field and ͑low͒ temperature. It is argued that the underlying mechanisms are mainly governed by the s-d scattering for the bulks and the spin transport for the thin films.…”