Phase transitions in spherical particles of a cubic ferroelectric are considered within Landau-Ginzburg-Devonshire theory. Concentrating on effects of the depolarizing field, we study competition between states with homogeneous polarization and vortex structures. For large radii of the sphere ($R>{R}_{c}$), the phase transition is into a vortex state while for $R<{R}_{c}$ it might be into an homogeneous state. ${R}_{c}$ is proportional to the square root of the dielectric constant of the environment. If this constant is of the order of unity, a transition into homogeneous state is practically impossible. The obtained results are applied to a discussion of the formation of ``polar nanoclusters'' in relaxors.

Magnetism in multiferroic Pb5Cr3F19 Robert Blinc,1,* Pavel Cevc,1 Gašper Tavčar,1 Boris Žemva,1 Valentin Laguta,2 Zvonko Trontelj,3,† Marko Jagodič,3 Damir Pajić,4 Armandas Balčytis,5 and James F. Scott6 1Jožef Stefan Institute, Ljubljana, Slovenia 2Institute of Physics AS CR, Prague, Czech Republic 3Institute of Mathematics, Physics and Mechanics, Ljubljana, Slovenia 4Department of Physics, Faculty of Science, University of Zagreb, Bijenička c. 32, 10000 Zagreb, Croatia 5Vilnius University, Vilnius, Lithuania 6Cavendish Laboratory, Department of Physics, Cambridge University, Cambridge, United Kingdom (Received 17 August 2011; revised manuscript received 30 December 2011; published 16 February 2012)

für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution 4.0 International License. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz. 1 H 1 7 0 Nuclear Quadrupole Double Resonance in DL-Proline

The electrocaloric (EC) effect in normal ferroelectric copolymer P(VDF-TrFE) and in relaxor terpolymer P(VDF-TrFE-CFE) is described in the framework of a phenomenological appoach. The change of the dipolar entropy in the copolymer, derived earlier from dielectric measurements by means of the Maxwell relation, is analyzed in terms of the mean field expression for entropy, and is shown to be consistent with the dielectric data. In the terpolymer case, the entropy change determined recently by direct (EC) measurements is found to be in qualitative agreement with the predictions of the mesoscopic spherical model of relaxor ferroelectrics.

The new eight pseudo-spin model recently introduced to discuss in a unified fashion the ferroelectric and antiferroelectric phase transitions in the hydrogen-bonded crystals of the KH2PO4 family is reviewed. The dielectric susceptibilities along the unique ferroelectric c-axis, and the a (b)-axis, perpendicular to the unique axis, are calculated. The application to the mixed crystals is suggested.

We report on the first observation of a magnetic transition in multiferroic Pb5Cr3F19. The system undergoes a ferroelectric transition at T-C = 545 K and probably an antiferromagnetic transition around T-N = 11 K. Between 50 K and T-N, the system is not paramagnetic but shows the presence of correlated spin clusters, indicating a superparamagnetic or spin glass state. The observable changes of the magnetic properties ongoing through ferroelectric phase transition show a magnetoelectric coupling that may be important for new devices.

We combine a wide variety of experimental techniques to analyze two heretofore mysterious phase transitions in multiferroic bismuth ferrite at low temperature. Raman spectroscopy, resonant ultrasound spectroscopy, electron paraelectric resonance, x-ray lattice constant measurements, conductivity and dielectric response, and specific heat and pyroelectric data have been collected for two different types of samples: single crystals and, in order to maximize surface/volume ratio to enhance surface phase transition effects, BiFeO${}_{3}$ nanotubes were also studied. The transition at $T$ $=$ 140.3 K is shown to be a surface phase transition, with an associated sharp change in lattice parameter and charge density at the surface. Meanwhile, the 201 K anomaly appears to signal the onset of glassy behavior.

We show that the flexomagnetic effect leads to new linear flexomagnetoelectric (FME) coupling in bulk and nanosized materials, where the polarization and (anti)magnetization vectors are spatially inhomogeneous due to external or internal forces. The linear FME coupling is proportional to the product of the gradients of (anti)magnetization, polarization, flexomagnetic, and flexoelectric tensors. Due to the gradient nature, the physical sources of the FME coupling are quite different from the well-known linear and quadratic magnetoelectric (ME) couplings. In nanosized systems with intrinsic spontaneous gradients of the (anti)magnetization and polarization the linear FME coupling induces the size-dependent linear ME coupling between the average polarization and magnetization. The linear ME coupling coefficient is inversely proportional to the characteristic size of the system (film thickness, particle radius, etc.), magnetization, and polarization extrapolation lengths. The flexomagnetic effect may lead to improper ferroelectricity in bulk and nanosized (anti)ferromagnetics via the new linear and nonlinear FME coupling. Thus we propose a new mechanism based of the flexomagnetic effect for creation of linear MEs, multiferroics, namely improper ferroelectrics-(anti)ferromagnetics. The FME coupling between the polarization and magnetization strongly influences the dielectric susceptibility and ME tunability of multiferroics. Using the symmetry theory we calculate the evident form of the flexomagnetic effect tensor for all 90 bulk magnetic classes and showed that 69 of them are flexomagnetic. Due to the symmetry lowering all 90 bulk magnetic classes become flexomagnetic in the vicinity of surfaces/interfaces. Consequently the flexomagnetic effect strongly increases the number of the ME multiferroic materials.

The static modulation structure of the phase front at the field-induced first-order transition point near the critical end point (CEP) was discussed on the basis of the Landau-type free-energy function. It was confirmed that the field E dependence of the thickness of the phase front is proportional to ( E CEP - E ) -1/2 , diverging at CEP ( E = E CEP ). It is pointed out that, in relaxor ferroelectrics, the boundary between the polar nanoregion (PNR) and the paraelectric matrix may be considered as a phase front, and that the contribution to the dielectric response of the phase front cannot be ignored.

Using the quantum-mechanical approach combined with the image charge method we calculated the lowest energy levels of the impurities and neutral vacancies with two electrons or holes located in the vicinity of flat surface of different solids. We obtained that the magnetic triplet state is the ground state of the impurities and neutral vacancies in the vicinity of surface, while the nonmagnetic singlet is the ground state in the bulk for e.g. He atom, Li+, Be++, etc. ions. The energy difference between the lowest triplet and singlet states strongly depends on the electron (hole) effective mass, dielectric permittivity of the solid and the distance from the surface. Pair interaction of the identical surface defects (two doubly charged impurities or vacancies with two electrons or holes) reveals the ferromagnetic spin state with the maximal exchange energy at the definite distance between the defects (~5-25 nm). We obtained that the nonmagnetic singlet state is the lowest one for a molecule with two electrons formed by a pair of identical surface impurities (like surface hydrogen), while its next state with deep enough negative energy minimum is the magnetic triplet. The metastable magnetic triplet state appeared for such molecule at the surface indicates the possibility of metastable orto-states of the hydrogen-like molecules, while they are absent in the bulk of material. We hope that obtained results could provide an alternative mechanism of the room temperature ferromagnetism observed in TiO2, HfO2, and In2O3 thin films with contribution of the oxygen vacancies.

The phase transitions in ferroelectric KH2PO4 (KDP) and antiferroelectric NH4H2PO4 (ADP) crystals are discussed on the basis of the eight-pseudospin model of KH2PO4-type crystals previously proposed, where eight pseudospins represent protons on hydrogen bonds connecting PO4 radicals in a unit cell. The dielectric susceptibilities along the c-axis, which is the unique ferroelectric axis, and a (b)-axis, which is perpendicular to the unique axis, are calculated.

The magnetic resonance spectroscopy of incommensurate ferroelectrics is reviewed with special emphasis on the study of critical fluctuations, the determination of the soliton density and the critical exponent for the amplitude of the modulation wave as well as the phason excitations in the “plane wave” case and the “multi—soliton lattice” case.

The electrocaloric effect (ECE), i.e., the conversion of electric energy into heat, is of great importance for application in new generation cooling or heating devices that would be friendlier to the environment. Here, utilizing direct measurements of the ECE change of the temperature ΔT via a high resolution calorimeter, we study the ECE as a function of the magnitude of the electric-field step E in the vicinity of the critical point in several bulk relaxor ferroelectric ceramic systems. Relatively large ΔT of ∼2 to 3 K were obtained at modest fields of 90 kV/cm, even in the case of ceramic materials. The effective responsivity ΔT/E as a function of the electric field shows a characteristic peak near the critical point, which demonstrates the importance of proximity to the critical point for the enhancement of the electrocaloric effect. Experimental results are in good agreement with the theoretical calculations based on the spherical random-bond random-field model.

Combining the ferroelectric and magnetic components into the solid solution, it is expected to obtain the magneto-electric multiferroic material. Novel bulk ceramic compounds xPZT+(1-x)PFW, where PZT = Pb(Zr0.575Ti0.425)O-3 and PFW = Pb(Fe2/3W1/3)O-3, for x = 0.15, 0.20, 0.25, 0.50, 0.70, 0.80 were synthesized and their magnetic investigation was performed, including the measurement of zero-field cooled (ZFC) and field-cooled (FC) magnetization curves, magnetic hysteresis loops and AC susceptibility.In the low temperature region (below 20 K) splitting between the ZFC and FC curves together with the frequency dependent peaks in AC susceptibility point to the glassy freezing/blocking of magnetic moments and magnetic relaxor-like behaviour.

We review work on multiferroic magnetic fluorides with an aim to correct the popular opinion that magnetic ferroelectrics are rare in nature. After a qualitative summary describing the main families of magnetic fluorides that are piezoelectric and probably ferroelectric, we discuss in detail the most popular recent groups, namely the K3Fe5F15 and Pb5Cr3F19 families.

The physical properties of ferroelectric polymers, like polyvinylidene fluoride and its copolymers are briefly discussed. It is shown that polar nanoclusters are induced either by compositional disorder, as in the case of the terpolymer P(VDF‐TrFE‐CFE) or by electron irradiation as in P(VDF‐TrFE) giving rise to relaxor behaviour. A spherical random‐field–random‐bond model of relaxor polymers is presented and the electrostrictive coefficients are calculated as well as the linear and non‐linear dielectric properties. It is shown that the main difference between inorganic and polymeric relaxors is the physical character of polar nanoregions, the nature of the interaction between them and the origin of the random fields.

Physical upper bounds on the electrocaloric effect (ECE) in bulk polar solids are derived using thermodynamic and statistical mechanics arguments. It is shown that the maximum ECE temperature change ΔT under saturation can be estimated from the dielectric data, such as the saturation polarization and effective Curie constant, as well as from the orientational degeneracy Ω of the elementary dipolar entities in the system and the specific heat of the material. Also obtained is a universal relation for the theoretical maximum value of ΔT, which depends only on Ω and the molar specific heat.

Muon spin rotation (µSR) and relaxation has been used to study the local magnetic structure of K3Fe5F15. A collinear F-µ+-F "hydrogen bond-like" symmetric double minimum type complex with a F...F distance of 2.8 Å and a separation between the two minima of 0.8 Å has been found in the paramagnetic phase. The apparent central position of the muon seems to be the result of fast muon tunneling between two equivalent minima in the F-µ+-F bond.

Solar power plants positioned in space for terrestrial electricity use have been proposed due to the ever-rising world energy consumption and its environmental impacts. This idea is analysed here in the context of sustainability of such power generation. To that end we have performed some new economic, environmental and social effects analysis of electricity generation by solar space power plants of both photovoltaic and solar thermal types power using the best currently available technology. The plants in the analysis were assumed to be in different Earth orbits, or on the Moon built by a robotised factory. One of our results is that both economically and environmentally the best scenario may be to launch a thermal solar power plant to the geostationary orbit from the Moon. Electricity produced in this way could be economically competitive to that generated by fossil fuels on Earth already for as few as 100 space power plants of about 5-10 GW each. This option is also deemed socially responsible with its capacity to reduce poverty with large amounts of cheap clean energy, and environmentally friendly, because it produces more than a hundred times less emissions than the same amount of electricity produced from fossil fuels on Earth. (C) 2010 Elsevier Ltd. All rights reserved.