Electronic Structure,Magnetic And Half-Metallic Properties Of Mn₂CoAl?001?Surface:A First-principles Study

With the miniaturization of electronic devices,the developments of spintronic devices have attracted great attentions.Spintronic devices require efficient injections of spin-polarized electrons from magnetic electrodes to semiconductors;therefore,half-metallic materials and spin gapless semiconductors,which have 100%spin po-larization at Fermi levels,are considered to be ideal sources for the injection of spin-polarized electrons.Half-metallic materials exhibit semiconducting property in one of spin directions and show metallic feature in the other spin direction,resulting a 100%spin polarization.In the spin gapless semiconductor,the Fermi level sits at a band gap for one of spin directions,while there is a zero energy gap at the Fermi level for the other spin direction,where the conduction band minimum and the valence band maximum touch at the Fermi level.In the experimental studies of half-metallic materials,the measured spin polarizations of the materials did not reach the value of the theoretical prediction.It can be ascribed to the surface effect,the electronic structure,magnetic and half-metallic properties at surfaces are quite different with the bulk.Therefore,investigations of the electronic structure,magnet-ic and half-metallic properties at surfaces are of great significant for developments of spintronic devices.In the thesis,the electronic structure,magnetic and half-metallic properties of the half-metallic Heusler Mn2CoAl(001)surfaces are studied by using the full potential linearized plane wave method(FLAPW)based on the density functional theory within the generalized gradient approximation(GCA).The optimized lattice constant of Mn2CoAl is 5.717 A.For Mn2CoAl,the valence band maximum(at the point)and the conduction band minimum(at the X point)touch at the Fermi level in the spin-up band,while the Fermi level located inside a band gap in the spin-down band,exhibiting the half-metallic and spin gapless semiconducting characters.For Mn2CoAl(001)surfaces,we considered eight types of possible surfaces,namely,AlAl,AlMn,MnA-void,Mn(A),Mn(B),CoMn,CoCo,and MnB-void surfaces.The calculated surface phase diagram of Mn2CoAl(001)shows that stable AIMn,AlAl,Mn(A),Mn(B)and CoCo surfaces can be obtained at appropriate thermodynamic conditions.By analyzing the density of states of the surfaces,it is found that the half-metallic character is destroyed at the CoCo surface,the CoMn surface,the AlAl surface,the Mn(A)surface,the MnA-void surface,and the MnB-void surface,where-as the AlMn surface and the Mn(B)surface retain the half-metallic character.The spin polarizations of AlAl surface,Mn(A)surface,MnA-void surface,and MnB-void surface are about 70%,and the spin polarizations of the CoCo surface and CoMn surface are smaller than 40%,whereas the AlMn surface and the Mn(B)surface have 100%spin polarization.Due to the formation of surfaces,the magnetic moments of atoms at the surface are changed significantly with respect to that of the bulk.All the eight types of Mn2CoAl(001)surfaces lose the spin gapless semiconducting character.