Masanori Ochi

In Situ Tuning of Magnetization and Magnetoresistance in Fe3O4 Thin Film Achieved with All-Solid-State Redox Device

An all-solid-state redox device composed of Fe3O4 thin film and Li(+) ion conducting solid electrolyte was fabricated for use in tuning magnetization and magnetoresistance (MR), which are key factors in the creation of high-density magnetic storage devices. Electrical conductivity, magnetization, and MR were reversibly tuned by Li(+) insertion and removal. Tuning of the various Fe3O4 thin film properties was achieved by donation of an electron to the Fe(3+) ions. This technique should lead to the development of spintronics devices based on the reversible switching of magnetization and spin polarization (P). It should also improve the performance of conventional magnetic random access memory (MRAM) devices in which the ON/OFF ratio has been limited to a small value due to a decrease in P near the tunnel barrier.

Effect of Ionic Conductivity on Response Speed of SrTiO3-Based All-Solid-State Electric-Double-Layer Transistor.

An all-solid-state electric-double-layer transistor (EDLT) with a Y-stabilized ZrO₂ (YSZ) proton conductor/SrTiO₃ (STO) single crystal has been fabricated to investigate ionic conductivity effect on the response speed, which should be a key parameter for development of next-generation EDLTs. The drain current exhibited a 4-order-of-magnitude increment by electrostatic carrier doping at the YSZ/STO interface due to ion migration, and the behavior strongly depended on the operation temperature. An Arrhenius-type plot of the ionic conductivity (σ(i)) in the YSZ and t(c)⁻¹, which is a current-rise time needed for charge accumulation at the YSZ/STO interface, shows a synchronized variation, indicating a proportional relationship between the two parameters. Analysis of the σ(i)-t(c) diagram shows that, in contrast to conventional EDLTs, the response speed should reach picosecond order at room temperature by using extreme miniaturization and superionic conductors. Furthermore, the diagram indicates that plenty of solid electrolytes, which have not been used due to the lack of criteria for evaluation, can be a candidate for all-solid-state EDLTs exceeding the carrier density of conventional EDLTs, even though the response speed becomes comparably lower than those of FETs.

Electron–Ion Mixed Conduction of BaCe0.90Y0.10O3−δ Thin Film Generated by Ru Substitution

The structural and electrical properties of a c-axis-oriented BaCe0.85Ru0.05Y0.10O3−δ (BCRY) thin film on an Al2O3(0001) substrate depending on film thickness have been studied. The lattice constant of the c-axis decreases with increasing film thickness. The electrical conductivity is higher in the thin film with a small lattice constant. The activation energy (EA) of the dry BCRY thin film with a high conductivity is 0.26 eV, which corresponds to half of that of the bulk ceramic. The BCRY thin film exhibits electron–ion mixed conduction with a small EA of 0.18 eV below 400 °C in H2O atmosphere. The Ce3+ state created by oxygen vacancies, which locates at the top of the valence band, plays an important role in the electron–ion mixed conduction or proton conduction of the BCRY thin film.

Electrical and structural properties of BaCe0.85Ru0.05Y0.10O3−δ thin film prepared by RF magnetron sputtering

The a- and c-axes-oriented BaCe0.85Ru0.05Y0.10O3−δ (BCRY) thin films have been deposited on Nb-SrTiO3(100) substrates by radio frequency (RF) magnetron sputtering. Such BCRY thin films have mixed valence states of Ce4+ and Ce3+. The activation energies (E A) for the conductivity of films thicker than 200 nm are 0.23–0.26 eV, which corresponds to half E A of bulk ceramics, below 400 °C. The BCRY thin films exhibit ion conduction at the bulk region and electron–ion mixed conduction at the surface region. Proton conduction is also observed in the surface state in addition to the mixed conduction. The Fermi levels (E F) locate at the middle position in the band gap region, although E F of the BaCe0.90Y0.10O3−δ thin films locates on the valence band side. These results indicate that the Ru5+ ions and protons act as donor ions in BCRY thin films.

Electronic structure of c-axis controlled α-Fe2O3 thin film probed by soft-X-ray spectroscopy

We have prepared c-axis controlled α-Fe2O3 thin films on Al2O3 substrates by RF magnetron sputtering and studied their electronic structure by soft-X-ray spectroscopy. The lattice constant of c-axis increases with increasing film thickness due to the relaxation of lattice mismatch between α-Fe2O3 and Al2O3 and formation of oxygen vacancies. The electrical conductivity is higher in thicker thin film. The valence band consists of t2g- and eg-subbband of Fe 3d state hybridized with O 2p state. The band gaps of ~25 and ~95 nm thicknesses of Fe2O3 thin film are ~1.8 and 1.4 eV, respectively, which correspond to the activation energy of electron conductivity. The above results indicate that the band gap and the conductivity of α-Fe2O3 thin film directly affect the change of the lattice constant of c-axis and formation of oxygen vacancies.

Comparison of subthreshold swing in SrTiO3-based all-solid-state electric-double-layer transistors with Li4SiO4 or Y-stabilized-ZrO2 solid electrolyte

SrTiO3 (STO)-based all-solid-state electric-double-layer transistors (EDLTs) with a Li4SiO4 (LSO) lithium ion conductor (i.e., electrolyte) or Y-stabilized-ZrO2 (YSZ) proton conductor were fabricated. While the LSO device showed significant drain current enhancement at room temperature, the YSZ device needed high temperature to achieve comparable drain current enhancement due to the difference in ionic conductivity between the two electrolytes. Subthreshold swing (S), which is a parameter used to evaluate the steepness of drain current enhancement in field-effect transistors (FETs), was calculated to be 66 and 227 mV/dec, respectively, for LSO and YSZ EDLTs. The 66 mV/dec is very close to the theoretical limit (60 mV/dec) for conventional FETs, indicating that LSO is more suitable for STO-based EDLTs and that the type of solid electrolyte used greatly affects EDLT switching characteristics.