W. Kleemann

Diffuse phase transition in BaTi1−xSnxO3 ceramics: An intermediate state between ferroelectric and relaxor behavior

Dielectric relaxation and polar structures of BaTi1−xSnxO3 ceramics, x=0.10–0.20, are investigated by means of dielectric spectroscopy and piezoresponse force microscopy. A transition regime between “normal” ferroelectric and relaxor behaviors is encountered. In the compositions with x=0.10, a complex domain pattern confirming the ferroelectric state is observed. Strong dielectric relaxation around Tm is attributed to domain wall motion. On the other hand, the dielectric spectra in the sample with x=0.20 are very similar to those observed in relaxor ferroelectrics. Analysis of the relaxation spectra at the intermediate concentration, x=0.15, reveals both domain wall response and an additional contribution related to mesoscale polar structures. The appearance of relaxor behavior in BaTi1−xSnxO3 is discussed within the framework of the random field model.

Evolution of the Polar Structure in Relaxor Ferroelectrics Close to the Curie Temperature Studied by Piezoresponse Force Microscopy

We present results of piezoresponse force microscopy studies on “uniaxial” relaxors Sr x Ba1− x Nb2O6 (SBN) and on “cubic” relaxors Pb[Mg1/3Nb2/3]1− x Ti x O3 (PMN-PT). The chosen compositions exhibit a spontaneous transition from the ergodic relaxor into the ferroelectric state. In both materials static regions of non-zero piezoresponse were found above the corresponding transition temperature. We attribute them to large polar nanosized regions (PNRs), which are “frozen” on the experimental time scale. The temperature evolution of the observed structures was investigated. It has been found that in PMN-PT these quasi-static PNRs are larger and exist in a broader temperature range than those in SBN.

Aging, rejuvenation, and memory effects in the domain state of Sr0.75Ba0.25Nb2O6

The dielectric susceptibility of strontium–barium niobate, Sr0.75Ba0.25Nb2 O 6, reveals strong hole-like aging behaviour in its ferroelectric state. The isothermal evolution of the linear susceptibility satisfactorily follows a stretched exponential law. After aging the sample at a selected temperature both rejuvenation and memory effects are observed in temperature cycle experiments. They seem to indicate cluster glass-like properties of the domain state involved.

Ferroelectric Phase Transitions and Electroconducting Properties of Ceramic BIMEVOX Solid Solutions (Me = La, Zr)

Ceramic solid solutions Bi 4 (V 1−x Zr x ) 2 O 11−z (I), (Bi 1−y La y ) 4 V 2 O 11−z (II) and (Bi 1−y La y ) 4 (V 0.96 Zr 0.05 ) 2 O 11−z (III) with x < 0.30, y < 0.20, were prepared by the solid state reaction method and were investigated by means of various experimental techniques. The low temperature ferroelectric α-phase exists in the solid solutions with x, y ≤ 0.05. The phase transition to paraelectric β-phase was revealed in these compositions by the dielectric spectroscopy, SHG and DTA/DSC methods. Dielectric permittivity measurements confirmed an effect of the domain walls “pinning” due to the presence of oxygen vacancies inherent to the bismuth vanadate based structures. Switching of ferroelectric domains by an external electric field was observed in Piezoresponse Force Microscopy experiments.

Spin cluster glass and magnetoelectricity in Mn-doped KTaO3

In ceramics of KTaO3 doped with 3 at.u2009% of Mn the dielectric response is dominated by the polydispersive behavior of Mn2+ centered polar regions, whereas the magnetic and magnetoelectric (ME) behaviors reflect an intimate coupling between A-site substituted Mn2+ ions and minute amounts of Mn3O4 precipitates mediated by the polar host material. This becomes apparent by the common onset at Tc≈42u2002K of the ordering of ferrimagnetic Mn3O4 and of a spin cluster glass, which is characterized by memory and rejuvenation effects. The composite magnetic system exposed to external magnetic and electric dc fields shows large third order ME susceptibility with a sharp anomaly at Tc and 1/T2 dependence as T→0.

Precise measurements of lattice strains of BaMnF4

Abstract High accuracy x-rays measurements confirmed that the average structure of BaMnF4 in the incommensurate phase is monoclinic. Also the results of accurate measurements of the lattice strains of BaMnF4 are reported. It is conspicuous that there is slight change in volume thermal expansion above and below Ti.

The Last Classical Physicist (Guest Editorial)

It is a great honor and responsibility to be Guest Editors for the special issue of Ferroelectrics, dedicated to the 90th Birthday of Nobel Prize Winner Prof. Vitaly Ginzburg. We have entitled our Guest Editorial “The Last Classical Physicist,” because his creative work practically covers all regions of the modern physics: astronomy, astrophysics, cosmic rays, high spin elementary particles, quantum electrodynamics, solid state physics, including superconductivity and superfluidity, superdiamagnetism and ferrotoroics, crystallooptics and last (but for this journal not least) the theory of ferroelectricity and soft mode conception. Ginzburg often says that Landau was born too late: quantum theory and relativistic physics were almost completed when Landau started as a scientist. Ginzburg is eight years younger than Landau. One can, say, that Ginzburg and Feynman worked “on the tail of the physics revolution.” But the universal character of their interests and discoveries permit one to consider them as classicists. The phenomenological theory of ferroelectricity was born on July 31, 1945, the day that the Editorial Board of the Journal of Experimental and Theoretical Physics (JETP) received the paper entitled “Dielectric properties of Ferroelectrics and Barium Titanate,” which was written by Ginzburg after preliminary discussions with Landau. In this paper, Ginzburg, using the Landau theory of the second order phase transition, developed a theory of the ferroelectric phase transition of the second order that was also applicable to transitions of the first order in the vicinity of a thermodynamic critical point (Curie critical point in the old terminology). This paper explained the experimental results, obtained in 1945 by Vul and Goldman in the same Institute (Lebedev Physics Institute of the Soviet, now Russian, Academy of Sciences, where Ginzburg began his work in 1938, and is still working). Later, in 1949, Ginzburg published two more well known papers, where he extended the scalar theory, which applied to a strictly uniaxial polarization, a vector formulation valid for the more complex polarization states in the case of BaTiO3 and the many perovskite ferroelectrics discovered since. In 1949 A. F. Devonshire independently developed essentially the same approach to the BaTiO3 dielectric properties, but omitted some important invariants with P6 (P is the order parameter) necessary in the case of a first-order transition. Devonshire cited the Ginzburg paper of 1949 and his manuscript came to Philosophical Magazine almost one year later. Ginzburg’s conclusion, that the ferroelectric phase transition in BaTiO3 takes place in the vicinity of the critical Curie point could not be confirmed experimentally at that time. Later it was shown that this conclusion is correct, but the critical point, which occurs at high pressures, was not demonstrated experimentally until it was discovered in SbSI in 1968 and even later in BaTiO3. What is most remarkable, perhaps, is how enduring and versatile is the formalism established in Ginzburg’s two brief papers, even after the passage of six decades of intensive experimental research on wide range of ferroic systems and the development of increasingly sophisticated theories. The connection between the ferroelectric phase transition and lattice dynamics and conception of soft mode was developed by Ginzburg in the above mentioned two articles published in 1949. It was done in frame of phenomenological mean free theory. Nevertheless the formulas obtained by Ginzburg for dispersion of the dielectric constant ε = ε (ω,T),

Optical activity of BaMnF4

Abstract A gyration component g23 of BaMnF4 was found to manifest a sharp peak at the incommensurate transition point. The phenomenon may be considered as caused by the coupling between an electronic polarization, which is condensed at Ti, and a complex order parameter.

The optical study of BaMnF4 in incommensurate phase

Abstract Accurate measurements of birefringence, optical activity and rotation of the optical indicatrix of BaMnF4 have been made by using HAUP method near the incommensurate transition point of 250 K. The IC phase is optically active below Ti and the indicatrix rotates below Ti around all the crystallographic axes.

Optical studies of the pyroelectric antiferromagnet BaMnF4

Abstract The apparent circular dichroism (CD) of BaMnF4 exhibits features, which are well-known from the linear optical anisotropies of this pyroelectric, antiferromagnetic compound: (i) the spectral and angular dependences resemble those of the linear dichroism (LD), (ii) the temperature dependence of the CD in the transparent region varies like the linear birefringence (LB) and is sensitive to the structural and the magnetic phase transitions. It is shown that the CD signal is generated from LD and LB by virtue of the optical activity of the crystal as a second order effect.