Electronic Structure And Magnetic Properties Of Heusler Alloys Co₂FeSi_1-xGa_x

The Half-metallic ferromagnets (HFMs) have attracted a mount of attention in recent years. For the HMF, its spin-up (or the majority-spin) band strides over the Fermi level (εF) and shows a typical metallic behavior, while the spin-down (or the minority spin) band falls into the gap between the valence band and conduction band to present semiconducting properties, resulting in the spin polarization of100%at εF.Therefore, the HMFs could efficiently inject the electrons with different spin characteristics into the semiconductors to achieve the spin transportation, and promise potential application in the spintronic devices, such as the magnetic sensor, the tunnel junction, the spin valve, as well as the primary materials in the electrode.Since the half-metallicity of the half-Heusler alloy NiMnSb was first predicted theoretically by de Groot et al using the first-principles calculations in1983, more and more theoretical investigations revealed that half-Heusler and Heusler alloys possess such half-metallicity and be classified into half-metallic ferromagnets (HMFs). Among those Heusler alloys, much attention has been paid to Co2FeSi owing to its Curie temperature and magnetic moment even reached up to1100K and6μB in per unit. Studies on electronic structure and magnetism of Co2FeSi revealed that such Heusler compound has half-metallicity, however, the Fermi level εF of Co2FeSi is located at the edge of the spin-down gap, therefore the atomic antisite, atomic disorders, crystal defects etc. related to the temperature would cause a slight shift of εF, leading to a remarkable change of electronic structure and magnetism, which is response for the diminishing of the spin polarization significantly even the loss of half-metallicity. Moreover, recent band structure calculations have been revealed that the classical exchange correlation potential could not accurately describe the electronic structure and magnetism due to the on-site correlation of transition metals d-electrons. Using GGA+U (UCo=2.1eV, UFe=2.5eV) scheme based on the density functional theory (DFT), we investigate the electronic structure, magnetism and half-metallic stability of Ga-doped Heusler compound Co2FeSi.In details, the main results of our studies are summarized as follows: 1. We optimized the Co2FeSi cell using GGA+U scheme, and through our tests, we found that when UFe=2.5eV, Uco=2.1eV, the calculated lattice constant (5.6777A) and magnetic moment (5.98μB) are very close to measured values of5.64A and6μB respectively.2. Our studies on electronic structure of Co2FeSi1-xGax indicate that when the doping concentration rises up to x=0.5, the εF of the corresponding alloy Co2FeSio.5Gao.5is located at slightly above the middle of the gap and hence is predicted to have stable half-metallicity.3. Our studies on magnetism of Co2FeSi1-xGax indicate that the RKKY-type exchange mechanism plays a dominating role in determining the magnetism of HMFs.