Effect of nonstoichiometry on the half-metallic character of Co2MnSi and its application to the spin sources of spintronic devices

Abstract

To take full advantage of the half-metallic character of Co-based Heusler alloys, the effect of defects associated with nonstoichiometry has to be understood. In this chapter, recent progress in understanding the effect of nonstoichiometry on the half-metallicy of Heusler alloys, in particular of Co2MnSi (CMS), and its application to highly efficient spin sources for magnetic tunnel junctions (MTJs) and for spin injection into semiconductors is described. The effect of nonstoichiometry on the half-metallic character is experimentally investigated through the saturation magnetization per formula unit (μs) of Co2MnαSiβ thin films and the tunnelingmagnetoresistance (TMR) ratio of fully epitaxial CMS/MgO/CMS MTJs (CMS MTJs) having Co2MnαSiβ electrodes with various values of α. It was found that the μs value was in good agreement with the half-metallic Slater-Pauling value when α was increased to a Mn-rich composition. It was also shown that the TMR ratios at 4.2 and 290K systematically increased with increasing α for the range from a Mn-deficient composition to a certain Mn-rich composition. A site-specific formula unit (SSFU) composition model, which assumes the formation of antisite defects, not vacancies, to accommodate nonstoichiometry, is described. The experimental α dependencies of μs and the TMR ratio have been consistently explained by firstprinciples calculations based on the SSFU composition model. These findings show that harmful defects in Co2MnSi can be suppressed by appropriately controlling the film composition; i.e., CoMn antisites detrimental to the half-metallicity can be suppressed with a Mn-rich composition. By applying Mn-rich CMS electrodes for fully epitaxial MgO-based MTJs, giant TMR ratios of up to 1995% at 4.2 K and up to 354% at 290K were demonstrated for CMS MTJs. Furthermore, an efficient spin injection into GaAs was demonstrated for lateral spin-transport devices by applying a half-metallic Mn-rich CMS electrode as a highly effective spin source. In addition, a nuclear field acting on the electron spins being produced by the dynamic nuclear polarization was electrically detected through the observation of transient oblique Hanle signals. Samples with a CMS spin source exhibited higher spin injection efficiency and a larger nuclear field compared to samples with a Co50Fe50 (CoFe) spin source, suggesting that the spin polarization of CMS is higher. These findings demonstrate that controlling defects through the film composition is critical to retain the half-metallicity of CMS. It is also clear that the half-metallic Co2MnSi electrodes are promising as a highly efficient spin source for future spintronic devices.