Heusler alloy films on Semiconductor Templates

Integration of ferromagnetic properties into semiconductor devices has become a focus of worldwide research. It promises access and control of the information provided by the electron spin, additionally to its charge currently used in the transistor, and accordingly advanced applications in the fields of a future spin- and magnetoelectronics. For device applications ferromagnetic electrodes are required that are efficient spin-injectors into semiconductor substrates (e. g., GaAs and Si) and/or exhibit a high tunneling magnetoresistance (TMR) effect when applied in magnetic tunneling junctions (MTJs). Promising electrode materials are therefore the so-called half-metallic ferromagnets. Ideally, they exhibit up to 100% spin-polarization at the Fermi level, thus enabling injection of a fully spin-polarized current and also a high TMR effect. A prominent materials class containing half-metallic ferromagnets are the Heusler alloys, which are attracting intense research interest particularly since the last decade. Heusler alloys are predominantly ternary compounds crystallizing in a lattice that consists of four interpenetrating fcc sublattices. In the case of the full Heusler alloys with formula X2 YZ two of them are occupied by X-atoms (e. g., Co2 MnSi), for the semi-Heusler alloys XYZ (e. g., NiMnSb) one fcc sublattice remains unoccupied. Due to the complex electronic structure of Heusler alloys various magnetic configurations are observed, like intinerant and localized magnetism, ferro-, antiferro- and helimagnetism, Pauli paramagnetism and even heavy fermionic behavior. Therefore, in addition to the technological relevance of Heusler alloys, they are also very interesting from a fundamental point of view. The proposed work focuses on the development of half-metallic Heusler alloy films on the two technologically leading semiconductor substrates, GaAs(001) and Si(001), with the ultimate goal of realizing (i) efficient spin- injectors and (ii) MTJs with a high TMR ratio for application in spin- and magnetoelectronics. The criteria for our materials` selection therefore are a good lattice-matching with the semiconductor substrates and a Curie temperature well above room temperature. In particular, it is aimed at an understanding of the mechanisms involved in the molecular beam epitaxy (MBE) of ternary compounds in order to succeed in achieving high-quality epitaxial growth with optimum crystallographic order and abrupt interfaces. In that respect the evolution of the mechanical film stress during growth and post-growth annealing will serve as sensitive in-situ tool for exploring the evolution of film structure and morphology. Furthermore, the magnetic properties of the Heusler films will be investigated in-situ as a function of stoichiometry, crystallographic order, and interface composition. Of particular interest are magnetic anisotropies and the magnetoelastic coupling. Since the latter is intimately related to the spin- orbit interaction, valuable insight on the role of structural disorder is expected. For evaluation of progress and success of our endeavors, a direct method for room-temperature measurements of spin efficiency and TMR effect of the spin-polarized current will be developed.

Integration of ferromagnetic properties into semiconductor devices has become a focus of worldwide research. It promises access and control of the information provided by the electron spin, additionally to its charge currently used in the transistor, and accordingly advanced applications in the fields of a future spin- and magnetoelectronics. For device applications ferromagnetic electrodes are required that are efficient spin-injectors into semiconductor substrates (e. g., GaAs and Si) and/or exhibit a high tunneling magnetoresistance (TMR) effect when applied in magnetic tunneling junctions (MTJs). Promising electrode materials are therefore the so-called half-metallic ferromagnets. Ideally, they exhibit up to 100% spin-polarization at the Fermi level, thus enabling injection of a fully spin-polarized current and also a high TMR effect. A prominent materials class containing half-metallic ferromagnets are the Heusler alloys, which are attracting intense research interest particularly since the last decade. Heusler alloys are predominantly ternary compounds crystallizing in a lattice that consists of four interpenetrating fcc sublattices. In the case of the full Heusler alloys with formula X2 YZ two of them are occupied by X-atoms (e. g., Co2 MnSi), for the semi-Heusler alloys XYZ (e. g., NiMnSb) one fcc sublattice remains unoccupied. Due to the complex electronic structure of Heusler alloys various magnetic configurations are observed, like intinerant and localized magnetism, ferro-, antiferro- and helimagnetism, Pauli paramagnetism and even heavy fermionic behavior. Therefore, in addition to the technological relevance of Heusler alloys, they are also very interesting from a fundamental point of view. The proposed work focuses on the development of half-metallic Heusler alloy films on the two technologically leading semiconductor substrates, GaAs(001) and Si(001), with the ultimate goal of realizing (i) efficient spin-injectors and (ii) MTJs with a high TMR ratio for application in spin- and magnetoelectronics. The criteria for our materials` selection therefore are a good lattice-matching with the semiconductor substrates and a Curie temperature well above room temperature. In particular, it is aimed at an understanding of the mechanisms involved in the molecular beam epitaxy (MBE) of ternary compounds in order to succeed in achieving high-quality epitaxial growth with optimum crystallographic order and abrupt interfaces. In that respect the evolution of the mechanical film stress during growth and post-growth annealing will serve as sensitive in-situ tool for exploring the evolution of film structure and morphology. Furthermore, the magnetic properties of the Heusler films will be investigated in-situ as a function of stoichiometry, crystallographic order, and interface composition. Of particular interest are magnetic anisotropies and the magnetoelastic coupling. Since the latter is intimately related to the spin-orbit interaction, valuable insight on the role of structural disorder is expected. For evaluation of progress and success of our endeavors, a direct method for room-temperature measurements of spin efficiency and TMR effect of the spin- polarized current will be developed.