J. M. Kiat

Dielectric evidences of core-shell-like effects in nanosized relaxor PbMg1∕3Nb2∕3O3

Temperature and frequency dependence of dielectric permittivity in nanotextured ceramics of PbMg1∕3Nb2∕3O3 has been performed in a wide range of grain sizes and shown to display strong variation. In particular, frequency relaxation follows the Vogel Fulcher law for the high grain sizes, whereas for intermediate sizes, two Debye processes are evidenced. This behavior as well as a diminution of the maximum of the real dielectric constant emax could be explained within the mechanism of relaxation based on the existence of polar nanoregions with a core-shell structure for each grain.

Size effects in a relaxor: further insights into PMN

Dielectric measurements of PbMg1/3Nb2/3O3 (PMN) powder and dense ceramics with grain sizes between 15 nm and two microns were carried out in a broad frequency range (20 Hz-1 GHz). Clear grain size dependence of relaxor behavior was evidenced. A progressive transformation from Vogel-Fulcher behavior towards the Arrhenius process in the PMN with reduction of grain size in both ceramics and powder was observed. In the case of ceramics we were able to extract deeper information from the distributions of relaxation times and an analysis using the Vogel-Fulcher law, revealing two main contributions: a fast part of distribution of relaxation times with a maximum close to 10(-11) s, which is almost grain-size independent and has a non-polar origin; whereas, a process with long relaxation times (in the time range of 10(-8) to 10(-5) s) is associated with the dynamics of the polar nanoregions and is strongly suppressed with reduction of grain size. The results of dielectric investigations are confirmed by Nuclear Magnetic Resonance experiments.

SHG Microscope Observations of Domain Structures of Multiferroic BiFeO3 Single Crystal

Temperature dependences of optical second harmonic generation (SHG) and light transmissivity are measured on a single crystal of multiferroic BiFeO3. SHG intensity exhibits a peak around the Neel temperature. The origin of this anomalous behavior could be attributed to rearrangements of ferroelectric micro-domain structures caused by spin-lattice coupling enhanced by ferroelectric order parameter.

DIELECTRIC BEHAVIOUR OF A NANOGRAIN PMN POWDERS

ABSTRACT Dielectric permittivity of 150 nm mean grain size Pb(Mn1/3Nb2/3)O3 (PMN) nanopowder has been investigated by dielectric spectroscopy in the 20 Hz–1 MHz frequency range and 100 K–400 K temperature range. The broad and diffused dielectric dispersion has been observed, but the dispersion region and the maximum of the real part of dielectric permittivity is shifted to lower temperatures compare to PMN single crystal and ceramics. The mean relaxation time, obtained from fits of the frequency dependences of the dielectric permittivity with Cole-Cole formula, changes according to the Vogel-Fulcher law with the freezing temperature T 0 = 88 K which is much lower than in bulk PMN materials.

Non-ergodicity and polar features of the transitional phase in lead zirconate

The so-called “intermediate” phase in lead zirconate (PbZrO3) is shown to exhibit a peculiar dielectric signature, both as a function of time and of frequency, on the contrary to the low temperature antiferroelectric and high temperature paraelectric phases. This signature is present over a temperature range that overlaps with the paraelectric phase and depends on whether the sample is cooled or heated. In addition, the time dependence of the dielectric constant indicates that PbZrO3 evolves isothermally toward a state that is neither the one measured while heating nor while cooling. We ascribe the observed non-ergodicity to the establishment of a modulated phase stabilized by polar defects.

JDN22: International School on “Crystallography and Neutrons” and 22nd Meeting of the French Neutron Society in the Oléron Island

Neutron News Volume 26 • Number 1 • 2015 21 T annual meeting of the French Neutron Society (SFN), the 22nd “Journées de la Diffusion Neutronique” (JDN22), attracted about eighty scientists and students in Oléron, a small island close to the Atlantic French coast, from September 21–26, 2014. The wonderful weather, unusually warm for late September in this region of France, was ideal for biking along the sea and enjoying during free time the outdoor swimming pool of the CNRS conference center. In the context of the International Year of Crystallography, the thematic school preceding the “Rossat-Mignod” meeting of the SFN was dedicated to “CRYSTALLOGRAPHY AND NEUTRONS.” This international school (September 21–24) was mainly supported by the CNRS (“Centre National de la Recherche Scientifi que”), the SFN, the Laboratoire Léon Brillouin (LLB), the Institut Laue-Langevin (ILL) and NMI3 (Neutron and Muon Initiative), with contributions from the “Ecole Centrale de Paris” and the “Triangle de la Physique.” The school focused on the specifi c strengths of neutron scattering (magnetism, light elements, contrast methods) for the study of various kinds of materials, with regards to other investigative techniques. The school began on Sunday afternoon with two lectures on the fundamentals of crystallography (O. Perez, CRISMAT) and the bases of diffraction (P. Becker, ECP), followed by an after-dinner lecture on Neutron Sources and Neutron Optics (P. Courtois, ILL). The two next days were devoted to lectures and training on nuclear and magnetic structure determinations from experimental neutron diffraction data by two eminent experts, V. Petricek (Prag), author of the JANA program and J. Rodriguez-Carvajal (ILL) who created the world-known FULLPROF suite. The characteristics of neutron diffractometers for powder and single crystal diffraction were described by N. Qureshi (ILL) before the fi rst tutorial. In addition to classical structure determination, A. Gukasov (LLB) gave an overview of the applications of neutron diffraction under extreme conditions and polarised neutron diffraction for spin density determinations and studies of magnetic anisotropy. Introductive lectures were also provided on diffuse scattering (I. Mirebeau, LLB), inelastic scattering (S. Petit, LLB) and refl ectometry (E. Kentsinger, Jülich). A book gathering the lectures of the school will be published in the collection SFN/EDP Sciences and will be open access on http://www.neutron-sciences.org in the year 2015. The last half-day of the school was common with the Rossat-Mignod meeting (September 24–26) and began with a session dedicated to crystallography, followed by the SFN prize session. The fi rst session was introduced by an invited talk on “Crystallography in Superspace” by B. Toudic (Rennes) followed by oral communications. The SFN prize was awarded to M. Deutsch for his thesis work “Experimental modelling of molecular magnets: Combined studJDN22: International School on “Crystallography and Neutrons” and 22nd Meeting of the French Neutron Society in the Oléron Island

Syntheses of (Pb-Sn-Ti)O 3 thin films by PLD method and their structural and ferroelectric properties

(Pb-Sn-Ti)O<inf>3</inf> (PST) thin films are fabricated by the pulse laser deposition method with three ceramic targets of PbO, SnO and TiO<inf>2</inf> with consecutive sequences of deposition. Lattice parameters of PST thin film are a=b=0.3993 nm, c=0.4048 nm. This fact together with TEM-EDX results suggests that PST films are the mixed state of PbTiO<inf>3</inf>-SnTiO<inf>3</inf>-PbSnO<inf>3</inf>. Real part of dielectric constant and remanent polarization are determined to be about 1000 and 35µC/cm² at room temperature, respectively. X-ray diffraction reveals that a structural phase transition takes place at 600 °C. Piezoelectric response is confirmed by a piezo-force scanning microscope.

Multi-ferroicity of thin-film-stabilized hexagonal YbFeO 3

Conditions for epitaxial growth of hexagonal YbFeO3 (h-YbFO) using the PLD method are examined. Ferroelectricity of h-YbFO is confirmed at room temperature by dielectric, dilatometric, SHG and TEM techniques which show anomalous temperature behaviors around 350K. SQUID reveals ferrimagnetic spin configuration along the c-axis in low temperature.