Electronic Structure And Magnetic Properties Of Half-Heusler Alloys CoMn_1-xNb_xSb: First-principles Study

Since de Groot and his team discovered that half-Heusler alloy NiMnSb is half-metallic ferromagnetic material for the first time,in 1983.Heusler alloy has been studied extensively.Recently,a lot of new physical phenomena have also been observed in these materials,such as strong ferromagnetism,the large magnetostriction,magneto-structural phase transition,as well as shape memory controlled by temperature and magnetism.Due to the above novel characters,this kind of compounds have a wide range of applications in magnetic recording,the realization of spin electronic devices(i.e,magnetic tunnel junction or the low-field magnetoresistive sensor),aerospace,medical,engineering and other fields.Also,because the Curie temperature of these alloys is mostly higher than room temperature,they are regarded as an important part of both condensed matter physics and materials science.Among Heusler alloy,Half-Heulser alloy CoMnSb is known as having the Curie temperature(To) of up to 490 K that has good application prospect.Theoretical research related to the lattice structure,electronic structure and magnetic properties of CoMnSb shows that it is semi-metallic since Mn atom has a larger local magnetic moment because of d-d hybrid.Based on Half-Heulser alloy CoMnSb as a basic configuration,we doped the Nb element of different concentrations at the location of Mn,and then calculated and analyzed the lattice structure,electronic structure and magnetic properties in the electron-doped case using density functional theory(DFT).In this thesis,we firstly introduce the research background,and then give a general discussion on the basic principles as well as the latest development in the first-principles methods.Finally,through the use of the generalized gradient approximation under density functional theory,we calculate lattice constants, electronic structure,spin distribution and magnetic moment of the Half-Heusler CoMnl-xNbxSb(x= 0,0.25,0.5,0.75,1)alloy.Through the geometry optimization and energy calculation,the lattice constant, electronic structure,spin distribution,as well as the size of magnetic moment of the material have been obtained.It was found that for doping level x＜0.75,the CoMn_1-xNb_xSb system exhibited a half-metallic ferromagnetism character and the spin moment decreased linearly according to the increase of the Nb doping level,which obeys the Slater-Pauling curve quite well;when the concentration of Nb atoms increases to x=1,the system changed from a ferromagnetic half-metallic to a common Pauling metallic.