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Study of proper and improper microwave dielectric losses by means of teraherz and infrared spectroscopy
Get PDFTitle: Study ol"proper and improper microwave dielectric tosses by means of tcrahcrz and infrared speetroseopy Author: Ondfej Tkac Department: Department of Dielectrics. Institute of Physics, Academy of Sciences of C/ech Republic Supervisor: RNDr. Slanislav Kamba.C'Sc.. Na Slovance 2. Prague 8 Supervisor's e-mail adress: kambawT/u.c/ Abstract: In the llrst part of this thesis there is described relevant theory, especially we pay our attention to the principles of infrared speetroseopy and fitting models, then to the explanation of the terms proper and improper dielectric losses and we also discuss dielectrics and ferroelectrics. By means of Fourier transform infrared speetroseopy we measured infrared spectra at room temperature in the range of 30 - 3000 em"1. We lilted the spectra and then we extrapolated the final complex permit!vity into the microwave range. This enabled us to estimate the proportion of improper dielectric losses on total microwave losses. By the ceramic SiVxPlxCeVfinO-^ (.\0 to 9) we studied the impact of amount of lead on dielectric properties. We found out that the substitution of strontium with lead causes the increase of permitivity up lo several thousands and forces ferroelectric transition for x>2. With perovskitc Ca/n, jNb^ :,-xVx(.)1 (.v=0 to 0,01) we examined the inlluence of..
Infrared Spectroscopy of Multiferroics
No full textInfrared Spectroscopy of Multiferroics Author: Veronica Goian Institute: Department of Dielectrics, Institute of Physics of the Academy of Sciences, Na Slovance 2, 182 21 Prague 8 Abstract: We have investigated numerous multiferroic and magnetoelectric materials mainly using infrared (IR) spectroscopy. Nevertheless, the studies were frequently combined with radio-frequency, microwave, THz, Raman and structural measurements provided by our colleagues, as well as by magnetic and elastic investigations, where we participated. Our main aim was the complex study of quantum-paraelectric antiferromagnet EuTiO3 in the form of crystals, ceramics and thin films. Near 300 K we have discovered an antiferrodistorive phase transition from cubic mPm3 to tetragonal I4/mcm structure in bulk EuTiO3 and explained its low-frequency dielectric properties by anomalous polar phonon behavior. Large and anisotropic magnetodielectric effect, which we found in EuTiO3, was successfully explained and experimentally confirmed by observation of tuning of phonon frequency with magnetic field. Our IR studies of tensile strained EuTiO3 thin films revealed a displacive ferroelectric phase transition near 250 K. Our American colleagues revealed the ferromagnetic order below 4.2 K in the same strained EuTiO3 thin film. In such way we have..
Static and dynamic magnetoelectric coupling in multiferroics
No full textTitle: Static and Dynamic Magnetoelectric Coupling in Multiferroics Author: André Maia Institute: Department of Dielectrics, Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21, Prague 8 Supervisor of the doctoral thesis: RNDr. Stanislav Kamba, CSc., DSc. Abstract: This thesis explores the multiferroic properties and magnetoelectric coupling in different compounds. The first study investigates BiMn3Cr4O12, where the ferroelectric transition induces an antiferromagnetic phase. The ferroelectric critical temperature corresponds to the Néel temperature below which the Cr3+ spins exhibit G-type antiferromagnetic order, and an increase in ferroelectric polarization is observed at the second Néel temperature of the Mn3+ spins, indicating enhanced magnetoelectric coupling. In the second study, the focus is on BiMn7O12 ceramics, revealing two structural phase transitions above room temperature related to ferroelectricity. Dielectric anomalies and Raman spectroscopy support the presence of static magnetoelectric coupling at the low-temperature magnetic phase transitions. The third study examines the effects of Mn3+ substitution with Fe3+ in TbMnO3. Different mechanisms for static and dynamic magnetoelectric coupling are observed, along with electromagnon and crystal-field excitations coupled..
Magnetodielectric coupling in EuTiO3
No full textTeoretická část práce se zabývá fonony, dielektrickými a magnetickými vlastnosti EuTiO3 a rozborem mechanismů vedoucích k existenci magnetodielektrické vazby v něm. Druhá část práce představuje zejména infračervenou a terahertzovou spektroskopii, tedy techniky, které byly použity k měření vlastností EuTiO3 keramik, a dále vysvětluje způsoby vyhodnocování spekter. Výsledky měření celkem osmi keramik (včetně jedné Eu2Ti2O7) jsou obsaženy v poslední kapitole. V teplotním intervalu 5 - 150 K se podařilo prokazatelně pozorovat měknutí dvou polárních fononů, které je příčinou vzrůstu statické permitivity s ochlazováním. Na základě pozorování dielektrické anizotropie (při 1 kHz) ve vnějším magnetickém poli předpovídáme rozštěpení měkkých fononů v přítomnosti tohoto pole. Rozštěpení bude o řád větší než u nedávno předpovězeného fononového Zeemanova jevu. Toto rozštěpení se v blízké budoucnosti chystáme ověřit pomocí terahertzové spektroskopie.Theoretical part of the thesis deals with phonons, dielectric and magnetic properties of EuTiO3 and with analysis of mechanisms leading to existence of magnetodielectric coupling in it. Following part of the thesis introduces mostly infrared and terahertz spectroscopy, which means techniques, that were used for investigation of EuTiO3 ceramics, and also explains ways of spectra evaluation. Experimental results obtained on eight ceramics (including one Eu2Ti2O7) are summarized in the last chapter. In temperature range of 5 - 150 K, softening of two polar phonons was provably observed, which implicates an increase of static permittivity during cooling. Based on observation of dielecric anisotropy (at 1 kHz) in external magnetic field we predict soft phonon splitting in this field, namely one order higher than in case of recently predicted Zeeman's effect. We are going to verify this splitting in near future using the terahertz spectroscopy
Study of multiferroics with hexaferrite crystal structure
No full textPráce se zabývá charakterizací fyzikálních vlastností polykrystalických hexaferitů strukturních typů M, Y a Z, které vykazují magnetoelektrický jev potenciálně využitelný v pamětech pro informační technologie. Infračervené reflektivity byly měřeny v závislosti na teplotě od 300 do 10 K, THz spektra byla studována v závislosti na teplotě i na magnetickém poli do 7 T. Pozorovali jsme jak mřížkové, tak i spinové excitace s výrazně odlišným chováním v závislosti na strukturním typu hexaferitu. V případě (Ba0,2Sr0,8)3Co2Fe24O41 (Z-hexaferitová struktura) jsme navíc provedli statická magnetická a magnetodielektrická měření, která kvalitativně prokázala magnetoelektrický jev. Také jsme změřili infračervenou reflektivitu monokrystalu Ba0,5Sr2,5Co2Fe24O41 a porovnali ji s reflektivitou keramiky.The work deals with characterization of physical properties of polycrystalline hexaferrrites of the structural types M, Y and Z, which exhibit magnetoelectric effect, potentially applicable in the memories for IT. Infrared reflectivities were measured from 300 to 10 K. THz spectra were studied depending on temperature and magnetic field up to 7 T. Lattice and spin excitations with different behaviour dependent on the crystal structure were observed. In case of (Ba0.2Sr0.8)3Co2Fe24O41 with Z-hexaferrite structure, we performed static magnetic and magnetodielectric measurements to qualitatively prove magnetoelectric effect. We also measured infrared reflectivity of Ba0.5Sr2.5Co2Fe24O41 single crystal and compared it with spectra of ceramic
Study of the spin and lattice dynamics in ferroics, multiferroics, and magnetoelectrics
Get PDFDisertační práce obsahuje výsledky kandidátova experimentálního výzkumu zaměřeného na dynamické vlastnosti krystalických a magnetických mřížek feroických a multiferoických materiálů s potenciálně silnou magnetoelektrickou vazbou. Tyto byly studovány za použití spektroskopických technik v infračervené, terahertzové, mikrovlnné, milimetrové a nízkofrekvenční oblasti a také pomocí metody generace druhé harmonické frekvence. Jádro práce je tvořeno šesti vědeckými publikacemi a manuskripty, které shrnují výsledky kandidátova výzkumu na vybraných zástupcích tří materiálových skupin perovskitových oxidů, hexagonálních feritů a van der Waalsovských halogenidů.The doctoral thesis contains results of the candidates experimental research focused mainly on the dynamic properties of the crystalline and magnetic lattices of ferroic and multiferroic materials with a potentially strong magnetoelectric coupling. They were studied using the infrared, terahertz, microwave, millimeter-wave, and low-frequency spectroscopic techniques as well as the second harmonic generation method. The core of the thesis consists of six scientific publications and manuscripts which summarize the results of the candidates research on representatives of three material groups - perovskite oxides, hexagonal ferrites, and van der Waals halides
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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