Study On The Magnetic Phase Transition Of Hexagonal Mn-based Ternary Alloys

Magnetic refrigeration,which is based on the magnetocaloric effect?MCE?,is regarded as a promising alternative to gas-compression cooling due to its high efficiency and environmental friendliness.Recently,intermetallic compounds based on MM'X?M,M'=Mn,Co,Ni and X=Si,Ge?have draw increasing attention for their tunable magnetostructural transitions?MSTs?and the associated large MCEs.Stoichiometric MM'X experiences a structural transition between a low-temperature orthorhombic phase and a high-temperature hexagonal phase in a paramagnetic region.By tuning stoichiometry,substituting of foreign-atoms with different radius and valence electron number,or doping with interstitial atoms,the magnetic and structural transitions can be modified to coincide near RT,resulting in the MSTs and considerable MCEs.As a member of MM'X family,MnNi Si has a high Curie temperature and low material cost.However,it is quite difficult to realize the coupling of magnetic and structural transitions near room temperature?RT?by single-element substitution for MnNi Si.In this work,we investigated the effect of co-substitution of Co/Fe and Ge/Ga at three different sites of MnNi Si.Co-substitution of Co/Fe for Mn,Ni and Ge/Ga for Si drastically reduces the structural transition temperature from 1200 K to below RT in the Mn_1-x-x Ni_1-x-x Co_2xx Si_1-x-x Ge_x and Mn_1-x-x Ni_1-x-x Fe_2xx Si_1-x-x Ga_x alloys.The magnetostructural transitions occurring between ferromagnetic and paramagnetic states with sharp jumps in magnetization were realized,which are tunable in broad Curie-temperature windows spanning RT by altering the content of Co/Fe and Ge/Ga.The observed maximum entropy changes at an external field change of 5T are-20.7 J/kg K and-38.1 J/kg K for Mn1-x Ni1-x Co2x Si1-x Gex and Mn1-x Ni1-x Fe2x Si1-x Gax alloys,respectively.The refrigerant capacities reach up to 209.5 J/kg and 205 J/kg for Mn_1-xNi_1-x-x Co_2xx Si_1-xGe_x and Mn_1-x-x Ni_1-x-x Fe_2xSi_1-xGa_x alloys,respectively.The magnetic and structural transitions were investigated for a?Co MnSi?_1-x?Ni₅Ge₃?_x system isostructural alloying system.Co MnSi has a double helical antiferromagnetic configuration at low temperature and undergoes a structural transition between orthorhombic and hexagonal phases around 1165 K,while paramagnet Ni5Ge3 has a stable hexagonal structure at RT.By chemically alloying Co MnSi with Ni5Ge3,magnetostructural transitions occurring between ferromagnetic and paramagnetic states,which can be induced by magnetic field,were obtained near RT.A relatively large magnetic entropy change?21 J/kg K?was observed under a field of 5 T in?Co MnSi?_0.86?Ni₅Ge₃?_0.14.The excellent characters of?Co MnSi?1-x?Ni5Ge3?x alloys make them promising multi-functional materials for application.