Magnetic ordering in La-substituted (Bi1-xLax)0.5Pb0.5Fe0.5Ti0.5O3 is investigated for 0.0 ≤ x ≤ 0.5 samples using magnetization measurement as a function of field and temperature. Occurrence of M-H hysteresis loops in all the samples at low temperature suggests existence of magnetically ordered clusters in an overall paramagnetic host. The magnetic ordering temperature is found to increase with increasing x. A decrease in the c/a ratio with x resulting in a reduction of average Fe3+-O-Fe3+ distance along the c direction is argued to enhanced cooperative magnetic interaction of Fe+3-O-Fe+3.
Pressure-induced phase transition studies in nanomaterials are important to comprehend thermodynamics at the nanoscale. Raman spectroscopic studies at high pressure in a diamond anvil cell were performed up to 40 GPa on rutile tetragonal phase of SnO2 nanoparticles (NPs) of sizes ∼2, 4, and 25 nm to investigate their phase stability and phonon anharmonicity. In 25 nm NPs, evidence of phase transitions was observed at ∼11 and ∼24 GPa, 4 nm NPs indicated a cubic phase transition ∼21 GPa, and the 2.4 nm quasi-nanocrystals were found to stable up to 30 GPa. Raman spectra down to 90 K indicated that phonons of 2.4 nm NPs were more anharmonic. The analysis of total Raman intensity with increasing pressure suggested propagation of disorder from the surface to the central core of the NPs under pressure. Pressure-induced effects on 25, 4, and 2.4 nm NPs reduced their average diameters to 6.4 ± 2.6, 4.04 ± 1.36, and 3.85 ± 0.9 nm, respectively. Using Raman mode Grüneisen parameters γj, the thermal expansion coefficient α of the 25, 4, and 2.4 nm SnO2 NPs at 300 K was estimated as 1.674 × 10–6, 1.178 × 10–6, and 1.690 × 10–6 K–1, respectively.
Phonons and magnetic and ferroelectric ordering in La-substituted (Bi1−xLax)0.5Pb0.5Fe0.5Ti0.5O3 for samples with 0.0 ≤ x ≤ 0.5 are investigated using Raman, magnetization, and polarization measurements as a function of temperature. The system is tetragonal for pure Bi0.5Pb0.5Fe0.5Ti0.5O3 with a large c/a ratio. The anisotropy is reduced when Bi is partially replaced by La (0 ≤ x ≤ 0.5), and it turns cubic for x ≥ 0.4. All the properties are found to change significantly with changes in the c/a ratio. Evidence of spin-glass-like magnetic ordering at low temperature is found in the case of x = 0.2. A mechanism for the systematic change of magnetic ordering temperature as a function of doping is also discussed. The phonon frequencies and line widths exhibit discontinuous changes across the tetragonal-cubic transition. Large polarization and forbidden Raman scattering in the cubic phase are explained on the basis of symmetry breaking due to the formation of a polar nano region, which leads to relaxor behavior.
Structure, phonon, and energy storage density were studied in Sr 2+ -substituted lead-free ferroelectric Ba 1-x Sr x TiO 3 (BST x ) ( x = 0.1, 0.3, and 0.7) samples, prepared using solid state reaction techniques, utilizing X-ray diffraction, Raman, and ferroelectric polarization measurement techniques. The system is tetragonal for x = 0.1 with large c/a ratio at room temperature. The tetragonal anisotropy decreases with increasing x, and becoming cubic for x > 0.3. All these structural properties change due to change in the c/a ratio upon rising temperature. Temperature dependent phonon spectroscopic results (80-500 K) indicate reduction in tetragonal to cubic phase transition temperature upon increasing x due to reduction in tetragonal anisotropy (c/a). Overdamping of ~90 cm -1 E-soft phonon mode and a dramatic decrease in the intensity of ~160 cm -1 A-mode were observed at around the tetragonal ferroelectric to cubic paraelectric phase. Using UV-Vis optical absorption spectra, the optical band gaps of these compounds were calculated as 1.73, 1.7, and 1.68 eV as a function of increasing the dopant concentration ( x ). The energy storage densities of these electrically polled BSTx capacitors were estimated using the polarization hysteresis loops. For x = 0.7, a large energy storage density (U re ) of 20 Joule/cm 3 with the efficiency of 57 % was estimated at applied electrical field of 88 kV/cm. Close to T c large dielectric constant and high energy density values of BST x capacitors indicate its possible application in high energy storage and pulse power devices. These results will be presented in detail at the meeting.
In situ Raman spectroscopic measurements have been carried out at high pressure up to 33 GPa using a diamond anvil cell to investigate the structural transitions in relaxor ferroelectric 0.85Pb(Zn1/3Nb2/3)O3-0.15PbTiO3. Raman modes are found to be broad due to substitutional disorder at the B-site of the perovskite. Evolution of spectra with pressure gives evidence for structural instabilities around 2.2, 6.3, and 14.6 GPa. New modes at 343 and 376 cm−1 appear across the transition at 6.3 GPa, characteristic of the high pressure antiferrodistortive rhombohedral phase (PII). The pressure dependence of mode frequency, width of the Raman bands, and integrated intensity of structurally sensitive A1(TO) mode at 272 cm−1 are obtained; their effect on polar ordering and structural transitions are discussed. The disappearance of the mode around 200 cm−1 and the appearance of a new one around 120 cm−1 are evident around 14.6 GPa, and these are attributed to a possible new phase PIII. The reported pressure-induced suppression of diffuse x-ray scattering on Pb-based relaxors is consistent with the observed Raman features.
Highly oriented 0.90[PbZr0.53Ti0.47]0.10[La0.80Sc0.20]O3-δ (PLZTS) thin films deposited on La0.67Sr0.33MnO3 (LSMO) coated MgO (100) substrates were grown by pulsed laser deposition technique. Temperature dependent dielectric measurements on metal-ferroelectric-metal Pt/PLZTS/LSMO thin film capacitors were carried out at several frequencies which exhibit high dielectric constants (450–580) with the diffuse peak around 400 K, and the diffusivity parameter γ was obtained as 1.96 for 100 kHz data. The slim polarization-electric field hysteresis loop was observed with less remanent polarization (∼7–10 μC/cm2) indicating its relaxor behavior. Temperature dependent Raman spectra measured between 80 and 550 K show softening of the symmetric E(LO2) band that disappeared at 300 K, corroborating the tetragonal-cubic phase transition. From the analysis of the measured hysteresis loops, the recovered energy density Ure ∼ 19 J/cm3 with the efficiency η ∼ 66% was estimated, suggesting its possible application in energy density capacitors.
Magnetoelectric multiferroics, because of their coupled electric and magnetic ordering in a single phase, are eminent for applications in logic and nonvolatile memory devices. Recent advances in computational modeling, synthesis, and characterization techniques are spurring significant advances in the study of these materials. During last several years, our group at University of Puerto Rico has been involved in synthesizing novel single-phase room temperature multiferroic materials that included not only solid solutions of lead zirconate titanate with lead iron tungstate, lead iron niobate, and lead iron tantalate, but also recently discovered palladium substituted room temperature multiferroics, namely PbPd0.3Ti0.7O3 and Pb(Zr0.2Ti0.8)Pd0.3O3. In this review, we have reported thin films of the above materials that were synthesized using pulsed laser deposition technique on various substrates at Speclab, University of Puerto Rico. We describe in detail thin film fabrication, structural diffraction analyses (XRD, TEM), dielectric, ferroelectric, piezo-response force microscopy, and magnetization results on recently grown Pd-substituted lead titanate and lead zirconate titanate thin films in an elaborate manner. The films were phase pure and stabilized in c-axis oriented tetragonal phase (P4mm) with surface roughness RQ of 2–4 nm. Temperature-dependent dielectric studies on metal-dielectric-metal heterostructure capacitors indicate that ferroelectric to paraelectric phase transition is above 500 K. A proof of ferroelectricity was confirmed from strong domain switching responses using piezo-response force microscopy. These films showed ferromagnetic ordering below 395 K. The origin of magnetic ordering in these materials was attributed to the existence of Pd2+ and Pd4+ mixed oxidation states of palladium dispersed in the polar matrices. We conclude with the above findings that these multiferroics may have potential applications.in nonvolatile memory and other multifunctional devices.
We report the crystal structure, dielectric, magnetic, and magneto-electric properties of (1−x) BiFeO3-xYMnO3 (0.00 ≤ x ≤ 0.2) multiferroic nanoceramics prepared by auto-combustion technique. YMnO3 substitution is found to induce a structural phase transition from R3c to R3c+Pbnm after x ≈ 0.1 using Rietveld refinement technique. Field emission scanning electron micrographs show decrease in grain size with increase in YMnO3 content. The dielectric permittivity and loss tangent are found to be increased with composition x. The anomalies noticed from the temperature dependent dielectric analysis reveal the signature of magneto-electric coupling in the system. A decrease in magnetic ordering temperature as a function of composition is found from dielectric study. At room temperature, the dielectric permittivity of all the YMnO3 modified samples decrease with increasing magnetic field. The maximum value of magneto-electric coupling coefficient (ε(H)-ε(0))/ε(0) is found to be ∼ −5.5% at H = 2 T for x = 0.2. The behaviour of the magnetic hysteresis loop observed at room temperature suggests the suppression of space modulated spin structure.
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