Function Of Dc Bias Field Research Articles

Electromechanical properties of uniaxial polar ionic plastic crystal [(C2H5)4N][FeBrCl3

Abstract Ferroelectric plastic crystals are an emerging class of materials that combine room temperature ferroelectricity and piezoelectricity with a high temperature plastic mesophase prior to melting. These materials offer possibilities for accessing different property parameter spaces from the state-of-the-art metal oxide and polymer ferroelectrics. Tetraethylammonium bromotrichloroferrite, [(C2H5)4N][FeBrCl3], has a unipolar wurtzite-like structure and thus may have potential for small but stable piezoelectric coefficients like the iso-symmetrical AlN. In this study, density functional theory was used to compute elastic compliance, piezoelectric coefficients, and dielectric constant values. Single crystals grown from aqueous solutions were evaluated via single crystal synchrotron x-ray diffraction, impedance spectroscopy and high and weak-field electromechanical characterization. Diffraction studies revealed that the anion tetrahedra orientated preferentially so that the Br− ion had a 30% alignment with the polarization vector. Electromechanical measurements found piezoelectric coefficients in the 5–9 pC N−1 and pm V−1 range. The piezoelectric coefficient (d 33) was most stable with 3.4% variation between 0.4 and 90 Hz and 0.5 and 3 V. Additional piezoelectric stability measurements were made as a function of DC bias field and temperature. Impedance measurements indicate contributions from either intrinsic effects unique to ionic plastic crystals, such as molecular rotation, or the extrinsic effect of electrode interfaces, both of which can play a role in the electromechanical response of the materials. The results show that [(C2H5)4N][FeBrCl3] has potential as a small signal piezoelectric that has a softer elastic moduli than AlN but a stiffer moduli than polyvinylidene fluoride, and thus occupies a unique parameter space.

Thermally Induced Magnetic Anisotropy in Nickel Films on Surface Acoustic Wave Devices

The combination of magnetostrictive thin films with surface acoustic wave (SAW) devices enables the magnetic sensors that exploit the AE-effect (magnetically induced change in modulus) without the need for free-standing structures that are susceptible to vibration and damage. The performance of these sensors is strongly influenced by magnetic anisotropy and, therefore, the state of stress in the magnetostrictive film. We report the fabrication and characterization of magnetic SAW devices comprised of a magnetostrictive layer and interdigital transducers on a piezoelectric substrate. The state of stress in the magnetostrictive layer is varied by annealing. Vibrating-sample magnetometer measurements indicate that the annealed devices show a strong uniaxial anisotropy with the easy axis parallel to the direction of acoustic propagation. The induced magnetic anisotropy is attributed to the anisotropic thermal expansion in the substrate. The change in frequency of one-port resonators and two-port SAW oscillators is measured as a function of dc bias field and ambient temperature. The normalized change in frequency (Δf/f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> ) is on the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> . These results demonstrate that the film stress can be used to optimize the performance of magnetic SAW devices and the SAW devices can be used to characterize the state of stress and magnetic anisotropy of magnetostrictive films.

Hysteresis loss analysis of soft magnetic materials under direct current bias conditions

Direct current bias related hysteresis loss characteristics of three commercially available magnetic materials: (1) an iron based Metglas tape core, (2) a Sendust powder core, and (3) a Mn-Zn based ferrite in both un-gapped and gapped configurations were studied. The measurements are conducted for a fixed external field Hext, a fixed flux swing (ΔB), and a fixed maximum forward magnetization (Bmax) as a function of the external bias field. In all the measurements, a direct correlation is found between permeability and measured loss values as a function of dc bias field. Increased hysteresis losses are measured in the magnetization rotation region in which classical domain theory predicts minimal losses. The observed trends are discussed within the frame work of classical domain theory.

Electric-field dependence of dielectric properties of sol-gel derived Ba(Zr0.2Ti0.8)O3 ceramics

The effect of a dc bias field on the diffuse phase transition and nonlinear dielectric properties of sol-gel derived Ba(Zr0.2Ti0.8)O3 (BZT) ceramics are investigated. Diffuse phase transitions were observed in BZT ceramics and the Curie–Weiss exponent (CWE) was γ∼2.0. The dielectric constant versus temperature characteristics and the γ in the modified Curie–Weiss law, e−1=em−1[1+(T−Tm)γ/C1](1≤γ≤2), as a function of the dc bias field was obtained for BZT ceramics. The results indicated that γ is a function of dc bias field, and the γ value decreased from 2.04 to 1.73 with dc bias field increasing from 0 to 20 kV/cm. The dielectric constant decreases with increasing dc bias field, indicating a field-induced phase transition. The dc bias field has a strong effect on the position of the dielectric peak and affects the magnitude of the dielectric properties over a rather wide temperature range. The peak temperature of the dielectric loss does not coincide with the dielectric peak and an obvious minimum value for the dielectric loss at the temperature of the dielectric peaks is observed. At room temperature, 300 K, the high tunability (K=80%), the low loss tangent (≈0.01) and the large FOM (74), clearly imply that these ceramics are promising materials for tunable capacitor-device applications.

High dielectric constant and tunability of strontium titanate ceramics modified bychromium doping

The incorporation of chromium on the strontium site of the perovskite lattice ofSrTiO3 ceramics and its influence on the structural, microstructural andlow-frequency dielectric properties is studied in this work. DenseSr1−1.5xCrxTiO3 (x = 0, 0.0005,0.0010, 0.0015) ceramics are prepared from powders obtained by a sol–gel method. The lattice parameter∼3.906 Å and averagegrain size ∼2.5 µm are almost identical for all the ceramic samples under study. On theother hand, the dielectric constant increases from approximately 7000 forx = 0 to>16 000 for x = 0.0005. Detailed investigations of the low-temperature dielectric properties show a remarkableincrease of the tunability and nonlinearity of the dielectric constant as a function of dc biasfield in strontium titanate ceramics modified by chromium doping but no ferroelectric orrelaxor-like behaviour. These results imply the possible incorporation of small Cr amountson the Sr site of strontium titanate ceramics and expand the tunable applications ofSrTiO3-based materials.

Bias Field Effect on Dielectric Response in the Region of Phase Transitions in (Pb,La)(Zr,Sn,Ti)O3 Ceramics

The dielectric response of (Pb 0.97 La 0.02 ) (Zr 0.53 Ti 0.12 Sn 0.35 )O 3 (PLZTS-2/53/12/35) ceramics was measured as a function of DC bias field E =. As the bias field intensity increases the values of ϵ′ max and ϵ” max are found to shift towards a lower temperature. A possible coexistence of the ferroelectric and antiferroelectric phases over a wide range of temperatures in the PLZTS-2/53/12/35 samples is assumed to explain the observed features of ϵ′ (T), ϵ” (T), and ϵ′ (E=) curves.

Tunable Dielectric Characteristics of 0.9Pb(Fe1/2Nb1/2)O3/0.1CaTiO3

Tunable dielectric characteristics of 0.9Pb(Fe1/2Nb1/2)O3/0.1CaTiO3 relaxor ferroelectric ceramics were investigated as the function of DC bias field and temperature. High tunability (more than 20%) was obtained in the present ceramics under a relatively weak DC bias field (1.4 kV/cm). The value of tunability changed from negative to positive with increasing DC field. For the simulation of the dielectric constant under DC bias field, modified model needs to be constructed for the relaxor ferroelectrics. The dielectric constant curve as a function of temperature without and under DC field was well fitted using the equation of diffuse phase transition. The lower emax, higher Tmax and higher diffuseness parameter under DC field were observed and only the lower emax contributed to positive tunability.

Effect of dc bias on the Curie–Weiss exponent in 0.76Pb(Mg1∕3Nb2∕3)O3–0.24PbTiO3 ferroelectric single crystal

Permittivity versus temperature characteristics and Curie–Weiss exponent (CWE) γ in the universal Curie–Weiss law [ε−1=εm−1[1+(T−Tc)γ∕(2δ2)](1⩽γ⩽2)] as a function of dc bias field were obtained for ⟨001⟩, ⟨011⟩, and ⟨111⟩ oriented 0.76Pb(Mg1∕3Nb2∕3)O3–0.24PbTiO3 single crystals. Results indicated that γ is a function of dc bias field and three different oriented crystals show slight different γ values but the similar dc field dependence.

Field dependence of dielectric properties of BST single crystals

(Ba1-xSrx)TiO3 single crystal fibers with x = 0.35 and 0.40 were grown, using the laser heated pedestal growth (LHPG) technique, in order to investigate the dielectric properties under dc bias for potential tunable capacitor application at room temperature. Temperature dependences of the dielectric constant and loss factor as a function of dc-bias field were measured. At room temperature and 5000V/cm, the changes on the dielectric constant were 45% and 20% for x = 0.35 (Tc≈20°C) and x = 0.40 (Tc≈4°C), respectively. Low dielectric loss (10−3-10−2) around RT, with and without the bias field, were observed for the composition x = 0.40. This composition although, showed distribution of phases generated during the melt growth process and that disturbed, to some extent, the field dependence behavior in the para-electric phase.

Inductive measurement of ultrafast magnetization dynamics in thin-film Permalloy

An inductive technique for the measurement of dynamical magnetic processes in thin-film materials is described. The technique is demonstrated using 50 nm films of Permalloy (Ni81Fe19). Data are presented for impulse- and step-response experiments with the applied field pulse oriented in the plane of the film and transverse to the anisotropy axis. Rotation times as short as 200 ps and free oscillations of the magnetization after excitation are clearly observed. The oscillation frequency increases as the dc bias field parallel to the anisotropy axis increases as predicted by classical gyromagnetic theory. The data are fitted to the Landau–Lifshitz equation, and damping parameters are determined as a function of dc bias field. Damping for both impulse and step excitations exhibits a strong dependence on bias field. Damping for step excitations is characterized by an anomalous transient damping which rapidly increases at low dc bias field. Transformation of the data to the frequency domain reveals a higher order precessional mode which is also preferentially excited at low dc bias fields. A possible source for both phenomena is precessional mode saturation for large peak rotations. The technique has the potential for 20 ps resolution, although only 120 ps resolution is demonstrated due to the limited bandwidth of the waveguides used.

Flux trapping and its effect on AC susceptibility of granular superconductors

Experimental data on the real (x′) and imaginary (x″) parts of AC magnetic susceptibility as a function of DC bias field (H) shows the effect of trapped flux in granular highT c superconductors. The aim was to substantiate our recent theoretical findings on the basis of a two-component critical state model suitable for granular highT c superconductors. Stress has been given to understanding the origin of hysteresis inx′(H) andx″(H). It was seen in the experimental data that above a certain value of DC field range irreversibility appears inx′(H) andx″(H) creating hysteresis like loops. Comparison of data with calculated loops shows good agreement.x′(H) andx″(H) curves show considerable asymmetry in presence of trapped flux.

Electrostrictive Coefficients of Pb(Mg1/3Nb2/3)O3 Ceramics

The electrostrictive coefficients Q33 and Q13 of Pb(Mg1/3Nb2/3)O3 ceramics have been precisely determined over a wide temperature range from -110°C to 100°C, using three different methods: strain gauge, piezoelectric resonance and interferometric techniques. A quadratic relation between induced strain and electric polarization holds even in the ferroelectric phase, as well as in the paraelectric phase. The electrostrictive coefficients are nearly temperature-independent with values of Q33=2.3(±0.2)×10-2 m4C-2 and Q13=-0.64(±0.05)×10-2 m4C-2. The field-induced piezoelectric coefficients d33 and d31 were also measured at room temperature as a function of dc bias field. The values of d33/ε3 and d31/ε3 are proportional to the induced electric polarization. The electrostrictive coefficients calculated from the slopes are in good agreement with those obtained by the strain-gauge method.