Our recent progress on magnetic entropy change（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 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 S was observed in a single crystal.The △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 B2 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（Co1-xFex）5 Mn36.6In13.4 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 TM in Ni51Mn49-xInx metamagnetic systems,which must be related to different nucleation mechanisms of structural transition under different external perturbations.
A series of CeMn2（Si1-xGex）2（x = 0.2, 0.4, 0.6, 0.8） compounds are prepared by the arc-melting method. All the samples primarily crystallize in the Th Cr2Si2-type structure. The temperature dependences of zero-field-cooled（ZFC） and FC magnetization measurements show a transition from antiferromagnetic（AFM） state to ferromagnetic（FM） state at room temperature with the increase of the Ge concentration. For x = 0.4, the sample exhibits two kinds of phase transitions with increasing temperature： from AFM to FM and from FM to paramagnetic（PM） at around TN-197 K and T C-300 K,respectively. The corresponding Arrott curves indicate that the AFM–FM transition is of first-order character and the FM–PM transition is of second-order character. Meanwhile, the coexistence of positive and negative magnetic entropy changes can be observed, which are corresponding to the AFM–FM and FM–PM transitions, respectively.
