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2.
Systematic Raman scattering experiments were performed on Pb(Mg 1/3Nb 2/3)O 3 (PMN) single crystals to resolve the low‐wavenumber dynamics in the crystal. Careful checking of the angular dependence of the Raman spectra indicated that the intense peak around 45 cm −1, which always smears the low‐wavenumber spectra, stems from the F2g mode due to Mg:Nb = 1:1 chemically ordered Fm3 m region. A proper scattering configuration for eliminating the strong F2g mode allowed the observation of the lowest wavenumber soft mode dynamics in PMN. The results revealed the softening of the mode towards Tc with underdamped oscillation. The soft mode becomes overdamped in a wide temperature range above Tc, suggesting the development of cluster dynamics due to polar nanoregions. It recovers the underdamped oscillation and hardens in the high‐temperature region. Finally, the physical picture of the relaxor phenomenon in PMN is discussed in terms of lattice dynamics by comparison with the typical displacive‐type ferroelectric phase transition. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
3.
Orthorhombic EuPdSb is known to undergo two magnetic transitions, at 12 K and at T
N≃ 18 K, and in phase III ( T < 12 K), single crystal magnetisation data have shown that the spin structure is collinear antiferromagnetic, with magnetic
moments along the crystal a axis. From a 151Eu M?ssbauer absorption study, we show that, at any temperature within phase III, all the moments have equal sizes, and that
in phase II (12 K< T <18 K) the magnetic structure is modulated and incommensurate with the lattice spacings. The modulation is close to a pure
sine-wave just below T
N = 18 K, and it squares up as temperature is lowered. We measured the thermal variations of the first and third harmonics
of the moment modulation, and we could determine the first and third harmonics of the exchange coupling. We furthermore show
that the antiferromagnetic-incommensurate transition at 12 K is strongly first order, with a hysteresis of 0.05 K, and that
the incommensurate-paramagnetic transition at 18 K is weakly first order. Finally, we present an explanation of the spin-flop
transition observed in the single crystal magnetisation data in phase III when || in terms of an anisotropic molecular field tensor.
Received 17 January 2001 and Received in final form 20 March 2001 相似文献
4.
Proton NMR second moment and spin—lattice relaxation times T 1 and T 1p have been studied for polycrystalline guanidinium hexachloroantimonate C(NH 2) 3SbCl 6 in a wide temperature range. A dynamic inequivalence of two cations has been detected in spite of their crystal-lographical equivalence. Activation parameters for C 3 reorientation and self-diffusion of the more mobile cation have been determined. It was shown that the para–ferroelastic phase transition at 351 K is connected with abrupt changes in the dynamics of the two cations. The weaker, second-order transition at 265 K is thought to be related to a change in the dynamics of one of the cations. 相似文献
5.
The behavior of the low-temperature specific heat C( T) for YBa 2Cu 3O 6 + x
single crystals with the doping level corresponding to the normal phase has been studied by the relaxation technique at different
values of the applied magnetic field. It has been found that the C( T)/ T plot exhibits such an anomaly as a steep increase with decreasing temperature from T about 4 K down to T ≤ 2 K (the minimum temperature value accessible in the experiment). The applied magnetic field as high as 9 T inverts this
anomaly and leads to a sharp drop in C( T)/ T during cooling within the same temperature range. A model involving the Schottky-type centers formulated in this work and
the data on spin correlation functions has allowed us to calculate the temperature dependence of the specific heat, which
fits the experimental curves quite well. 相似文献
6.
High quality orthorhombic and tetragonal SrRuO 3 thin films were grown by pulsed laser deposition on SrTiO 3(001) and Ba 0.75Sr 0.25TiO 3 buffered LaAlO 3(001) substrates. Resistivity vs. temperature curves showed a slope change at a Curie temperature of 147.5 ± 2 K for 40 nm thick films irrespective of crystalline symmetry. The Hall resistivity of both films contained an anomalous Hall contribution. The anomalous Hall coefficient was positive throughout the whole temperature range for the tetragonal film, whereas it showed a sign change at 143 K for the orthorhombic film. This is a strong indication that the Berry‐phase mechanism is the dominant anomalous Hall effect mechanism in SrRuO 3. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
7.
SrRuO 3 is an orthogonally distorted perovskite (Pbnm) structure whose ferromagnetism is often viewed as an itinerant ferromagnet. Although SrRuO 3 has been studied for more than half a century, its structure, magnetism and transport properties are still poorly understood. In this paper, the structure and magnetic evolution of SrRuO 3 are discussed in depth through the substitution of Ca 2+ for Sr 2+ at A sites. The results show that as the Ca substitution increases, the lattice constant decreases, the orthogonal distortion becomes larger, and the saturation magnetization M S, Curie temperature T C and Weiss temperature θ p decrease accordingly. Eventually, the ferromagnetic SrRuO 3 changes to paramagnetic CaRuO 3. The critical exponent β of samples with different substitution contents was obtained by fitting the experimental results, and the value for SrRuO 3 (β = 0.55) was similar to that obtained by mean field theory. However, the value increases with the substitution x of Ca, which can't be explained by any scaling theory. The results show that the increase in the value of β is related to the magnetic disorder caused by different magnetic interactions. Analysis using the Rhodes-Wohlfarth criterion indicates that Sr 1-xCa xRuO 3 has both itinerant-electron and localized-electron magnetism, which is consistent with the theoretical predictions. 相似文献
8.
The local conductivity of SrTiO 3 thin films epitaxially grown on SrRuO 3‐buffered SrTiO 3 single crystals has been investigated in detail with an atomic force microscope equipped with a conducting tip (LC‐AFM). These experiments demonstrate that the conductivity of SrTiO 3 thin films originates from nanoscale well‐conducting filaments connecting the surface to the SrRuO 3 bottom electrode. The electrical conduction of the filaments is shown to be reversibly modulated over several orders of magnitude by application of an appropriate electrical field. We analyze the resistive switching by addressing individual filaments with the AFM tip as well as by scanning areas up to the µm scale. Temperature dependent measurements reveal that resistive switching on a macroscopic scale can be traced down to the insulator‐to‐metal transition of the independently switchable filaments. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
9.
The attenuation and the velocity of ultrasonic waves in single crystalline high- T
c YBa 2Cu 3O 7–x have been measured in the temperature range from 3 to 280 K for the C 11-, C 33-, C 44- and C 66-mode. At the superconducting transition a pronounced peak in the absorption is observed for the longitudinal C 11-mode. The longitudinal C 11- and C 33-modes and the transverse C 44-mode exhibit a plateau in the temperature dependence of the velocity. The temperature dependence of the absorption of the transverse C 44-mode reveals a change in the slope at T
c. The data are compared with the predictions derived from a thermodynamic treatment and with the results of measurements on coarse grain and fine grain sintered and sinter-forged materials. 相似文献
10.
Nanolayered superlattices composed of ferromagnetic SrRuO 3 and antiferromagnetic SrMnO 3 layers were grown on SrTiO 3 (100) substrates by pulsed laser deposition. Multilayers were grown under predetermined synthesis conditions resulting in growth of SrRuO 3 and SrMnO 3 by step flow and layer-by-layer modes, respectively. The growth of SrMnO 3 was observed to occur through the layer-by-layer during the entire deposition process despite the expected increase in surface roughness because of the incorporation of SrRuO 3 upper layers. Monitoring by reflection high-energy electron diffraction (RHEED) revealed that the growth of every SrMnO 3 layer consisted of a pre-stage during which the gaps on the relatively rough SrRuO 3 surface were filled before the actual growth of the SrMnO 3 layer, which resulted in incomplete half oscillation and change from spot patterns to streaky patterns. The in-plane lattice constant did not show any considerable change in the case of SrRuO 3 and SrMnO 3 layers, despite the considerable lattice mismatch between the two materials (SrRuO 3, SrMnO 3) and SrTiO 3. On the other hand, the RHEED patterns showed the existence of lattice mismatch effects in the out-of-plane lattice constant, which showed significant strains of opposite signs in the different layers, indicating a strong dependence on the composition of the layers and superlattice periodicity. In this paper, the growth characteristics of a SrRuO 3/SrMnO 3 multilayer along with its magnetic properties will be discussed. 相似文献
11.
AbstractIn this work, we have studied on double-layered perovskite (Ruddlesden–Popper) manganite structure in Pr 1.75Sr 1.25Mn 2O 7 synthesised by sol–gel method. The crystal structure of the double-layered perovskite is found as tetragonal from the X-ray diffraction analysis with I4/mmm space group. A high Curie temperature, TC = 305 K is observed from the temperature dependence of magnetisation measurement. The isothermal magnetisation curves showed that magnetic phase transition is second order due to the positive slope of the Arrott plots. Maximum magnetic entropy change (Δ SM) and adiabatic temperature change (Δ Tad) values are calculated as 3.99 J kg ?1 K ?1 and 2.1 K under external magnetic field of 70 kOe, respectively. Since our double-layered perovskite manganite sample has desired TC value and relatively high Δ SM, it can be a potential candidate as a magnetocaloric material for room temperature magnetic cooling systems. 相似文献
12.
We present the studies of the phase transition behaviors of V 2O 3 thin film using temperature‐dependent Raman scattering spectroscopy. Our results show that in both the cooling and heating processes of V 2O 3 thin film, the phase transition occurs gradually but not suddenly, contrary to that in single crystal. The coexistence of both the metal and insulator phases with co‐phasing Δ Tc larger than 30 K is observed in both the cooling and heating processes. We discuss that this large co‐phasing Δ Tc should be distinguished with the large hysteresis Δ Th reported in nanostructures. In addition, our discussions indicate that co‐phasing Δ Tc and hysteresis Δ Th would be mainly correlated with stress and defect states in sample, respectively. Furthermore, our Raman analyses suggest that stress would also induce phase transitions in V 2O 3, and the stress (pressure)‐induced phase transitions would behave differently comparing with the temperature‐induced transitions under normal pressure. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
13.
The SrRuO 3 films (50 nm thick) grown by laser evaporation on (001)(LaAlO 3) 0.3 + (Sr 2AlTaO 6) 0.7 substrates were under partially relaxed biaxial compressive mechanical stresses. The films consisted of crystallites with
lateral dimensions of 40–100 nm and a relative azimuthal misorientation of about 0.9°. Ferromagnetic ordering of spins in
the SrRuO 3 films was manifested by a change in the slope of the temperature dependence of their electrical resistivity ρ at T ≈ 155 K. For a magnetic field H parallel to the measuring current, the maximum values (∼7.5%) of the magnetoresistance MR = [ρ(μ 0
H = 5 T) − ρ(μ 0
H = 0)]/ρ(μ 0
H = 0) were observed at temperatures of about 100 K. At T = 95 K (μ 0
H = 5 T), the anisotropic magnetoresistance of the films was 8% and increased by a factor of approximately 1.5 with decreasing
temperature to 4.2 K. 相似文献
14.
The heat capacity (C
P), the thermal diffusion (η), the thermal conductivity (κ), and the electrical resistance of the La0.825Sr0.175MnO3 single crystal have been measured in the temperature range 80–350 K in magnetic fields to 40 kOe. Dependences C
P(T), κ(T), and η(T) have anomalies near T
C, which are suppressed in magnetic field. The minima in dependences κ(T) and η(T) near T
C are explained by the phonon scattering on fluctuations of the magnetic order parameter. Dependences κ(T) and η(T) have anomalies near T
S = 200 K related to the structural transition from the rhombohedral (R) to the orthorhombic (O*) phase. 相似文献
15.
The relaxation dynamics of the lattice and low-energy quasiparticles in a YBa 2Cu 3O 7−x superconductor are studied by the light reflection technique with femtosecond temporal resolution in a wide temperature range. It is shown that, for T> T c, there exist two temperature regions with qualitatively and quantitatively different excitation dynamics, and the transition between these regions is of a hysteretic nature. It is also found that the character of changes observed in the charge carrier relaxation dynamics in the superconducting state testifies to the presence of an anisotropic gap with nodes at the Fermi surface. 相似文献
16.
The magnetic properties of Ho 2Fe 17???xMn x compounds (x = 0–2) of ferromagnetic ordering temperatures up to T C ~344 K have been investigated by DC magnetization and Mössbauer effect measurements. The nature of the magnetic phase transitions and the critical behaviour around T C has been investigated by analysis of the magnetisation data and the critical exponents β, γ and δ determined. The critical exponents are found to be similar to the theoretical values of the mean-field model for which β?=?0.5 and γ?=?1.0, indicating the existence of a long-range ferromagnetic interactions. The isothermal entropy changes ΔS around T C have been determined as a function of temperature in different magnetic fields. 相似文献
17.
Temperature‐dependent magnetization and magnon Raman spectra were measured for anti‐ferromagnetic NiO‐nanoflowers. The results show several new magnetic behaviors, including the appearance of a ferromagnetic phase, a reduced Néel temperature ( TN) and a reduced Curie temperature ( TC). The temperature dependencies of the double magnon (2 M) Raman wavenumber and intensity are similar to those of magnetization. A magnetic granules model (MGM) consisted of a crystalline core enclosed by a shell is proposed. The model suggests that the large quantity of spins induced by specific surface effect in the shell plays a key role in nano‐magnetism. Based on the MGM, the micro‐mechanism of the observed new magnetic behavior is understood by the magnon Raman spectra. The MGM is based on the general features of magnetic nano‐particles, and thus it should be generally applicable to common magnetic nano‐particles. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
18.
The temperature dependence of the electric-field gradient of 111Cd in single crystalline LiTaO 3 was studied from room temperature to 1040 K in the ferroelectric and paraelectric phases. The data taken at room temperature
show unambiguously the presence of two quadrupole interaction frequencies, ν Q1=230 MHz and ν Q2=242 MHz, with nonzero asymmetry parameters, while above the Curie temperature (T C=878 K) the data are well described by a unique frequency. The electric field gradient shows a usual temperature dependence,
increasing aproximately in a linear fashion until T C and then decreasing faster. The initial increase is explained mostly by the lattice expansion, while above T C it is necessary to consider Li and O displacements.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
19.
(1 – x)Pb(Hf 1–yTi y)O 3– x Pb(Mg 1/3Nb 2/3)O 3 ( x = 0.1 ~ 0.25, y = 0.555) ternary piezoelectric ceramics were prepared using the two‐step precursor method. Morphotropic phase boundary (MPB) compositions, located at x = 0.18 ~ 0.22, were confirmed using X‐ray diffraction and by their dielectric, piezoelectric and ferroelectric properties. The optimum dielectric and piezoelectric properties were achieved for the MPB composition 0.8Pb(Hf 0.445Ti 0.555)O 3–0.2Pb(Mg 1/3Nb 2/3)O 3, with dielectric permittivity εr, piezoelectric coefficient d33, planar electromechanical coupling kp and Curie temperature TC being on the order of 2800, 680 pC/N, 70% and 276 °C, respectively. Of particular significance is that the new ternary ceramics exhibit comparable piezoelectric and electromechanical properties to commercial PZT5H ceramics, but with much improved TC, showing a potential for applications at elevated temperature. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
20.
Measurements of magnetic susceptibility χ as a function of temperature T and of magnetisation M as a function of applied magnetic field H at a number of fixed temperatures were made on polycrystalline samples of Cu 2FeGeSe 4. The χ versus T data show that an antiferromagnetic transition occurs at 20 K and that a second transition occurs at 8 K, indicating a transition to weak ferromagnetic form. The M versus H curves indicated that at all temperatures below 70 K bound magnetic polarons (BMP) occur, in the paramagnetic, antiferromagnetic and weak ferromagnetic ranges. Below 8 K, the M versus H curves exhibited magnetic hysteresis, and this is attributed to the interaction of the BMPs with tetragonally anisotropic matrix. The B versus H curves were well fitted by a Langevin-type of equation, and the variation of the fitting parameters determined as a function of temperature. These showed that above 20 K the total BMP magnetisation fell almost linearly with increasing temperature and effectively disappeared at 70 K. The number of BMPs remained practically constant with temperature having a mean value of 6.55×10 18/cm 3. The analysis gave a value of 213 μ B for the average magnetic moment of a BMP, corresponding to 42.4 Fe atoms. Using a simple spherical model, this gives the radius of a BMP as 12.0 Å. 相似文献
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