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J. Kreisel

Publications and source records attributed to J. Kreisel.

31 records · Page 2Linked to original sources
arXiv2010

Temperature-dependent Raman scattering of KTa1-xNbxO3 thin films

We report a Raman scattering investigation of KTa1-xNbxO3 (x = 0.35, 0.5) thin films deposited on MgO and LaAlO3 as a function of temperature. The observed phase sequence in the range from 90 K to 720 K is similar to the structural phase transitions of the end-member material KNbO3. Although similar in the phase sequence, the actual temperatures observed for phase transition temperatures are significantly different from those observed in the literature for bulk samples. Namely, the tetragonal (ferroelectric) to cubic (paraelectric) phase transition is up to 50 K higher in the films when compared to bulk samples. This enhanced ferroelectricity is attributed to biaxial strain in the investigated thin films.

cond-mat.mtrl-sci↗
arXiv2010

Temperature-dependent Raman scattering of DyScO3 and GdScO3 single crystals

We report a temperature-dependent Raman scattering investigation of DyScO3 and GdScO3 single crystals from room temperature up to 1200 °C. With increasing temperature, all modes decrease monotonously in wavenumber without anomaly, which attests the absence of a structural phase transition. The high temperature spectral signature and extrapolation of band positions to higher temperatures suggest a decreasing orthorhombic distortion towards the ideal cubic structure. Our study indicates that this orthorhombic-to-cubic phase transition is close to or higher than the melting point of both rare-earth scandates (\approx 2100 °C), which might exclude the possibility of the experimental observation of such a phase transition before melting. The temperature-dependent shift of Raman phonons is also discussed in the context of thermal expansion.

cond-mat.mtrl-sci↗
arXiv2009

Phase transitions and ferroelectrics: revival and the future in the field

It appeared worthwhile to us to present a state-of-the-art look at the field of ferroelectrics. We are certainly not attempting to provide a complete review of all aspects of the field of ferroelectrics over the last years but we wish to transport a flavour of the current excitement in the field through the (subjective) choice of four specific examples of current interest: (i) Piezoelectrics and the morphotropic phase boundary, (ii) Multiferroics, (iii) The effect of high pressure on ferroelectrics and (iv) Strain-engineering in ferroelectric oxide thin films. For each topic we will try to work out both current interesting approaches and an outlook into future challenges. Throughout our discussion, the reader is referred to a list of significant review articles, books and papers in the field.

cond-mat.mtrl-sci↗
arXiv2009

Raman scattering of perovskite DyScO3 and GdScO3 single crystals

We report an investigation of DyScO3 and GdScO3 single crystals by Raman scattering in various scattering configurations and at various wavelengths. The Raman spectra are well-defined and the reported spectral signature together with the mode assignment sets the basis for the use of Raman scattering for the investigation of RE-scandates. The observed positions of Raman modes for DyScO3 are for most bands in reasonable agreement with recent theoretical ab initio predictions of the vibrational spectrum for the same material. Further to the phonon signature, a luminescence signal is observed for both scandates. While the luminescence is weak for DyScO3, it is very intense for GdScO3 when using a 488 or 514 nm excitation line, which in turn inhibits full analysis of the phonon spectrum. We show that a meaningful phonon Raman analysis of GdScO3 samples can be done by using a 633 nm excitation.

cond-mat.mtrl-sci↗
arXiv2009

Effect of high pressure on multiferroic BiFeO3

We report experimental evidence for pressure instabilities in the model multiferroic BiFeO3 and namely reveal two structural phase transitions around 3 GPa and 10 GPa by using diffraction and far-infrared spectroscopy at a synchrotron source. The intermediate phase from 3 to 9 GPa crystallizes in a monoclinic space group, with octahedra tilts and small cation displacements. When the pressure is further increased the cation displacements (and thus the polar character) of BiFeO3 is suppressed above 10 GPa. The above 10 GPa observed non-polar orthorhombic Pnma structure is in agreement with recent theoretical ab-initio prediction, while the intermediate monoclinic phase was not predicted theoretically.

cond-mat.mtrl-sci↗
arXiv2009

Coupling between phonons and magnetic excitations in orthorhombic Eu_{1-x}Y_xMnO_3

In this work we present a detailed study of the structural and lattice dynamic properties of Y-doped EuMnO3 ceramics (Eu1-xYxMnO3, with 0<x<0.55). A thorough analysis towards the correlation between both structural and Raman modes parameters has been undertaken. Our results provide evidence for two main structural distortions of MnO6 octahedra, arising from a cooperative Jahn-Teller and rotational distortions in these compounds. The temperature dependence of the B1g symmetric stretching mode of the MnO6 units has revealed either a positive or negative shift regarding the pure anharmonic temperature dependence of the phonon frequency, which strongly depends on the Y-concentration. This frequency renormalization is explained in terms of a competition between ferro and antiferromagnetic interactions. The pronounced frequency renormalization of the symmetric stretching mode for x=0.2 evidences the predominance of the ferromagnetic interactions against the antiferromagnetic ones, which is in good correlation with the spontaneous magnetization measured in this compound. The emerging of the shifts referred well above the Neel temperature is likely associated with the coupling between phonons and dynamical fluctuations of the magnetic system, which provides further grounds for the existence of strong spin-phonon coupling in this compound.

cond-mat.mtrl-sci↗
arXiv2008

Raman scattering investigation across the magnetic and MI transition in rare earth nickelate RNiO3 (R = Sm, Nd) thin films

We report a temperature-dependent Raman scattering investigation of thin film rare earth nickelates SmNiO3, NdNiO3 and Sm0.60Nd0.40NiO3, which present a metal-to-insulator (MI) transition at TMI and an antiferromagnetic-paramagnetic Neel transition at TN. Our results provide evidence that all investigated samples present a structural phase transition at TMI but the Raman signature across TMI is significantly different for NdNiO3 (TMI = TN) compared to SmNiO3 and Sm0.60Nd0.40NiO3 (TMI =/ TN). It is namely observed that the paramagnetic-insulator phase (TN < T < TMI) in SmNiO3 and Sm0.60Nd0.40NiO3 is characterized by a pronounced softening of one particular phonon band around 420 cm-1. This signature is unusual and points to an important and continuous change in the distortion of NiO6 octahedra (thus the Ni-O bonding) which stabilizes upon cooling at the magnetic transition. The observed behaviour might well be a general feature for all rare earth nickelates with TMI =/ TN and illustrates intriguing coupling mechanism in the TMI > T > TN regime.

cond-mat.mtrl-sci↗
arXiv2007

Pressure-induced phase transitions in the multiferroic perovskite BiFeO3 studied by far-infrared micro-spectroscopy

We present the results of pressure-dependent far-infrared reflectivity measurements on the multiferroic perovskite BiFeO3 at room temperature. The observed behavior of the infrared-active phonon modes as a funtion of pressure clearly reveals two structural phase transitions around 3.0 and 7.5 GPa, supporting the results of recent Raman and x-ray diffraction studies under pressure. Based on the pressure-dependent frequency shifts of the infrared-active phonon modes we discuss the possible character of the phase transitions.

cond-mat.other↗
arXiv2007

Structural distortion and magnetism of BiFeO3 epitaxial thin films: a Raman spectroscopy and neutron diffraction study

A previous study of the growth conditions has shown that single-phase BiFeO3 thin films can only be obtained in a narrow pressure-temperature window and that these films display a weak magnetic moment. Here we study in more detail the structure and the magnetism of single-phase BiFeO3 films by means of reciprocal space mapping, Raman spectroscopy and neutron diffraction. X-ray and Raman data suggest that the BiFeO3 structure is tetragonal for 70 nm-thick films and changes to monoclinic for 240 nm-thick films, thus remaining different from that of the bulk (rhombohedral) structure. In the 240 nm monoclinically distorted film neutron diffraction experiments allow the observation of a G-type antiferromagnetic order as in bulk single crystals. However, the satellite peaks associated with the long-wavelength cycloid present in bulk BiFeO3 are not observed. The relevance of these findings for the exploitation of the magnetoelectric properties of BiFeO3 is discussed.

cond-mat.mtrl-sci↗
arXiv2006

Infrared and THz studies of polar phonons and improper magnetodielectric effect in multiferroic BFO3 ceramics

BFO3 ceramics were investigated by means of infrared reflectivity and time domain THz transmission spectroscopy at temperatures 20 - 950 K, and the magnetodielectric effect was studied at 10 - 300 K, with the magnetic field up to 9 T. Below 175 K, the sum of polar phonon contributions into the permittivity corresponds to the value of measured permittivity below 1 MHz. At higher temperatures, a giant low-frequency permittivity was observed, obviously due to the enhanced conductivity and possible Maxwell-Wagner contribution. Above 200 K the observed magnetodielectric effect is caused essentially through the combination of magnetoresistance and the Maxwell-Wagner effect, as recently predicted by Catalan (Appl. Phys. Lett. 88, 102902 (2006)). Since the magnetodielectric effect does not occur due to a coupling of polarization and magnetization as expected in magnetoferroelectrics, we call it improper magnetodielectric effect. Below 175 K the magnetodielectric effect is by several orders of magnitude lower due to the decreased conductivity. Several phonons exhibit gradual softening with increasing temperature, which explains the previously observed high-frequency permittivity increase on heating. The observed non-complete phonon softening seems to be the consequence of the first-order nature of the ferroelectric transition.

cond-mat.mtrl-sci↗
arXiv2006

EXAFS study of lead-free relaxor ferroelectric BaTi(1-x)Zr(x)O3 at the Zr K-edge

Extended X-ray absorption fine structure (EXAFS) experiments at the Zr K-edge were carried out on perovskite relaxor ferroelectrics BaTi(1-x)Zr(x)O3 (BTZ) (x = 0.25, 0.30, 0.35), and on BaZrO3 for comparison. Structural information up to 4.5 A around the Zr atoms is obtained, revealing that the local structure differs notably from the average Pm-3m cubic structure deduced from X-ray diffraction. In particular, our results show that the distance between Zr atoms and their first oxygen neighbors is independent of the Zr substitution rate x and equal to that measured in BaZrO3, while the X-ray cubic cell parameter increases linearly with x. Furthermore, we show that the Zr atoms tend to segregate in Zr-rich regions. We propose that the relaxor behavior in BTZ is linked to random elastic fields generated by this particular chemical arrangement, rather than to random electric fields as is the case in most relaxors.

cond-mat.mtrl-sci↗
arXiv2003

High pressure phases in highly piezoelectric Pb(Zr0.52Ti0.48)O3

Two novel room-temperature phase transitions are observed, via synchrotron x-ray diffraction and Raman spectroscopy, in the Pb(Zr0.52Ti0.48)O3 alloy under hydrostatic pressures up to 16 GPa. A monoclinic (M)-to-rhombohedral (R1) phase transition takes place around 2-3 GPa, while this R1 phase transforms into another rhombohedral phase, R2, at about 6-7 GPa. First-principles calculations assign the R3m and R3c symmetry to R1 and R2, respectively, and reveal that R2 acts as a pressure-induced structural bridge between the polar R3m and a predicted antiferrodistortive R-3c phase.

cond-mat↗