arXiv subjects

Stanislav Kamba

Publications and source records attributed to Stanislav Kamba.

28 records · Page 2Linked to original sources

arXiv2016

Electromagnon in the Z-type hexaferrite $({\rm Ba}_{x}{\rm Sr}_{1-x})_3\rm Co_2Fe_{24}O_{41}$

We studied experimentally the high-temperature magnetoelectric $({\rm Ba}_{x}{\rm Sr}_{1-x})_3\rm Co_2Fe_{24}O_{41}$ prepared as ceramics (x = 0, 0.2) and a single crystal (x = 0.5) using inelastic neutron scattering, THz time-domain, Raman and far-infrared spectroscopies. The spectra, measured with varying temperature and magnetic field, reveal rich information about the collective spin and lattice excitations. In the ceramics, we observed an infrared-active magnon which is absent in $E^ω\perp z$ polarized THz spectra of the crystal, and we assume that it is an electromagnon active in $E^ω \| z$ polarized spectra. On heating from 7 to 250 K, the frequency of this electromagnon drops from 36 to 25 cm$^{-1}$ and its damping gradually increases, so it becomes overdamped at room temperature. Applying external magnetic field has a similar effect on the damping and frequency of the electromagnon, and the mode is no more observable in the THz spectra above 2 T, as the transverse-conical magnetic structure transforms into a collinear one. Raman spectra reveal another spin excitation with a slightly different frequency and much higher damping. Upon applying magnetic field higher than 3 T, in the low-frequency part of the THz spectra, a narrow excitation appears whose frequency linearly increases with magnetic field. We interpret this feature as the ferromagnetic resonance.

cond-mat.mtrl-sci↗

arXiv2016

Far infrared and terahertz spectroscopy of ferroelectric soft modes in thin films: A review

Far-infrared and terahertz spectroscopy of ferroelectric soft and central modes in thin films on substrates is reviewed. In addition to classical displacive proper ferroelectrics, also incipient and relaxor ferroelectrics and multiferroics are discussed. Special attention is paid to differences between the soft-mode behavior in thin films and bulk materials (ceramics and single crystals) and their influence on the low-frequency permittivity. Particularly the effects of the thin film strains and depolarizing electric fields of the probing waves on the grain boundaries are emphasized. The soft-mode spectroscopy is shown to be a very sensitive tool to reveal the thin film quality.

cond-mat.mtrl-sci↗

arXiv2016

No clear evidence of ferroelectric order in tensile-strained anatase-TiO2 thin films grown on (110) NdGaO3 substrates

Very recently there was a report of the discovery using piezoelectric force microscopy (PFM) of a switchable ferroelectric polarization in 1.6 % tensile strained TiO2 thin film with anatase crystal structure (see N. Deepak et al. Adv. Funct. Mater. 2014, 24, 2844). The polarization disappeared only above 450 K, which was assigned as a Curie temperature, Tc. Here we have performed X-ray diffraction, second-harmonic generation (SHG) and infrared investigations of the same films. Phonon frequencies exhibit less than a 10 % shift down with the tensile strain and no anomaly near expected TC. SHG experiment did not reveal any signal characteristic for inversion symmetry breaking, as expected in ferroelectric phase, and the c-lattice parameter exhibits no anomaly on heating near expected TC. Based on these results, we can summarize that we were not able to confirm the previously discovered ferroelectricity in tensile-strained anatase-TiO2 thin films.

cond-mat.mtrl-sci↗

arXiv2014

Possible coupling between magnons and phonons in multiferroic CaMn7O12

Spin and lattice dynamics of CaMn7O12 ceramics were investigated using infrared, THz and inelastic neutron scattering (INS) spectroscopies in the temperature range 2 to 590 K, and, at low temperatures, in applied magnetic fields of up to 12 T. On cooling, we observed phonon splitting accompanying the structural phase transition at Tc = 450K as well as the onset of the incommensurately modulated structure at 250 K. In the two antiferromagnetic phases below T_N1 = 90K and T_N2 = 48 K, several infrared-active excitations emerge in the meV range; their frequencies correspond to the maxima in the magnon density of states obtained by INS. At the magnetic phase transitions, these modes display strong anomalies and for some of them, a transfer of dielectric strength from the higher-frequency phonons is observed. We propose that these modes are electromagnons. Remarkably, at least two of these modes remain active also in the paramagnetic phase; for this reason, we call them paraelectromagnons. In accordance with this observation, quasielastic neutron scattering revealed short-range magnetic correlations persisting within temperatures up to 500K above T_N1.

cond-mat.mtrl-sci↗

arXiv2014

Peculiar Bi-ion dynamics in Na1/2Bi1/2TiO3 from terahertz and microwave dielectric spectroscopy

Dynamics of the main dielectric anomaly in Na1/2Bi1/2TiO3 (NBT) was studied by time-domain THz and microwave spectroscopy, using also previously published data and their new overall fits. Above the dielectric maximum temperature Tm ~ 600 K, the response consists of coupled sub-THz oscillator and a relaxation mode, assigned to strongly anharmonic Bi-ion vibrations and hopping, whose slowing down explains the paraelectric-like permittivity increase to Tm. Below Tm, the main relaxation continues slowing down and additional relaxation, assigned to quasi-Debye losses, appears in the 10^11 Hz range. The oscillator hardens on cooling and takes over the whole oscillator strength. The permittivity decrease below Tm is caused by the reduced strength of the relaxations due to dominance of the rhombohedral phase within the coexistence region with the tetragonal phase. The anharmonic dynamics of Bi is supported by previous structural studies. NBT represents a hybrid between standard and relaxor ferroelectric behaviour.

cond-mat.mtrl-sci↗

arXiv2013

Electromagnon in ferrimagnetic eps-Fe2O3 nanograin ceramics

Electromagnons are known from multiferroics as spin waves excited by the electric component of electromagnetic radiation. We report the discovery of an excitation in the far-infrared spectra of eps-Fe2O3 which we attribute to an electromagnon appearing below 110 K, where the ferrimagnetic structure becomes incommensurately modulated. Inelastic neutron scattering shows that the electromagnon energy corresponds to that of a magnon from the Brillouin zone boundary. Dielectric measurements did not reveal any sign of ferroelectricity in eps-Fe2O3 down to 10 K, despite its acentric crystal structure. This shows that the activation of an electromagnon requires, in addition to the polar ferrimagnetic structure, a modulation of the magnetic structure. We demonstrate that a combination of inelastic neutron scattering with infrared and / or terahertz spectroscopies allows detecting electromagnons in ceramics, where no crystal-orientation analysis of THz and infrared spectra is possible.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic and dielectric properties of multiferroic Eu0.5Ba0.25Sr0.25TiO3 ceramics

Dielectric and magnetic properties of Eu0.5Ba0.25Sr0.25TiO3 are investigated between 10 K and 300 K in the frequency range from 10 Hz to 100 THz. A peak in permittivity revealed near 130 K and observed ferroelectric hysteresis loops prove the ferroelectric order below thistemperature. The peak in permittivity is given mainly by softening of the lowest frequency polar phonon (soft mode revealed in THz and IR spectra) that demonstrates displacive character of the phase transition. Room-temperature X-ray diffraction analysis reveals cubic structure, but the IR reflectivity spectra give evidence of a lower crystal structure, presumably tetragonal I4/mcm with tilted oxygen octahedra as it has been observed in EuTiO3. The magnetic measurements show that the antiferromagnetic order occurs below 1.8 K. Eu0.5Ba0.25Sr0.25TiO3 has three times lower coercive field than Eu0.5Ba0.5TiO3, therefore we propose this system for measurements of electric dipole moment of electron.

cond-mat.mtrl-sci↗

arXiv2012

Polar phonon anomalies in single crystalline TbScO3

Polarized infrared reflectivity spectra of a (110)-oriented TbScO3 single crystal plate were measured down to 10 K. The number of observed polar phonons active along the crystallographic c axis at low temperatures is much higher than predicted by factor-group analysis for the orthorhombic Pbnm space group. Moreover, the lowest frequency phonons active in E||c as well as in E||[1-10] polarized spectra exhibit dramatic softening tending to a lattice instability at low temperatures. The dielectric permittivity at microwave frequencies does not show any ferroelectric-like anomaly, but the dielectric loss exhibits a maximum at 100 K. The origin of the discrepancy between the number of predicted and observed polar phonons as well as the tendency toward lattice instability are discussed. Magnetic measurements reveal an antiferromagnetic phase transition near 3 K.

cond-mat.mtrl-sci↗

arXiv2008

Spectroscopic properties and lattice dynamics of ferroelectric and related functional oxide ceramics

Dielectric response as a function of frequency for high-permittivity dielectric and ferroelectric materials is discussed emphasizing the dynamic behaviour in the microwave and infrared range. After introducing the usual modelling of the polar phonon response and anharmonic hopping in locally dynamically disordered solids, including discussion of displacive and order-disorder ferroelectric phase transitions, we summarised the experimental data for selected ferroelectric, incipient ferroelectric and antiferroelectric perovskite ceramics (STO, BTO, BST, PZO), discussing the dielectric grain size effect due to a low-permittivity (dead) grain-boundary layer. Other important structural types of ferroelectrics (Aurivillius compounds SBT, pyrochlores CNO, PMN) are mentioned, as well. Attention is then paid to relaxor ferroelectrics (PLZT, PMN, PMT, PST, NBT of perovskite structure and SBN and a novel BLNTN solid solution system and SLTN of tungsten-bronze structure) and their extremely broad and complex dielectric dispersion. Then some new results on magnetoelectric multiferroics (BiFeO3, BiFe_(1/2)Cr_(1/2)O3, EuTiO3) are summarised and finally our results on microwave ceramics (used for microwave applications) are noted, discussing mainly the problem of extrapolating the microwave dielectric properties from the infrared and THz range. A glossary is added, briefly explaining the meaning of several concepts which might not be quite familiar for the reader.

cond-mat.mtrl-sci↗

arXiv2008

Ferroelectric and Incipient Ferroelectric Properties of a Novel Sr_(9-x)PbxCe2Ti2O36 (x=0-9) Ceramic System

Sr_(9-x)PbxCe2Ti12O36 system is derived from the perovskite SrTiO3 and its chemical formula can be written as (Sr_(1-y)Pby)0.75Ce0.167TiO3. We investigated dielectric response of Sr_(9-x)PbxCe2Ti12O36 ceramics (x = 0-9) between 100 Hz and 100 THz at temperatures from 10 to 700 K using low- and high-frequency dielectric, microwave (MW), THz and infrared spectroscopy. We revealed that Sr9Ce2Ti12O36 is an incipient ferroelectric with the R-3c trigonal structure whose relative permittivity e' increases from 167 at 300 K and saturates near 240 below 30 K. The subsequent substitution of Sr by Pb enhances e' to several thousands and induces a ferroelectric phase transition to monoclinic Cc phase for x>=3. Its critical temperature Tc linearly depends on the Pb concentration and reaches 550 K for x=9. The phase transition is of displacive type. The soft mode frequency follows the Barrett formula in samples with x<=2 (typical for quantum paraelectrics) and the Cochran formula for x>=3. The MW dispersion is lacking and quality factor Q is high in samples with low Pb concentration, although the permittivity is very high in some cases. However, due to the lattice softening, the temperature coefficient of the permittivity is rather high. The best MW quality factor was observed for x=1: Q*f=5800 GHz and e'=250. Concluding, the dielectric properties of Sr_(9- x)PbxCe2Ti12O36 are similar to those of Ba_(1-x)SrxTiO3 so that this system can be presumably used as an alternative for MW devices or capacitors.

cond-mat.mtrl-sci↗