Magnetostructural Transition And Magnetocaloric Effect In Co-and Ga-doped MnNiSi Alloys

The MnM'X-based(M' = Ni or Co,X = Ge or Si)intermetallic compounds have received considerable attention in recent years thanks to their tunable magnetostructural transitions and the associated large magnetocaloric effect.As a member of MnM'X family,stoichiometric MnNiSi alloy has some interesting structural and magnetic properties,which can result in sufficient flexibility for the design of magnetocaloric materials.However,MnNiSi-based systems cannot achieve stable magnetostructural coupling through the traditional methods,such as single element doping or applying pressure.Using the method of isostructural alloying,magnetostructural coupling has been successfully realized in some MnNiSi-based systems by binary element co-substitution.Researches now are mainly focused on obtaining a wide temperature interval where magnetostructural coupling can take place and achieving a large associated magnetocaloric effect,which both are desired in practical applications.In this work,we decided to introduce Co and Ga atoms into MnNiSi matrix and analyze the structural and magnetic properties of several Mn-Co-Ni-Si-Ga polycrystalline alloys through the use of X-ray diffraction,differential scanning calorimetry and vibrating sample magnetometer.This work includes study of structural properties,structural phase transition temperatures,annealing treatment,magnetic properties and magnetocaloric effect.The research has shown that the structural transition temperature of Mn1-yCoyNiSi1-xGax(y1 = 0.4,y2 = 0.5),Mno.7Coo.6Nio.7Si1-xGax,Mno.7Coo.7Nio.6Si1-xGax and Mn1-yCo2yNi1-ySi1-xGax(y1 = 0.25,y1 = 0.20,y3 = 0.15)alloys can be efficiently reduced from 1200 K to below room temperature with increasing the Ga content(x).In all studied systems increased substitution level of Co resulted in a stronger stabilization of high-temperature hexagonal phase.By studying Mn1-yCoyNiSi1-xGax(y1 = 0.4,0.030 ?x1? 0.100;y2 = 0.5,0?x2?0.11)alloys,we found that introducing Co only at Mn sites is insufficient to realize the coupling between the structural and magnetic transformations.Analysis of structural and magnetic properties of Mn0.7Co0.6Ni0.7Si1-xGax and Mn0.7Co0.7Ni0.6S1-xGax(0.01?x?0.11)alloys shown that substitution of Co for both Mn and Ni atoms not only can reduce the structural phase transition temperature of stoichiometric MnNiSi alloy,but also results in a coupling between the structural and magnetic transformations.By studying Mn0.85Co0.3Ni0.85Si1-xGa,(0.10 ?x? 0.20)alloys,we found that the structural transition temperature can be efficiently reduced with a low level of Co substitution.Based on this improvement,the Curie temperature and the magnetization of the orthorhombic phase both maintain at a high values.The tunable magnetostructural coupling can be established in wide temperature interval of 303 K.The upper limit of this temperature interval is as high as 542 K,which is the highest among the reported MnM,X-based compounds.The magnetostructural transformation is accompanied with a relatively large magnetocaloric effect and broad working temperature ranges.