Theoretical Study Of Magnetic Interactions And Magnetocaloric Effects For Heusler Alloys

As the energy and environment problem become increasingly prominent in the life,the room temperature magnetic refrigeration as a green refrigeration technology has been highly emphasized by various countries.The key to the development of room temperature magnetic refrigeration technology is to search for magnetic refrigeration materials that have large magnetocaloric effects at room temperature and are environmental friendly and inexpensive.In recent years,Heusler alloy Ni-Mn-X?X=Ga,In,Sn?have been found to have giant magnetocaloric effects near room temperature.Compared to other magnetocaloric materials,such as rare earth manganese oxides or rare earth alloy compounds,Heusler alloys are inexpensive and environmental friendly.So it is considered as potential magnetic refrigeration materials at room temperature.In this thesis,the magnetic exchange interactions and magnetocaloric effects of NiMn-based Heusler alloys are studied.Firstly,the optimized lattice parameters of the alloys are calculated by first-principles method.Then the magnetic moments and exchange coupling parameters are calculated by the KKR-Green?s function mode.Finally,The thermodynamic properties of alloys such as specific heat capacity,phase transition temperature,and magnetic entropy change are simulated using the Monte Carlo method.Main contents are as follows:?1?The magnetocaloric effect of Ni_2-xMn_1+xGa?0.2?x?0.75?Heusler alloy is studied.The results show that the excess of Mn atoms will increase the magnetocaloric effect of the alloy.At x=0.75,the maximum value of magnetic entropy change at second-order phase transition is?S_mag=-1.81 J/?mol·K?for the field change of?H=5 T.?2?The magnetic properties and entropy change values of Ni-Mn-In Heusler alloys are investigated.We consider Ni₂Mn_1+xIn_1-x?x=0.38,0.40?alloys and Ni₂Mn_1.4In_0.6-xR_x?R=Ag,Al,B,Ge,Si,Fe,Co,Gd?alloys with R substituting for In atoms.The results show that the structural phase transition temperature and the Curie temperature of Ni₂Mn_1.4In_0.6.6 are very close,and the entropy change values around the structure phase transition are larger than those around the magnetic phase transition.In the Ni₂Mn_1.38In_1.62.62 alloy,the magnetic entropy changes at the magnetic phase transition are larger than those at the structural phase transition.In the case of Ni₂Mn_1.4In_0.6-xR_x?x=0.04-0.2?,the magnetic entropy changes decreases with increasing x for R=B and Si,while the magnetic entropy changes will increase with the increase of x for R=Al,Ag,Ge,Fe,Co.?3?The magnetic properties and magnetocaloric effect of Ni-Mn-In alloys are studied.The magnetic moments and the magnetic exchange interactions in Ni₂Mn_1+xSn_1-x-x alloys are first calculated.Then,we discuss the effect of a partial substitution of In atoms by Si or Fe atoms on the magnetic moments and thermodynamic properties of the systems.It is found that the addition of Fe can lead to the increase of the total magnetic moments,and also give rise to the increase in the magnetic entropy change values of the alloys.