Sakyo Hirose

Mutual control of magnetization and electrical polarization by electric and magnetic fields at room temperature in Y-type BaSrCo2−xZnxFe11AlO22 ceramics

Magnetoelectric (ME) properties were investigated for BaSrCo2−xZnxFe11AlO22 ceramics with the Y-type hexaferrite structure. This system exhibits a magnetic ordering above room temperature (RT) and high electrical resistivity exceeding 500 MΩ·cm by annealing under 10 atm O2. It is found that the samples with x ≤ 0.4 exhibit the robust ME effect with a reversal of electrical polarization induced by a low magnetic field at RT. Moreover, the electric-field-induced magnetization switching is also observed at RT. These results clearly demonstrate the mutual control of magnetization and electrical polarization at RT in the Y-type hexaferrite system.

Multilevel magnetization switching by electric field in c-axis oriented polycrystalline Z-type hexaferrite

Direct and converse magnetoelectric (ME) effects, namely, magnetic-field (B) induced electric polarization (P) and electric-field (E) induced magnetization (M), respectively, were investigated at room temperature for c-axis oriented polycrystalline specimens of a Z-type hexaferrite, Sr3Co2Fe24O41. The B profile of the linear ME coefficient obtained from the converse effect well coincides with that obtained from the direct effect. Furthermore, M-E curves show a substantial hysteretic behavior, which allows reversal and multilevel switching of M by applying pulsed E. The present results demonstrate the feasibility of nonvolatile memory elements by using the ME Z-type hexaferrite at room temperature.

Direct electrocaloric measurement of 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 films using scanning thermal microscopy

We show that scanning thermal microscopy can measure reversible electrocaloric (EC) effects in <40 μm-thick ceramic films of the relaxor ferroelectric 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3, with the substrate present. We recorded roughly the same non-adiabatic temperature change (±0.23 K) for a thinner film that was driven harder than a thicker film (±31 V μm−1 across 13 μm versus ±11 V μm−1 across 38 μm), because the thicker film lay relatively closer to the substantially larger adiabatic values that we predicted by thermodynamic analysis of electrical data. Film preparation was compatible with the fabrication of EC multilayer capacitors, and therefore our measurement method may be exploited for rapid characterisation of candidate films for cooling applications.

Influence of distribution of additives on electrical potential barrier at grain boundaries in ZnO-based multilayered chip varistor

The influence of the distribution of additives on the electrical potential barrier at grain boundaries in a ZnO-based multilayered chip varistor was investigated by scanning surface potential microscopy (SSPM) and transmission electron microscope (TEM). The leakage current strongly depended on the distribution of additives, and a more uniform distribution improved the electrical nonlinearity. SSPM images revealed that the grain-boundary resistance of a uniformly distributed sample was higher than that of a poorly distributed sample. The decrease in the electrical nonlinearity due to the poor distribution was attributed to a decrease in the potential barrier height instead of an increase in the inactive grain boundary. The TEM analysis indicated that the poor distribution of Pr ions interfered with the formation of the potential barrier and resulted in the decrease in its height.

Progress on electrocaloric multilayer ceramic capacitor development

A multilayer capacitor comprising 19 layers of 38 μm-thick 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 has elsewhere been shown to display electrocaloric temperature changes of 2.2 K due to field changes of 24 V μm−1, near ∼100 °C. Here we demonstrate temperature changes of 1.2 K in an equivalent device with 2.6 times the thermal mass, i.e., 49 layers that could tolerate 10.3 V μm−1. Breakdown was compromised by the increased number of layers, and occurred at 10.5 V μm−1 near the edge of a near-surface inner electrode. Further optimization is required to improve the breakdown strength of large electrocaloric multilayer capacitors for cooling applications.

Resistance switching and retention behaviors in polycrystalline La-doped SrTiO3 ceramics chip devices

Resistance switching and retention behaviors in polycrystalline La-doped SrTiO3 ceramics have been investigated. La-doped SrTiO3 ceramics exhibits resistance switching as large as that of thin-film devices and exhibits a long-term memory effect of over 5 h. By means of a complex impedance analysis, it was clarified that these resistance changes can be reasonably attributed to the change in the electrical potential barrier at the interfaces and the resistance of the grain boundary changes remarkably by the application of voltage pulses. From the resistance retention properties at various temperatures, it was found that the high-resistance state is very stable even at 125 °C, whereas the resistance in the low-resistance state increases with time and its relaxation speed becomes remarkably faster with increasing temperatures. These results imply that the migration of the point defects could change the distribution of the space charge near the interface, resulting in a change in the interface resistances.

Effect of local electrical properties on the electrostatic discharge withstand capability of multilayered chip ZnO varistors

The local electrical properties at individual grain boundaries of multilayered chip varistors composed of ZnO–Bi2O3 (Bi–ZnO) and ZnO–Pr6O11 (Pr–ZnO) ceramics have been investigated using a scanning probe microscope (SPM) to clarify their effect on the electrostatic discharge (ESD) withstand capabilities. Pr–ZnO varistors exhibit a higher ESD withstand capability compared to Bi–ZnO varistors, although both types of devices exhibit similar electrical nonlinearity and surge current withstand capabilities. Bi–ZnO varistors exhibit asymmetric current-voltage (I-V) characteristics after the application of ESD pulses; their breakdown voltage decreases from 9 to 4 V and the leakage current increases. This indicates that the ESD pulses destroy some electrical potential barriers. Scanning surface potential microscopy (SSPM) measurements of a Bi–ZnO varistor reveal the existence of high electrical potential barriers at grain boundaries and electrode interfaces, and high-resistance secondary phases. In contrast, SSPM...

Degradation of potential barriers in ZnO-based chip varistors due to electrostatic discharge

Degradation of potential barriers in ZnO-based varistors due to the electrostatic discharge (ESD) was investigated using scanning probe microanalysis and capacitance-time and isothermal capacitance transient spectroscopies. Pr6O11-ZnO (Pr-ZnO) varistors exhibit excellent ESD withstand capability compared with Bi2O3-ZnO (Bi-ZnO) varistors. After the application of ESD, asymmetrically degraded double Schottky barriers were observed in both Pr-ZnO and Bi-ZnO varistors, and the Schottky barrier in Bi-ZnO was found to be destroyed. The potential barriers of both types of varistors can respond to an ESD pulse, whose rise time is ∼1 ns, but after application of the ESD pulse, the Bi-ZnO varistor takes more time to recover its initial capacitance than does the Pr-ZnO varistor. Such difference in the transient behaviors of potential barriers is attributed to differences in the energy and distribution of interfacial states of Pr-ZnO and Bi-ZnO varistors. Experimental results clearly indicated a strong correlation b...

Improvement in Resistance Switching and Retention Properties of Pt ∕ TiO2 Schottky Junction Devices

Resistance switching and retention properties of Pt/TiO 2 Schottky junction devices are improved by doping anatase Ti0 2 thin films with Co ions. Co-doped TiO 2 devices exhibit excellent rectifying current―voltage characteristics and resistance switching compared to undoped TiO 2 devices. Conventional Schottky junction devices such as Pt/Nb:SrTi0 3 and Pt/TiO 2 devices exhibit very poor resistance retention properties in low resistance state (LRS); the resistance in LRS increases with time and the resistance relaxation accelerates with an increase in temperature. Co-doped Ti0 2 devices show excellent resistance retention properties even at 100°C. Experimental results indicate that in Schottky junction devices, resistance switching originates from the formation of local tunneling paths, and the oxygen vacancies near the electrode interface play an important role in resistance switching. These results are very important from the point of view of understanding the resistance switching mechanism and improving the resistance switching properties of Schottky junction devices.

Electric field and temperature dependence of dielectric permittivity in strontium titanate investigated by a photoemission study on Pt/SrTiO3:Nb junctions

Schottky junctions made from platinum and niobium-doped strontium titanate (SrTiO3:Nb) were investigated by hard X-ray photoemission (HXPES) and through a band bending behavior simulation using a phenomenological model, which assumes a decrease in dielectric constant due to an electric field. Thus, we confirmed that the observed HXPES spectra at relatively high temperatures, e.g., >250 K, were well simulated using this phenomenological model. In contrast, it was inferred that the model was not appropriate for junction behavior at lower temperatures, e.g., <150 K. Therefore, a reconstruction of the phenomenological model is necessary to adequately explain the dielectric properties of SrTiO3.