Magnetoelectric and magnetoelastic properties of easy-plane ferroborates with a small ionic radius

The magnetic, magnetoelectric, and magnetoelastic properties of RFe₃(BO₃)₄ ferroborates are studied. The measurement of the field dependences of the magnetoelectric polarization along the a axis in holmium ferroborate and in the mixed composition Ho_0.5Sm_0.5Fe₃(BO₃)₄ revealed the following dependences for easy-plane ferroborates: (a) the longitudinal and transverse magnetoelectric effects have the opposite signs and (b) magnetically induced polarization changes its sign in a field close to the field of exchange between rare-earth and iron ions. These dependences agree well with theoretical predictions based on the symmetry of the compounds. The relatively low f-d exchange field in holmium ferroborate (about 20 kOe), which magnetizes the rare-earth subsystem, causes smaller polarization jumps (about 30 ??C/m²) in fields lower than 10 kOe as compared to the jumps in other easy-plane ferroborates (R = Sm, Nd). The increase in the electric polarization induced in HoFe₃(BO₃)₄ in magnetic fields higher than 100 kOe (200?C300 ??C/m²) is found to be significantly smaller than in neodymium ferroborate, which indicates a substantial dependence of the magnetoelectric effects on the electronic structure of a rare-earth ion.     

A Kinetic Model of Quantum Jumps

A new class of models describing the dissipative dynamics of an open quantum system S by means of random time evolutions of pure states in its Hilbert space is considered. The random evolutions are linear and defined by Poisson processes. At the random Poissonian times, the wavefunction experiences discontinuous changes (quantum jumps). These changes are implemented by some non-unitary linear operators satisfying a locality condition. If the Hilbert space of S is infinite dimensional, the models involve an infinite number of independent Poisson processes and the total frequency of jumps may be infinite. We show that the random evolutions in are then given by some almost-surely defined unbounded random evolution operators obtained by a limit procedure. The average evolution of the observables of S is given by a quantum dynamical semigroup, its generator having the Lindblad form.⁽¹⁾ The relevance of the models in the field of electronic transport in Anderson insulators is emphasised.     

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.     

Magnetic aftereffects and the barkhausen effect in a magnesium manganese ferrite single crystal

The temperature dependence of the critical magnetic field [H₀(T)] of a magnesium manganese ferrite single crystal is analyzed, together with the changes taking place in the critical magnetic field with time, H₀(t), the temperature dependence of the domain-boundary stabilization field H_st(T), and the time required to establish the equilibrium state of the domain boundaries; the domain structure is examined, and the induced anisotropy constant is calculated; so is the activation energy of the process leading to the stabilization of the domain boundaries. The magnetic aftereffects and the Barkhausen jumps accompanying them are of a diffusion nature.     

Effect of the length of action of a steady electric field on the polarization buildup process in triglycene sulfate and barium titanate single crystals

During the investigation of the Barkhausen effect and the reversal current [1] in ferroelectric crystals, it was observed that, together with other factors, the length of time during which the specimens had been kept in a steady electric field has a substantial effect on the polarization reversal process. In this connection, it is of interest to investigate parallely the Barkhausen effect and the reversal current in ferroelectric crystals at various lengths of action of the electric field. In this paper we have investigated the number of Barkhausen jumps, the charge being reversed, the buildup time of the total and irreversible polarization, as well as the starting field for jumps and reversal current as a function of the length of action of the field on specimens of TGS and BaTiO₃ single crystals during the quasistatic polarization reversal process.     

Neutron scattering study of NH4+ Motion in NH4+ β-alumina

Incoherent inelastic neutron scattering has been used to study the motion of NH₄⁺ ions in NH₄⁺ β-alumina. The results establish that jump reorientation of NH₄⁺ ions is rapid compared to translational diffusion: The data are consistent with thermally activated jumps between equivalent NH₄⁺ orientations with a proton jump frequency of $\text{~1.0 × 10}{}^{12}\text{sec}$ at room temperature.The data are inconsistent with either free rotation or unrestricted rotational diffusion. The residence time between translational diffusion jumps is >6 × 10^?11 sec at temperatures less than 473°K.     

Distinctive field dependence of the longitudinal muon polarization for Mu* in polycrystalline silicon

A distinctive longitudinal magnetic field dependence of the muon polarization for anomalous muonium in polycrystalline semiconductor targets has been predicted. The polarization exhibits a cusp,i.e., a discontinuous jump in the slope from negative to positive. Measurements of the longitudinal polarization for polycrystalline silicon in fields up to 0.5 T, and temperatures 53 and 200 K have been made at LAMPF. A cusp in the field dependence indeed occurs at 0.345 T, in excellent agreement with the prediction. No cusp is observed at 200 K because Mu^* has been thermally ionized.     

Fatigue of lead titanate and lead zirconate titanate thin films

The fatigue of lead titanate and lead zirconate titanate ferroelectric thin films, i.e., a change in the polarization as a function of the number of switching cycles in an external electric field, is investigated experimentally. The threshold numbers of switching cycles are determined to be 10¹⁰–10¹¹ for the lead titanate films and 10⁹–10¹⁰ for the lead zirconate titanate films. It is shown that a change in the temperature does not substantially affect the threshold number of switching cycles at which the switched polarization decreases drastically. However, an increase in the external field strength leads to a noticeable decrease in the threshold number of switching cycles. The process of fatigue is accompanied by an increase in the coercive field and the internal bias field. It is established that, as the number of switching cycles increases, the internal bias field changes more significantly as compared to the coercive field. The absence of a change in the phase composition in repeatedly switched samples indicates that the fatigue processes have a nonchemical nature. The anomaly observed in the frequency dependence of the permittivity in the frequency range 10⁶–10⁷ Hz due to the domain structure disappears after multiple switching cycles. This suggests that the observed fatigue phenomenon has a domain nature.     

Temperature dependences of the dielectric properties of lithium-titanium ferrite ceramics

Temperature dependences of the real ?′ and imaginary ?″ parts of the complex permittivity of lithium-titanium ferrite ceramics are measured in the frequency range 10²?10⁶ Hz at different test-signal amplitudes and dc bias voltages. It is found that the dielectric characteristics of the ceramic samples drastically change in narrow temperature ranges. The assumption is made that relaxators whose reorientation is due to tunneling transitions of electrons inside “bivalent iron ion-trivalent metal ion” pairs are involved in polarization processes. Under certain conditions, the reorientation of relaxators can have collective character.     

Non-nearest-neighbor jumps in 2D diffusion: Pd on W(110)

A variety of jumps has in the past been identified in diffusion of atoms on 1D channeled surfaces. To establish the jump processes important in diffusion on a 2D surface, the movement of individual Pd atoms has been examined on W(110). From the distribution of displacements of Pd at high temperatures, double jumps are found along the close-packed <111>. For the first time, sizable differences are also observed between the mean-square displacements along x and y, which demonstrate unexpected contributions from jumps along <110>, but not along <001>. These jumps proceed over activation barriers higher than for single jumps, under conditions predicted from previous work with Pd on the channeled W(211).     

Polymer creep in a static magnetic field

By noting changes in the color of samples in a polarization microscope with varying degree of birefringence, the effect of a weak static magnetic fields (2 and 4 kOe) on molecules of glassy polymers has been observed. Variations in the nonmonotonicity of the rate of discontinuous creep at +18° were studied interferometrically. It is shown that the abruptness of the deformation jumps varies in a magnetic field, where this abruptness is assumed to be due to the existence of strong physical junctions between the kinetic units of deformation. The reactivity of the polymers to the magnetic field stands in satisfactory correspondence with their magnetic susceptibilities. Results show that such nonmagnetic materials as glassy polymers can noticeably alter their deformational properties in response to the action of magnetic fields. Fiz. Tverd. Tela (St. Petersburg) 39, 1690–1692 (September 1997)     

Nonlinear optical polarization effects in quantum-dot materials

The amplitude and dispersion of the cubic susceptibility-tensor components χ _ijkl ⁽³⁾ (?Ω; Ω, ω, ?ω) and of their combinations responsible for the Kerr and Faraday optical effects, as well as for the self-rotation of the polarization ellipse, are calculated within a simple model of a material with semiconducting spherical quantum dots without the inclusion of the excitonic effects and of processes of higher orders in field. It is shown that the susceptibilities increase smoothly in amplitude below the two-photon absorption threshold while remaining negative. Above the threshold, the susceptibilities exhibit sharp maxima associated with two-photon resonances. The regions with positive and negative real parts of the χ⁽³⁾ tensor components alternate. The magnitude of the induced linear or circular birefringence obtained suggest the possibility of using the nonlinear optical polarization effects in these materials to control light with short, high-intensity light pulses.     

Self-oscillations of electronic and nuclear polarization during optical excitation of an Si/CaF2 nanostructure in a transverse magnetic field

We examine theoretically low-frequency and high-frequency self-oscillations of electronic and nuclear polarization in an Si/CaF₂ nanostructure in a transverse magnetic field. We show that the low-frequency self-oscillations are stable in zero field, and the analogous high-frequency oscillations are stable beyond the region of the maximum on the Hanle curve. The frequency of the low-frequency oscillations is 0.001–0.500 of the reciprocal nuclear longitudinal relaxation time; the frequency of the high-frequency oscillations is 10⁸–10⁹ Hz, and their amplitude reaches 50% of the initial electronic spin polarization. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 363–369, May–June, 2006.     

A semiclassical model for the polarization on inclusively produced Λ0-particles at high energies

We present a semiclassical model for inclusive Λ⁰-production which is able to explain the observed polarization effect in proton-proton collisions. It is well known that hard scattering processes, when calculated in perturbative QCD, do not give rise to large polarization effects. The mechanism producing the polarization in this model is basically a soft process, where quark-antiquark pairs are produced by a tunneling process in the colour field and where perturbatice QCD is not applicable.     

The magnetic field dependence of the electron spin polarization in consecutive spin correlated radical pairs in type I photosynthetic reaction centres

The magnetic field/microwave frequency dependence of the spin polarized EPR spectra of the sequential spin correlated radical pairs P⁺A^? ₁ and P⁺F^? _x in type I photosynthetic reaction centres is investigated. Experimental data are presented for photosystem (PS) I and reaction centres of heliobacteria at × band (9.7 GHz) and K band (24 GHz). In photosystem I at ambient temperatures the lifetime of A ^? ₁ is ~290 ns and both states are observable by transient EPR. In heliobacteria, electron transfer to F_x occurs within ~600 ps and only the state P⁺F^? _x is observed. The experimental data show a net polarization of P⁺ in the state P⁺F^? _x, which displays a clear dependence on the strength of the external field. The net polarization generated in sequential radical pairs is expected to pass through a maximum as a function of the Zeeman energy when the characteristic time of singlet-triplet mixing is comparable with the lifetime of the precursor. In PS I, the precursor lifetime (290ns) is much longer than the characteristic time of singlet-triplet mixing at × band (9 GHz, 3 kG) and K band (24 GHz, 8 kG). As a result, the observable net polarization decreases with the field strength in this region. In contrast, in heliobacteria, the precursor lifetime (600 ps) is much shorter than the characteristic time of singlet-triplet mixing, and the net polarization increases in the same range of Zeeman energies. The polarization patterns in these two systems can be described using the specific limiting cases of a short lived and long lived precursor radical pair and written as a sum of several contributions. The spectra are simulated on this basis using parameters derived entirely from independent experimental data, and good agreement between the experimental polarization patterns is obtained. The calculated polarization patterns are sensitive to spin dynamics on a timescale much shorter than the spectrometer response time, and the expected influence of a 10 ns component in the electron transfer, as observed optically in some PS I, preparations is discussed. No significant influence from such a component is found in the spin polarization patterns of PS I from the cyanobacterium Synechocystis 6803.     

A theoretical study of low-energy position diffraction

In an exploratory study of the diffraction of slow positrons from atoms and single-crystal surfaces, theoretical intensity and spin polarization results from a W crystal-atom and a W(001) surface are compared to corresponding electron diffraction results obtained with and without an exchange potential. In contrast to e^- diffraction, significant spin polarization effects are found for e⁺ only at energies above about 100 eV. The computing time for e⁺ is about half of the time required for e^-.     

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.     

Effects of weak magnetic fields on post-implantation damage in superconducting oxides

Experimentally verifiable effects of weak permanent magnetic fields (PMF’s) acting during ion implantation in high-T_c superconducting (HTSC) materials at T ? 300K on post-implantation damage (PID) and material parameters are considered. The presence of PMF’s of H ? 10³Oe during ion implantation can enlarge substantially the PID in HTSC materials implanted with ions of moderate energies (e.g. 200–400 keV) and dosage (10¹¹-10¹² cm^-3) at room temperature. The PMF-induced increase in the radiation damage causes the corresponding enhancement in the material resistivity R and reduction in the critical current j_cir (measured after the cooling of the HTSC material down to T^(L) < T_c after the ion implantation). This is an extension of the PMF effects found experimentally (and explained theoretically) in semiconductors in our previous work [7]. The experimentally verifiable PMF effects on the defect (atomic) migration and radiation damage is a generic consequence of the kinetic electron-related theory of atomic rate processes in solids. The theory links the PMF effects with electron transitions occurring in the nanometer vicinity of atoms overcoming energy barriers which affect exponentially rates of atomic (defect) diffusion. The magnetic field can enhance the number of downward electron transitions that accompany atomic (defect) jumps over energy barriers and synchronize with the jumps. This enhances exponentially the rates of defect migration out of thermal spikes that prevents the defects from fast recombination, and thus, the PMF increases the PID and changes correspondingly R and j_cir.