Significantly enhanced reversibility and mechanical stability in grain-oriented MnNiGe-based smart materials

Abstract

Materials that undergo a magnetostructural transition (MST) usually exhibit fascinating magnetoresponsive properties, making them an important class of smart materials. However, practical application of this class of smart materials has been hindered by structural degradation as well as large irreversibility of the MST during consecutive thermal and field cycles. Here we report a significant improvement of the reversibility and mechanical stability in grain-oriented MnNiGe-based alloys that were fabricated using a directional solidification method. The preferred grain orientation enables synergistic deformations between neighboring grains during the MST, leading to a substantial reduction in the transition-induced stress concentration. As a result, in situ and ex situ microscopic observations demonstrate a good mechanical stability of the textured alloys across the MST, in strong contrast to conventional MM'X (M, M’ = Mn, Fe, Co, Ni; X = Si, Ge) materials with randomly-oriented grains. The detailed transition stages of the MST have also been observed at the microscopic scale, and are reported for the first time in the MM'X family. Besides, a low thermal hysteresis (ΔThys) of ∼ 4 K was obtained in the textured alloys, which is the lowest ΔThys in the MM'X material family. Textured MnNiGe-based alloys show a large reversible isothermal entropy change (≥ 35.9 Jkg−1K−1) in a 5 T field change, which is the highest among typical magnetocaloric materials. Consequently, this work provides a promising strategy for enhancing the cyclic stability of materials with a MST, which may boost their practical applications in solid-state refrigerators, energy harvesters and high-precision actuators.