First-principles Studies On Surface/Interface Properties And Doping Effects Of Co-based Heusler Alloys

As a carrier of the charge, the electron also bears intrinsical spin properties. For the traditional electrocircuit, due to the degeneration of spin, the electrons with spin-up or spin-down can not be discriminated, while it can be controlled in half-metallic ferromagnets (HMFs). Because of the appearance of exchange splitting, two of energy bands of the HMF respectively present metallic and semiconducting character, resulting in polarization of100%at Fermi level, which opens the door for manipulating spin freedom of the electron. The HMFs are therefore regard as promising candidates for application in magnetic tunnel junctions (MTJs), spin valve, and magnetic random access memory (MRAM) etc.. As one of HMFs, Co-based Heusler alloys, due to relatively higher spin polarization and magnetism, Curie temperature over than room temperature, as well as extremely excellent matching to some of nonmagnetic insulators like MgO, GaAs and Al-O etc., are focused intensively. However, investigations from experiments of MTJs revealed a significant reduction of tunnel magnetoresistance ratio (TMR) against temperature in Heusler electrodes, which weakens the performances of application in MTJs severely. Hence, understanding fully all kinds of depolarization origins and putting forward a reasonable scheme on overcoming the limitation that is key point for the Co-based Heusler film studies. To start with the bulk properties of Co-based Heusler alloys, we comprehensively investigated their structures, origins of magnetisms and half-metallicities, and the half-metallic stability depending on doping of sp-elements, which is also a feasible method for developing MTJs. Moreover, for Co-based Heusler surface, the thermodynamic stability, atomic disorder effect, electronic structure and spin polarized behavior are systemically investigated, and the possible preserved surface half-metallicity is proved firstly. Finally, the Co-based Heusler heterojunction jointed nonmagnetic semiconductor is emphasized. We investigated those properties which are closely related to MTJs, including the atomic interaction, electronic structure, magnetism and spin polarization of the interface. It may orientate the development of Co-based Heusler MTJs in experiments and devices.In details, the main results of our studies are summarized as follows:1. The structural, half-metallic and magnetic properties as well as their physical mechanism of the Co-based Heusler bulk phase are studied, the results revealed that owing to special geometric symmetry of Co-based Heusler Co2YZ alloy with L2I high-order structure, the hybridization between Co and Y-site transition metal atom mediated by Z-site sp atom leads to novel half-metallic energy bands and total magnetic moment per cell following Slater-Pauling rule. Moreover, calculated electronic structure also presented that the half-metallic gap is derived from d-band gap from Co atom in Co-based Heusler alloys. Due to the fact that the Fermi level locates at the edge of half-metallic gap, it implies instability of the half-metallic characters.Considering the importance of the half-metallicity of Co-based Heusler alloy for application in MTJs, we further studied the doping effects in quaternary Heusler alloys Co2MnGe1-xGax and Co2MnAlI-xGex, including into their crystal structure, half-metallic stability and magnetism. Within the computed accuracy range, the lattice constant and total magnetic moment follows the Vegard law and Slater-Pauling rule well, respectively. With the change of the doping concentration, the Fermi level can be adjusted. It is an effective way to find out the alloy with excellent the half-metallic stability as the Fermi level just locates in the middle of spin-minority gap. The physical mechanism behind such sp-element doping can be successfully explained by the d-electronic hybridization and RKKY-type indirect magnetic exchange.2. For Heusler surfaces, on the one hand, the structure, thermodynamic stability, atomic relaxation, magnetism, electronic structure and surface atomic spin polarization of Heusler alloy Co2MnZ(Z=Si, Ge, Sn)(100) surface are comprehensively investigated, the results revealed Z-site atomic dependence of the electronic and magnetic properties. The calculated phase diagram indicated that with increasing core electrons of Z atoms in Co2MnZ (100) the CoCo termination will be faded out of the thermodynamic equilibrium region gradually. Due to the difference of Co Z bonding the surface Co and Mn atoms prefer to move towards the slab and vacuum, respectively. The half-metallic gap observed in bulk has been destroyed by the surface states in deficient-Mn atomic terminated surface, only the terminations capped pure Mn atoms in Co2MnSi (100) and Co2MnGe(100) surfaces preserve spin-polarization of100%instead of the Co2MnSn(100) surface, which is a possible explanation for low experimental tunnel magnetoresistance (TMR) value in Co2MnSn(100)-based magnetic tunnel junctions (TMJs).On the other hand, the Co2MnGe1-xGax(100) surface are investigated. The results revealed that Co-Ge and Co-Ga bonding are more favourable than Co-Mn bonding and the terminations involving surface Mn atoms are more stable than CoCo terminations. By comparing with the bulk values, the surface Co and Mn magnetic moments are enhanced obviously. Moreover, due to the appearance of surface states at "ideal" surfaces, the half-metallicy has not been detected, which is a possible reason why the tunnel magnetoresistence steeply drops as temperature increases. However, in the pure atomic terminations, including MnMn, GeGe and GaGa, the surface properties can be slightly adjusted by the Ga-doped concentrations in bulk. As a result, in the MnMn termination of Co2MnGe0.5Ga0.5(100) the spin polarization of100%is detected, indicating that the spin polarization of100%at the (100) surface can remain if the corresponding bulk presents excellent half-metallic stability. Thus we predict that this thin film will present a higher potential for applications in ferromagnetic electrodes.3. Based on the studies above, we finally implemented the investigations on the Co-based Heusler Co2CrZ (Z=Ge, Ga) heterojunctions adjoined GaAs facets. The properties that atomic interaction, crystal distortion, interface state, magnetism and spin polarization etc. are emphasized, and the atomic disorder effect is also discussed in our work. By comparison between Co2CrGa(100)/Ga and Co2CrGe(100)/Ga, we found that in spite of their extremely similar crystal matching to GaAs (the ratio of mismatch is less than1%), the different spin polarized behavior of interface atoms are observed because of discrepancies of interface atomic interaction and electronic structure. For CoCo/Ga heterojunction, the Co atom shift towards interface orientation because of the bonding of Co-Ga, while the other Co atom without bonding is shrank into slab, which is similar to the behavior of surface Co atom. Due to the increase of interlayer space near by the interface atomic playe, Co atoms tend to be clustered. It not only enhances RKKY-type magnetic coupling of Co atoms, but also increases the direct exchange interaction of Co-Cr sharply, resulting in spin inversion of Co without bonding. Moreover, the calculated interface atomic density of stats (DOS) indicated that the interface states from spin-minority band of Co atoms appear around the Fermi level, which destroy the half-metallicity observed in bulk and exhibit a lower spin polarization for the film. However, for CrZ/Ga and CrCr/Ga heterojunction, the higher spin polarizations of interface Cr and Co atoms are preserved and closed to95%. Hence, the higher TMR value of Co2CrGa-CrGa/Ga/Co2CrGa-CrGa MTJ is predicted in our work and focused expectantly in the later experimental researches.