We focus on study of dielectric and vibration spectra of ferroelectrics in the form of single crystals, ceramics, thin films and multilayers in a very broad spectral and temperature range (1 mHz – 150 THz, 5 – 950 K).
Head | |
Stanislav Kamba | infrared and broad-band dielectric spectroscopy |
Staff | |
Viktor Bovtun | microwave and broad-band dielectric spectroscopy |
Elena Buixaderas | Raman and infrared spectroscopy |
Ivan Gregora | Raman spectroscopy, PFM microscopy connected with Raman spectroscopy |
Martin Kempa | microwave and broad-band dielectric spectroscopy |
Dmitry Nuzhnyy | infrared and terahertz spectroscopy |
Tetyana Ostapchuk | infrared spectroscopy |
Jan Petzelt | infrared and broad-band dielectric spectroscopy, structural phase transitions |
Jan Pokorný | Raman spectroscopy |
Maxim Savinov | low-frequency dielectric spectroscopy |
Vladimír Vorlíček | Raman spectroscopy |
PhD. students | |
Fedir Borodavka | Raman spectroscopy |
Veronica Goian | infrared spectroscopy of multiferroics |
Iegor Rafalovski | Raman spectroscopy |
Our studies of phonon dynamics are frequently supplemented by inelastic neutron scattering studies in Institute of Laue-Langevin and inelastic X-ray scattering in ESRF (both in Grenoble, France).
Infrared spectroscopy with external magnetic fields we conduct in GHMFL (Grenoble, France). Magnetic and magnetocapacitive experiments are performed in the Departement of magnetic and low-temperature physics in our Institute (equipped with PPMS14 and SQUID Magnetometer Quantum Design).
We have investigated the magnetoelectric effect in BiFeO3 and observed rather a large change of permittivity with magnetic field at high temperatures, where the sample becomes partially conducting due to defects. However, in this case the magnetoelectric effect is not intrinsic, i.e. due to coupling of polarization and magnetization, but due to combination of magnetoresistance and Maxwell-Wagner polarization effect (see S. Kamba et al., Phys. Rev. B 75, 024403 (2007)). It means that in BiFeO3 one cannot expect switching of magnetization with an electric field and vice versa, although the electric control of antiferromagnetic domain structure was reported. Baettig and Spaldin predicted from ab initio calculations that the chemically ordered double perovskite Bi2FeCrO6 should have higher magnetization than BiFeO3 and comparably high polarization. We have investigated Bi2FeCrO6 thin films and shown that if the B-site cations are ordered, this system actually belongs to the rare high-temperature multiferroics, but its magnetization and polarization is comparable to BiFeO3 and critical temperatures are far above room temperature (TN > 600 K and TC > 900 K). For details see S. Kamba et al., Phys. Rev. B 77, 104111 (2008)).
Strong magnetodielectric effect was observed in EuTiO3 crystal, which exhibits quantum paraelectric behavior similar to SrTiO3: its permittivity ε′ increases on cooling and finally ε′ saturates below ~30 K. In contrast to SrTiO3, EuTiO3 undergoes an antiferromagnetic phase transition at TN = 5,5 K, and the phase transition is accompanied by a sharp drop down of ε′ below TN (without magnetic field). In a static magnetic field ε′ increases so that the drop down disappears for fields above 1 T. The magnetoelectric effect is huge - about 7%. We have shown that the temperature dependence of ε′ between 300 and 6 K is due to a soft optic phonon (vibration of magnetic Eu2+ cation) which reduces its frequency on cooling and finally the soft mode frequency saturates below 30 K. For details see [Kamba et al., Europhys. Lett. 80, 27002 (2007) and Goian et al., Eur. Phys. J. B 71, 429 (2009)]. Observed tuning of ε′ with magnetic field should be caused by tuning of the soft phonon frequency, which we experimentally confirmed.
Temperature dependence of the permittivity of magnetoelectric EuTiO3 for various magnetic fields. The change of the permittivity with magnetic field is due to a magnetic field-induced frequency shift of the polar phonon.
Other important results on multiferroics were published in 2010 in Nature and Nature Materials. Comments on these results can be found here.
Detailed temperature dependence of ferroelectric soft modes and a broad dielectric relaxations were investigated not only in lead based piezoelectrics [e.g. PLZT - see E. Buixaderas et al., Appl. Phys. Lett. 94, 052903 (2009)], but mainly in lead-free piezoelectrics like KNN [see E. Buixaderas et al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 56, 1843 (2009)] and NBT-BT [Hlinka et al., submitted to Ferroelectrics]
Teplotní závislost permitivity v Sr9-xPbxCe2Ti12O36 (x=0-9). Teploty maxim odpovídají teplotám feroelektrického fázového přechodu.
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