Molecular beam epitaxial growth and characterization of the manganese-based Heusler alloy films for application in spintronics.

摘要

This thesis studied the molecular beam epitaxial (MBE) growth of the Manganese-based Heusler alloy films, Ni2MnAl and Co2MnGe, on III-V semiconductor AlxGa1-xAs, characterized the structural, magnetic and electrical properties of these epitaxial films, achieved spin injection from Co2MnGe into a Al0.1Ga 0.9As/GaAs spin-polarized light emitting diode (spin-LED) heterostructure.; It has been a major challenge to obtain the Heusler alloy Ni2MnAl in a single ferromagnetic L21 phase that is not mixed with the antiferromagnetic B2 phase. In Chapter 3 of this thesis, single layer of either ferromagnetic Ni2MnAl in a L21 phase or antiferromagnetic Ni2MnAl in a B2 phase has been successfully grown by changing the substrate temperature during the MBE growth. Higher growth temperature (T ≥ 400°C) gave ferromagnetism and lower temperature (T ≤ 180°C) brought antiferromagnetism to the films.; The Heusler alloy Co2MnGe was predicted to be half-metallic from first-principles calculations. Chapter 4 shows that the epitaxial Co 2MnGe films grown on GaAs were ferromagnets at room temperature with a saturation magnetization of 5.1 muB/formula unit. The resistivity of these films were as low as 11 muO·cm at low temperature, which is an order of magnitude lower than the amount reported previously, indicating higher quality of the films with fewer defects. Co2MnGe/Al 0.1Ga0.9As/GaAs spin-LED heterostructures were grown and processed into devices. Electroluminescence measurements detected that the spin polarization of the electrons at the time of recombination was 13% from the quantum well (QW) detector and 14% from the bulk detector at 2 K. Hanle effect measurements characterized the efficiency of a bulk LED detector, resulting in the injected spin polarization from Co2MnGe being 27% at 2 K.; Chapter 5 studied the composition effect on the structural, magnetic and electrical properties and spin polarization of the Co2MnGe films. Although the Co2.2MnGe and Co1.8MnGe films were still single crystals, magnetic properties dramatically changed compared to the stoichiometric film, including a decrease of the saturation magnetization to 2/3 of the original amount (1000 em/cm3) and doubling the coercivity. Spin injection measurements showed that the injected spin polarization dramatically decreased compared with the stoichiometric Co2MnGe films.