Abstract
The purpose of this review was to investigate the correlation between magnetism and crystallographic structures as it relates to the martensite transformation of Ni2MnGa type alloys, which undergo martensite transformation below the Curie temperature. In particular, this paper focused on the physical properties in magnetic fields. Recent researches show that the martensite starting temperature (martensite transformation temperature) TM and the martensite to austenite transformation temperature (reverse martensite temperature) TR of Fe, Cu, or Co-doped Ni–Mn–Ga ferromagnetic shape memory alloys increase when compared to Ni2MnGa. These alloys show large field dependence of the martensite transformation temperature. The field dependence of the martensite transformation temperature, dTM/dB, is −4.2 K/T in Ni41Co9Mn32Ga18. The results of linear thermal strain and magnetization indicate that a magneto-structural transition occurred at TM and magnetic field influences the magnetism and also the crystal structures. Magnetocrystalline anisotropy was also determined and compared with other components of Ni2MnGa type shape memory alloys. In the last section, magnetic field-induced strain and magnetostriction was determined with some novel alloys.
Highlights
The purpose of this review was to investigate the correlation between magnetism and crystallographic structures as it relates to the martensite transformation of Ni2MnGa type alloys, which undergo martensite transformation below the Curie temperature
Indicated that the martensite transformation takes place in the ferromagnetic phase, and the decrease in magnetization observed at fields between 0 and 1 T is due to the strong magnetocrystalline anisotropy of the martensite phase in association with the multi-domain structure of the martensite state
Ni2Mn1−xCuxGa, it is considered that the thermal hysteresis of the thermal strain that decreases with increasing magnetic field is an indication of strong magneto-structural coupling in Ni41Co9Mn31.5Ga18.5
Summary
Ferromagnetic shape memory alloys (FSMAs) have been extensively studied as potential candidates for smart materials. Among FSMAs, Ni2MnGa is the most familiar alloy [1] It has a cubic L21 Heusler structure (space group Fm 3 m) with lattice parameter a = 5.825 Å at room temperature, and it orders ferromagnetically at the Curie temperature TC ≈ 365 K [2,3]. Several studies on Ni–Mn–Ga alloys have addressed martensite transformation and correlation between magnetism and crystallographic structures [6,7,8,9,10,11,12,13,14,15,16,17,18]. Indicated that the martensite transformation takes place in the ferromagnetic phase, and the decrease in magnetization observed at fields between 0 and 1 T is due to the strong magnetocrystalline anisotropy of the martensite phase in association with the multi-domain structure of the martensite state. The Curie temperature and magnetization saturation reflect the degree of the atomic order of the alloy, but seem to be linked to the particular order of the Mn sub-lattice
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