By introducing spin degrees of freedom in materials and devices, we can create new functionalities. This article overviews the studies on the control of spins (creation, injection, accumulation, relaxation, transport, and detection) and relations of spin-related properties with magnetic, electrical, optical, and thermal properties. Developments of semiconductor-based materials with spin-related functions are reviewed. Future issues and prospects for device applications are also described.
Realizing an electronic phase competition in a material is a promising approach to induce a large electronic change by a small external field. One of typical materials showing the phase competition is the half-doped perovskite manganite, in which the ground state drastically changes between the antiferromagnetic-insulating (AFM-I) and ferromagnetic-metallic (FM-M) states depending on the slight difference in the band width. Near the phase boundary, the two phases coexist and compete with each other, giving rise to a gigantic field response like the colossal magnetoresistance. We show in this paper the observation of a self-organized anisotropic phase separation with use of a microwave impedance microscope for a thin film of Nd0.5Sr0.5MnO3, which locates on the verge of the phase boundary. We also show an artificially tailored phase separation and its magnetic control realized in superlattices composed of Pr0.5Ca0.5MnO3 (AFM-I) and La0.5Sr0.5MnO3 (FM-M).
Semiconductor spintronics is an extensively studied field toward next generation devices. For room temperature operation of semiconductor spintronics, high temperature ferromagnetic semiconductor is needed. This paper reviews recent progress of one of the high temperature ferromagnetic semiconductors, ferromagnetic oxide semiconductor, mainly Co-doped TiO2.
We analyzed microstructures of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with Co-based Heusler alloy by a transmission electron microscope. Based on the relationship between the microstructure and transport properties, we conclude that the structural and electrical matching between the spacer material and ferromagnetic electrode are important for the high GMR.
Electrons convey not only an electric charge but also a magnetic charge. Spintronics aims to achieve electronic devices with new capabilities by using both the charge and spin of electrons in solids. This area is now developing with advances in surface science. With the reduction of junction sizes into nanometer-scale, it has become impossible to improve spintronic devices without the investigation of surface/interface structures. In this article, our studies on exploration of correlation between surface/interface structures in magnetic tunnel junctions yielding high tunnel magnetoresistance ratio and their magneto-transport properties by using a scanning tunneling microscopy are presented. Then, recent studies on development of new ordered alloys with perpendicular magnetic anisotropy by using a monoatomic alternative deposition technique are introduced topically.
Considerable attention has recently been paid to the active control of spin degrees of freedom in organic semiconductors (OSCs). There are, however, some discrepancies in experimental results of magnetoresistance in sandwich structures composed of ferromagnetic metals and OSCs, partly due to the difficulties in preparation of well-defined interfaces between electrodes and OSCs. This article introduces the current status of the research filed of organic spintronics.
We investigated the growth of quaterrylene thin films on substrates with different surface energy: silicon dioxide (SiO2) and an octadecyltrichlorosilane self-assembled monolayer (OTS-SAM). We clearly elucidated that a lattice strain induced by the molecular-substrate interaction was essential factor for determining overall growth process and evolving structures. Surface modification by SAMs drastically changed the overall growth process from a Stranski-Krastanov (S-K) mode (layer-plus-island) on the SiO2 surface to a Frank-van der Merwe (F-M) mode (layer-by-layer) on the OTS surface. Detailed structural analysis by X-ray diffraction techniques revealed that the S-K mode was induced by lattice strain in the initial wetting layers on the SiO2 surface. On the other hand, strain-free initial layers were already formed at the beginning of growth on the OTS surface, thereby suppressing island formation. Moreover, the films on the SiO2 surface were found to incorporate high microstrain driven by crystal defects such as dislocations and a mosaic structure. In contrast, few crystal defects were present in the films on OTS surface, demonstrating that OTS treatment efficiently improves the molecular alignment.