MgO Insulator Research Articles

RETRACTED: Magnetic properties of iron-based soft magnetic composites with MgO coating obtained by sol–gel method

Soft magnetic composites with a thin MgO insulating layer were produced by a sol-gel method. Energy dispersive X-ray spectroscopy, X-ray analysis, Fourier transform infrared spectroscopy, density measurement and compositional maps confirmed that thin layers of MgO covered the iron powders. Coercivity measurement showed that the stress relaxation and reduction of hysteresis loss efficiently occurred at 600 °C. At this temperature, the phosphate insulation of commercial SOMALOY™ samples degrade and their electrical resistivity, magnetic permeability and operating frequency decreases noticeably. The results show that the MgO insulation has a greater heat resistance than conventional phosphate insulation, which enables stress-relief at higher temperatures (600 °C) without a large increase in eddy current loss. The results of annealing at 600 °C show that the electrical resistivity and ferromagnetic resonance frequency increased from 11 μΩ m and 1 kHz for SOMALOY™ samples to 145 μΩ m and 100 kHz for the MgO insulated composites produced in this work.

First-principles study of magnetism driven by intrinsic defects in MgO

The d 0 ferromagnetism of MgO containing neutral point defects has been investigated using the first-principles method. The generalized gradient approximation (GGA), local density approximation (LDA) and local density approximation with on-site effect U (LDA+ U) schemes were employed to describe the exchange correlation energy. Undoped MgO is found to be nonmagnetic. Neutral oxygen vacancies in the supercell induce no ferromagnetism, while neutral Mg vacancies could bring about large magnetic moments. The spin polarization of 2p electrons of oxygen atoms is responsible for the induced magnetic moments. The ferromagnetic moments enhance with an increase of the concentration of V Mg 0 vacancies. Ferromagnetism depends sensitively on the distribution of Mg vacancies. There may be a preferable occupation for V Mg 0 vacancies in the MgO lattice. The LDA+ U calculation indicates that a concentration of 6.25% V Mg 0 vacancies could convert the MgO insulator to half-metal. Our results may be valid for the region near the surface of nanograins.

MAGNETORESISTANCE IN Fe/MgO/Fe MAGNETIC TUNNEL JUNCTIONS

The magnetoresistance of Fe/MgO/Fe magnetic tunnel junctions (MTJs) was studied taking into consideration image forces. For MTJs with an MgO insulator, explanations are given of the giant tunneling magnetoresistance (TMR) effect and the effect of the increasing TMR with an increase in MgO insulator thickness. It is demonstrated that the electron current density through MTJs can be high enough to switch the magnetization of a ferromagnetic electrode.

Magnetoresistance of magnetic tunnel junctions with low barrier heights

The magnetoresistance of low-barrier magnetic tunnel junctions (MTJs) was studied within a two-band model of free electrons in ferromagnetic electrodes, taking into consideration image forces. For MTJs with an MgO insulator, explanations are given of the giant tunneling magnetoresistance (TMR) effect and the effect of increasing TMR as the width of the MgO barrier increases. It is shown that TMR and the electron current density through MTJs depend strongly on the dielectric constant of the MgO insulator. It is found that the TMR of low-barrier MTJs reaches a maximum at a particular value of the applied bias voltage. It is demonstrated that the electron current density through low-barrier MTJs can be high enough to switch the magnetization of a ferromagnetic electrode.

High Performance Soft Magnetic Composite (SMC) Produced by Improved Process

Soft magnetic composites (SMC’s) are produced compacting iron powder coated with a thin insulator and then heat-treating the greens. In this study, to improve magnetic properties of SMC’s, formation of heat resistant MgO insulation layer on iron powder particles was examined. It was found that the MgO coating can be made by a dry process with two steps, i.e., oxidation of the iron particle surfaces and reduction of iron oxide by magnesium vapor. The SMC fabricated from the MgO-coated powder exhibits high resistivity even after heat treatment at 600, which leads to low iron losses comparable to those of the as-punched silicon-iron laminations. This can be ascribed to complete relief of stress introduced in the compaction process.

絶縁被覆鉄粉を用いた複合軟磁性材料の腐食特性

Soft magnetic composites (SMC) have attracted a growing interest since they have the positive points in isotropic magnetic properties and high electric resistivity. They also have the advantage in fabricating complex shape products easily. In this experiment, water atomized iron powder with phosphate insulator coating around its surface was mixed with binder and three kinds of SMC specimens were prepared. The first one is prepared without binder (Sample A), the second one is prepared by using silicon resin as binder (Sample C), and the third one by using thermosetting polyimide (Sample D). In addition, the water atomized iron powder with MgO insulator coating (Sample B) was used, where the silicon resin and Bi-B system glass with low-temperature melting point were used as binder. Using these four kinds of samples, their corrosion behavior were investigated by electrochemical method. In the active zone, Sample D indicated the highest corrosion resistance due to the prevention of anodic reaction. It is because the added thermosetting polyimide prevents the penetration of corrosion solution into the sample. On the other hand, in the passive zone, Sample B showed the highest corrosion resistance due to the formation of uniform passive state film with good quality. It is because the added Bi-B system glass prevents both grain separation and exposure of newly-formed surface to corrosion solution.

Properties of Soft Magnetic Composite with Evaporated MgO Insulation Coating for Low Iron Loss

MgO insulation coating with thickness of ca. 50nm was evaporated on the surface of iron powder and fabricated soft magnetic composite(SMC) with low iron loss. The MgO insulation coating had greater heat resistance than conventional phosphate insulation coating, which enabled stress relieving annealing at higher temperature (600C). Electrical and magnetic properties of SMC was examined. The electrical resistivity of the SMC annealed at 700C was 21μΩm. The iron loss at 50Hz for Bm = 1.5T was 5.34W, which was 40% of conventional SMC and was at the same level with laminated steel plate (t0.35). These results show that MgO insulation coating has enough heat resistance and adhesiveness to powder surface to obtain SMC with low iron loss.

ＭｇＯ系耐熱絶縁皮膜鉄粉を用いた低鉄損圧粉磁心の磁気特性

To decrease the hysteresis losses of soft magnetic composite (SMC) cores, it is effective that the coercivity of cores is reduced by the high temperature stress relieving annealing to the green compacts. It is reported about magnetic properties of SMC, which MgO system evaporation insulation coating on the surface of iron powders had greater heat-resistance than the conventional phosphate insulation coating. We found that it can be obtained new SMC core with lower iron loss made from iron powders with evaporated MgO insulation coating. The iron loss of them at 50 Hz for Bm = 1.5 T was lower 30% of the conventional SMC core and was almost the same with the laminated steel (50H600) core. It became clear that typical MgO insulating films had microcrystallines have the thickness of about 50 nm and enough heat resistance.

Large spin effects in Coulomb blockade ofFe∕MgO∕Fetunnel junctions

The Coulomb blockade across Fe and Pd nanoislands grown on single-crystalline MgO insulator films on Fe(001) was investigated with scanning tunneling microscopy. In $I(U)$ spectra of individual islands, the characteristic steps of the Coulomb staircase were observed as a function of island size. A detailed analysis of the Coulomb staircase energetics in terms of the island capacitance reveals the expected linear behavior for electrostatic interactions. For ferromagnetic Fe islands, however, a significant offset of 1 eV was found, which is missing in the case of nonmagnetic Pd islands. This effect is explained by the spin dependence of the electronic transport across the MgO barrier in combination with a quasihalf-metallic density of states of the Fe islands, as corroborated by first-principles electronic-structure calculations.

Examination of MgO insulator thin films for high-Tc superconducting devices

We have examined feasibility of MgO thin films as an insulating layer in high-Tc SFQ digital devices. When MgO thin films were prepared at 350°C by RF magnetron sputtering on Y0.9La0.2Ba1.9Cu3Oy (La–YBCO) thin films, they showed a rough surface and existence of (111) orientation. Employing a 10-nm-thick BaZrO3 buffer layer on the La–YBCO films, we successfully fabricated MgO thin films with a flat surface. The prepared MgO thin film on the La–YBCO film had a dielectric constant of 11.5. La–YBCO thin films on the MgO thin films had (00l) orientation and in-plane alignment. It was also found that the MgO thin film had a lower etching rate than those for La–YBCO films and conventional insulating thin films such as SrSnO3 and CeO2.

Radiation-induced thermoelectric sensitivity in the mineral-insulated cable of magnetic diagnostic coils for ITER

Magnetic coils are indispensable for plasma position control in tokamaks. For ITER, non-inductive voltages generated under irradiation have to be reduced to well below 1 μV in order to support long pulse (3000 s) operation. In situ measurements of the differential voltage between the ends of two magnetic coils wound with mineral-insulated (MI) cable have been carried out at the JMTR fission reactor. The MI cables of the two magnetic coils had copper center conductor with diameter 0.5 and 0.8 mm, respectively, stainless steel outer sheath and MgO insulation. The measured differential voltage for the two MI cables increased with neutron fluence, reaching 4.5 and −0.7 μV in the two coils, at a fast neutron fluence of 1.26 × 10 23 n/m 2. The magnitude of the measured voltage can be explained by thermoelectric potentials, enabled mainly by non-uniform transmutation and displacement damages of the copper core of the cable.

Microstructural and electrical properties of MgO thin films grown on p-InP (100) substrates at low temperature

The growth of MgO thin films on p-InP (100) substrates by using electron-beam deposition at a relatively low temperature (∼200°C) was performed in order to produce high-quality MgO/p-InP (100) heterointerface and MgO insulator gates with dielectric constants of low magnitude. Atomic force microscopy and X-ray diffraction measurements showed that the MgO films grown on the InP substrates were polycrystalline thin layers with very smooth surfaces. Transmission electron microscopy and selected-area diffraction measurements showed that the grown MgO films were polycrystals with small domains and that the MgO/InP (100) heterointerface had no significant interdiffusion problem. Room-temperature current–voltage and capacitance–voltage ( C– V) measurements clearly revealed a metal–insulator–semiconductor behavior for the diodes with MgO insulator gates, and the interface state densities at the MgO/p-InP interfaces, as determined from the C– V measurements, were approximately 4.5×10 11 eV −1 cm −2 at an energy of about 0.7 eV below the conduction-band edge. The dielectric constant of the MgO thin films, as determined from the C– V measurements, was as low as 9.67. These results indicate that the MgO layers grown on p-InP (100) substrates at low temperature hold promise for potential electronic devices based on InP substrates.

Performance of all-NbN quasi-optical SIS mixers for the terahertz band

We have designed, fabricated, and tested terahertz superconductor-insulator-superconductor (SIS) mixers having a self-compensated NbN/AlN/NbN tunnel junction and an epitaxially grown NbN/MgO/NbN microstripline. The junction as a distributed element was 0.8-/spl mu/m wide and 2.4-/spl mu/m long, and its current density was 50 kA/cm/sup 2/. The microstripline consisted of a 200-nm-thick NbN ground plane, a 180-nm-thick MgO insulator, and a 350-nm-thick NbN wiring layer. An investigation of the mixer in our receiver configuration showed flat noise characteristics from 870 to 960 GHz. The lowest receiver noise temperature of about 550 K was obtained at 909 GHz, including a 9-/spl mu/m-thick Mylar beam splitter loss and other optical losses. These characteristics suggest that SIS mixers with self-compensated NbN/AlN/NbN junctions and NbN/MgO/NbN microstriplines are appropriate for used in wideband and low-noise operations at terahertz frequencies.

Microstructural and electrical properties of MgO thin films grown on p-GaAs (100) substrates

MgO thin films grown on p-GaAs (100) substrates by using electron-beam deposition at a relatively low temperature (∼250°C) was performed in order to produce high-quality MgO/p-GaAs (100) heterointerfaces and MgO good insulator gates. Atomic force microscopy and X-ray diffraction measurements showed that the MgO films grown on the GaAs (100) substrates were (110) oriented epitaxial layers with smooth surfaces. Transmission electron microscopy measurements showed that the MgO/GaAs (100) heterointerface had no significant interdiffusion problem. Room-temperature current-voltage and capacitance-voltage (C—V) measurements clearly revealed a metal-insulator-semiconductor behavior for the diodes with MgO insulator gates. The dielectric constant and the interface state density at the MgO/p-GaAs interface were 9.06 and 1.9 × 10 11 eV −1 cm −2 at an energy of about 0.7 eV below the conduction-band edge, respectively. These results indicate that the MgO epitaxial films grown on p-GaAs (100) substrates at low temperature hold promise for potential applications in high-frequency electronic devices.

Irradiation effects on magnetic probes made of mineral insulated cable

A magnetic probe made of mineral insulated (MI) cable in Japan Material Testing Reactor (JMTR) was irradiated as one of the irradiation tests on diagnostics components for experimental thermonuclear reactors such as International Thermonuclear Experimental Reactor (ITER). Magnetic probes have to be installed inside the vacuum vessel to measure the magnetic field, which is very important for the position control of the plasma. The magnetic probe made of MI cable is regarded as the most reliable magnetic sensor, and proposed to ITER. The irradiated coil of the magnetic probe was made of MI cable with 1.6 mm outer diameter, 0.5 mm center conductor diameter and MgO insulator. The fast neutron flux (>1 MeV) and fast neutron fluence were 9×10 16 n m −2 s −1 and 6×10 23 n m −2, respectively. The probe inductance was analyzed with the equivalent circuit where the sheath is modeled as a secondary coil in order to remove the shielding effect of the sheath. A weak dependence of the inductance on the temperature was found, which is explained by the thermal expansion of the coil. There is no irradiation effect on the electrical characteristics of the probe up to the fluence of 6×10 23 n m −2, except RIEFM of 5–6 V between the sheath and the center conductor.

Analyses of High Frequency Capacitance–Voltage Characteristics of Metal-Ferroelectrics-Insulator-Silicon Structure

Capacitance–voltage (C–V) characteristics of the metal-ferroelectrics-insulator-silicon (MFIS) structure have been calculated by the conventional metal-oxide-silicon (MOS) analysis method. The potential profile is obtained by using the Poisson equation of D–E hysteresis, and its capacitance is estimated as combination of SiO2-Si capacitance and ferroelectric film capacitance obtained from dielectric constant corresponding to the internal field. PbZrxTi1-xO3 (PZT) and SrBi2Ta2O9 (SBT) ferroelectrics, SiO2, MgO, SrTiO3 (STO) and CeO2 insulators, and silicon semiconductor, were used as independent parameters for the analyses. The dependence of MFIS C–V characteristics on ferroelectric properties, film thickness and impurity concentration of the semiconductor is calculated. Film thicknesses of ferroelectrics and insulators affect the memory window width markedly, and have to be optimized in order to obtain a large memory window width. On the other hand, MFIS C–V characteristics and memory window width are not influenced by Pr because the effective Pr is small in the MFIS structure.

In situ measurement of radiation induced conductivity in oxide insulators during neutron irradiation

An experimental investigation of the in situ electrical conductivity of Wesgo Al995 polycrystalline alumina at approximately 450 °C has been performed at the high flux beam reactor at Brookhaven National Laboratory. The measured radiation induced conductivity (RIC) was about 10−8 S/m at an ionizing dose rate of 6000 Gy/s. No evidence for permanent radiation induced electrical degradation was observed for an applied electric field of 147 V/mm up to a dose level of ≈1.8 displacements per atom. The effect of neutron irradiation on the electrical properties of two mineral insulated cables was also investigated. The RIC in the MgO insulation of a coaxial and a triaxial cable was measured to be in the range of 6–20 ×10−8 S/m at an ionizing dose rate of ≈ 6000 Gy/s.

Effect of MgO Addition on the Bondability and Interfacial Reaction between Dielectric and Glass-Ceramics for the Fabrication of Multi-Component Ceramic Substrates

Abstract Bondability and interfacial reaction between dielectric and insulator layers have been examined to obtain a basic understanding of bonding mechanisms. Lead-containing complex perovskite was used as a dielectric material. Two kinds of glass-ceramics were used as insulator material; lead borosilicate glass containing Al2O 3 (insulator A) and the same containing Al2O3 and MgO (insulator B). Dielectric and insulator layers did not bond when insulator A was used. When insulator B was used, however, strong bonding was achieved between the two layers by firing the powder compacts at temperatures between 800° and 1000°C. Addition of MgO to lead borosilicate glass increased the thermal expansion coefficient to that of the dielectric and enhanced the formation of reaction layers, resulting in good bonding. Two reaction layers were identified. The main reaction products were enstatite and bredigite for one layer contacting the dielectric, and enstatite and a compound with the same diffraction pattern as tha...

Fabrication and Characterization of Nb–(Nb Nanoconstrictions)–NbN Short Weak Links

We report the fabrication and characterization of thin-film Nb–(Nb nanoconstrictions)–NbN short weak links prepared by applying an electric field to an insulating Nb/MgO/NbN edge sandwich with an area of 0.2 µm2. The growth of Nb nanoconstrictions in the MgO ( ∼7 nm thick) insulator layer connecting two superconducting banks is assisted by electric field in liquid helium. For an interpretation of device quasiparticle characteristics, the Blonder, Tinkham and Klapwijk (BTK) theory [Phys. Rev. B 25 (1982) 4515] is used. The current-voltage curves and excess current-normal resistance products are found to agree closely with the BTK predictions by assuming two independent (series) normal metal-superconductor contacts. The two-level fluctuation (TLF) of conductance was apparently observed in the samples with Andreev reflection probability of A (E)≪1. For the samples with A (E)∼1, i.e., nearly clean contacts, no apparent TLF was observed in the frequency band up to 100 kHz.

Superconducting Ba1−xKxBiO3 thin films and junctions

Superconducting Ba1−xKxBiO3 (BKBO) thin films and heteroepitaxial multilayers were deposited by in situ off-axis sputtering on various substrate at 400 °C. A typical superconducting BKBO thin film on the SrTiO3 and MgO substrates had a very sharp zero resistance transition at a temperature of 22–24 K as measured by transport four point and mutual inductance probes. The normal state resistivity exhibited metallic to semiconductive characteristics depending on film composition with the lowest room temperature resistivity of about 0.25 mΩ cm. The film orientation was highly epitaxial to both SrTiO3 and MgO substrates. Normal metal-insulator-superconductor (NIS) type tunnel functions with native insulator layer and deposited MgO layer were reproducibly fabricated. The superconducting energy gap of about 3.6 meV was obtained from those NIS junctions. Heteroepitaxial superconductor- insulator-superconductor (SIS) trilayer junctions were also fabricated using MgO insulator layers. For a 5 nm thick insulator layer, quasiparticle SIS tunnel junctions with nonhysteric I-V characteristics were obtained. By reducing MgO insulator layer thickness down to 3 nm, SIS tunnel junctions with Josephson supercurrent were obtained with hysteric I-V characteristics.