Hysteresis phenomena in the course of polarization evolution in a sinusoidal electric field in lead magnesium niobate crystals

The processes of polarization evolution in single crystals of the PbMg_1/3Nb_2/3O₃ model ferroelectric relaxor in a sinusoidal electric field are investigated at temperatures near and above the temperature T _d 0 of destruction of the induced ferroelectric state upon heating in zero electric field. The polarization switching current loops are measured in the ac electric field applied along the 〈111〉 and 〈110〉 pseudocubic directions. The electroluminescence intensity loops are obtained under the combined action of ac and dc electric fields applied along the 〈100〉 direction. In a certain temperature range above T _d 0 and the freezing temperature T _f in lead magnesium niobate, there are electric current anomalies, that correspond to the dynamic formation and subsequent destruction of the ferroelectric macroregions throughout each half-cycle of the ac electric field. The measurements of electroluminescence hysteresis loops demonstrate that the observed depolarization delay (related to the ac electric field amplitude) increases with an increase in the dc electric field and decreases as the ac field amplitude increases. The nature of the observed phenomena is discussed.     

Polarization kinetics of a photosensitive relaxor ferroelectric

Polarization switching in alternating quasi-static electric fields of frequency 10^?4 Hz and polarization relaxation in dc fields were studied in a photosensitive La-and Ce-doped barium-strontium niobate relaxor ferroelectric. Experimental data obtained in the thermal activation stage of the relaxation were used to reconstruct the relaxation time distribution spectrum. The characteristics of the polarization kinetics of an illuminated and a dark crystal are compared. It is shown that, in the crystal illuminated by light, the photoconductivity compensates for random electric and depolarization fields, thereby giving rise to a growth in amplitude of the dielectric hysteresis loops in the polarization versus field relation and to longer polarization relaxation times or increased heights of the potential barriers separating stable states from metastable states.     

Multiferroic magnetoelectric coupling and relaxor ferroelectric behavior in 0.7BiFeO3-0.3BaTiO3 nanocrystals

The structural, microstructural, polarization, magnetization, dielectric constant, and relaxor characteristics of 0.7BiFeO₃-0.3BaTiO₃ (BF-BT) nanocrystals have been studied. BF-BT nanocrystals were prepared by a chemical route using polyvinyl alcohol as surfactant. The phase structure is confirmed by X-ray diffraction and average particle size by transmission and scanning electron microscopy. The magnetoelectric coupling is studied by polarization hysteresis loops under the influence of applied magnetic field and the phase transition anomaly. The diffuse phase transition is studied by modified Curie-Weiss law and relaxor characteristics by Vogel-Fulcher relation.     

Anomalies of polarization processes in relaxor ferroelectrics

The anomalous polarization of relaxor ferroelectrics (relaxors), such as solid solutions of barium-strontium niobate (SBN) and lead magnesium niobate (PMN), is studied in ac low-frequency (up to 10^?4 Hz) and dc electric fields. The results of studying dielectric hysteresis loops, polarization relaxation, the coercive-field distribution spectrum, nonlinear polarization for various scales of inhomogeneities, and polarization in a photosensitive SBN relaxor subjected to illumination are presented. All of the anomalous polarization properties of relaxors substantially distinguish them from ordinary homogeneous ferroelectrics and serve as signs and a measure of structural disorder. This article presents a brief review of our recent studies and some new results obtained as a result of their analysis.     

Investigation of ferroelectric properties of strontium barium niobate crystals by second harmonic generation technique

The ferroelectric properties of Sr _x Ba_{1 − x} Nb₂O₆ (SBN-x) crystals have been investigated using noncollinear (diffuse) second harmonic generation of IR radiation converted by a disordered domain structure. The switching characteristics have been determined from the dependences of the second harmonic intensity I _2ω(t) and I _2ω(E) for the SBN-0.75 crystals. These characteristics are in good agreement with the results of measurements by traditional methods and exhibit a number of specific features (long polarization relaxation times, irreproducibility of the shape of hysteresis loops, low-frequency dependence of the quantity E _c ) associated with the relaxor nature of SBN solid solutions. The kinetics of the diffuse second harmonic generation can be described in the framework of the phenomenological approach to polarization relaxation processes in relaxor ferroelectrics. The intensity of the diffuse second harmonic in switching is determined by the parameters (duration, amplitude) of the switching field pulse.     

Effect of illumination on magnetization and microwave absorption of La1−xCaxMnO3 (x<0.2) films

Light-induced changes of the hysteresis loops of magnetization and microwave absorption are investigated in low-doped La_1−xCa_xMnO₃ (x<0.2) thin films. The width of the hysteresis loops decreases clearly under illumination with visible or near-infrared light at temperatures below 50 K. The microwave conductivity has a minimum value at magnetic fields corresponding to the magnetization reversal and is shifted towards weaker fields under illumination. These effects show complex nonexponential time evolution and dependence on strength of the magnetic field. The results can be explained by assuming that small ferromagnetic metallic regions exist within the insulating ferromagnetic phase of the sample, and that these regions are expanded by optically induced charge transfer between Jahn–Teller split e_g states of neighboring Mn³⁺ ions. Decrease of the Mn³⁺ XPS core level spectrum is observed in the samples under illumination with a HeNe laser.     

Relaxor behaviour in PZN-PT-BT ferroelectric ceramics

The relaxor behaviour of the (Pb_0.8Ba_0.2)[(Zn_1/3Nb_2/3)_0.7Ti_0.3]O₃ ferroelectric ceramic is presented. A strong dispersion of the maximum of the dielectric permittivity (ε′) below the transition temperature (T_m) is observed, which shift towards higher temperatures with increasing frequency. There is a strong deviation from the Curie-Weiss law. The results are fitted by using the Volger-Fulcher relationship, showing typical behaviour of a spin glass system. The hysteresis loops suggest relaxor behaviour.     

Reversible permittivity of a photosensitive relaxor ferroelectric

The reversible permittivity of barium strontium niobate, a photosensitive relaxor ferroelectric, doped with lanthanum and cerium is investigated in the dark and under illumination with a power density of 0.22 mW/cm². The permittivity is measured under a combined effect of a weak ac electric field at a frequency of 1 MHz and a slowly varying periodic field E _b with an amplitude of ±2.3 kV/cm. It is shown that the illumination significantly increases the permittivity, changes the dependence of the permittivity on the periodic field E _b , and eliminates the possible unipolarity of the crystal, thus significantly improving the reproducibility of the permittivity during repeated measurements.     

Synthesis and characterizations of BNT–BT–KNN ceramics for energy storage applications

Dielectric, ferroelectric and piezoelectric properties of the (0.94–x) Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–xK_0.5Na_0.5NbO₃/BNT–BT–KNN ceramics with x = 0.02 and 0.05 (2KNN and 5KNN) were studied in detail. Dielectric study and temperature-dependent polarization hysteresis loops indicated a ferroelectric-to-antiferroelectric transition at depolarization temperature (T_d). The low T_d in both the ceramic samples suggested the dominant antiferroelectric ordering at room temperature (RT), which was also confirmed by RT polarization and strain hysteresis loops studies. Antiferroelectric-to-paraelectric phase transition temperature (T_m) was nearly same for both systems. The 5KNN ceramic samples showed the relaxor behaviour. The values of the dielectric constant, T_d, and maximum strain percentage increased, whereas the coercive field and remnant polarization decreased with the increase of the KNN percentage in the BNT–BT–KNN system. High-energy storage density ～0.5 J/cm³ at RT hinted about the suitability of the 5KNN system for energy storage applications.     

Voltage shift of hysteresis loops of SrBi2Ta2O9 thin films under unipolar stress

Voltage shifts of hysteresis loops of metalorganic decomposition (MOD)-derived SrBi₂Ta₂O₉ (SBT) thin films, known as imprint, have been observed after exposing the thin-film capacitors to unipolar pulses. The voltage shift changes with cumulative total time at maximum voltage, following a relationship with no pulse-width dependence. The origin of the voltage shift is briefly discussed in terms of an internal bias field induced by injected electrons trapped at positive polarity. The pulse-measurement responses are greatly affected by the internal bias field, even though no imprint failure was observed up to 10¹⁰ unipolar pulses. The voltage shift and asymmetric properties can be removed easily by applying bipolar pulses of saturation amplitude. Received: 27 June 2000 / Accepted: 16 August 2000 / Published online: 5 October 2000     

Polarization and depolarization of relaxor ferroelectric strontium barium niobate

The anomalies of polarization (mismatch of hysteresis loops for several first repolarization cycles, the absence of specific coercive field, etc.) in quasi-static and dc electric fields have been found in the strontium barium niobate relaxor ferroelectric. The anomalies are associated with a highly inhomogeneous structure of the crystal, which is considered a clearly defined nonergodic system with a random distribution of strong local internal fields. The energy distributions of potential barriers for the polarization and the depolarization are obtained at different electric field strengths and temperatures.     

Phase transition properties for the kinetic Ising model in an oscillating magnetic field

The phase transition properties for the kinetic Ising model in a sinusoidally oscillating magnetic field are investigated by means of the mean-field calculations. The integrated phase diagrams for not only the amplitude of the oscillating field versus temperature but also the frequency of the oscillating field versus temperature have been calculated respectively, in which the discontinuous and continuous transitions are observed. Meanwhile the hysteresis loops in relation to the two kinds of the phase diagrams have been analyzed systematically, and the effects of the amplitude and the frequency of the oscillating field on the hysteresis loops are illustrated.     

Magnetic,magnetoelectric and dielectric behavior of CoFe2O4–Pb(Fe1/2Nb1/2)O3 particulate and layered composites

Magnetic, magnetoelectric and dielectric properties of multiferroic CoFe₂O₄–Pb(Fe_1/2Nb_1/2)O₃ composites prepared as bulk ceramics were compared with those of tape cast and cofired laminates consisting of alternate ferrite and relaxor layers. X-ray diffraction analysis and Scanning Electron Microscope observations of ceramic samples revealed two-phase composition and fine grained microstructure with uniformly distributed ferrite and relaxor phases. High and broad maxima of dielectric permittivity attributed to dielectric relaxation were found for ceramic samples measured in a temperature range from −55 to 500 °C at frequencies 10 Hz–2 MHz. Magnetic hysteresis, zero-field cooled (ZFC) and field cooled (FC) curves, and dependencies of magnetization on temperature for both magnetoelectric composites were measured with a vibrating sample magnetometer in an applied magnetic field up to 80 kOe at 4–400 K. The hysteresis loops obtained for composites are typical of a mixture of the hard magnetic material with a significant amount of the paramagnet. The bifurcation of ZFC–FC magnetizations observed for both composites implies spin-glass behavior. Magnetoelectric properties at room temperature were investigated as a function of dc magnetic field (0.3–7.2 kOe) and frequency (10 Hz–10 kHz) of ac magnetic field. Both types of composites exhibit a distinct magnetoelectric effect. Maximum values of magnetoelectric coefficient attained for the layered composites exceed 200 mV/(cm Oe) and are almost three times higher than those for particulate composites.     

Effects of size and surface anisotropy on thermal magnetization and hysteresis in the magnetic clusters

Based on Monte Carlo simulation, the spin configurations, thermal magnetization and hysteresis loops of the clusters coated by the surface shell with radial anisotropy are studied. Interestingly, a new multidomain containing a few of subdomains whose easy directions are along those of the configurational anisotropy, a magnetization curve in steps and a first order phase transition from the single domain to the multidomain in the thermal and field magnetization processes, are found, which is as a result of the interplay of the configurational anisotropy, the size effect, the surface anisotropy, the applied field and the thermal fluctuation. In this first order transition, we find a critical temperature, a critical surface anisotropy and a critical size. The simulated temperature dependence of the coercivity of the cluster with the surface anisotropy can be fitted by H_c (T)=H_c (0)(1-C_αT^α) with low value of α, which explains well the experimental results of the nanoparticles. Moreover, it is found that the hysteresis loops and coercivity are strongly affected by the cluster size and the thickness of the surface layer.     

Nucleation field, hysteresis loop, coercivity mechanism and critical thickness in composite multilayers with perpendicular anisotropy

Analytical expression for the nucleation field has been derived for a hard/soft multilayer system with anisotropy perpendicular to the film plane, which depends on the soft thickness L^s, the interface exchange coupling constant Jⁱ and the intrinsic material parameters. Both nucleation field and coercivity decrease as L^s increases. For very small L^s, the coercivity mechanism is pure nucleation and the hysteresis loops are square. As L^s rises, the coercivity mechanism changes from nucleation to pinning gradually, where the hysteresis loops have to be calculated numerically. The critical thickness at which the mechanism varies has been discussed in detail on the basis of easy axis orientation and the interface exchange coupling constant.     

Polarization switching process of soft lead zirconate titanate bulk ceramics

The domain switching dynamics was investigated in Nb-doped Pb(Zr_0.52Ti_0.48)O₃ bulk ceramics through observing systematically the evolution of the hysteresis loops at frequencies ranging from 0.01 to 100 Hz and in a field of 7.5–35 kV/cm. The experimental results indicate that the hysteresis loops are remarkably dependent on the fields and the frequencies. The data are analyzed by the Merz equation. The analysis results reveal that the requirement to satisfy the Merz equation is saturated loops. Based on these results, we divided the polarization switching process into three regions.     

Anomalies in the slow polarization kinetics of a ferroelectric relaxor in the temperature region of a diffuse phase transition

It is shown experimentally in the specific example of Cr-doped barium-strontium niobate that the anomalies in the infralow-frequency dielectric properties characteristic of a ferroelectric relaxor persist over the whole temperature region of the diffuse phase transition and decrease gradually with increasing temperature. Experimental data are presented on the anomalous quasi-static dielectric hysteresis loops, the slow polarization kinetics, and the anomalously broad energy-distribution spectra of potential barriers. The anomalies are a signature and a quantitative measure of the structural disorder typical of a relaxor.     

Structural and Dielectric Properties of Organic Ferroelectric 2-Methylbenzimidazole

2-methylbenzimidazole single crystals and films are obtained by evaporation from an ethanol solution. The films have a texture with a predominant orientation of the pseudotetragonal axis in the film plane. The blocks are split crystals of the spherulite type, in which the pseudotetragonal axis rotates around the block center. Dielectric hysteresis loops are investigated in the crystals and films in the temperature range of 290–350 K. In the films, loops are observed for both in-plane and out-of-plane electric-field orientation. The maximum value of polarization in the films is close to that observed in single crystals P _s ~ 5 μC/cm². The difference in the temperature behavior of the hysteresis loops in the crystals and films is associated with the specific structure of the blocks.     

Influence of parameters of the harmonic magnetic field on the dynamic hysteresis loops and domain structure of a ferrite garnet film

The dynamic properties and the domain structure of an epitaxial (111) garnet ferrite film with perpendicular anisotropy have been considered in a harmonic magnetic field with an amplitude in the range 0–170 Oe and a frequency in the range 0.2–7.0 kHz. A direct correspondence between the obtained images of dynamic domain structures and particular sections of the hysteresis loops is set up. It has been established that variations in the parameters of the magnetic field lead to qualitative changes observed in the domain structure and, correspondingly, in the shape and area of the hysteresis loops.     

Slow polarization processes in relaxor ferroelectrics

The general features of the slow polarization kinetics in relaxor ferroelectrics, observed and studied by us in the specific example of crystals of barium-strontium niobate (SBN) solid solutions of various compositions and of lead magnesium niobate (PMN), are considered. The dielectric hysteresis loops and spectra of the polarization relaxation time distribution in quasi-static and static electric fields were found to reveal characteristic anomalies related to a random internal electric field in the bulk of a relaxor ferroelectric. Such a field caused by structural disorder accounts for the anomalously broad spectrum of potential barriers for domain walls. The part played by free charge carriers in the formation of giant barriers is demonstrated. The paper presents some quantitative data characterizing the specific features of the structure and physical properties of the relaxors.