Magnetic entropy change involving martensitic transition in NiMn-based Heusler alloys

Our recent progress on magnetic entropy change (Δ S ) involving martensitic transition in both conventional and metamagnetic NiMn-based Heusler alloys is reviewed. For the conventional alloys, where both martensite and austenite exhibit ferromagnetic (FM) behavior but show different magnetic anisotropies, a positive Δ S as large as 4.1 J·kg −1 · K −1 under a field change of 0–0.9 T was first observed at martensitic transition temperature T M ∼ 197 K. Through adjusting the Ni:Mn:Ga ratio to affect valence electron concentration e/a, T M was successfully tuned to room temperature, and a large negative Δ S was observed in a single crystal. The −Δ S attained 18.0 J·kg −1 ·K −1 under a field change of 0–5 T. We also focused on the metamagnetic alloys that show mechanisms different from the conventional ones. It was found that post-annealing in suitable conditions or introducing interstitial H atoms can shift the T M across a wide temperature range while retaining the strong metamagnetic behavior, and hence, retaining large magnetocaloric effect (MCE) and magnetoresistance (MR). The melt-spun technique can disorder atoms and make the ribbons display a B 2 structure, but the metamagnetic behavior, as well as the MCE, becomes weak due to the enhanced saturated magnetization of martensites. We also studied the effect of Fe/Co co-doping in Ni 45 (Co 1− x Fe x ) 5 Mn 36.6 In 13.4 metamagnetic alloys. Introduction of Fe atoms can assist the conversion of the Mn—Mn coupling from antiferromagnetic to ferromagnetic, thus maintaining the strong metamagnetic behavior and large MCE and MR. Furthermore, a small thermal hysteresis but significant magnetic hysteresis was observed around T M in Ni 51 Mn 49− x In x metamagnetic systems, which must be related to different nucleation mechanisms of structural transition under different external perturbations.