Makoto Takayanagi

Electrical-pulse-induced resistivity modulation in Pt/TiO2−δ/Pt multilayer device related to nanoionics-based neuromorphic function

Resistivity modulation behavior in Pt/TiO2−δ/Pt multilayer devices was investigated in terms of nanoionics-based neuromorphic function. The current relaxation behavior, which corresponds to short-term and long-term memorization in neuromorphic function, was analyzed using electrical pulses. In contrast to the huge difference in ionic conductivity for bulk crystal materials of TiO2−δ and WO3, the difference in the relaxation behavior was small. Rutherford backscattering spectrometry and hydrogen forward scattering spectrometry revealed that the TiO2−δ thin film contained 5.6 at. % of protons. This indicates that the neuromorphic function in TiO2−δ-based devices is caused by extrinsic proton transport, presumably through the grain boundary.

Unexpected metal-insulator transition in thick Ca1-xSrxVO3 film on SrTiO3 (100) single crystal

Epitaxial Ca1-xSrxVO3 (0 ≦ x ≦ 1) thin films were grown on (100)-oriented SrTiO3 substrates by using the pulsed laser deposition technique. In contrast to the previous report that metal-insulator transition (MIT) in Ca1-xSrxVO3 (CSVO) was achieved only for extremely thin films (several nm thick), MIT was observed at 39, 72, and 113 K for films with a thickness of 50 nm. The electronic structure was investigated by hard and soft X-ray photoemission spectroscopy (HX-PES and SX-PES). The difference between these PES results was significant due to the variation in an escape depth of photoelectrons of PES. While HX-PES showed that the V 2p3/2 spectra consisted of four peaks (V5+, V4+, V3+, and V2+/1+), SX-PES showed only three peaks (V5+, V4+, and V3+). This difference can be caused by a strain from the substrate, which leads to the chemical disorder (V5+, V4+, V3+, and V2+/1+). The thin film near the substrate is affected by the strain. The positive magnetoresistance is attributed to the effect of electron-el...

Electron Conduction of Nd0.6Sr0.4FeO3−δ Thin Film with Oxygen Vacancies Prepared by RF Magnetron Sputtering

We have firstly studied the electrical conductivity and the electronic structure of the Nd0.6Sr0.4FeO3−δ (NSFO) thin film on Al2O3(0001) substrate deposited by RF magnetron sputtering. The prepared thin film has larger lattice constant than the bulk crystal due to the stress from the substrate and the oxygen vacancies. The Fe 2p photoemission spectrum exhibits the mixed valence states of Fe2+ and Fe3+. The electrical conductivity exhibits thermal activation-type behavior and increases with increasing film thickness. The band gap of charge transfer type is in a good agreement with the activation energy estimated from the Arrhenius plot. These results indicate the conducting carrier of the NSFO thin film is mainly electron, although the conductivity does not depend on oxygen gas partial pressure.

Resonant photoemission and X-ray absorption spectroscopies of lithiated magnetite thin film

Resonant photoemission spectroscopy (RPES) and X-ray absorption spectroscopy (XAS) were used to investigate the effect of lithiation on the electronic structure of Fe3O4 thin film relevant to the operation mechanism of nanoionic devices to enable magnetic property tuning. Comparison of the Fe 2p XAS spectrum for lithiated Fe3O4 (Li–Fe3O4) with that for pristine Fe3O4 clearly demonstrated that the number of Fe2+ ions at octahedral B sites is increased by lithiation. The valence band RPES spectra of Li–Fe3O4 further showed that lithiation increases the density of states near the Fermi level originating Fe2+ ions at octahedral B sites. These findings agree well with the observed decrease in the saturation magnetization in the magnetization–magnetic field (M–H) loop of Li–Fe3O4 thin film, indicating that minority spins (down spins) increase (i.e., total spins decrease) due to lithiation. The variation in the number of Fe2+ ions at B sites is suggested to be an underlying operating mechanism of a nanoionics-based magnetic property tuning device.