The dielectric behaviour of single-crystal MgO,Fe/MgO and Cr/MgO

The dielectric constants and loss factors,, for pure single-crystal MgO and for Fe-and Cr-doped crystals have been measured at frequencies, , from 500 Hz to 500 kHz at room temperature. For pure MgO at 1 kHz the values of and the loss tangent, tan , (9.62 and 2.16×10^–3, respectively) agree well with the data of Von Hippel; the conductivity, , varies as ⁿ withn=0.98±0.02. In Fe-doped crystals increases with Fe-concentration (at any given frequency); for a crystal doped with 12800 ppm Fe, was about four times the value for pure MgO. At all concentrations the variation of log with log was linear andn=0.98±0.02. A decrease in with increasing Fe-concentration was also observed. A similar, although less pronounced, behaviour was found in Cr-doped crystals. The effects are discussed in terms of hopping mechanisms.     

Structure and properties of dispersion-strengthened CrMgO,CrY2O3, CrY and CrCu condensates

Two-phase condensates of chromium with a dispersed phase of MgO, Y₂O₃, yttrium or copper were prepared by condensation from a vapor phase. A study of their structure, microhardness and mechanical properties as a function of the dispersant indicated the presence of a minimum in the hardness and of a maximum in the mechanical properties such as the tensile strength and the ductility at a critical concentration of the dispersant. At this concentration the average crystallite size of the matrix was equal to the mean free path between the dispersed particles. The yttrium- and yttria-containing condensates exhibited a better resistance to oxidation than that of pure chromium.     

Effects of MgO/CaO on the structural, thermal and dielectric properties of aluminoborosilicate glasses

Glasses with compositions xMgO-(20?x)CaO-10Al₂O₃-20B₂O₃-50SiO₂ (x = 0, 5, 10, 15 and 20 mol%) were prepared by conventional melting method. X-ray photoelectron spectroscopy (XPS) results indicated that the proportion of non-bridging oxygen increased with increasing MgO content. Nuclear magnetic resonance spectra showed that the fraction of the four-coordinated boron ions N_4(B) and aluminum ions N_4(Al) decreased with increasing MgO/CaO. Thus, higher field strength cations were expected to weaken the glass network. However, the increase in the glass transition temperature (T_g) indicated that the magnesium ions strengthened the glass network. The decrease in dielectric constant ε_r and loss tanδ could be attributed to the increase in the rigidity of the glass network as the MgO content increased.     

High tunnel magnetoresistance in epitaxial Co/sub 2/Cr/sub 0.6/Fe/sub 0.4/Al/MgO/CoFe tunnel junctions

We have fabricated fully epitaxial magnetic tunnel junctions (MTJs) using a Co-based full-Heusler alloy Co/sub 2/Cr/sub 0.6/Fe/sub 0.4/Al (CCFA) thin film and an MgO tunnel barrier. The CCFA thin film for the lower ferromagnetic electrode was deposited by magnetron sputtering on an MgO-buffered MgO single-crystal substrate, and the MgO tunnel barrier was formed by electron beam evaporation. The microfabricated epitaxial CCFA/MgO/CoFe MTJs showed high tunnel magnetoresistance ratios of 42% at room temperature and 74% at 55K.     

Electrophysical properties of Cu/Cr and Fe/Cr film systems within elastic and plastic deformation range

We have investigated the electrophysical properties of metallic thin films based on Cu/Cr and Fe/Cr systems. We find that the longitudinal gauge factor of two-layer films is significantly greater as compared with one-layer films, which have the same thickness as the total thickness of a two-layer film. Interface and intensive grain–boundary electron scattering explain such an increase in the longitudinal gauge factor. We find that the longitudinal gauge factor increases in transition from elastic to plastic zone.     

Structure and mechanical properties of rapidly solidified Al-(Fe,Cr) and Al-Mg-(Fe,Cr) alloys

For Al-(Fe,Cr) and Al-Mg-(Fe,Cr), by employing a combination of X-ray diffraction, metallography and transmission electron microscopy, detailed understanding of microstructural transformations that occur during rapid solidification and consolidation has been achieved. A major decrease in the solid solubility extension of Fe in an Al-Mg system with an increase of Mg concentration has been found. The decrease in solubility of Fe results in the reduction of strength and hardness of the Al-Mg-Fe alloys in comparison with Al-Mg-Cr alloys.     

Magnetic, magnetocapacitance and dielectric properties of Cr doped bismuth ferrite nanoceramics

BiFe_1−xCr_xO₃ (x = 0, 0.04, 0.06 and 0.08) nanoceramics were prepared by sol-gel method. Nanoceramics were calcined at 450 °C. Calcined powders were leached in diluted nitric acid to get single phase. TEM analysis shows the particle size to be ∼80 nm. Thermogravimetric analysis of as prepared powder indicates that the single phase is formed at around 450 °C. Magnetization was found to increase as the concentration of Cr was increased. Dielectric constant and dielectric loss were found to decrease with increase in frequency for all the compositions. Magnetocapacitance was found to increase with magnetic field. For BiFe_1−xCr_xO₃ (x = 0.04, 0.06 and 0.08) nanoceramics, the change of dielectric constant induced by magnetic field may be well approximated by Δ?/? = γM², here, γ (magnetoelectric interaction) is small and positive. A linear fit gave the value of γ of ∼18.4 × 10⁻², 12.3 × 10⁻² and 3.3 × 10⁻² for BiFe_1−xCr_xO₃ (x = 0.04, 0.06 and 0.08) nanoceramics, respectively.     

纳米MgO制备方法对MgO/LDPE纳米复合材料介电性能的影响

本文对比分析了传统法和微波辅助法合成的Mg(OH)2对所制备的纳米MgO晶粒尺寸和表面形貌的影响,以及纳米MgO不同制备方法对MgO/LDPE纳米复合材料空间电荷、体积电阻率及直流击穿场强的影响.结果表明,传统法制备的纳米MgO晶粒尺寸为22.74 nm,微波辅助法制备的纳米MgO的晶粒尺寸为12.76 nm,MgO质量分数为2%时有效降低了复合材料内部空间电荷的积聚,对复合材料的体积电阻和击穿场强均有提高.     

Thermal properties of ball-milled nanocrystalline Fe,Co and Cr powders

Ball-milled Fe, Co and Cr powders have been investigated by differential scanning calorimetry to study the stored enthalpy and the origin of the enhanced specific heat in these materials. The samples were prepared of high purity powder of 40–50 μm initial particle size and milling times up to 1 month have been applied. The results of calorimetric measurements have been correlated with structural parameters (average grain size and dislocation density) obtained by high resolution X-ray diffractometry.     

Interfacial interaction in Fe/MgO/Fe magnetic tunneling junctions

We present a study on interfacial structures and tunneling magnetoresistance (TMR) in Fe/MgO/Fe junctions using a MgO(111) film with {100} facets. It is shown using X-ray photoelectron spectroscopy that a FeO layer occurs at MgO/Fe rather than Fe/MgO interface, which could be used to tune the TMR effect. At the Fe/MgO interface, such a change in electronic structure is attributed to the band bending associated with a change in thickness of Fe films. The present study provides a new understanding on the Fe/MgO/Fe interfacial behavior and metal/oxide barriers involving electron transport.     

Interface characterization of epitaxial Fe/MgO/Fe magnetic tunnel junctions

Following predictions by first-principles theory of a huge tunnel magnetoresistance (TMR) effect in epitaxial Fe/MgO/Fe magnetic tunnel junctions (MTJs), measured magnetoresistance (MR) ratios of about 200% at room temperature (RT) have been reported in MgO-based epitaxial MTJs. Recently, a MR ratio of about 600% has been reported at RT in MgO-based MTJs prepared by magnetron sputtering, using amorphous CoFeB as the ferromagnetic electrode. These MTJs show great potential for application in spintronic devices. Fully epitaxial MTJs are excellent model systems that enhance our understanding of the spin-dependent tunneling process as the interface is well defined and can be fully characterized. Both theoretical calculations and experimental results clearly indicate that the interfacial structure plays a crucial role in the coherent tunneling across a single crystal MgO barrier, especially in epitaxial MgO-based MTJs grown by molecular beam epitaxy (MBE). Surface X-ray diffraction, Auger electron spectroscopy, X-ray absorption spectra, and X-ray magnetic circular dichroism techniques have been reported previously for interface characterization. However, no consistent viewpoint has been reached on the interfacial structures (such as FeO layer formation at the bottom Fe/MgO interface), and it is still an open issue. In this article, our recent studies on the interface characterization of MgO-based epitaxial MTJs by X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and spin-dependent tunneling spectroscopy, will be presented.     

Analysis of the diffusion processes in Al/Cr, Al/Fe and Cr/Fe multilayers using the MRI model

One of the methods for synthesis of intermetallic films consists of two steps: deposition of a multilayer film containing monometallic layers, and consequential annealing to induce a diffusion process and alloy formation. Though the diffusion coefficients are generally known for couples of common metals, they can considerably differ for films deposited by PVD due to their specific microstructure. Bimetallic multilayers Al/Cr, Al/Fe and Cr/Fe were analysed as the first step for formation of the complex metallic alloy Al₄(Cr,Fe). The films were deposited by triode sputtering with a total thickness of about 250 nm and consisted of 6 layers. Annealing was performed in vacuum at temperatures 240-650 °C.The depth distributions of elements in the films annealed at various temperatures were measured by Auger electron spectroscopy. Detailed analysis of the profile was conducted using the MRI model, which takes into account interface broadening of the measured profile due to three reasons: ion-induced atom mixing, roughness, and information depth of analysed electrons. Thus we reconstructed the true depth profile of the as-deposited samples and profiles of the annealed samples that allowed us to extract the diffusion coefficients.     

Low frequency dielectric properties of human blood

Nano- and microparticles in human blood affect drastically the charge transfer CT through the viable fluid. Low-frequency dielectric properties of the blood, characterized by CT of β - dispersion type, are widely investigated with special stress on physical mechanisms. However, no publication in the literature deals with the presence of α - dispersion in human blood. In the present study, a mechanism associated with α-, β-, and γ- dispersions is presented. The dielectric losses (electrical conduction) phenomenon is also discussed with special emphasis on the distribution of relaxation times. These losses cause problems with low-frequency dielectric measurements, thus, they are correlated with the famous empirical Col-Cole factor, (1-α). A Gaussian distribution of relaxation times is suggested and found to be exponentially related with (1-α). The results suggest new diagnostic and therapeutic methods for blood disorders.     

氮杂碳包Fe、Co、Ni材料的合成及介电特性

采用交流电弧法,在氮气氛下合成了氮杂碳包金属Fe、Co、Ni纳米晶.采用热重分析(TG)法测量了合成材料中Fe、Co、Ni等金属的含量.借助于透射电子显微镜(TEM)、粉末衍射法(XRD)、X光电子能谱法(XPS)等分析手段进行了合成材料的形貌观察和结构表征.采用反射-传输网络参数法在HP8510B矢量网络分析仪上进行了材料复介电常数测量.结果表明氮杂碳包金属Fe、Co、Ni纳米晶在8.2～12.4GHz频率范围内具有复介电特性,其损耗角正切tgδε=ε"/ε′平均值分别达到0.9,0.31和0.30,显示出氮杂碳包金属Fe、Co、Ni纳米晶对微波具有较好的介电损耗特性.     

Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions

The tunnel magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJs) is the key to developing magnetoresistive random-access-memory (MRAM), magnetic sensors and novel programmable logic devices. Conventional MTJs with an amorphous aluminium oxide tunnel barrier, which have been extensively studied for device applications, exhibit a magnetoresistance ratio up to 70% at room temperature. This low magnetoresistance seriously limits the feasibility of spintronics devices. Here, we report a giant MR ratio up to 180% at room temperature in single-crystal Fe/MgO/Fe MTJs. The origin of this enormous TMR effect is coherent spin-polarized tunnelling, where the symmetry of electron wave functions plays an important role. Moreover, we observed that their tunnel magnetoresistance oscillates as a function of tunnel barrier thickness, indicating that coherency of wave functions is conserved across the tunnel barrier. The coherent TMR effect is a key to making spintronic devices with novel quantum-mechanical functions, and to developing gigabit-scale MRAM.     

Spin filtering effects in monocristalline Fe/MgO/Fe magnetic tunnel junctions

Single-crystal magnetic tunnel junctions employing bcc (1 0 0) Fe electrodes and MgO(1 0 0) insulating barrier are elaborated by molecular beam epitaxy. Two extreme regimes have been investigated. First, for extremely thin MgO thickness we show that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions, mediated by the tunneling of the minority spin interfacial resonance state. Second, for large MgO barrier thickness, the tunnel transport validates specific spin filtering effect in terms of symmetry of the electronic Bloch function and symmetry-dependent wave function attenuation in the single-crystal barrier. Within this framework, we present giant tunnel magnetoresistive effects at room temperature (125–150%). Moreover, we illustrate that the interfacial chemical and electronic structure plays a crucial role in the filtering. We show that the insertion of carbon impurities at the Fe/MgO interface changes radically the voltage response of the tunnel magnetoresistance. Moreover, we provide experimental evidence for the electronic interfacial resonance states contribution to the spin polarized tunnel transport.     

频率对VHB4910介电弹性体材料介电性能的影响

介电弹性材料是一种新型的电活性聚合物,在电激励下能产生较大的机械变形,可被广泛的用作致动器和传感器.其中3M公司的VHB4910系列丙烯酸薄膜是研究最广泛的介电弹性材料,其关键的电学参数,特别是介电常数,将影响整体的机电耦合性能.然而到目前为止,人们对于VHB4910介电材料介电常数取值的认识尚不统一,特别是频率对介电...     

Temperature and frequency dependent dielectric properties of lithium orthophosphate LiBaPO4

The lithium barium orthophosphate LiBaPO₄ compound has been synthesized by the classic ceramic method and characterized by X-ray diffraction (XRD) technique. The electrical conductivity and modulus characteristics of the system have been investigated in the temperature and the frequency range 681–872 K and 200 Hz–1 MHz respectively by means of impedance spectroscopy. The activation energy value of LiBaPO₄ sample is bigger than of the LiMPO₄ (M = Mn, Co, Ni, Fe) compounds. The frequency dependent conductivity of the present system shows the power law feature. Dielectric data were analyzed using complex electrical modulus M* at various temperatures. The peak positions ω_m of the above spectra shift towards higher frequencies with increase in temperature. The above spectra have been characterized in terms of Kohlrausch–Williams–Watts (KWW) relaxation function to understand relaxation behavior. The activation energy responsible for relaxation calculated from the modulus spectra is found to be almost the same as the value obtained from temperature variation of dc conductivity. The electrical modulus and its scaling behavior are also investigated and the relationship between power-law exponent n and stretched exponential exponent β is found.