Structure and Lattice Dynamics of Ferroelectric Superlattices




НазваниеStructure and Lattice Dynamics of Ferroelectric Superlattices
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Plenary Lectures



Structure and Lattice Dynamics of Ferroelectric Superlattices



Yu.I. Yuzyuk


Faculty of Physics, Southern Federal University, Zorge 5, Rostov-on-Don 344090, Russia


e-mail: yuzyuk@rambler.ru


Artificially fabricated superlattices (SLs) constructed by alternate layers of different polar and non-polar perovskites oxides exhibit superior properties, which make them attractive for thin-film device applications.

We report x-ray diffraction and micro-Raman scattering investigations of highly constrained epitaxial ferroelectric thin films (Ba,Sr)TiO3 (BST) and series of superlattices BaTiO3/(Ba,Sr)TiO3 (BT/BST), BaTiO3/BaZrO3 (BT/BZ) and (Ba,Sr)TiO3/BiFeO3 (BST/BF). To clarify role played by epitaxial strains we have studied size effect in the set of single BST films deposited under similar condition onto MgO substrates.The two-dimensional (2D) stresses in epitaxial thin films change the entire phase transition sequence, creating new phases that are not present in bulk materials. The upward shift of the Curie temperature due to the 2D stress was systematically studied in several (Ba,Sr)TiO3/(001)MgO thin films by means of x-ray diffraction and Raman spectroscopy as a function of film thickness. Significant transformation of the E(TO) soft mode was found in the thickness-dependent Raman spectra of (Ba,Sr)TiO3/MgO films at the transition from c-domain tetragonal phase (thick films) to monoclinic phase when the film thickness was below 80 nm.

The in-plane and out-of-plane lattice parameters and polarized Raman spectra of BT/BST and BT/BZ superlattices were studied as a function of modulation period. The specific lattice parameters dependences on Sr content in BST layers for BT/BST-x SLs and on the modulation period for BT/ST SLs have been analyzed. It was found that when ≥ 76 Å the a and c parameters show no dependence on the stacking periodicity implying the appearance of the stress only near interfaces and the relaxation of the entire stress of layers. For BT/ST SLs the obtained results are in agreement with Raman data where soft mode depends slightly on the periodicity. Alternatively, for BT/BST-x SLs lattice parameters can be varied significantly even at larger modulation period ( = 110 Å). We found that the parameters of BST layer depend obviously on Sr concentration. The obtained results are in a good agreement with the Raman data obtained on the set of similar SLs where the soft mode frequency is shifted markedly when Sr content is varied from 0 to 1 in BST layers. Thus, the fabrication of the artificial SLs with variable composition of the constituting layers even with large modulation period allows strong variation of the ferroelectric properties comparing to the BT/ST SLs with variable layers thickness. Strain engineering in artificial superlattices allowed modifying their dielectric properties by varying either the Ba/Sr in BST layer concentration or the modulation periodicity in the SLs.

Thin films of multiferroic BiFeO3/MgO revealed no significant transformation of their Raman spectra, while BST/BF SLs exhibited drastic changes of the Raman response as a function of layers thickness due significant epitaxial strains between alternating layers.

Acknowledgment. This work was supported by Russian Foundation for Basic Research (Grant № 12-02-91051 CNRS_а)



Polarons in Magnetoelectric Fluorides


C. Filipič, G. Tavčar, E. Goreshnik, B. Žemva, and A. Levstik


Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia


e-mail: cene.filipic@ijs.si


The vast majority of all known inorganic ferroelectrics and multiferroics are transitional metal oxides with perovskite structures. But some of the fluorides and oxyfluorides also show ferroelectric properties and are potential multiferroic systems. We studied several fluorides system, i. e., K3Fe5F15, K3Fe2Cr3F15, (NH4)2FeF6, and Pb5Cr3F19, and all of them at higher temperatures undergo ferroelectric/antiferroelectric phase transition, and some of them also ferromagnetic transition at intermediate or at low temperatures. But the common characteristic of the studied fluorides systems is the appearance of small polarons which govern the charge transport at low temperatures.

In all studied fluoride systems the complex dielectric constant/ac electrical conductivity was investigated as a function of the frequency and temperature. In some substances at higher temperatures the charge transport is governed by a thermally activated process, while at lower temperatures, in all studied systems, the real part of the complex ac electric conductivity was found to follow the universal dielectric response  s, being typical for hopping or tunneling of localized charge carriers. A detailed analysis of the temperature dependence of the UDR parameter s in terms of the theoretical model for tunneling of small polarons revealed that, at low temperatures, this mechanism governs the charge transport in all studied fluoride systems.


Evolution Of Nanodomain Structures In Uniaxial Ferroelectrics
And Achievements Of Nanodomain Engineering



V.Ya. Shur1,2


1Ferroelectric Laboratory, Ural Federal University, Ekaterinburg, 620000, Russia

2Labfer Ltd., Ekaterinburg, 620014, Russia

e-mail: vladimir.shur@usu.ru


Nanodomain engineering represents one of the most important targets of the ferroelectric science and technology nowadays. The periodical tailored domain structures are successively used for spatial modulation of the electro-optic and nonlinear optical properties for manufacturing various devices with upgraded performance. The further development of the poling process for fabrication of sub-micron-pitch gratings and precise engineered domain structures for the photonic applications needs the study of the domain evolution with submicron spatial resolution.

The application of the modern high-resolution experimental methods for studying the domain structure formation in nanoscale has allowed to reveal and to investigate the important role of the formation and evolution of the ensembles of isolated nanodomains. The reviewed resent achievements in research of the nanodomain kinetics in highly non-equilibrium switching conditions discovered the important role of the “earlier invisible” nanodomains and formation of self-assembled nanodomain structures.

The single crystals of lithium niobate (LN) and lithium tantalate (LT) family were chosen for investigation both as the model ferroelectrics and the most important for application nonlinear optical materials. These uniaxial crystals possess simple domain structure which can be studied by various experimental techniques, and ability to change in wide range the bulk screening rate by heating.

The nanodomain kinetics has been studied in the samples with the surface layer modified by proton exchange and during switching by pyroelectric field during cooling after fast pulse laser heating. Various types of the nanoscale domain structures and scenarios of their evolutions have been singled out. The analysis of nanodomain images at different depth in the bulk obtained using scanning laser confocal Raman microscopy allows us to obtain the unique information about formation of the nanodomain structures.

The obtained knowledge opens the new approach to development of nanodomain engineering. The high-efficient crystals with periodical domain structure for laser light frequency conversion in wide spectral range have been manufactured.


Acknowledgment. The equipment of the Ural Center for Shared Use “Modern Nanotechnology”, Institute of Natural Sciences, Ural Federal University has been used. The research was made possible in part by RFBR (Grants 10-02-96042-r-Ural-а, 10-02-00627-а, 11-02-91066-CNRS-а); by Ministry of Education and Science (Contracts P1262 and 16.552.11.7020), by OPTEC LLC and in terms of Ural Federal University development program with the financial support of young scientists.


References

1. V.Ya. Shur, Nano- and Micro-domain Engineering in Normal and Relaxor Ferroelectrics, in Handbook of Advanced Dielectric, Piezoelectric and Ferroelectric Materials, ed. by Z.-G. Ye (Woodhead Publishing Ltd., Cambridge 2008), pp. 622-669.

2. V.Ya. Shur, M.S. Nebogatikov, D.O. Alikin, et al. J. Appl. Phys. 110, 052013 (2011).

3. V.Ya. Shur, D.K. Kuznetsov, E.A. Mingaliev, et al. Appl. Phys. Lett. 99, 082901 (2011).


Magnetodielectric Effect and Magnetoelectricity in Multiferroics

and Heterogeneous Systems: Modeling and Experiment


A.V. Turik


Department of Physics, Southern Federal University, 344090 Rostov-on-Don, Russia

e-mail: turik@sfedu.ru


Quantitative measures of magnetodielectric effect (MDE) are magnetodielectric coefficient MD and magnetoelectric coefficient of dielectric losses ML (magnetodielectric and magnetolosses [1])



where ε = ε′ - i ε′′ is complex dielectric permittivity of the material, e(H) and e(0) are dielectric permittivities measured at alternating electric field with strength Е and induction D in presence and in absence of the constant magnetic field H and induction B, respectively.

For explanation of MDE a number of models have been offered. It is possible to mention the models of antiferromagnetic spin fluctuations, spin-dependent polarization due to accumulation of a spatial charge [2], a combination of magnetoresistance effect and Maxwell-Wagner relaxation [1], magnetostriction and electrostriction [3], occurrence of the Hall effect on rough surfaces metal-dielectric, change of polarization of oxygen octahedrons owing to interaction of the magnetic field with magnetic moments of Fe ions. For lead ferroniobate PbFe1/2Nb1/2O3 (PFN) ceramics the model not considered earlier, in which MDE connects with a shift TC in the magnetic field of ferro-paraelectric phase transition temperature, is of our main interest.

For research of influence of electric field E on magnetoelectric permittivity = D/H the formulae [4] were used. For mutually perpendicular and parallel orientation of Е3 and H1


D3 = 330E3 + 31H1, D3 / E3 = 33H = 330 + H1 31 / E3, (1)

D3 = 330E3 + 33H3, D3 / E3 = 33H = 330 + H3 33 / E3. (2)


As is seen from (1) and (2), MDE is responsible for 31 and 33 dependence on E3. As /E = 2D/EH = /H, a sign /E is defined by a sign of MD, i.e. by a sign of /H. The value and a sign of /H close TC strongly depend on temperature, and /H (TC)  0.

For BaTiO3 crystal TC = 0.1–0.3 C at B = 10–20 T. In semiconductor ferroelectrics displacement of the Curie temperature in the magnetic field is by one or two order of magnitude greater than in BaTiO3 (for example, in (PbGe)Te TC  1 C at B = 3 T). Displacement TC is connected with the magnetoelectric contribution -1/2ijPi2Mj2 [5] in the free energy (λij = 2CklQe,kiQm,lj are connection factors, C are elasticity modules, Qe and Qm are magnetostriction and electrostriction coefficients, Pi and Mj are electric polarization and magnetization). This contribution in multiferroics can be much more, than in BaTiO3. Strengthening MDE in PFN ceramics is connected with found out at T > 300 K spontaneous magnetization Ms due to presence of weak ferromagnetism, probably, owing to micro impurities PbFe12O19.

References

[1] G. Catalan. Appl. Phys. Lett. 88, 102902 (2006).

[2] Y. Chen, X-Y. Zhang, C. Vittoria, V.G. Harris. Appl. Phys. Lett. 94, 102906 (2009).

[3] H.Y. Hwang, S-W. Cheong, N. Ong, B. Batlogg. Phys. Rev. Lett. 77, 2041 (1996).

[4] A.V. Turik, A.V. Pavlenko, K.P. Andryshin et al. Phys. Sol. State 54, 947 (2012).
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