Fyzikální ústav Akademie věd ČR

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).

People

Head
Stanislav Kamba	infrared and broad-band dielectric spectroscopy
Staff
Viktor Bovtun	microwave and broad-band dielectric spectroscopy
Fedir Borodavka	Raman spectroscopy
Elena Buixaderas	Raman and infrared spectroscopy
Veronica Goian	infrared spectroscopy of multiferroics
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
Iegor Rafalovski	Raman spectroscopy
Jakub Vít
Stella Skiadopoulou

What is original and unique in our lab?

• We developed unique experimental methods for dielectric measurements of high-permittivity and high-loss ferroelectric materials (bulk and thin films) in the microwave range from 10 to 400 K.

• Time domain THz transmission spectra (0.1-2.4 THz) can be measured between 5 and 900 K, THz reflectance below 300 K. A new recent acquisition allows us to measure the THz transmission spectra in a magnetic field up to 7 T (temperatures 1.5 to 300 K). All this in collaboration with the THz spectroscopy group.

• Infrared dielectric response can be obtained not only on bulk samples but also on ultra-thin films with thickness down to 20 nm.

• Micro-Raman spectrometer (488 nm excitation) in combination with AFM microscope allows us to study phonons with high spatial resolution together with AFM topography. Additional ultraviolet laser (325 nm excitation) is suitable for measurements of ultrathin films.

• In collaboration with other groups from our Institute we can study magnetoelectric and magnetic properties of multiferroics.

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).

Equipment

• Dielectric analyzer Alpha_AN (Novocontrol), frequency range 3x10^-6Hz–20 MHz, 10–900K
  (contact: M. Savinov, ext. 2641, room 135)
• Impedance analyzer HEWLETT-PACKARD 4192A, frequency range 100 Hz – 5 MHz, 10 – 900 K
  (contact: M. Savinov, ext. 2641, room 135)
• Impedance analyzer AGILENT 4291B, frequency range 1 MHz – 1.8 GHz, coaxial technique, 100 – 550 K
  (contact: V. Bovtun, ext. 2618, room 143)
• Network analyzer AGILENT E8364B, frequency range 50 MHz – 50 GHz, coaxial technique suitable for dielectric measurements of high-permittivity high loss thin films and bulks, temperature range 10 – 400 K
  (contact: V. Bovtun, ext. 2618, room 143)
• Custom made setup for time-domain THz spectroscopy; spectral range: 5 – 80 cm^-1 (0.1 – 2.5 THz), temperature range 10 – 950 K, magnetic fíeld up to 7 T (in collaboration with the THz spectroscopy group)
• Fourier spectrometer BRUKER IFS113v, spectral range 15-10.000 cm^–1, temperature range 5 – 950 K, transmission and specular reflection measurements (two instruments)
  (contact: S. Kamba, ext. 2957, room 133)
• Micro-Raman spectrometer RM 1000 (RENISHAW), multichannel detection, temperature range 10 – 1450 K, polarizing microscope (excitation @ 514 or 633 nm)
  (contact: I. Gregora, ext. 2654, room 128)
• In-Via Reflex Raman Microscope (RENISHAW) combined with NTEGRA Spectra AFM Upright Microscope (NT-MDT). optical AFM head – for Raman and AFM mapping (excitation @ 488 or 325 nm)
  (contact: I. Gregora, ext. 2654, room 128)

Some recent results

New approach to preparation of highly tunable microwave dielectrics

Sr_n+1Ti_nO3_n+1 has layered crystal structure (see Figure below) and it has excellent dielectric properties in the form of 50 nm thin films with mechanical tensile strain. These properties are highly promising for applications in microwave electronics. The paper, describing unique properties and preparation of biaxially strained thin films of Sr_n+1Ti_nO_3n+1, was published in Nature journal in 2013.

Modern microwave filters and resonators used in cellular phones are made from materials, whose permittivity is tuned by electric field. Ferroelectrics reach highest tunability near temperature of the phase transition from paraelectric to ferroelectric phase, because the permittivity is the highest at this temperature. Nowadays, mainly Ba_1-xSr_xTiO₃ is commercially used, because it exhibits high tuning of permittivity. Unfortunatelly, this material has undesirable high dielectric losses. Scientists from Department of Dielectrics in cooperation with international team theoretically predicted, physically prepared and experimentally characterized layered perovskite system Sr_n+1Ti_nO_3n+1 with n=1-6. Although this system is non-ferroelectric in the bulk crystals or ceramics, their thin films become ferroelectric under tensile strain and their critical temperature (T_c) increases with n. Owing to this their permittivity and tunability increase. The best properties were obtained in Sr₇Ti₆O₁₉, where high tunability and exceptionally low dielectric losses were observed. Sr7Ti6O19 has one order of magnitude better microwave properties than widely used Ba_1-xSr_xTiO₃ in tunable dielectric devices. Dielectric losses are usually caused by structural defects, which are always present in the materials. We found out that unique layered nanostructure of Sr_n+1Ti_nO_3n+1 „absorbs“ defects and therefore it exhibits low dielectric losses. Since only polar phonons contribute to microwave dielectric loss and ferroelectric soft mode is sensitive on electric field, microwave dielectric loss is anomalously low and tunability is very high.

See details in C. H. Lee et al., Nature 502, 532 (2013) .

Figure of layered crystal structure of Sr_n+1Ti_nO_3n+1 with n=1-6 and n=∞, and temperature dependence of dielectric permittivity in thin films of Sr_n+1Ti_nO_3n+1. Temperatures of permittivity maxima correspond to paraelectric-ferroelectric phase transition temperature.

This paper is result of a long-term collaboration between Institute of Physics ASCR and Cornell University in USA. Authors from Institute of Physics ASCR are marked in bold. For details contact S. Kamba (kambafzu [dot] cz)

Explanation of magnetodielectric effect and discovery of incommensurately modulated phase in EuTiO₃

EuTiO₃ is an antiferromagnet below T_N = 5.3 K. Temperature dependence of its permittivity ε exhibits quantum paraelectric behavior, i.e. ε increases on cooling and saturates at low temperatures due to quatum fluctuations, which inhibit the creation of long-range ferroelectric order. Below T_N, ε remarkably decreases, but in magnetic field it again increases by 7 % due to a very strong spin-phonon coupling. Thanks to this is possible to induce ferroeletricity in tensile strained thin films of EuTiO₃, which we experimentally proved two years ago.

This year we have measured compressively strained EuTiO₃ film deposited on LSAT substrate (LSAT = (LaAlO₃)_0.29-(SrAl_1/2Ta_1/2O₃)_0.71). Thanks to 0.9 % compressive strain, the phonons have higher frequencies and the permittivity of the film is lower than in the bulk samples. The lowest-frequency polar phonon exhibits a rather sharp feature so that it was possible to measure very accurately its shift with magnetic field. We have obtained about 1.5 % shift down of the phonon frequency, which is responsible for almost 3% increase in ε with magnetic field. In this way we have experimentally elucidated the origin of magnetodielectric effect in EuTiO3 and we believe that such mechanism should work as well in other electrically nonconducting magnetic materials.

Up to now, the bulk EuTiO₃ was assumed to crystallize in the cubic phase down to the lowest temperatures. Our detailed X-ray and electron diffraction together with dynamical mechanical analysis and infrared reflectivity studies revealed near 300 K an antiferrodistortive phase transition in the EuTiO₃ ceramics. The perovskite structure changes from cubic to tetragonal one due to an antiphase tilting of oxygen octahedra. Moreover, the electron diffraction on EuTiO₃ single crystal revealed that the tetragonal structure is incommensurately modulated.

See details in Kamba et al., Phys. Rev. B 85, 094435 (2012) and Goian et al., Phys. Rev. B 86, 054112 (2012).

a) IR reflectance spectra of EuTiO3 thin film measured at 1.9 K and at various magnetic fields. The shift of phonon frequency is clearly seen. Inset shows the incommensurate satellites (layer 1/2kl) observed at 300 K in electron diffraction of EuTiO₃ single crystal. b) Magnetic field dependence of static permittivity obtained from IR spectra of EuTiO₃ film deposited on the LSAT substrate.

Magnetoelectric multiferroics

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 (T_N > 600 K and T_C > 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 T_N = 5,5 K, and the phase transition is accompanied by a sharp drop down of ε′ below T_N (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.

Relaxor ferroelectrics

Relaxor ferroelectrics exhibit broad dielectric relaxations, therefore we use a broad-band dielectric spectroscopy (100 Hz – 100 THz) for investigation of these materials. Recently we have investigated coarse-grain and fine-grain ceramics of PbMg_1/3Nb_2/3O₃-35%PbTiO₃ [V. Bovtun et al., Phys. Rev. B 79, 104111 (2009)]. ε^*(f,T) in coarse-grain ceramics exhibits relaxor behavior at high temperatures and a sharp anomaly at the ferroelectric phase transition. The fine-grain ceramics exhibit mainly relaxor ferroelectric behavior with a smaller dielectric constant. The difference is explained by different relaxational dynamics of polar nanoclusters, which appear to be more stabilized at high temperatures in the fine-grain ceramics by pinning at grain boundaries. Below T_C, the growth of ferroelectric domains is suppressed in fine-grain ceramics as supported also by a second harmonic generation and therefore the macroscopic ferroelectric phase transition does not occur.

Dynamics of ferroelectric phase transitions in piezoelectrics

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]

New ferroelectric phase transition

V oddělení dielektrik byl objeven feroelektrický přechod v Sr_9-xPb_xCe₂Ti₁₂O₃₆ (x=0-9). Bylo ukázáno, že vzorky s nízkou koncentrací olova (x<3) jsou tzv. incipientní feroelektrika, tj. že se s ochlazováním blíží k feroelektrickému stavu, ale kvantové fluktuace mu zabrání vzniknout. Vzorky s vyšší Pb koncentraci se stávají feroelektrické a jejich kritická teplota lineárně roste s koncentrací olova. Komplexní dielektrická, terahertzová, infračervená a Ramanova spektra ukázala, že fázové přechody jsou čistě posuvného typu, protože se pozoroval jasný měkký feroelektrický mód fononového původu. Strukturní měření ukázala, že paraelektrická fáze má trigonální strukturu, zatímco feroelektrická krystaluje v monoklinické struktuře. Obsáhlý článek byl publikován v prestižním časopise Chemistry of Materials [S. Kamba et al., Chem. Mat., 21, 811 (2009)].

Teplotní závislost permitivity v Sr_9-xPb_xCe₂Ti₁₂O₃₆ (x=0-9). Teploty maxim odpovídají teplotám feroelektrického fázového přechodu.