Kohei Fujiwara

5d iridium oxide as a material for spin-current detection

Devices based on pure spin currents have been attracting increasing attention as key ingredients for low-dissipation electronics. To integrate such spintronics devices into charge-based technologies, electric detection of spin currents is essential. The inverse spin Hall effect converts a spin current into an electric voltage through spin-orbit coupling. Noble metals such as Pt and Pd, and also Cu-based alloys, have been regarded as potential materials for a spin-current injector, owing to the large direct spin Hall effect. Their spin Hall resistivity ρSH, representing the performance as a detector, is not large enough, however, due mainly because of their low charge resistivity. Here we report that a binary 5d transition metal oxide, iridium oxide, overcomes the limitations encountered in noble metals and Cu-based alloys and shows a very large ρSH~38 μΩ cm at room temperature.

Dual field effects in electrolyte-gated spinel ferrite: electrostatic carrier doping and redox reactions.

Controlling the electronic properties of functional oxide materials via external electric fields has attracted increasing attention as a key technology for next-generation electronics. For transition-metal oxides with metallic carrier densities, the electric-field effect with ionic liquid electrolytes has been widely used because of the enormous carrier doping capabilities. The gate-induced redox reactions revealed by recent investigations have, however, highlighted the complex nature of the electric-field effect. Here, we use the gate-induced conductance modulation of spinel ZnxFe3−xO4 to demonstrate the dual contributions of volatile and non-volatile field effects arising from electronic carrier doping and redox reactions. These two contributions are found to change in opposite senses depending on the Zn content x; virtual electronic and chemical field effects are observed at appropriate Zn compositions. The tuning of field-effect characteristics via composition engineering should be extremely useful for fabricating high-performance oxide field-effect devices.

Observation of rebirth of metallic paths during resistance switching of metal nanowire

To clarify the mechanism of resistance-switching phenomena, we have investigated the change in the electronic structure of a Ni nanowire device during resistance-switching operations using scanning photoelectron microscopy techniques. We directly observed the disappearance of density of state (DOS) at the Fermi level (EF) in a high-resistance state and recovery of a finite DOS at EF in a low-resistance state. These results are direct evidence that the Ni nanowire is fully oxidized after switching to the high-resistance state and that Ni-metal conductive paths in the oxidized nanowire are recovered in the low-resistance state.

Identification of Giant Mott Phase Transition of Single Electric Nanodomain in Manganite Nanowall Wire

In the scaling down of electronic devices, functional oxides with strongly correlated electron system provide advantages to conventional semiconductors, namely, huge switching owing to their phase transition and high carrier density, which guarantee their rich functionalities even at the 10 nm scale. However, understanding how their functionalities behave at a scale of 10 nm order is still a challenging issue. Here, we report the construction of the well-defined (La,Pr,Ca)MnO3 epitaxial oxide nanowall wire by combination of nanolithography and subsequent thin-film growth, which allows the direct investigation of its insulator-metal transition (IMT) at the single domain scale. We show that the width of a (La,Pr,Ca)MnO3 nanowall sample can be reduced to 50 nm, which is smaller than the observed 70-200 nm-size electronic domains, and that a single electronic nanodomain in (La,Pr,Ca)MnO3 exhibited an intrinsic first-order IMT with an unusually steep single-step change in its magnetoresistance and temperature-induced resistance due to the domains arrangement in series. A simple model of the first-order transition for single electric domains satisfactorily illustrates the IMT behavior from macroscale down to the nanoscale.

Fabrication of three-dimensional epitaxial (Fe,Zn)3O4 nanowall wire structures and their transport properties

We have established a unique technique to fabricate three-dimensional (3D) well-defined transition-metal oxide epitaxial nanostructures. Fabrication of epitaxial spinel ferrite Fe2.2Zn0.8O4 (FZO) nanowall wires with a tunable width down to 20 nm was achieved. Cross-sectional transmission electron microscopy revealed the existence of an epitaxially matched lateral interface between the FZO nanowall wire and the side surface of 3D-MgO substrate. Magnetoresistance measurements showed ferromagnetic properties of the FZO nanowall wire at 300 K. The role of antiphase boundaries on the functionalities of the FZO nanoconfined wire is discussed.

Estimation of dc transport dynamics in strongly correlated (La,Pr,Ca)MnO3 film using an insulator-metal composite model for terahertz conductivity

Temperature-dependent conductivities at dc and terahertz (THz) frequency region (σTHz(ω,T)) were obtained for a strongly correlated (La0.275Pr0.35Ca0.375)MnO3 (LPCMO) film using THz time domain spectroscopy. A composite model that describes σTHz(ω,T) for LPCMO through the insulator-metal transition (IMT) was established by incorporating Austin-Mott model characterizing the hopping of localized electrons and Drude model explaining the behavior of free electrons. This model enables us to reliably investigate the dc transport dynamics from THz conductivity measurement, i.e., simultaneously evaluate the dc conductivity and the competing composition of metal and insulator phases through the IMT, reflecting the changes in microscopic conductivity of these phases.

Discrimination between gate-induced electrostatic and electrochemical characteristics in insulator-to-metal transition of manganite thin films

We propose a method of discriminating whether the gating effect is electrostatic or electrochemical on an electric double-layer transistor (EDLT), which can be a good guideline for examining the nature of EDLTs. The electrochemical effect between a channel and an ionic liquid depends on the gate voltage (VG). Our study on the dependence of the transport properties of a (La0.525Pr0.1Ca0.375)MnO3 (LPCMO) EDLT on VG revealed that the electrostatic effect is dominant below |VG| = 2 V, whereas the electrochemical effect is dominant at higher voltages. The modulation of the insulator-to-metal transition property of LPCMO was realized through the electrostatic effect.

Artificial three dimensional oxide nanostructures for high performance correlated oxide nanoelectronics

We report a strategy for controlling nanoscopic electronic domains to produce gigantic Mott metal–insulator transition phenomena in strongly correlated oxides by fabricating oxide micro-nano-wires, nanowalls, nanoboxes. We investigated the dependence of spatial dimensionality on wire width for a disordered configurations of metallic domains in VO2 microwires to nanowires on TiO2(001) and Al2O3(0001) substrates with well-positioned alignment by a nanoimprint (NIL) technique. We observed a temperature-induced steep multistep metal–insulator transition in artificial VO2 micro/nano-wires. With further development, we report a new bottom-up fabrication method for the formation of extremely small transition-metal oxide nanostructures employing a combination of NIL and pulsed laser deposition (PLD) techniques, called the three-dimensional nanotemplate-PLD method, to demonstrate functional oxide nanowall wires, nanoboxes, and hetero-nanowall oxide devices with widths of 20–120 nm and excellent size controllability.

Fabrication of tetragonal FeSe–FeS alloy films with high sulfur contents by alternate deposition

We report the synthesis of tetragonal

Nonvolatile Transport States in Ferrite Thin Films Induced by Field-Effect Involving Redox Processes

mathrm{FeS}_xmathrm{Se}_{1-x}