Nonlinear piezoelectricity in epitaxial ferroelectrics at high electric fields

Nonlinear effects in the coupling of polarization with elastic strain have been predicted to occur in ferroelectric materials subjected to high electric fields. Such predictions are tested here for a PbZr0.2Ti0.8O3 ferroelectric thin film at electric fields in the range of several hundred MV/m and strains reaching up to 2.7%. The piezoelectric strain exceeds predictions based on constant piezoelectric coefficients at electric fields from approximately 200 to 400 MV/m, which is consistent with a nonlinear effect predicted to occur at corresponding piezoelectric distortions.     

超宽带脉冲电场测试技术研究

电磁脉冲测试尤其是宽带脉冲电场测试己成为电子设备抗扰度试验研究的重要手段，国内外不少学者开展了这方面的研究，但是研制宽频带、大动态、高灵敏度的脉冲电场测试仪始终是该研究领域的一个难点．文章作者在国家自然科学基金资助下，对国内外研究动态进行了调查分析，综合运用相关先进技术，研制成功超宽带脉冲电场测试系统，其技术指标达到：带宽为3．5Hz--1GHz，电场强度从V／m量级到MV／m量级，动态范围达到60dB，从而为脉冲电场的测试和电磁防护研究及电子设备抗扰度试验提供了强有力的手段．     

Fe_3O_4单晶薄膜磁性电场调控的微磁学仿真研究

磁性薄膜磁学特性电场调控的相关研究对开发新型低功耗磁信息器件具有突出意义.本文基于电场调控磁性的基本理论,以OOMMF(Object Oriented Micro-Magnetic Frame)微磁学仿真软件为工具,研究了电场对生长于PZN-PT单晶衬底上Fe_3O_4单晶薄膜磁学特性的调控.研究结果显示:无外加电场时,薄膜表现出典型的软磁特性;沿衬底[001]方向施加的外加电场可以使得薄膜矫顽力、矩形比等磁学特性发生显著改变:当外加磁场沿[100]([010])时,施加正值(负值)电场强度可以显著增大薄膜的矫顽力与矩形比,当电场强度不小于0.6 MV/m时薄膜矩形比达到1.这是因为外加电场导致薄膜产生单轴应力各向异性,使得薄膜的等效磁各向异性发生了从无外电场下的面内四重磁晶各向异性向高电场下的近似单轴磁各向异性的过渡.外加1 MV/m与-1 MV/m的电场时等效易磁化轴分别沿[100]与[010]方向.另外,外加1 MV/m(-1 MV/m)的电场强度可以使得铁磁共振的频率增大(减小)接近1 GHz.     

Fast polarization reversal in thin copolymer films of vinylidene fluoride-trifluoroethylene

In this paper we present new experiments on the switching of the polarization in thin (<1 m) ferroelectric copolymer films of P(VDF-TrFE). We observed an unexpectedly short switching time for these thin films: The polarization of a 0.2 m thick film could be reversed by poling fields of 600 MV/m in less than 100 ns. This result which may revive interest in ferroelectric data storage is discussed in terms of a switching process which also involves the ohmic conductivity of the sample.     

Zero—field—cooled and field—cooled magnetizations and magnetic susceptibility of itinerant ferromagnet SrRuO3

Zero-field-cooled(ZFC) magnetization,field-cooled(FC) magnetization,ac magnetic susceptibility and major hysteresis loops of itinerant ferromagnet SrRuO3 have been measured at magnetic ordering temperatures ranging from 5 to 160K.An empirical model is proposed to calculate the measured ZFC magnetization.The result indicates that the calculated ZFC magnetization compares well with the measured one.Based on the generalized Preisach model.both the ZFC and FC curves are reproduced by numerical simulations.The critical temperature and critical exponents are determined by measuring the ac magnetic susceptibility in different bias magnetic fields at temperatures in the vicinity of the point of phase transition.     

复合外场中磁光材料(BiTm)3(FeGa)5O12薄膜畴壁运动特性的研究

通过对BiTm)₃(FeGa)₅O₁₂膜施工加低频交变磁场,匀速率增加的直流磁场和同时对该膜施加这两种磁场(复合外场),用照相划线读数方法和通过电荷耦合器件(CCD)-计算机作数字化处理获得磁畴壁(DW)的相对百分数。结果表明:(1)复合外场下DW运动规律中存在交互作用项;(2)低频交变磁场幅值(140A/m)远低于等效阻力场(10³A/m)时,DW可以运动,但不同步,频率大于1Hz时明显滞后,(3)利用图像转换有利于提高实验结果的分辨率。     

Ni/Au层对碳纳米管薄膜强流脉冲发射稳定性的影响

采用酞菁铁高温裂解法在镀有镍金缓冲层的硅基底上生长了碳纳米管薄膜(Ni/Au-CNT),并采用二极结构在相同的主Marx电压下研究了其强流脉冲发射稳定性.结果表明:在脉冲电压峰值为1.60～1.74 MV(对应的脉冲电场峰值为11.43～12.43 V/μm)时,Ni/Au-CNT薄膜首次发射的电流峰值可达331.2A;Ni/Au层不仅能提高CNT薄膜的强流脉冲发射电流峰值,还能提高其发射稳定性;当冷阴极重复脉冲发射7次时,Ni/Au-CNT的脉冲电流峰值衰减到初值的72％,而Ni-CNT和Si-CNT脉冲电流峰值分别衰减到初值的62％和32％.     

Commensurability effects in magnetic properties of superconducting Nb thin films with periodic submicrometric pores

Pinning properties in 100 nm thick continuous and porous superconducting Nb films are examined by ac susceptibility and dc magnetization measurements. The Nb film was deposited on a smooth Si substrate, while the porous film, Nb_P, was deposited on an anodized Al oxide substrate. Pores or “antidots” 40 nm in diameter, 100 nm apart, form a triangular array. The porous film presents commensurate or matching field effects for applied magnetic fields where the magnetic flux threading each unit cell is an integer number of the flux quantum, where ac shielding capability and dc diamagnetic magnetization show an abrupt increase. The response to ac fields as a function of temperature and dc field provided a way to determine that Nb_P sample has higher pinning than the continuous one, and that T_C suppression due to fluxoid quantization is not relevant for the investigated temperature range.     

Skin dose assessment in unmodulated and intensity-modulated radiation fields with film dosimetry

Superficial dose from 6- and 18-MV photon beams has been studied by measuring surface dose and shallow build-up dose using radiographic film EDR2, radiochromic film EBT2 and plane-parallel chamber. Measurements have been made for intensity- and non-intensity-modulated beams.The results show that the surface dose was found to be 19.8% and 10% of maximum dose in unmodulated fields for 6 and 18 MV photon beams, respectively. The study further showed that intensity modulation decreased surface dose by 1.1% and 0.7% for the same field size at 6 and 18 MV, respectively, and surface dose was dropped by magnetically sweeping contaminating electrons. EDR2 and EBT2 films show in good agreement in shallow build-up region.This study demonstrated the capability of EDR2 film, in addition to radiochromic film, to measure surface and build-up dose in case of treatment planning system uncertainties with regard to skin toxicity or shallow target coverage.     

Vortex Pinning due to Dynamic Spin-Vortex Interaction in aSuperconductor/Ferromagnet Multilayer

We investigate the mutual interaction between superconductivity and ferromagnetism in a Nb/Ni81 Ee19 multilayer by ac susceptibility measurements. Compared with a pure superconducting Nb film, the critical current density of the multilayer is apparently enhanced in a low magnetic field region but remains nearly the same in high magnetic fields, which indicates that a continuous ferromagnetic layer with in-plane magnetization can produce strong vortex pinning in a low field region. We interpret this unusual vortex-pinning phenomenon as a consequence of dynamic spin vortex interaction which induces a spin rotation following vortex movement. In addition, we propose that this dynamic interaction could be used for spin manipulation via a superconductor.     

Low field DC investigation of hot carrier trapping in silicon dioxide films

The experimental method used in this work is based upon the idea of nonavalanche injection of carriers heated by direct electric field. The structure consisted of an n-channel MOS transistor and two p-n junctions. The process of charge injection in this structure was investigated by studying the dependence of gate current on heating voltage. The trapping properties of the SiO₂ film were studied by monitoring the charging of the film during injection of electrons. The capture cross-sections, the trap centre concentrations and the dependence of the capture cross section on the electric field for fields between 1 MV/cm and 2.5 MV/cm were determined.     

Bump formation on a glass surface with a transparent coating using femtosecond laser processing

The morphology of a glass surface having a transparent coating processed with focused femtosecond laser pulses is investigated. The transparent coating is formed of poly-methyl methacrylate (PMMA). When the glass was coated with a PMMA film with a thickness of 2.8 μm, bumps were formed over a wide range of axial focus positions. The same laser pulse energy produced cavities when processing bare glass with no coating. The bumps were formed as a result of suppressing material emission from the glass surface by a shielding effect of plasma generated by ablation of the PMMA film and by physical blocking of the PMMA film. A thinner film with a thickness of 0.7 μm produced a reduced shielding effect, forming an exploded bump with a small pit at its center and debris around the periphery. PACS 44.10.+i; 61.80.Ba; 79.20.Ds     

Solution of near-field thermal radiation in one-dimensional layered media using dyadic Green's functions and the scattering matrix method

A general algorithm is introduced for the analysis of near-field radiative heat transfer in one-dimensional multi-layered structures. The method is based on the solution of dyadic Green's functions, where the amplitude of the fields in each layer is calculated via a scattering matrix approach. Several tests are presented where cubic boron nitride is used in the simulations. It is shown that a film emitter thicker than 1 μm provides the same spectral distribution of near-field radiative flux as obtained from a bulk emitter. Further simulations have pointed out that the presence of a body in close proximity to an emitter can alter the near-field spectrum emitted. This algorithm can be employed to study thermal one-dimensional layered media and photonic crystals in the near-field in order to design radiators optimizing the performances of nanoscale-gap thermophotovoltaic power generators.     

Thermally stimulated current in polypropylene films charged in strong electric fields

We have observed a unique feature of the thermally stimulated depolarization (TSD) current in polypropylene (PP) films polarized in strong electric fields (up to 1–2 MV/cm). The direction of the TSD current is the same as the direction of the charging (polarizing) current, and the shape of the TSD current curve is much different from the standard curve. We show by computer model that these features can be explained by charge carrier injection in the film by charging from both electrodes. Both the charging curve shape and TSD current direction are explained by a model in which injected carriers form charged regions inside the film at a distance from the electrodes of one-fourth the thickness of the dielectric. Saint Petersburg State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 12–20, April, 1997.     

High-frequency electric-field trap for micron and submicron particles

Summary Four e-beam-processed, planar electrodes with gaps between 0.5 and 4 μm were used to create quadrupole electric-field trap. The electrodes were immersed in an aqueous particle suspension and driven by kHz to MHz signals of several volts amplitude. Micron and submicron particles could be stably trapped by negative dielectrophoresis. Latex beads of 1000, 600, 100 and 14 nm diameter could be concentrated between the electrodes (positive dielectrophoresis) or levitated as condensed cloud (negative dielectrophoresis). The results are surprising since polarisation forces depend on the volume of the particle and, up to now, it was expected that thermal forces would dominate the behaviour of particles with diameters <100 nm. However, micron-scaled electrode configurations allow the application of extremely strong fields (up to 20 MV/m) and open up new perspectives for microparticle handling and macromolecule trapping.     

Magnetic interactions and Preisach distributions of nanostructured barium hexaferrite

Barium hexaferrite (phase M) samples with different nanostructures were studied. Sample M1 is composed of nanocrystals of BaFe₁₂O₁₉ produced after milling the elemental oxides (Fe₂O₃ and BaCO₃) and heating in air atmosphere. Two more samples were produced by milling the same oxides and a 20% excess of -Fe. The resulting powder (composed of phase M and a 20% hematite) was heat-treated in different conditions, resulting in samples MF1 (with a partially sintered structure) and MF8 (with almost completely sintered structure).

In order to have an insight into the interactions in each sample, Preisach distributions were obtained using first-order reversion curves (FORCs) measurements. The Preisach distribution corresponding to M1 is a Gaussian-shaped one, with a maximum around 4.1 kOe. The distribution of MF1 has a narrow and high peak at 5.3 kOe, a number of overlapping small peaks down to 2.6 kOe and a distinct and low-intensity peak at 2 kOe. MF8 has a Preisach distribution with a succession of equally spaced distinct peaks from 4.2 to 1.5 kOe.

The found Preisach distributions suggest that the interactions occur among nanocrystals inside conglomerates with different number of particles.     

Effect of an electric field on the fracture of ferroelectric ceramic materials

This paper reports on the experimental results of measuring the time elapsed between the loading and the fracture of ferroelectric ceramic specimens under the action of a static electric field and mechanical stresses that differ in magnitude. The dependence of the durability of the specimens on the applied stress is determined for electric fields in the range from 0 to 5 MV/m. It is shown that, in the time range 1–10³ s, the durability of the ferroelectric ceramic material substantially increases in weak electric fields (the hardening effect) and significantly decreases in strong electric fields. The results obtained can be explained in terms of the fact that the load and the electric field affect the same defects (fracture nuclei) in the ferroelectric ceramics.     

Relation between the microstructural properties of electrical steels and the Preisach modelling

In this paper, the relation of the microstructural properties of electrical steels and the material parameters in the dynamic Preisach model is established. Particular experimental work was performed in order to separate the influence of grain size from the crystallographic texture. First, the parameters defined in the dynamic Preisach model has been fitted using the hysteresis loss characteristics. Next, we identified and separated the effect of average grain size from the crystallographic texture on each material parameter defined in the Lorentzian–Preisach distribution function. Here, the same texture dependence was identified for all kinds of magnetisation processes, i.e. for the irreversible and reversible parts of the quasi-static magnetisation and for the AC excess loss behaviour. Finally, the Preisach distribution function is rewritten explicitly including the grain size and crystallographic texture dependence.     

Magnetoelectric effect from the direct coupling of the Lorentz force from a brass ring with transverse piezoelectricity in a lead zirconate titanate (PZT) disk

A novel composite of brass ring and PZT disk shows a high dc magnetic field (H_dc) response when using the product effect of the Lorentz force effect from a metal ring in a dc magnetic field applied with ac electrical current, and the piezoelectric effect of piezoelectric material. The output voltage between the two faces of PZT shows a good linear response to the dc magnetic field (<1 kOe) under different ac electrical current inputs (<300 mA). The magnetoelectric coefficient is about ∼33.2 mV/T A. Simultaneously, the magnitude of its magnetoelectric coefficient can be manually controlled by an applied electrical current. This composite has the potential for applications in magnetoelectric transducers and sensors that work without coils even for static magnetic fields. PACS 85.80.Jm; 77.84.-S; 75.80.+q; 77.84.Dy; 77.65.-j     

Harmonic generation in superconducting Bi(Pb)SrCaCuO thick films

We present a detailed study of harmonic generation in Bi(Pb)SrCaCuO superconducting thick film prepared by screen printing technique. A comparative study of amplitude of harmonics of two films of different J_c have been carried out. The variation of amplitudes of the harmonics are studied as a function of magnitude of ac and dc field. The temperature dependence of amplitude of third harmonic (V³) is studied with increasing amplitude as well as frequency of ac field. These results are analyzed in the frame work of critical state model. V₃ -T curve also indicates the presence of two phases in Bi(Pb)SrCaCuO film.