Papers by Author: Manabu Gomi

Abstract: The magnetic properties of an artificial multiferroic material, a Cr₂O₃/LiNbO₃/Cr₂O₃ hetero structure with various-thickness LiNbO₃ layers, were investigated. All samples showed ferromagnetism due to the existence of oxygen deficiency or an excess of the Cr₂O₃ layer in the LNO/Cr₂O₃ interface. The ferromagnetism of the samples was affected by the crystal orientation of the LiNbO₃ layer, and seemed to have a major impact on the magneto-electric behavior of this hetero system.

Abstract: Resistance random access memory (RRAM) is attractive as a next-generation form of nonvolatile memory. We investigated an electric field-induced resistance change of SrFeO2+x film as a candidate for RRAM material. SrFeO2.5-x film prepared at 300 oC showed hysteresis in its current-voltage curve and distinct pulse-switching properties. On the other hand, the sample prepared below 280 oC showed hysteresis in its current-voltage curve but didn’t show pulse-switching properties. The amount of oxygen in the sample and easiness of oxygen migration play important roles in the resistance-switching properties.

Abstract: We proposed a new type of multi-ferroic system using magnetoelectric (ME:Cr2O3) material/ultra-thin ferroelectric materials (FE)/a magnetoelectric material hetero structure. This ME and FE heterosystem showed a general Polarization – Electric field hysteresis curve. When a magnetic and electric field (ME field) was applied at the same time, a decrease of polarization was observed. This might be due to the antiferromagnetic domain alignment of Cr2O3 by the ME field.

Abstract: We investigated the influence of an external magnetic field for the carrier injection process of a metal (Au) / insulator (Cr2O3/Fe/CeO2) / semiconductor (Si) (MIS) capacitor, in which the insulator consists of magnetic materials. By applying an electric field, electrons propagating through the CeO2 layer from Si were injected into the Fe or an oxygen deficiency layer formed around the Fe layer. When a magnetic field was applied, the hysteresis window width of this capacitor was reduced. I-V curve analyses under a magnetic field revealed that this reduction was more likely due to the magnetic state of the Fe layer and the interaction between Fe and Cr2O3.

Abstract: We have investigated the relationships between the electric field-induced resistance change and the strength of the exchange interaction of the Cr2O3/ La0.7Sr0.3MnO3 (LSMO) magnetic hetero system. The hetero system subjected to field cooling (FC) showed a positive shift in the magnetization curves due to an exchange bias. The exchange bias field changed depending on the FC field. Resulting from the exchange behaviors, the resistance of LSMO film was changed by the application of an electric field to the Cr2O3 gate. This resistance change is more likely due to the interface interaction strength between the Cr2O3 and LSMO film

Abstract: We investigated the magnetic interaction between antiferromagnetic, magnetoelectric Cr2O3 film and ferromagnetic La0.7Sr0.3MnO3 film, which shows colossal magneto-resistance. Magnetic properties and structural analyses revealed that a decrease in the thickness of La0.7Sr0.3MnO3 films gives rise to a change in magnetic anisotropy from perpendicular to in-plane. The exchange interaction between Cr2O3 and La0.7Sr0.3MnO3 also changed depending on the magnetic properties of La0.7Sr0.3MnO3. This suggests that the crystallographical control and the selection of a suitable thickness of La0.7Sr0.3MnO3 are important to obtain an exchange interaction in the magnetic hetero system using colossal magneto-resistance material.

Abstract: We investigated the magnetic and dielectric properties of a metal (Pt)/insulator (Cr2O3)/semiconductor (Si) (MIS) capacitor composed of magneto-electric (ME) materials. The capacitor has anti-ferromagnetic properties and a very small electrically induced magnetic moment. It also shows capacitance-voltage (C-V) properties typical of a Si-MIS capacitor without any hysteresis. By inserting a thin Cr2O3-x layer, the C-V curve has a hysteresis window with a clockwise trace, which indicates that electrons have been injected into the Cr2O3-x layer. These results indicate that this MIS capacitor contains a floating gate and an ME insulating layer in a single system.

Abstract: Magnetic, dielectric properties and magnetoelectric effect of ceramics with a composition of 0.9 BaTiO3-0.1 LaMnO3 have been investigated. X-ray diffraction measurements showed that the sintered ceramics are composites containing a small amount of (La, Ba)MnO3 phase in the BaTiO3 matrix. These composites were multiferroic, having ferromagnetic and ferroelectric Curie temperatures of 330 K and 392 K respectively. We found that the composite sintered at 1150 °C exhibits a reduction of spontaneous magnetization as large as 55 % at room temperature when an electric field of 1.4 KV/mm is applied. This reduction is probably ascribed to a change of hole concentration distribution in the precipitated ferromagnetic (La, Ba)MnO3 and the resultant decrease of ferromagnetic Curie temperature.

Abstract: We investigated the magneto-electric properties and electric properties of Cr2O3 thin films. Cr2O3 thin films were prepared on a thermal-oxidized Si substrate and a c-Al2O3 substrate. The sample prepared on the thermal-oxidized Si substrate had poly-crystalline structure. On the other hand, the sample prepared on the c-Al2O3 substrate has preferentially (006) oriented structure. Both samples had high enough resistivity to get a high induced magnetic moment. The induced magnetic moment by applying external voltage was observed on the oriented sample, although there was no induced magnetic moment on the same voltage for the poly-crystalline sample. This difference may be due to the random distribution of crystal and the leakage current caused by the Pool-Frenkel type defect in the poly-crystalline film.