M. G. Karkut

Domain-enhanced interlayer coupling in ferroelectric/paraelectric superlattices.

We investigate the ferroelectric phase transition and domain formation in a periodic superlattice consisting of alternate ferroelectric (FE) and paraelectric (PE) layers of nanometric thickness. We find that the polarization domains formed in the different FE layers can interact with each other via the PE layers. By coupling the electrostatic equations with those obtained by minimizing the Ginzburg-Landau functional, we calculate the critical temperature of transition Tc as a function of the FE/PE superlattice wavelength Lambda and quantitatively explain the recent experimental observation of a thickness dependence of the ferroelectric transition temperature in KTaO3/KNbO3 strained-layer superlattices.

Ferroelectric transition in an epitaxial barium titanate thin film: Raman spectroscopy and x-ray diffraction study

We have performed x-ray diffraction and Raman spectroscopy measurements in the temperature range of 300–873 K on a single phase epitaxially oriented BaTiO3 thin film grown by pulsed laser deposition on a single crystal MgO substrate. The θ–2θ room temperature diffraction measurements and asymmetric rocking curves indicate that the film is very weakly tetragonal with the c-axis parallel to the plane of the film. X-ray diffraction measurements up to high temperature reveal only a change in slope in the perpendicular to the plane lattice parameter around 450 K (in bulk Tc=395 K) indicating that a diffuse-like of phase transition is taking place. Room temperature polarized Raman spectra show that the film is indeed tetragonal with C4v symmetry and with the a-axis perpendicular to the film plane. Monitoring of the overdamped soft mode and the 308 cm−1 mode confirms that the phase transition is taking place over a wide temperature range according to the x-ray results. The increase of the phase transition temper...

Ferroelectric Q-phase in a NaNbO3 epitaxial thin film

Epitaxial NaNbO3 thin films have been grown by pulsed laser deposition on cubic (00l) MgO substrate with epitaxial (La0.5Sr0.5)CoO3 buffer layer. Micro-Raman spectroscopy studies revealed that the ferroelectric Q phase (Pmc21, Z=4) is stable in a 250-nm-thick film in contrast to the antiferroelectric phase P (Pbma, Z=8) known to exist in the bulk single crystals and ceramics of undoped stoichiometric NaNbO3. Temperature-dependent Raman spectra indicate that the Q phase is stable over a wide temperature range (at least from 80 to 600 K), while the low-temperature ferroelectric rhombohedral phase N, typical for NaNbO3 single crystals, is not observed.

ENHANCED MAGNETIZATION IN BiFeO3/BaTiO3 MULTILAYERS: AN INTERFACE EFFECT?

ABSTRACT We have used pulsed laser deposition to grow BiFeO3/BaTiO3 multilayers on SrTiO3 substrates. The samples are characterized by Reflection High Energy Electron Diffraction (RHEED), Atomic Force Microscopy (AFM), and by X-ray diffraction measurements and modelling. The two multilayers investigated here have modulation periods A = 90Å and 180Å, while the total thickness of 1800Å is the same for both samples. Hence, twice as many interfaces are encountered for the A = 90Å than for the A = 180Å sample. SQUID magnetometer measurements performed on these multiferroic samples show that the magnetization is significantly larger for the smaller wavelength sample thus pointing to a multilayer effect between the magnetic susceptibility and the number of interfaces per unit volume.

Epitaxial growth and magnetoelectric relaxor behavior in multiferroic 0.8Pb(Fe1/2Nb1/2)O3–0.2Pb(Mg1/2W1/2)O3 thin films

We present electric and magnetic properties of 0.8Pb(Fe1/2Nb1/2)O3–0.2Pb(Mg1/2W1/2)O3 films epitaxially grown on (001) SrTiO3 substrates using pulsed laser deposition. A narrow deposition window around 710 °C and 0.2 mbar has been identified to achieve epitaxial single-phase thin films. A typical Vogel–Fulcher relaxorlike dielectric and magnetic susceptibility dispersion is observed, suggesting magnetoelectric relaxor behavior in these films similar to the bulk. We determine a magnetic cluster freezing temperature of 36 K, while observing weak ferromagnetism via magnetic hysteresis loops up to 300 K.

Absence of a PbTiO3 phase transition in PbTiO3/BaTiO3 superlattices

Abstract We have performed X-ray diffraction and Raman spectroscopy on three PbTiO 3 /BaTiO 3 superlattices (PTO/BTO) from room temperature (RT) to 600°C. The superlattices were 4000 A thick and the modulation wavelengths Λ, with equally thick layers of PTO and BTO, were 136, 187 and 357 A. The samples were laser deposited onto single crystal MgO substrates buffered with 50 A of SrTiO 3 (STO). We observe no X-ray evidence of a phase transition in the PTO layers of these three superlattice samples up to 600°C. This result was reinforced by X-ray modeling of the samples. We also monitored the E(1TO) or “soft “ mode by Raman spectroscopy up to 550°C and found only a linear decrease of mode frequency with temperature for all three superlattices: another sign that no phase transition is taking place in this temperature range.

Polarization Rotation in Ferroelectric Tricolor PbTiO3/SrTiO3/PbZr0.2Ti0.8O3 Superlattices.

In ferroelectric thin films, controlling the orientation of the polarization is a key element to controlling their physical properties. We use laboratory and synchrotron X-ray diffraction to investigate ferroelectric bicolor PbTiO3/PbZr0.2Ti0.8O3 and tricolor PbTiO3/SrTiO3/PbZr0.2Ti0.8O3 superlattices and to study the role of the SrTiO3 layers on the domain structure. In the tricolor superlattices, we demonstrate the existence of 180° ferroelectric stripe nanodomains, induced by the depolarization field produced by the SrTiO3 layers. Each ultrathin SrTiO3 layer modifies the electrostatic boundary conditions between the ferroelectric layers compared to the corresponding bicolor structures, leading to the suppression of the a/c polydomain states. Combined with the electrostatic effect, the tensile strain induced by PbZr0.2Ti0.8O3 in the PbTiO3 layers leads to polarization rotation in the system as evidenced by grazing incidence X-ray measurements. This polarization rotation is associated with the monoclinic Mc phase as revealed by the splitting of the (HHL) and (H0L) reciprocal lattice points. This work demonstrates that the tricolor paraelectric/ferroelectric superlattices constitute a tunable system to investigate the concomitant effects of strains and depolarizing fields. Our studies provide a pathway to stabilize a monoclinic symmetry in ferroelectric layers, which is of particular interest for the enhancement of the piezoelectric properties.

Microwave dielectric dispersion in a multiferroic Pb(Fe1/2Nb1/2)O3 thin film

We present the dielectric dispersion in a Pb(Fe1/2Nb1/2)O3 (PFN) thin film grown on (001) SrTiO3 substrate from 20 MHz to 20 GHz in the temperature range of 270 to 340 K. In the radio frequency region, the hopping charge transport and associated polar nanoregion and/or domain-wall motion contribute to the spectrum. In the microwave region, relaxational dispersion was observed with a dielectric contribution of Δe ≈ 600 at room temperature having a characteristic frequency (fm)0.4 = 9.8 · (T−T0) Hz, where T0 is 660 K. It is associated with possible mode-softening behavior, related to the onset of polar nanoregions at T0. The dielectric permittivity shows similarities with 1-dimensional Ising model behaviour.

Ferroelectric Phase in Barium Titanate Epitaxial Thin Film

X-ray diffraction and Raman spectroscopy were used to examine the structure and the ferroelectric state of highly oriented barium titanate thin film deposited by the pulsed laser deposition on (001)MgO substrate. It is found that the film is weakly tetragonal and domains are a-axis oriented. The temperature dependencies of the out plane lattice parameter and the tetragonal Raman peak at 308 cm m 1 show that the ferro-paraelectric phase transition occurs above the the bulk transition temperature and can be considered as diffuse. This behaviour is attributed to the stress effect.

Strain effect in PbTiO3/PbZr0.2Ti0.8O3 superlattices: From polydomain to monodomain structures

Ferroelectric symmetric superlattices consisting of alternating layers of PbTiO3 and PbZr0.2Ti0.8O3, were grown by pulsed laser deposition on SrTiO3 and SrRuO3-coated SrTiO3 substrates. The superlattices, with wavelengths Λ ranging from 20 A to 200 A, were analyzed using x-ray diffraction (θ−2θ diffraction scans, rocking curves, and reciprocal space mapping), high resolution transmission electron microscopy, and piezoforce scanning microscopy. For large-period superlattices, the strain is relieved by the formation of an a/c polydomain structure which propagates through the whole film. We investigate the influence of the wavelength on the a-domain volume fraction, the lattice parameters, the in-plane strain exx, and the mosaicity of the samples. We show that with decreasing the wavelength, a reduction of the a-domain volume fraction is observed as well as a reduced tensile in-plane strain and a lower mosaicity. A concomitant improvement of the local ferroelectric response is detected. Below a critical wave...