Field Dependence Of Dielectric Permittivity Research Articles

Effect of Al3+ substitution on the magnetodielectric properties of the one-dimensional frustrated Ca3Co2O6

Ca3Co2-xAlxO6 (x = 0, 0.02, 0.04, 0.06, 0.08 and 0.1) were prepared via the conventional sol-gel method. The structure, magnetic, dielectric and magnetodielectric properties were investigated. The Rietveld structural refinement confirms the rhombohedral structure of the samples. The positive Curie-Weiss temperature θ and magnetic coupling exchange constants J indicate the dominant intrachain ferromagnetic interactions in the spin chains. The obvious inhibition effect to equally spaced magnetization steps demonstrates that Al3+ could release the magnetic frustration. With Al3+ substitution, both the magnetization intensity and the critical field of the ferrimagnetism to ferromagnetism transition decrease. The samples present obvious dielectric anomalies around the Curie temperature Tc and the freezing temperature Tf. Meanwhile, the magnetic field dependence of dielectric permittivity decreases suddenly at critical magnetic fields. These results reveal strong magnetodielectric coupling effect in Ca3Co2-xAlxO6. The same sign of Δεr and Δtanδ suggests that the magnetodielectric coupling effect is intrinsic.

Relaxor ferroelectric nature and magnetoelectric coupling of the one dimensional frustrated Ca3CoMnO6

The typical ↑↑↓↓ spin order of the one-dimensional frustrated Ca 3 CoMnO 6 breaks the inversion symmetry and results in the ferroelectricity. But the previous calculations predict a much larger ferroelectric polarization than the experimental observation. To comprehend the divergence, Ca 3 CoMnO 6 ceramics were prepared and the magnetic, dielectric properties were studied. In accordance with the formation of short range magnetic correlation, around the antiferromagnetic transition temperature T N the Ca 3 CoMnO 6 ceramics present weak relaxor ferroelectric nature. It implies the existence of polar nanoregions. Thus, the destruction of long range ferroelectric order and the formed polar nanoregions should be responsible for the suppressed macroscopic ferroelectric polarization. The activation energy and static freezing temperature of Ca 3 CoMnO 6 are also calculated. The obtained values are E a = 6.31 × 10 −3 eV and T f = 1.5 K, respectively. Both of them are lower about two orders of magnitude than the conventional relaxor ferroelectrics. Magnetic field dependent dielectric permittivity manifests the large magnetodielectric coupling of Ca 3 CoMnO 6 . Under the application of field of 7 T, the dielectric permittivity reduces by about 10%. • Around the antiferromagnetic transition temperature T N the Ca 3 CoMnO 6 ceramics present weak relaxor ferroelectric nature. • The Ca 3 CoMnO 6 ceramicspresent weak relaxor ferroelectric nature. • Magnetic field dependent dielectric permittivity manifests the largemagnetodielectric coupling.

Description of electric field-dependent dielectric permittivity in PMN ceramics

The temperature dependences of the linear and nonlinear components of the dielectric permittivity of PMN ceramics were determined as a function of bias electric fields. The influence of the applied bias electrical field, superimposed to the small signal oscillating electrical field, is investigated. A decrease in the real dielectric permittivity with increasing the applied bias electrical field was obtained for the investigated temperature range. The temperature dependence of the nonlinear dielectric response reveals an anomalous behavior, indicating a phase transition induced by the bias electric field. The present investigation revealed the influence of the bias electric field on the freezing temperature and activation energy. The results were discussed according to a spherical random bond–random field glassy model for relaxors.

Magnetoelectric properties of Co-doped BiFeO3 nanoparticles

The magnetoelectric (ME) properties of sol–gel grown BiFe[Formula: see text]Co[Formula: see text]O3 (BFCO) nanoceramics, with different sizes, were investigated at room-temperature. X-ray diffraction (XRD) measurement was performed to investigate structural properties of the samples understudy. Magnetic field-dependent dielectric permittivity has been systematically investigated in the frequency range of 20 Hz to 1 MHz. To ensure the origin of magnetodielectric response, the magnetoimpedance (MI) spectroscopy was adopted using equivalent circuit model. The a.c. conductivity was found to obey the Jonscher’s universal power law. The modifications in spiral spin structure in the BFCO nanoparticles with size less than [Formula: see text]62 nm significantly affect the ME coupling parameters.

Dielectric tunability in Pb(Sc1/2Ta1/2)O3 single crystals

The dc field dependence of dielectric permittivity in Pb(Sc1/2Ta1/2)O3 (PST) has been investigated near room temperature. The phase boundary on the temperature–field phase diagram in PST has been determined, where the ferroelectric critical endpoint (CEP) under the dc field applied along the [111]c direction (in the cubic coordinate) has been estimated to be between 20 and 25 °C. A high dielectric tunability of 92% in the field of 20 kV/cm along the [111]c direction has been obtained at 20 °C. The dielectric tunability above the critical temperature in the case of the first-order phase transition has been discussed on the basis of the Landau theory.

Observation of Resonant Quantum Magnetoelectric Effect in a Multiferroic Metal-Organic Framework.

A resonant quantum magnetoelectric coupling effect has been demonstrated in the multiferroic metal-organic framework of [(CH3)2NH2]Fe(HCOO)3. This material shows a coexistence of a spin-canted antiferromagnetic order and ferroelectricity as well as clear magnetoelectric coupling below TN ≈ 19 K. In addition, a component of single-ion quantum magnets develops below ∼ 8 K because of an intrinsic magnetic phase separation. The stair-shaped magnetic hysteresis loop at 2 K signals resonant quantum tunneling of magnetization. Meanwhile, the magnetic field dependence of dielectric permittivity exhibits sharp peaks just at the critical tunneling fields, evidencing the occurrence of resonant quantum magnetoelectric coupling effect. This resonant effect enables a simple electrical detection of quantum tunneling of magnetization.

Thermally assisted and magnetic field driven isostructural distortion of spinel structure and occurrence of polar order inCoCr2S4

We report appearance of polar order with a ordering temperature $({T}_{C})$ at 28 K, which is well below the ferrimagnetic order at 225 K $({T}_{N})$ for ${\mathrm{CoCr}}_{2}{\mathrm{S}}_{4}$. Intriguingly, the value of spontaneous electric polarization $(P)$ is $\ensuremath{\sim}122 \ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$ at 15 K, which is the second largest value in Cr octahedra-based spinels after ${\mathrm{CdCr}}_{2}{\mathrm{S}}_{4}$. Incidentally, the $P$ value is $\ensuremath{\sim}60$ times larger than the value of $P$ for the oxide counterpart ${\mathrm{CoCr}}_{2}{\mathrm{O}}_{4}$. The significant magnetoelectric coupling is verified from the magnetodielectric response and magnetic field dependent enhancement of $P$. We note that the field-dependent dielectric permittivity scales linearly to the squared magnetization in the low field regime below $\ensuremath{\sim}10$ kOe as described by the Ginzburg-Landau theory. Synchrotron diffraction studies over a wide temperature range, $15\text{--}300$ K, illustrate strong magnetoelastic coupling at ${T}_{N}$ and isostructural distortion at ${T}_{C}$. Analyses of the diffraction patterns reveal that the occurrence of polar order involves expansion of Co tetrahedra and contraction of Cr octahedra of the spinel structure and these distortions are further enhanced driven by the magnetic field. The delicate interplay between magnetoelastic, magnetoelectric, and electroelastic couplings in ${\mathrm{CoCr}}_{2}{\mathrm{S}}_{4}$ proposes the system as a potential candidate in multiferroics.

Self-consistent field model for strong electrostatic correlations and inhomogeneous dielectric media.

Electrostatic correlations and variable permittivity of electrolytes are essential for exploring many chemical and physical properties of interfaces in aqueous solutions. We propose a continuum electrostatic model for the treatment of these effects in the framework of the self-consistent field theory. The model incorporates a space- or field-dependent dielectric permittivity and an excluded ion-size effect for the correlation energy. This results in a self-energy modified Poisson-Nernst-Planck or Poisson-Boltzmann equation together with state equations for the self energy and the dielectric function. We show that the ionic size is of significant importance in predicting a finite self energy for an ion in an inhomogeneous medium. Asymptotic approximation is proposed for the solution of a generalized Debye-Hückel equation, which has been shown to capture the ionic correlation and dielectric self energy. Through simulating ionic distribution surrounding a macroion, the modified self-consistent field model is shown to agree with particle-based Monte Carlo simulations. Numerical results for symmetric and asymmetric electrolytes demonstrate that the model is able to predict the charge inversion at high correlation regime in the presence of multivalent interfacial ions which is beyond the mean-field theory and also show strong effect to double layer structure due to the space- or field-dependent dielectric permittivity.

Fatigue-free unipolar strain behavior in CaZrO3 and MnO2 co-modified (K,Na)NbO3-based lead-free piezoceramics

The unipolar fatigue behavior of CaZrO3 and MnO2 co-modified (K,Na,Li)(Nb,Ta)O3 lead-free piezoceramics was investigated systematically. The well-known charge agglomeration model is shown to explain the overall changes observed during unipolar fatigue, such as the development of bias field as well as the anisotropy in bipolar strain hysteresis and field-dependent dielectric permittivity. In addition, it is found that the unipolar strain exhibits only small degradation within 3% at the field amplitude of 2 kV/mm up to 107 cycles. This exceptionally good fatigue resistance is identified due to the presence of additional process, assigned as a “softening” effect that competes against the usual fatigue effect.

Magnetodielectric response of coexisting phases in half doped manganites

The magnetic field dependence of dielectric permittivity and resistivity has been measured simultaneously in two separate half doped manganites Pr0.75Na0.25MnO3 and Pr0.5Ca0.5Mn0.975Al0.025O3. A gigantic change in dielectric permittivity was detected near the percolation threshold. The hysteresis in dielectric permittivity as well as in resistivity supports the first order nature of the insulator-metal transition while the tunability is due to the coexisting of phases across the transition. A highly resistivity correlated dielectric behaviour has been observed throughout the insulator to metal transition evidencing the dielectric catastrophe phenomenon. The field dependent dielectric data are analyzed in the framework of Maxwell-Garnett theory in association with Mott-Hubbard model.

Dielectric Nanodroplets: Structure, Stability, Thermodynamics, Shape Transitions and Electrocrystallization in Applied Electric Fields

The response and properties of a dielectric nanodroplet with a 10 nm diameter made of formamide placed in a uniform electric field are explored with molecular dynamics simulations and an analytic free energy formulation. Increasing fields cause the initially spherical liquid droplet to undergo gradual prolate spheroidal elongation along the field direction that culminates in a shape instability and a first-order shape transition to a slender liquid needle at a field of ∼0.50 V/nm. This transition is accompanied by enhanced reorientation of the molecular dipoles along the field direction, with the elongated droplets exhibiting a field-induced ferroelectric state, unlike the state of the droplet under field-free conditions. For larger fields, we find further gradual enhancement of the molecular dipole reorientation and the onset of a first-order electro-crystallization transition at a field of 1.4375 V/nm. This transition is portrayed by a sharp increase in the positional order parameter, a significant decrease in the molecular diffusion, and further preferential reorientation of the dipoles. Both transitions exhibit hysteresis as the change in the electric field is reversed, that is, for decreasing fields. Data from the molecular dynamics simulation were used for determination of the field-dependent dielectric permittivity of the formamide droplet. An analytic dielectric continuum model is developed, and a free energy expression which includes polarization, surface, and saturation terms is formulated, yielding results for the variation of the droplet shape as a function of the applied field, or electric Bond number, in agreement with the results obtained from the molecular dynamics simulations. Proper inclusion of dielectric saturation effects in the continuum model, expressed in terms of an electric field-dependent dielectric permittivity, is found to play an important role.

Accurate Simulations of Electric Double Layer Capacitance of Ultramicroelectrodes

This paper aims to develop rigorous and accurate numerical tools for simulating electric double layers formed near ultramicroelectrodes. It also aims to assess the validity of existing models and to identify the dominant physical phenomena that must be accounted for. The electric double layer capacitance was numerically predicted for spherical ultramicroelectrodes of various radii in aqueous electrolyte. The model accounted for the Stern and diffuse layers, the finite size of ions, and the dependency of the electrolyte dielectric permittivity on the local electric field. This study reveals that models reported in the literature suffer from severe limitations. First, it demonstrates that the electrolyte field-dependent dielectric permittivity significantly affects the predicted Stern layer and total specific capacitances and must be accounted for. The finite ion size and the Stern layer also need to be considered in simulating electric double layers under high concentrations and surface potential. This study also establishes that the Helmholtz model predicts the Stern layer capacitance for all electrode radii if the electrolyte permittivity is assumed to be constant. However, it underestimates the Stern layer capacitance for sphere radii less than 40 nm when accounting for field-dependent permittivity. The electrode curvature was found to have negligible effect on the predicted specific capacitance for sphere radii larger than 40 nm. Then, the latter is equal to that of planar electrodes. Results of this study can be used to design electrodes for electrochemical sensors and electrical energy storage.

Modeling saturated and unsaturated ferroelectric hysteresis loops: An analytical approach

In ferroelectric materials, hysteresis behavior is very difficult to model due to its nonlinear and history-dependent characteristics. Among approaches that are able to describe unsaturated loops, many of them are either very complicated (numerical procedures must be employed) or the resulting loops contain some undesirable or defective features. In this work, a simple hysteresis model based on a special construction of the Preisach function is proposed. Explicit expressions for the polarization-field (P-E) responses under increasing and decreasing applied fields have been derived. The saturated and unsaturated P-E loops can be conveniently calculated by piecing together such responses. The technique is widely applicable to the modeling of ferroelectric hysteresis behavior of ceramics and polymers. As examples we study the applied field dependence of dielectric permittivity of a ferroelectric film and the remanent polarization of ferroelectric composites after ac poling. We find that the model predictions agree well with the experimental results.

Microstructure Evolution of Pulsed Laser‐Deposited (Ba, Sr)TiO3 Films on MgO for Microwave Applications

To develop low‐loss tunable microwave circuits, based on the field dependence of dielectric permittivity, phase pure (Ba0.5, Sr0.5)TiO3 doped with 1% W (BST) thin films 0.3‐μm thick were deposited on single crystal MgO wafers by pulsed laser deposition. The BST films were characterized by X‐ray θ–2θ scans and pole figure analysis, field emission scanning electron microscopy (FESEM), and cross‐sectional transmission electron microscopy (TEM), coupled with selected‐area electron diffraction (SAED). Although, the X‐ray θ–2θ scan indicated an epitaxial nature of BST with an out‐of‐plane orientation of (100), the pole figure analysis confirmed the presence (4–6%) of (111)‐oriented grains in a matrix of (100) textured grains. The columnar grains exhibited an in‐plane (i.e., along the plane perpendicular to the growth direction) grain size that was thickness‐dependent. The cross‐sectional TEM, coupled with SAED in the thickness direction, corroborated the pole figure analysis. Additionally, from X‐ray analysis, it was observed that the textured films were under in‐plane tension. The deposited film was characterized at microwave frequencies (1–20 GHz) using interdigitated electrodes deposited on top of the film. The film was characterized by a relatively low dielectric Q of 5–7. A 17% change in the capacitance was observed when applying a 40 V bias. From the observed microstructure, a preliminary understanding of its evolution and its relationship with the microwave dielectric properties is discussed, and some ideas to obtain truly epitaxial BST films are presented.

Orientation and direct current field dependent dielectric properties of bulk single crystal SrTiO3 at microwave frequencies

The dielectric properties of bulk single crystal SrTiO3 (STO) are studied in the temperature range 30–300 K, under dc field 0–10 kV/cm at 1.0 kHz and in the frequency range 0.3–2.0 GHz. Two sets of circular disk shaped parallel plate resonators have been fabricated and tested experimentally. In the first set thin YBa2Cu3O7 films have been epitaxially grown on both surfaces of [100] STO disks. In the second set, three crystal orientations of SrTiO3 [100], [110], and [111] have been used to make parallel-plate resonators with evaporated Cu/Ti electrodes. No peculiarities in field dependent dielectric permittivity are detected for STO with smaller densities of impurities. For samples with larger impurity densities double loop hysteresis is observed in the dc field dependent permittivity. For all orientations the losses are minimum at temperatures about 50–55 K. The losses at microwave frequencies increase with the applied dc field regardless of the orientation of the STO crystal. At relatively high dc fields (depending on the temperature) the losses start decreasing slightly.

Double loop hysteresis in direct current dependent dielectric permittivity of SrTiO3

Experimental double loop hysteresis in dc field dependent dielectric permittivity in bulk single crystal SrTiO3 is reported. Small signal measurements of the permittivity are performed in the frequency range 0.5–1.5 GHz at temperatures below 77 K using electrically thin circular parallel-plate resonators. The dielectric permittivity is extracted form the measured resonant frequency. The double loop hysteresis may be caused by field induced local paraelectric/ferroelectric phase transitions, and/or switching (polarization reversal) in local opposing ferroelectric domains associated with symmetry breaking impurities/structural defects.

Structural and electrical characteristics of Pb0.90La0.15TiO3 thin films on different bottom electrodes

Pb 0.90 La 0.15 TiO 3 (PLT15) thin films were deposited by the sol-gel method on Pt, Pt/Si, and RuO2 on Si and Pt/Si bottom electrodes. X-ray diffraction, micro-Raman spectroscopy, and atomic force microscopy techniques were used for structural characterization of these films. PLT15 films on different electrodes showed good surface morphology with dense and uniform microstructure. PLT15 films on solution derived RuO2 bottom electrodes show (100) preferred orientation of growth and result in larger crystallites. Films deposited on a Pt bottom electrode show sharp and intense Raman features indicating better crystallinity and insignificant film–electrode interactions. PLT15 film on a Pt bottom electrode exhibited higher dielectric constant (1300 at 100 kHz) and high values of Pm and Pr, 68 and 46 μC/cm2, respectively, compared to other electrodes. Films on a RuO2 bottom electrode showed relatively inferior dielectric and ferroelectric properties. The ac field dependence of dielectric permittivity at subswitching fields was fitted using the Rayleigh law. It was found that ∼22% of the total measured permittivity was due to irreversible domain wall displacement for the films on a Pt electrode. The reversible polarization components estimated from the capacitance–voltage (C–V) and quasistatic hysteresis measurements showed that Prev/Psat at Vmax for the case of Pt/Si (24%), was larger than that of Pt (11%) bottom electrodes. The observed results were correlated with the domain wall pinning at the disturbed film–electrode interface.

Effect of bottom electrodes on the structural and electrical characteristics of lanthanum doped lead titanate thin films

Ferroelectric Pb0.90La0.15TiO3 (PLT15) thin films were deposited by sol-gel method on Pt, Pt/Si, ruthenium oxide (RuO2)/Si and RuO2/Pt/Si bottom electrodes. X-ray diffraction, and micro-Raman spectroscopy techniques were used for structural characterization of these films. PLT15 films deposited on RuO2 electrodes show (100) preferred orientation of growth and result in larger crystallites. Films on Pt electrode exhibited higher dielectric constant (1300 at 100 kHz) and high values of Pm, Pr values, 68 and 46 μC/cm2 respectively. AC field dependence of dielectric permittivity at sub-switching fields was fitted using the Rayleigh law. The reversible polarization components estimated from the CV and quasi-static hysteresis measurements for films on Pt/Si (24%), was larger than that of Pt (11%) bottom electrodes. The observed results were correlated with the domain wall pinning at the disturbed film-electrode interface.

Fatigue and nonlinear dielectric response in sol-gel derived lead zirconate titanate thin films

The evolution of switching polarization (Pr s) suppression, in Pb(Zr, Ti)O3 films, induced by two different electric treatments is addressed in terms of the nonlinear dielectric response. The ac electric field dependence of dielectric permittivity was collected at different fatigue states and the Rayleigh law was used to interpret the fatigue dependence of the nonlinear response. This approach has enabled us to distinguish between two fatigue states induced by the different electric treatments.

Field-dependent dielectric permittivity of paraelectric superlattice structures

SrTiO 3/SrZrO 3 and SrTiO 3/BaTiO 3 superlattice structures have been grown by pulsed laser deposition, and their dielectric properties have been measured at room temperature and at 77 K. The electric field-dependence of the permittivity of both types of structures exhibits a different behavior from that observed in the corresponding solid solutions Sr(Ti x Zr 1− x )O 3 and (Ba x Sr 1− x )TiO 3, respectively. For the case of the SrTiO 3/SrZrO 3 superlattices, the data is compatible with the assumption that strain induces ferroelectricity in the SrTiO 3 films at room temperature.