Magnetic structure analysis of the L21-type austenite in Ni–Mn–In alloys

Abstract

In general, the magnetocaloric (MC) materials undergoing a magneto-structural transition (first order) have a higher MC effect than purely magnetic transition (second-order). However, the first order transformation displays thermal and magnetic hysteresis and reversibility problems. In this work we present an alternative way to improve the MC effect in Ni–Mn–In alloys, controlling magnetism in second-order transformation. The effect of Mn magnetism on different L21 austenite crystallographic positions in Ni45Mn37In18 alloy has been analyzed and compared with the stoichiometric Ni50Mn25In25 and the previously reported Ni50Mn34In16 alloys using macroscopicmagnetic measurements. Neutron scattering was used to study the atomic occupancies and the magnetic coupling in the austenitic phase. The stoichiometric alloy presents maximum order, with the Mn atoms located in 4a sites, making the coupling ferromagnetic and resulting in a saturation magnetization of 3.9 μB. The saturation magnetization increases with the increment of Mn atoms in the alloy. In this way, the saturation magnetization of Ni45Mn37In18 alloy is enhanced by ~25% with respect to the stoichiometric alloy due to the presence of Mn atoms in all crystallographic positions and being coupled ferromagnetically, as revealed by neutron powder diffraction. This magnifies the overall ordered magnetic moment, especially for the atoms located in the 4a position, in agreement with the Bethe-Slater curve. The magnetocaloric effect was analyzed by isothermalmagnetic measurements in the ferromagnetic-to-paramagnetic phase transition temperature region, revealing the enhancement of the maximum magnetic entropy change |ΔSM|max according to the increase of the Mn concentration in the composition. Also, the effective refrigerant capacity (RCeff) at μ0ΔH = 5 T was calculated obtaining 322.9 J/kg for Ni45Mn37In18, being one of the biggest values ever reported in rare-earth-free materials.