Elucidation of annealing effects on austenite–martensite and magnetic phase transitions in Mn2Ni1.6Sn0.4 ribbons synthesized from bulk alloys prepared by RF induction melting

Abstract

We investigate the consequences of annealing on the structure, microstructure, magnetic, and electrical transport of Mn2Ni1.6Sn0.4 ribbons. The 40–60 μm thick ribbons were formed by melt spinning of the bulk ingot synthesized by RF induction melting. Ribbons were annealed at 600 °C for 12 hrs and the other at 800 °C for 2 hrs in vacuum. All ribbons crystallize into a cubic symmetry (space group F4¯3m). The orthorhombic phase appears as the secondary one. In ribbons annealed at 600 °C, the crystallite size decreases from ∼ 47 nm to ∼ 29 nm, and annealing at 800 °C doubles the crystallite size to ∼ 96 nm. The least amount of microstrains (17x10−4) in the as-synthesized ribbon is consistent with the occurrence of the superlattice peaks (111) and (200). Lower intensities of the superlattice peaks in 600 °C and 800 °C ribbons are consistent with higher microstrains (25x10−4 and 24x10−4). Hence, a higher degree of Ni-Mn ordering improves magnetic properties in annealed samples. The as-synthesized ribbon has an austenite-phase Curie temperature of TCA∼303K, which enhances to TCA∼320K and TCA∼310K after annealing at 600 °C and 800 °C. The ribbon annealed at 600 °C has the highest saturation moment. All the ribbons show the exchange-bias (EB) effect, and the most pronounced effect is seen in the 800 °C annealed ribbon. Microstructure uniformization and relative strengths of the Ni-Mn and Mn-Mn interactions improve the magnetics. Near TC, MT=M0TC-Tβ is satisfied (β = 0.31, 0.36, and 0.37, for as-synthesized, 600 °C and 800 °C annealed ribbons). The low-temperature spontaneous magnetization obeys the Bloch law MT=M01-AT32, with a deviation in 600 °C annealed ribbon due to spin freezing. The ρ-T has been fitted in the different temperature ranges using the various combinations in ρT=ρ0+AphT+BsdTα. At T < 85 K, the resistivity of the as-synthesized ribbon follows ρT=ρ0+BsdTα, with α≈1.47, while for the 600 °C annealed ribbon, α≈1.49. The exponent α∼1.5 supports the existence of the spin glass state in the lower temperature region of as-synthesized and 600 °C annealed ribbons. In the high-temperature range, the linear ρ-T behavior is described by ρT=ρ0+AphT and consistent with the electron–phonon scattering.