Tunable Multifuctionality in Heusler Alloys by Extreme Conditions

The multifunctional materials have demonstrated various properties such as shape memory effect (SME), magneto caloric effect (MCE), magneto resistance (MR), piezoresistance (PR), exchange bias (EB), half metallic ferromagnetism (HMF), and spin polarization. Among many Heusler compounds, Ni-Mn-Ga alloys provide SME, MCE, PR, and MR behaviors. These properties can be tuned by some external/internal perturbations such as pressure, magnetic field, and chemical composition. These alloys are prepared using an arc melting furnace under by melting the high-purity starting elements (99.99%). The aim of the book chapter is to enhance the multicaloric properties (MCE and PR) nearer to ambient temperature by the application of some external parameters. Hence, we have chosen few Heusler alloys. These materials are investigated under extreme conditions (hydrostatic pressure, high magnetic field, and low temperature). All the doped and undoped Ni-Mn-Ga alloy series alloys exhibit conventional MCE. The application of external magnetic field increases the magnetization for both alloys. The hydrostatic pressure influences Ms and broadens the hysteresis width in both the samples. The observed metamagnetic transition at ambient pressure gets suppressed at higher pressure. Also, high pressure induces larger magneto crystalline anisotropy. The effect of pressure on MCE is decreased for both Ni2–xMn1+xGa (x = 0 and 0.15) alloys. These alloys exhibit –ve PR (x=0 @ 30 kbar) and +ve PR (x = 0.15@ 28 kbar) when subjected to hydrostatic pressure. The rate of change of T and resistivity with respect to pressure are calculated and show positive values for both the samples. The residual resistivity and electron-electron scattering factor are found to be decreased with pressure for x = 0, and it exhibits metallic behavior. However, both parameters increase for x = 0.15 alloy, and it may be related to static disorder effects and spin fluctuations.