The structural transitions, magnetic properties, and electronic structures of Co(Fe)-doped MnNiSi compounds are investigated by x-ray powder diffraction, differential scanning calorimetry (DSC), magnetic measurements, and first-principles calculations. Results indicate that all samples undergo a martensitic transition from the Ni 2 In-type parent phase to TiNiSi-type orthorhombic phase at high temperatures. The substitution of Co(Fe) for Mn in Mn 1Àx Co x NiSi (x ¼ 0.2, 0.3, and 0.4) and Mn 1Ày Fe y NiSi (y ¼ 0.26, 0.30, 0.36, 0.46, and 0.55) samples decreases the structural transition temperature and Curie temperature of martensite. The martensite phases show a typical ferromagnetic behavior with saturation field being basically unchanged with increasing Co(Fe) content, while the saturation magnetization shows a decreasing tendency. The theoretically calculated moments are in good agreement with the experimentally measured results. The orbital hybridizations between different 3d elements are analyzed from the distribution of density of states. V C 2015 AIP Publishing LLC.

In this study, we obtained a ferromagnetic Mn 0.36 Fe 0.6 Ni 0.98 Si compound in which magnetic and structural transitions are coupled together. By combining Fe doping and Mn depletion, the magnetostructural coupling was realized, with a magnetostructural transition happening at 337 K from paramagnetic hexagonal phase to ferromagnetic orthorhombic phase. The magnetocaloric effect was measured across the transition, showing a wide temperature span and a zero magnetic hysteresis loss. The maximum values of magnetic entropy change and refrigeration capacity near reverse martensitic transition (~367 K) are -2.52 J kg -1 K -1 and 108 J kg -1 , respectively, under a field change of ΔH = 0 ~ 50 kOe. Index Terms-magnetostructural transition, magnetocaloric effect, MnNiSi I INTRODUCTIONThe ferromagnetic martensitic transition (FM-MT)[1]-[3] as a kind of magnetostructural transition (MST), is paid continuous attention in ferromagnetic shape memory alloys. For those MSTs with a coupling of magnetic and structural transitions, many physical effects[4]- [7] have been achieved in different studied systems. The excellent magnetoresponsive properties, such as magnetic entropy change (ΔS m ), can also be achieved by tuning the MST. A large magnetization difference (ΔM) in the vicinity of MSTs is relevant to obtain the considerable field-driven effects. Therefore, the MSTs with change between paramagnetic (PM)/antiferromagnetic (AFM) and FM states are desired. However, first order phase transitions have two important drawbacks, namely, the narrowness of the ΔS m curve and the presence of hysteresis. The ΔS m with coupled magnetic and structural transitions is also accompanied by an magnetic hysteresis, leading to the decrease of low operation frequencies and cooling efficiency [8]. A compound, which has room-temperature MCE with small magnetic hysteresis, is desired for magnetic refrigeration.In recent years, MM'X compounds (M, M' = transition metals, X= carbon or boron group) have been studied intensively. The MM'X compounds transform between the high-temperature Ni 2 In-type hexagonal austenite (space group P6 3 /mmc, 194) and the low-temperature TiNiSi-type orthorhombic martensite (space group Pnma, 62) upon heating and cooling. In these MM'X compounds, the Curie temperature of martensite (T C M ) is generally higher than the Curie temperature of austenite (T C A ). In order to couple magnetic with structural transitions together, the temperature of MT (T t ) should be declined below T C M . In order to achieve this magnetostructural coupling, many effective methods have been implied, such as the application of chemical substitution[5], [9], [10], vacancy introduction[11]-[13], applying pressure[14]. Stoichiometric MnNiSi[15], [16] MM'X compound undergoes a hexagonal-to-orthorhombic MT at 1210 K in paramagnetic state. The martensitic phase is a typical ferromagnet with a high T C M at 622 K. Based on substitution of main-group elements[16], alloying MnNiSi with MnNiGe, the structural and magnetic properties were studied. Howeve...