High Bias Field Research Articles

Dynamics of glass-forming liquids. XIX. Rise and decay of field induced anisotropy in the non-linear regime.

For glycerol and three monohydroxy alcohols, we have measured the non-linear dielectric effects resulting from the application and removal of a high dc bias electric field. The field effects are detected by virtue of a small amplitude harmonic field, from which time resolved changes in the dielectric loss are derived. The changes in permittivity are dominated by modifications of the time constants (rather than amplitudes) which display two contributions: a heating-like decrease of relaxation times that originates from the time dependent field when the bias is switched on and off and a slowing down of the dynamics resulting from the field induced reduction of configurational entropy. As observed for the electro-optical Kerr effect, the rise of the entropy change is slower than its decay, a feature that we rationalize on the basis of the quadratic dependence of the entropy change on polarization. For glycerol, the observed steady state level of the field induced shift of the glass transition temperature (+84 mK) matches the expectation based on the entropy change and its impact on dynamics via the Adam-Gibbs relation (+88 mK). For the alcohols, these non-linear effects rise and decay on the time scales of the prominent dielectric Debye process, underscoring the relation of these features to polarization anisotropy, opposed to mechanical or enthalpy relaxation which are orders of magnitude faster in these systems. A model is discussed which captures the observed magnitudes as well as time dependences in a near quantitative fashion. It is demonstrated that the high bias field modifies the response of polarization to the ac field, including a temporary change in the low field susceptibility.

Effect of technological conditions on the magnetic and magnetoresistive properties of Fe20Ni80/Fe50Mn50 films

The effect several physical and technological factors have on the magnetic and magnetoresistive properties of Fe20Ni80/Fe50Mn50 multilayers is studied. The best technological conditions for preparing films with high exchange bias fields (∼30 Oe) and high anisotropy of the magnetoresistive effect (∼2%) are determined.

Exchange Bias Effect in Phase Separated La0.33Pr0.34Ca0.33MnO3 Thin Films

Exchange bias effect is observed in the phase separated La0.33Pr0.34Ca0.33MnO3 thin films. High exchange bias field of about 1 kOe is achieved at 4 K. The exchange bias effect in La0.33Pr0.34Ca0.33MnO3 thin films might originate from the intrinsic phase separation of the La0.33Pr0.34Ca0.33MnO3 or surface effect. The dependence of exchange bias effect on temperature, cooling field, and thickness is also investigated. This work would open an avenue to the application in the magnetic memory devices based on the phase separated manganites.

Enhancement of the magnetic interfacial exchange energy at a specific interface in NiFe/CoO/Co trilayer thin films via ion-beam modification

A series of ferromagnetic Ni80Fe20(55 nm)/antiferromagnetic CoO (25 to 200 nm)/ferromagnetic Co (55 nm)/SiO2(substrate) trilayer thin films were fabricated by ion-beam assisted deposition in order to understand the role of ion beam modification on the interfacial and interlayer coupling. The microstructural study using transmission electron microscopy, X-ray reflectometry, and polarised neutron reflectometry showed that ion-beam modification during the deposition process led to an oxygen-rich Co/CoO nanocomposite interface region at the bottom layer. This interface caused a high exchange bias field for the ferromagnetic cobalt. However, the exchange bias for top permalloy ferromagnet remained low, in line with expectations from the literature for the typical interfacial energy. This suggest that the ion-beam enhancement of the magnetic exchange bias is localized to the Co/CoO interface where local microstructural effects provide the dominant mechanism.

Magnetic behaviors in melt spun Fe52−xMn23+xGa25 (x = 0–3) ribbons

The effect of volume fraction of B2-type ferromagnetic (FM) phase on magnetic behavior of melt-spun Fe52−xMn23+xGa25 (x = 0, 1, 2, and 3) ribbons has been investigated. X-ray diffraction (XRD) results show that the volume fraction of the B2 phase is decreased, accompanied by the increased antiferromagnetic (AFM) fcc phase, with increasing x. The magnetization isotherms of these ribbons at different temperatures demonstrate that there exists a critical field HCR from AFM to FM state, and the HCR decreases and vanishes finally with increasing temperature. High exchange bias field (HEB) at 10 K through a cooling field at HCR from 300 K could be found and correlated to the volume fraction of B2-type FM phase. Both HCR and HEB are increased with decreasing the amount of B2-type FM phase. The HCR is increased from 20 kOe for x = 0 to 50 kOe for x = 3, and HEB is increased from 0.9 kOe for x = 0 to 2.5 kOe for x = 2. The latter phenomenon might be attributed to the change of exchange interactions at FM/AFM interfaces due to the change of proportion of FM and AFM phase at low temperature.

Estimation of intrinsic contribution to dielectric response of Pb0.92La0.08Zr0.52Ti0.48O3 thin films at low frequencies using high bias fields

Because most domain wall motion (extrinsic) is arrested at high bias fields, experiments were conducted to evaluate the lattice (intrinsic) contributions to the dielectric response of lead lanthanum zirconate titanate (PLZT) at 0-100 MV/m, 213-523 K, and 1-1000 kHz. The intrinsic contribution depended weakly on bias field and frequency, while the extrinsic contribution strongly depended on these same parameters as well as temperature. The threshold bias field required to suppress domain wall motion in PLZT thin films was ∼20-25 MV/m, and the intrinsic permittivity measured at those fields was ∼300-350, in agreement with literature values.

Theory of Low- and High-Field Transports in Metallic Single-Wall Nanotubes

Individual metallic single-wall carbon nanotubes show unsual non-Ohmic transport behaviors at high bias fields. For low resistance contact samples, the differential conductance dI/dV increases with increasing bias, reaching a maximum at $\sim$ 100mV. As the bias increases further, dI/dV drops dramatically [Yao et al., Phys. Rev. Lett. 84, 2941 (2000)]. The higher the bias, the system behaves in a more normal (Ohmic) manner. This so-called zero-bias anomaly is temperature-dependent (50--150K). We propose a new interpretation. Supercurrent runs in the graphene wall below $\sim$ 150K. The normal conduction-electron currents run outside the wall, which are subject to the scattering by phonons and impurities. The currents along the tube induce circulating magnetic fields and eventually destroy the supercurrent in the wall at high enough bias, and restore the Ohmic behavior. If the prevalent ballistic electron model is adopted, then the scattering effects cannot be discussed.

Ferroelectric, electrical, and magnetic properties of BiFe0.95Mn0.05O3 thin films epitaxially grown on conductive Nb:SrTiO3 and La0.7Sr0.3MnO3-buffered Nb:SrTiO3 substrates

BiFe0.95Mn0.05O3 (BFMO) thin films with different thicknesses have been epitaxially grown on -oriented Nb-doped SrTiO3 (NbSTO) and La0.7Sr0.3MnO3(LSMO)-buffered NbSTO substrates by pulsed laser deposition. At high bias field the space-charge-limited current (SCLC) is the dominant conduction mechanism for all BFMO films while at low bias field the Ohmic conduction is the predominant mechanism. An analysis of leakage current characteristics reveals that the ferroelectric properties are critically dependent on the density of defects in BFMO films. For the BFMO/LSMO/NbSTO structure, the coercive field of the BFMO film is much smaller than that of the BFMO film directly grown on the NbSTO substrate, which is attributed to the suppression of the substrate-induced clamping effect. For both BFMO/NbSTO and BFMO/LSMO/NbSTO structures, the ferroelectric hysteresis loops show no change under a magnetic field up to 9 T, which is explained in terms of weak ferromagnetic-ferroelectric coupling in the BFMO film and the very low magnetic-field-induced electric voltage drop across the LSMO layer.

Fabrication and dielectric property of ferroelectric PLZT films grown on metal foils

We have grown ferroelectric Pb 0.92La 0.08Zr 0.52Ti 0.48O 3 (PLZT) films on platinized silicon and LaNiO 3-buffered nickel substrates by chemical solution deposition using a sol–gel process based on acetic acid chemistry. The following measurements were obtained under zero-bias field: relative permittivity of ≈960 and dielectric loss of ≈0.04 on the PLZT film grown on Pt/Si substrates, and relative permittivity of ≈820 and dielectric loss of ≈0.06 on the PLZT film grown on LNO-buffered Ni substrates. In addition, a relative permittivity of 125 and dielectric loss of 0.02 were measured at room temperature under a high bias field of 1 × 10 6 V/cm on PLZT deposited on LNO-buffered nickel substrate. Furthermore, a steady-state leakage current density of ≈8.1 × 10 −9 A/cm 2 and mean breakdown field strength of 1.7 × 10 6 V/cm were measured at room temperature. Finally, remanent polarization ( P r ) of ≈2.0 × 10 −5 C/cm 2, coercive electric field ( E c ) of ≈3.4 × 10 4 V/cm, and energy density of ≈45 J/cm 3 were determined from room-temperature hysteresis loop measurements on PLZT/LNO/Ni film-on-foil capacitors with 250-μm-diameter platinum top electrodes.

Effect of graphene oxide nanomaterial in electroclinic liquid crystals

The collective dielectric relaxation studies have been carried out on the electroclinic liquid crystals (ELCs) doped with 0.1 to 0.2 wt % of graphene oxide (GO) in the frequency range of 20 Hz to 1 MHz. The GO favors for a good quality vertical alignment without any surface treatment of the substrates. The coupling of GO with indium tin oxide (ITO) substrate and ELC materials affects the molecular ordering and supports the ELC molecules to be aligned along the GO attached to the ITO surface in vertical direction. The vertical alignment can be changed to homogeneous by applying a high bias field to the sample and such converted homogeneous cell shows an additional dielectric relaxation peak in the low frequency side of Goldstone mode in SmC∗ phase due to presence of GO whereas in the pure material no such peak was observed. The frequency separation of both peaks (Goldstone mode and an additional peak) increases with temperature and low frequency peak vanishes near transition temperature.

Giant Persistent Photoconductivity of the WO3 Nanowires in Vacuum Condition.

A giant persistent photoconductivity (PPC) phenomenon has been observed in vacuum condition based on a single WO3 nanowire and presents some interesting results in the experiments. With the decay time lasting for 1 × 104 s, no obvious current change can be found in vacuum, and a decreasing current can be only observed in air condition. When the WO3 nanowires were coated with 200 nm SiO2 layer, the photoresponse almost disappeared. And the high bias and high electric field effect could not reduce the current in vacuum condition. These results show that the photoconductivity of WO3 nanowires is mainly related to the oxygen adsorption and desorption, and the semiconductor photoconductivity properties are very weak. The giant PPC effect in vacuum condition was caused by the absence of oxygen molecular. And the thermal effect combining with oxygen re-adsorption can reduce the intensity of PPC.

Terahertz emission characteristics of ErAs:InGaAs-based photoconductive antennas excited at 1.55 μm

We characterize ErAs:In0.53Ga0.47As superlattices as substrates for photoconductive terahertz emitters excited at 1.55 μm. The bandwidth of the emitted radiation is studied as a function of the superlattice period (or equivalently the electron lifetime) and the applied bias field. The results show that a variation in the electron lifetime from 0.2 to 6.3 ps does not considerably influence the bandwidth of the emitted radiation. However, the bandwidth increases linearly from 2.6 to 3.0 THz as the applied bias field is increased from 7 to 30 kV/cm. At higher bias fields, saturation is observed. The largest measured bandwidth is 3.1 THz.

Ionic Iridium(III) Complexes with Bulky Side Groups for Use in Light Emitting Cells: Reduction of Concentration Quenching

AbstractHere, the photophysics and performance of single‐layer light emitting cells (LECs) based on a series of ionic cyclometalated Ir(III) complexes of formulae $\left[ {{\rm Ir}\left( {{\rm ppy}} \right)_{\rm 2} \left( {{\rm bpy}} \right)} \right]^ + {\rm PF}_{\rm 6}^ -$ and $\left[ {{\rm Ir}\left( {{\rm ppy}} \right)_{\rm 2} \left( {{\rm phen}} \right)} \right]^ + {\rm PF}_{\rm 6}^ -$ where ppy, bpy, and phen are 2‐phenylpyridine, substituted bipyridine and substituted phenanthroline ligands, respectively, are reported. Substitution at the NˆN ligand has little effect on the emitting metal‐ligand to ligand charge‐transfer (MLLCT) states and functionalization at this site of the complex leads to only modest changes in emission color. For the more bulky complexes the increase in intermolecular separation leads to reduced exciton migration, which in turn, by suppressing concentration quenching, significantly increases the lifetime of the excited state. On the other hand, the larger intermolecular separation induced by bulky ligands reduces the charge carrier mobility of the materials, which means that higher bias fields are needed to drive the diodes. A brightness of ca. 1000 cd m−2 at 3 V is obtained for complex 5, which demonstrates a beneficial effect of bulky substituents.

Electrical properties of epitaxial SrTiO3 tunnel barriers on (001) Pt/SrTiO3 substrates

Coherent, epitaxial Pt films with root-mean-square surface roughness values of less than one unit cell were grown on (001) SrTiO3 substrates by dc sputtering. These Pt films served as bottom electrodes for epitaxial SrTiO3 tunnel barriers grown by rf magnetron sputtering. The SrTiO3 barriers were free of pinholes and showed mean surface roughness values of less than one unit cell. Barriers with a thickness of 4.5 nm showed excellent insulating properties and nonlinear current-voltage characteristics. At high bias fields, a reproducible hysteresis and deviation from the ideal tunneling behavior were observed.

MRI brain image segmentation and bias field correction based on fast spatially constrained kernel clustering approach

A fast spatially constrained kernel clustering algorithm is proposed for segmenting medical magnetic resonance imaging (MRI) brain images and correcting intensity inhomogeneities known as bias field in MRI data. The algorithm using kernel technique implicitly maps image data to a higher dimensional kernel space in order to improve the separability of data and provide more potential for effectively segmenting MRI data. Based on the technique, a speed-up scheme for kernel clustering and an approach for correcting spurious intensity variation of MRI images have been implemented. The fast kernel clustering and bias field correcting benefit each other in an iterative matter and have dramatically reduced the time complexity of kernel clustering. The experiments on simulated brain phantoms and real clinical MRI data have shown that the proposed algorithm generally outperforms the corresponding traditional algorithms when segmenting MRI data corrupted by high noise and gray bias field.

Influence of electric field on the ultrasound velocity in PZT ceramics

The paper deals with the influence of the electric bias field on the ultrasound velocity of various samples of Pb(Zr x Ti1−x )O3 (PZT) ceramics. The ultrasonic velocities were measured on commercial types of PZT ceramics as APC 841, APC 850, and APC 856, at room temperature. The comparison of the ultrasound velocities dependence on electric bias field was made for poled/unpoled soft ceramics. The ultrasonic pulse-echo technique was used in our experiment. The ultrasonic system is based on Matec Instruments, Inc. modules. The time domain response was recorded and time of flight between echoes was directly measured by digital oscilloscope, type Agilent 54622D. The high bias field was applied on disk samples by the high voltage amplifier type Trek 610D, using a special setup and sample holder. The sound velocity was found to change drastically near the coercive field for a PZT ceramics, where the velocity of longitudinal waves decreases with an increasing field while the velocity of shear waves increases, which is caused by the change of the elastic anisotropy under influence of the depolarization field. The consequent change of the piezoelectric contribution to effective elastic constant decreases the velocity of longitudinal waves and at the same time increases the velocity of shear waves around the coercive field.

Direct tunneling effective mass of electrons determined by intrinsic charge-up process

The electron effective mass for direct tunneling (EETM) in silicon dioxide (SiO 2) was determined by many authors using a high bias field for obtaining direct measurable direct tunneling current densities (TCDs), leaving some ambiguity to the electron kinetic energy at/within the SiO 2 barrier. A determination of the EETM by exploiting a long term intrinsic charge-up process of vacant defects adjacent to the SiO 2 barrier by capacitance–voltage ( CV) measurements is reported. The EETM is obtained by using the fixed charge sheet densities for iterating the TCD as a function of time. The zero bias field renders the average electron kinetic energy to converge on the thermal energy, leaving less ambiguity to the EETM. For an SiO 2 layer of d SiO 2 = 8.2 nm , an EETM of m eff = (0.3 ± 0.03) m 0 is obtained.

Self-Deflection of Dark Screening Spatial Solitons Based onHigher-Order Space Charge Field

The effects of higher-order space charge field on the self-deflection ofdark screening spatial solitons in biased photorefractive crystals arenumerically investigated under steady-state conditions. The expressionfor an induced space-charge electric field including higher-orderspace-charge field terms is obtained. Numerical results indicate thatdark solitons possess a self-deflection process during propagation, andthe solitons always bend in the direction of the c axis of thecrystal. The self-deflection of dark solitons can experienceconsiderable increase especially in the regime of high bias field strengths.

Dielectric behaviour of BaTiO3-based ceramic multilayer capacitors under high dc bias field

The dielectric behaviour of X7R-MLCCs with different active layers under high dc bias field (up to 6.25 MV m−1) has been investigated in the whole measurement temperature range from 213 to 453 K. The results show that as the dc bias field increases, the dielectric permittivity in the whole measurement range and the frequency dispersion at lower temperature are both suppressed, while dielectric loss initially decreases till the temperature reaches about 423 K and subsequently remains invariable. These results are attributed to the response of the core-shell structure in grains to the applied dc bias field. Furthermore, the dielectric permittivity of X7R-MLCCs with more dielectric layers is more significantly affected because of a more powerful pinning effect of the residual stress between the dielectric layer and the Ni electrode on the migration of the domain walls and the influence of created interfaces on dielectric properties of the dielectric materials close to the interface. The Curie peaks for two samples shift towards higher temperature at a rate 0.55 × 10−5 K m V−1 in the presence of the dc bias field. It is more significant that an interesting field-induced transition from paraelectric phase to ferroelectric phase in the shell part was found at about 323 K and Curie temperature.

A dielectric mode in electroclinic liquid crystals

The dielectric properties of electroclinic and ferroelectric liquid crystal materials have been investigated in the frequency range of 100Hz–10MHz. A dielectric mode has been predicted in electroclinic liquid crystals near the transition temperature of Sm-C*–Sm-A phase. It has been observed that the investigated material has nonlayer shrinkage near the transition temperature of Sm-C*–Sm-A phase and shows anomalous behavior of dielectric spectra, tilt, and texture which is entirely different from the behavior of ferroelectric liquid crystals (FLCs). The dielectric relaxation frequency and tilt angle are almost constant with respect to temperature near the transition temperature under high bias field, which is characteristically different from FLCs. The high dielectric permittivity near Tc owing to the presence of mode contribution leads to the fact that there is an intralayer phason variation few degrees before transition. The mode has been named random mode due to its origin from randomization of tilt near Tc. Tilt randomization has been considered as an order parameter because in both the phases molecules are tilted but it is disordered in Sm-A phase making its intralayer polarization zero.