First Principle Study of Co2FeSi (001) Surface and Co2FeSi/GaAs (001) Interface

An important issue in Spintronics is effective spin injection from the spin current source that is commonly a Ferromagnet, into the electronic devices almost are semiconductors. Most important advantages of half-metal Ferro magnets respect to metallic Ferromagnets, beside their higher spin polarization at Fermi level, is lower impedance mismatch between this kind of materials and semiconductors. Half-metallic Ferro magnets show metal like behavior in one spin channel while in the other spin channel have energy gap at Fermi level so as semiconductors. Hence, in principle one expects these materials have 100% spin polarization at Fermi level and so 100% spin-polarized electric current. But even if half-metallic character there exists really in bulk case, any change in electronic structure due to existence of defects, surface states and interface effects can cause this character is destroyed in application1-3. Full Heusler alloys are of materials promised to be half metal, and in this family those based on Co (Co2MnSi, Co2CrAl...) are of specific importance because of their higher magnetic moment and Curie temperature3,4. Recently experiments have proved highest value of these two quantities for Co2FeSi among Heusler alloys and also half-metals5,6. Obtaining integer magnetic moment per formula unit of this alloy in experiment, straightens probability of its half-metalicity. But low measured spin polarization both for bulk alloy and its interface with Al2O3 shows unstability of half-metallic character of this alloy7. Computational work done by Kandpal et.al also shows not only GGA can't obtain the correct magnetic and electronic properties of this alloy in theoretical lattice parameter (in good agreement with experimental lattice constant), but also using LDA+U, with U parameters that lead to magnetic moments just near the exact integer 6μB, don’t show half-metallic character for this alloy. On the other word, half-metalicity is very sensitive in this alloy; so accurate study of its surface and interface effects in the electronic and magnetic properties seems necessary. On the other hand in experiment, supplying high crystal order in this alloy is rather difficult, so it is not easy to distinguish that decreasing spin-polarization is due to practical limitations or basic reasons due to bondings etc. In this view computational investigation is considerable; Here we report our computational results for Co2FeSi(001) surface and its interface with GaAs(001) within Density Functional Theory (DFT) frame work. In this study, at first we calculated optimized structure using GGA, then LDA+U correction was used to calculate the more accurate lectronic structure and magnetic properties