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.

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.

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.

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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.