Reverse Barkhausen jumps

The magnetic parameters, forward and reverse magnetic reversal jumps in iron-core quartzite specimens and in magnetite single crystals, and the effect of thermal treatment on the magnetic and electric parameters of quartzites are studied. Reverse Barkhausen jumps are larger than the forward ones, but they are considerably fewer. The maxima of forward jumps correspond to the coercive force, while those of the reverse ones to fields exceeding the coercive force. Possible causes of the manifestation of reverse Barkhausen jumps are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fkika, No. 12, pp. 8–12, December, 1984.     

Stability of the single-domain state in single crystals of triglycine sulphate

The stability of the single-domain state in single crystals of triglycine sulfate is investigated using the Barkhausen effect. Barkhausen jumps are detected upon invariable macroscopic polarization. The parameters of the observed Barkhausen jumps are estimated and compared.     

Investigation of the fractal features of the polarization reversal processes in ferroelectric crystals under injection conditions

The features of polarization reversal processes of ferroelectric crystals are analyzed under the condition that electron beams of a scanning electron microscope are injected into the subsurface layer of a sample. The proposed mathematical model describes the dynamics of the polarization switching of a ferroelectric crystal using the injection contact technique. The simulation relies on the basic principles of scale invariance in domain structure evolution. The polarization switching current of ferroelectric crystals is investigated as the dynamic characteristic of a finite medium exhibiting fractal behavior. The application of fractal and multifractal analysis of a time series enables us to calculate the fractal dimension of the polarization reversal process under injection conditions.     

Barkhausen effect in magnetic thin films: General behaviour and stationarity along the hysteresis loop

Several aspects of the Barkhausen effect can be investigated by using a measurement equipment that records the Barkhausen signal as a function of time in digital form on computer tape. Measurements on thin magnetic films containing a small number of domain walls are presented. The behaviour of the wall jumps during a reversal is investigated. During the steep part of the loop the stationarity of the signal is measured with the magnetic field as a parameter. The influence of the coercive fieldH _c and the thicknessd _m is considered and compared with results obtained by others on bulk samples.     

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.     

Effects of oxygen partial pressure on the ferroelectric properties of pulsed laser deposited Ba0.8Sr0.2TiO3 thin films

The Ba_0.8Sr_0.2TiO₃ thin films were grown on the Pt–Si substrate at 700 °C by using a pulsed laser deposition technique at different oxygen partial pressure (PO₂) in the range of 1–20 Pa and their properties were investigated. It is observed that the PO₂ during the deposition plays an important role on the tetragonal distortion ratio, surface morphology, dielectric permittivity, ferroelectric polarization, switching response, and leakage currents of the films. With an increase in PO₂, the in-plane strain for the BST films changes from tensile to compressive. The films grown at 7.5 Pa show the optimum dielectric and ferroelectric properties and also exhibit the good polarization stability. It is assumed that a reasonable compressive strain, increasing the ionic displacement, and thus promotes the in-plane polarization in the field direction, could improve the dielectric permittivity. The butterfly features of the capacitance–voltage (C–V) characteristics and the bell shape curve in polarization current were attributed to the domain reversal process. The effect of pulse amplitude on the polarization reversal behavior of the BST films grown at PO₂ of 7.5 Pa was studied. The peak value of the polarization current shows exponential dependence on the electric field.     

Influence of cycling electric polarization on multiferroic behaviors in heterostructural films composed of ferroelectric and ferromagnetic oxides

The CoFe₂O₄/Pb(Zr_0.52Ti_0.48)O₃ bilayer films were prepared by a sol–gel process, and the influence of cycling electric polarization on the multiferroic behaviors of the bilayer films was studied. The ferroelectric polarization hysteresis loops under various choices of magnetic bias were measured by an integrating current method. The results showed that after undergoing cycling electric polarization the ferroelectric polarization of the bilayer films enhanced and the suppression of ferroelectric polarization by external magnetic bias remarkably weakened. Based on the measurements of activation energy and leakage current, we confirmed that the oxygen vacancy migration in the bilayer films occurred during cycling electric polarization. Furthermore, we analyzed the mechanism of the influence of cycling electric polarization on the multiferroic behaviors of the bilayer films and attributed it to the oxygen vacancy migration, which could cause a part of ferroelectric domains to be unpinned from the oxygen vacancies and become more active under electric field and magnetic bias.     

Influence of domain structure on the nonlinear polarization properties of A 2 BX 4-group crystals

Harmonic analysis of repolarization in A ₂ BX ₄-group crystals in the ferroelectric phase near phase-transition temperature T _c is used to separate the contributions to the harmonic-component amplitudes in the output signal of a Sawyer-Tower circuit from charging the reference capacitor with capacitive and conductive current through such a crystal. Harmonic analysis of dielectric hysteresis loops obtained by placing a specimen in an electric field with harmonically changing strength reveals general regularities in the evolution of the domain walls of A ₂ BX ₄-group crystals. It is established that the domain structure affects the nonlinear polarization properties of our specimens. The dependence of the potential relief of ferroelectric ion motion in a harmonically changing electric field is plotted on the basis of the experimental results.     

Anomalies of dielectric properties in a polycrystalline lithium-titanium ferrite

For the first time a dielectric hysteresis is observed in a polycrystalline lithium-titanium ferrite in the temperature range 475–575 K. A thermal Barkhausen effect is detected during heating and cooling of ferrite specimens prepolarized in an electric field. The results provide evidence on the formation of properties similar to ferroelectric ones in lithium-titanium ferrites under certain conditions. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 59–61, February, 2007.     

Electret states and the phase transition in a surface layer of the TlGaSe2 ferroelectric semiconductor

The electret polarization is investigated in the TlGaSe₂ ferroelectric semiconductor. It is proved for the first time that stable internal electric fields associated with residual electret polarization are induced in crystals of the TlGaSe₂ ferroelectric semiconductor at temperatures T < 200 K. It is experimentally established that the peak of the pyroelectric current measured in the vicinity of the phase transition to the ferroelectric polar phase depends substantially on the temperature at which the external electric field is switched off when the TlGaSe₂ ferroelectric crystal under investigation is preliminarily cooled from room temperature. The results obtained are discussed in the framework of a model according to which internal electret fields are induced by charges localized at different levels in the bulk and on the surface of the TlGaSe₂ ferroelectric crystal. These fields drastically change at temperatures in a narrow range near 135 K. The inference is made that a phase transition occurs in the surface layer of the TlGaSe₂ crystal at a temperature close to ～135 K.     

Study of the domain structure evolution in single crystals of relaxor ferroelectric Sr0.61Ba0.39Nb2O6:Ce1

Optical visualization with simultaneous recording of the switching current was used for studying of domain structure evolution in single crystals relaxor ferroelectric strontium barium niobate (Sr_0.61Ba_0.39Nb₂O₆) doped by cerium (0.22 mol %) (SBN61:Ce). It was shown that the maze domain structures can be observed during polarization reversal due to local change of refractive index induced by formation of microscale domains with charged domain walls. It was proposed, that optical inhomogeneities occur in the switching regions, while the optical uniform areas correspond to regions where the switching either has not started yet or has already completed. The original mathematical treatment of the obtained images and switching currents has been used for quantitative characterization of the polarization reversal process.     

Barkhausen jumps and switching current in triglycine sulfate crystals

The slow processes involved in establishment of the overall and discontinuous parts of the polarization of a triglycine sulfate crystal are compared on the basis of a joint study of the switching current and the Barkhausen jumps. Over a wide field range (10–10³ V/cm) the time required to establish the entire polarization is equal to the time required to establish its irreversible part due to jumps. The time dependences of the overall and discontinuous parts of the polarization are shown to be analogous. There is a threshold field above which the discontinuous repolarization processes play a decreasing role with increasing field.     

Electromechanical-induced antiferroelectricben ferroelectric phase transition in PbLa(Zr,Sn,Ti)O3 ceramic

Antiferroelectric—ferroelectric (AFE—FE) phase transition in ceramic Pb_0.97La_0.02(Zr_0.75Sn_0.136Ti_0.114)O₃ (PLZST) was studied by dielectric spectroscopy as functions of frequency (10²—10⁵ Hz) and pressure (0—500 MPa) under a DC electric field. The hydrostatic pressure-dependent remnant polarization and dielectric constant were measured. The results show that remnant polarization of the metastable rhombohedral ferroelectric PLZST poled ceramic decreases sharply and depoles completely at phase transition under hydrostatic pressure. The dielectric constant undergoes an abrupt jump twice during a load and unload cycle under an electric field. The two abrupt jumps correspond to two phase transitions, FE—AFE and AFE—FE.     

Ferroelectric-specific stark effect in stoichiometric LiNbO3:Fe at room temperature

By means of EPR spectroscopy of LiNbO(3):Fe at room temperature (RT) it is shown that the Stark effect in ferroelectric crystals can be different from that observed in other materials. Novel properties appear when an external E field reverses the direction of the spontaneous polarization, the direction of the linear Stark shift stays the same with a reversal of the E field. The corresponding spectral line shifts can occur over a long time scale (hours). These properties seem to be a general feature of the Stark effect in ferroelectric crystals when the external E field exceeds the coercive field.     

Pulse train measurement of ferroelectric switching in thin films of vinylidene fluoride/trifluoroethylene copolymer

Ferroelectric switching processes in thin films of a vinylidene fluoride/trifluoroethylene copolymer are investigated by pulse train measurement that involves the application of a train of unipolar on–off electric field pulses and the analysis of charge response. It is found that polarization reversal is considerably delayed as pulse width becomes shorter than the conventional switching time, but is eventually completed even if pulse width is reduced to 0.2 μs. The charge response in each on–off cycle was resolved into dielectric and ferroelectric components. The former is nonlinear as well as relaxational and reflects changes in dipolar fluctuation and orientation during polarization reversal. The latter is responsible for polarization reversal that progresses forward and backward depending on the on–off period. Net polarization reversal occurs first gradually and then rapidly, corresponding to nucleation and growth, respectively. The switching current responsible for these processes can be reproduced by the sum of power and lognormal functions. The implications of these results are discussed in relation to the switching process that occurs as a result of the rotation of chain molecules. PACS 42.70.Jk; 77.80.Fm; 77.22.Ej     

Electroluminescence study of polarization changes in lead magnoniobate crystals in pulsed electric fields

Electroluminescence is used to study changes in the polarization of crystals of a model ferroelectric relaxor, lead magnoniobate (PMN), in pulsed electric fields. The amplitude of the electoluminescence pulses produced during polarization and depolarization of PMN crystals is found to depend on the duration of the applied electric field pulses if this duration is shorter than the most probable time for polarization buildup. These data provide evidence of rapid changes in the polarization through realignment of the domain and heterophase structure at temperatures above the temperature for destruction of the induced macrodomain ferroelectric phase and evidence of an “excited” polarization state for short-lived pulses whose decay is accompanied by an enhancement in the amplitude of the depolarizing luminescence pulse and by a reduction in the time delay of its emission following the end of the field pulse. Fiz. Tverd. Tela (St. Petersburg) 39, 341–343 (February 1997)     

Variation of the soft-mode frequency in ferroelectric semiconductors in a static electric field

The action of a static electric field on the soft-mode frequency in narrow-gap ferroelectric semiconductors, as a result of the contribution of the electron subsystem, is investigated within the framework of the vibronic mechanism. It is shown that the frequency of the soft mode varies directly as the square of the electric field strength under the action of the field, and the soft-mode frequency is determined in the vibronic model by the electron distribution function, whose parameters also vary under the action of the electric field. The effect is stronger for crystals with a one-dimensional electron spectrum than for those with an isotropic three-dimensional spectrum; according to estimates for ferroelectric semiconductors of the type Pb_1–xGe_xTe, a frequency variation of the order of the soft-mode frequency itself is attained in fields of the order of tens of volts per centimeter.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 33–37, January, 1988.     

铁电双层膜的反转动力学行为

基于Landau-Khalatnikov运动方程,本文研究了含有表面过渡层和铁电界面耦合的反转动力学行为(包括平均极化、反转时间、反转电流和矫顽场).研究结果表明:在铁电双层膜系统中存在一个竞争的机理,即表面过渡层与界面耦合的竞争作用.我们发现在双层膜反转过程中出现了反常行为,这些反常行为归因于表面过渡层与界面耦合之间的竞争.表面过渡层与界面耦合的共同行为对铁电双层膜的动力学特性起到了决定性的作用.     

Acoustic emission from domain wall motion in ferroelectric lead germanate

Simultaneous visual observation and monitoring of the ultrasonic signals show that acoustic emission (AE) produced as Pb₅Ge₃O₁₁c-plate crystals undergo the ferroelectric hysteresis cycle results from domain wall nucleation and collapse. AE activity takes place predominantly at applied electric fields large enough to bring a crystal substantially towards its saturation polarisation. A notable feature observed in Pb₅Ge₃O₁₁ and Pb_5?xBa_xGe₃O₁₁ alloys is the existence of an abrupt threshold electric field, denoted by E_∞ in the high gain limit, for production of AE: as the electric field is increased beyond the threshold value, the amount of AE observed increases by several orders of magnitude. E_∞ depends linearly on the inverse of the c-plate sample thickness (1/d) and also upon sample temperature—falling to a minimum at about 70°C for Pb₅Ge₃O₁₁ and then rising steeply as the Curie temperature (178°C) is approached. E_∞ is also dependent on the rate of change of the applied electric field and as this is increased extrapolates to the high frequency limit of the coercive field. Measurements of crystal polarisation indicate a one-to-one correspondence between AE and the electrical Barkhausen pulses which occur during polarisation reversal—further evidence for domain nucleation and collapse as the source of AE. Optical and AE studies made simultaneously on gadolinium molybdate add confirmation that these particular processes are responsible for the AE produced by ferroelectric crystals.     

Barkhausen jumps in the motion of a single ferroelectric domain wall

It is shown that the motion of a domain wall in the improper ferroelectric-ferroelastic gadolinium molybdate Gd₂(MoO₄)₃ demonstrates a self-organized critical behavior under polarization reversal. Barkhausen pulses are measured experimentally during the jumpwise motion of a single plane domain wall in monocrystalline plates with artificial pinning centers of the “field inhomogeneity” type.