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Structural phase transition behavior of a material under temperature plays a key role in understanding its physical properties, instabilities, and design of new functions. However, there is extreme shortage of investigations on the temperature-driven phase transition behaviors of bismuth ferrite (BiFeO3). Herein, magnetic phase transitions within low-temperature regimes, ferroelectric phase transitions under high temperature, structural instability evolution, and spin-phonon coupling in 5% La-doped BiFeO3 ceramic (BFLa05) were systemically studied by employing X-ray diffraction, differential scanning calorimetry, Raman spectroscopy, and magnetic measurement, as well as the phenomenological theoretical analysis. Thanks to the outstanding stability of BFLa05 ceramic under high temperature, a complete phase transition sequence of α ↔ β ↔ γ was observed, simultaneously determining the crystalline symmetries for β and γ phases as orthorhombic Pnma and cubic Pm3m, respectively. Octahedral tilt as a typical str...

International audience; Ceramics of pure phase Yttrium (Y) doped BiFeO 3 prepared by a solid-state sintering route were characterized by X-ray diffraction, Raman spectroscopy, magnetic and electrical measurements. The results and analysis show that Y substitution greatly reduces the leakage current and enhances the multiferroic properties of BiFeO 3. Leakage conduction mechanism is shown to change from space-charge-limited conduction type in pure BiFeO 3 to a Poole–Frenkel emission behavior in Bi 0.90 Y 0.10 FeO 3. A Fowler-Nordheim tunneling mechanism in Bi 0.95 Y 0.05 FeO 3 ceramic is also evidenced under high electric fields. At the same time, enhanced magnetic properties due to Y-doping are confirmed by temperature dependent magnetometry and supported by Raman spectroscopy. An unexpected and sharp switching behavior in the magnetization under low magnetic fields observed in Bi 0.90 Y 0.10 FeO 3 ceramic, together with its improved ferroelectric property, may trigger such system for promising magneto-electric applications.

Ferroic domain walls could play an important role in microelectronics, given their nanometric size and often distinct functional properties. Until now, devices and device concepts were mostly based on mobile domain walls in ferromagnetic and ferroelectric materials. A less explored path is to make use of polar domain walls in nonpolar ferroelastic materials. Indeed, while the polar character of ferroelastic domain walls has been demonstrated, polarization control has been elusive. Here, we report evidence for the electrostatic signature of the domain-wall polarization in nonpolar calcium titanate (CaTiO3). Macroscopic mechanical resonances excited by an ac electric field are observed as a signature of a piezoelectric response caused by polar walls. On the microscopic scale, the polarization in domain walls modifies the local surface potential of the sample. Through imaging of surface potential variations, we show that the potential at the domain wall can be controlled by electron injection. This could enable devices based on nondestructive information readout of surface potential., Comment: 30 pages, 12 figures

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.

Ferroelectric domain structures of BiFe 0 3 single crystals are observed using second harmonic generation and polarization microscopes. Anisotropy of SHG intensities is measured on (111)c and (110)c plates and analyzed by the Sapriel theory. The observed domains keep the same pattern from room temperature to above T N of 643 K without essential change.

A narrow part of Fe2O3–Bi2O3 phase diagram was re-investigated in order to elaborate single crystals of the multiferroic BiFeO3 (BFO). Centimeter-size single crystals were successfully obtained by flux method, and present a preferred growth direction. X-ray diffraction studied have highlighted that the growth direction is along the polar axis [111] r of the structure. The stability of BFO versus temperature (reversible ferroelectric transition followed by multiple irreversible decompositions) is discussed in the light of Differential Scanning Calorimetry (DSC) analysis performed between 25 and 1400°C.

Using a simple methodology based on the classical Bragg–Brentano X-ray geometry we have evidenced at room temperature a pseudo-cubic phase and a monoclinic (Pm) phase in thin films of PbSc1/2Nb1/2O3 (PSN) and (PbSc1/2Nb1/2O3)0.57–(PbTiO3)0.43 (PSN–PT) respectively, the latter compound corresponding to a morphotropic phase boundary (MPB) composition. Depending on the substrate and the thickness, different domains configuration are obtained. Competition between epitaxial and thermal stress induces a complex structural evolution with temperature which is different from that of ceramic samples. Indeed PSN films transform from a pseudo-cubic (triclinic) phase towards a cubic phase whereas in PSN–PT, a succession of transformations from a pure Pm phase toward a tetragonal (possibly non polar) phase is observed, a cubic phase never being reached up to 950 K.

BiFeO${}_{3}$ exhibits a complex phase-transition sequence under pressure associated with changes in octahedron tilts and displacements of Bi${}^{3+}$ and Fe${}^{3+}$ cations. Here, we investigate the local structure of Fe${}^{3+}$ as a function of pressure through absorption crystal-field spectroscopy in the 0--18 GPa range. We focus on the influence of phase transitions on the Fe${}^{3+}$ off-center displacement through the energy ($E$) and oscillator strength (${f}_{d\ensuremath{-}d}$) of the ${}^{4}$T${}_{1}$ and ${}^{4}$T${}_{2}$ Fe${}^{3+}$ (3${d}^{5}$) bands observed below the band gap (${E}_{\mathrm{gap}}$ $=$ 2.49 eV) at 1.39 and 1.92 eV, respectively, at ambient conditions. Pressure induces linear redshift of both ${}^{4}$T${}_{1}$ and ${}^{4}$T${}_{2}$ bands, consistent with the compression of the FeO${}_{6}$ octahedron under pressure. On the other hand, the transition oscillator strength (${f}_{d\ensuremath{-}d}=3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$), enabled by both the exchange mechanism and the off-center Fe${}^{3+}$ distortion, slightly increases with pressure. The absence of notable anomalies in the variation of $E$($P$) and ${f}_{d\ensuremath{-}d}$($P$) through the phase sequence from the ferroelectric rhombohedral $R$3$c$ phase to the nonpolar orthorhombic Pnma phase suggests a persisting off-center position of the Fe${}^{3+}$. While this local polarity is correlated and expected in the ferroelectric $R$3$c$ phase, its presence in the high-pressure nonpolar Pnma phase indicates the presence of local polar instabilities.

International audience; The coexistence of ferroelectricity and magnetic order in epitaxial hexagonal YbFeO3 (h-YbFeO3) thin films deposited on yttria stabilized zirconia substrates is examined using an impedance analyzer, a second harmonic generation (SHG) microscope, an X-ray diffractometer, a transmission electron microscope and a superconducting quantum interference device magnetometer. The results show that h-YbFeO3 films exhibit ferroelectricity below 350K with threefold translational periodicity along the [110] direction similar to rare earth manganese oxides, and ferrimagnetism with a four-up and two-down spin configuration of Fe+3 and Yb+3 ions along the c-axis at low temperatures. In addition, a possibility of new ferroelectric phases with an electronic origin induced by the magnetic ordering is discussed based upon SHG and D-E hysteresis measurements.

Conditions for epitaxial growth of hexagonal YbFeO 3 (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.

Ferroelectric properties of ${\text{Bi}}_{1\ensuremath{-}x}{\text{Pb}}_{x}{\text{FeO}}_{3}$, through a structural approach, by structural Rietveld refinement and x-ray diffraction versus temperature have been carried out. This study allows us to draw a phase diagram for the system ${\text{Bi}}_{1\ensuremath{-}x}{\text{Pb}}_{x}{\text{FeO}}_{3}$. Although lead atom gets a lone pair and is commonly understood as having a ``ferroelectric'' character, we report in this paper that adding of lead atom in the Bi subsystem of ${\text{BiFeO}}_{3}$, destroys the long-range FE order induced by Bi lone pairs on contrary, and progressively breaks the ferroelectric ordering up to the point where the structure becomes cubic on average. The decreasing of ${T}_{C}$ with increasing Pb content also confirms that adding of Pb atoms destabilizes the ferroelectric rhombohedral order and stabilizes the cubic phase via a complex texture of mixed phases, occurring at a local scale.

International audience; An effective Hamiltonian scheme is developed to study finite-temperature properties of multiferroic BiFeO3. This approach reproduces very well (i)the symmetry of the ground state, (ii)the Néel and Curie temperatures, and (iii)the intrinsic magnetoelectric coefficients (that are very weak). This scheme also predicts (a)an overlooked phase above TC 1100K that is associated with antiferrodistortive motions, as consistent with our additional x-ray diffractions, (b)improperlike dielectric features above TC, and (c)that the ferroelectric transition is of first order with no group-subgroup relation between the paraelectric and polar phases. © 2007 The American Physical Society.

Regularly twinned domain structures are observed by scanning piezoforce microscopy on single crystalline grains of BiFeO3 ceramics being grown by a special low temperature sintering process. The domains are considerably larger than those observed in thin films. Their spontaneous polarization comes close to that predicted theoretically and overcomes restrictions hitherto being set to bulk single crystals. The observed ferroelastic twin domain structure resembles that of classic T domains in rhombohedrally distorted NiO, but is additionally superimposed by ferroelectric twin domain patterns.

International audience; The perovskite Bismuth ferrite BiFeO3 (BFO) is considered to be a model multiferroic and offers a rare multiferroelectric robustness since it presents a coexistence of ferroelectric and antiferromagnetic order up to unusually high temperatures. Perovskite-type materials are known for their common structural instabilities which can be driven by diverse external parameters like temperature, pressure, stress etc. Such instabilities and the associated structural distortions are often very subtle and difficult to detect by techniques probing the average structure such as X-ray diffraction. Here we present an investigation of the BFO phonon spectrum by the local probe Raman spectroscopy as a function of polarization, temperature and pressure. We review a recent temperature-dependent Raman investigation which illustrates a first-order structural phase transition at the ferroelectric Curie temperature. Our temperature dependence results further indicate a phonon-anomaly around the magnetic Néel temperature, which is discussed in the light of multiferroicity. We will further illustrate that BFO presents important pressure-induced structural instabilities and we discuss the role of such instabilities for the understanding of strained thin film BFO which show distinct properties compared to the bulk.

We have recently evidenced the existence of a complex path of polarisation rotation via two monoclinic phases in the giant-piezoelectric materials PSN–PT that is similar to the PMN–PT system. In this article we have presented the state of knowledge on the PSN–PT system and have given new results showing the close connection between the local inhomogeneous chemical order of Sc/Nb/Ti cations and the stability of different ferroelectric phases. The notion of morphotropic phase boundaries is discussed in relation with the gradual appearance of long-range polar order with increasing Ti content in PSN–PT. Comparison with the films and nanopowders/ceramics has also been made.

International audience; In this paper, we show that a pure single phase by doping Fe-site of BiFeO3 (BFO) using tetravalent Zr4+ ions can be achieved by introducing cation (Bi3+) vacancies. The structural analysis reveals that the ferroelectric nature of BFO should be weakly affected by 10% of Zr4+ doping as the c/a ratio and the Curie temperature T-C remain roughly unchanged compared to that of pure BFO. In contrast, the magnetic properties are affected as a weak ferromagnetism and a change of Neel temperature T-N are observed. Beyond the double-exchange interactions arising from the creation of Fe2+, we propose another simple model inducing a local ferromagnetic coupling rather than an antiferromagnetic which considers the replacement of the magnetically active Fe3+, time to time, by a nonactive Zr4+.

Highly oriented Pb(Sc1/2Nb1/2)O3 (PSN) and Pb(Sc1/2Nb1/2)O3/43%PbTiO3 (PSN/43%PT) thin films were fabricated on MgO and SrTiO3 (001) substrates by pulsed laser deposition (PLD) technique. La1/2Sr1/2CoO3 was deposited on the substrate as a bottom electrode layer for dielectric measurements. The orientation of the films was checked by X-ray diffraction method, and is found to be well oriented along the c-axis for PSN and PSN/PT. Film thickness was determined by the Laue oscillation of X-ray diffraction profile. This enables us to determine the number of pulses for stacking one layer of PSN and PSN/PT thin films. The temperature dependences of lattice constants of PSN and PSN/PT thin films were determined. They exhibit small but clear changes at almost the same temperature as that of the ferroelectric transition points of PSN and PSN/PT ceramics. The surface roughness of these films was observed using atomic force and scanning electron microscopes. Temperature change of dielectric constant ε and D-E hysteresis loop were determined for PSN and the real part of ε shows 850.