The unit cell parameters a, b, and c of [N(CH3)4]2ZnCl4 have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients αa, αb, and αc along the principal crystallographic axes and of the unit cell thermal expansion coefficient αV were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T1=161 K and T2=181 K and that the phase transition occurring at T3=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T1=161 K and T2=181 K in [N(CH3)4]2ZnCl4 were established to be first-order. 相似文献
We use inelastic neutron scattering to study the low-energy spin excitations of polycrystalline samples of nonsuperconducting CeFeAsO and superconducting CeFeAsO0.84F0.16. Two sharp dispersionless modes are found at 0.85 and 1.16 meV in CeFeAsO below the Ce antiferromagnetic (AF) ordering temperature of TNCe ? 4 K. On warming to above TNCe ? 4 K, these two modes become one broad dispersionless mode that disappears just above the Fe ordering temperature TNFe ? 140 K. For superconducting CeFeAsO0.84F0.16, where Fe static AF order is suppressed, we find a weakly dispersive mode center at 0.4 meV that may arise from short-range Ce-Ce exchange interactions. Using a Heisenberg model, we simulate powder-averaged Ce spin wave excitations. Our results show that we need both Ce spin wave and crystal electric field excitations to account for the whole spectra of low-energy spin excitations. 相似文献
The structural and dynamic properties of the three-component Zr47Cu46Al7 system are subjected to a molecular dynamics simulation in the temperature range T = 250–3000 K at a pressure p = 1.0 bar. The temperature dependences of the Wendt–Abraham parameter and the translation order parameter are used to determine the glass transition temperature in the Zr47Cu46Al7 system, which is found to be Tc ≈ 750 K. It is found that the bulk amorphous Zr47Cu46Al7 alloy contains localized regions with an ordered atomic structures. Cluster analysis of configuration simulation data reveals the existence of quasi-icosahedral clusters in amorphous metallic Zr–Cu–Al alloys. The spectral densities of time radial distribution functions of the longitudinal (C?L(k, ω)) and transverse (C?T(k, ω)) fluxes are calculated in a wide wavenumber range in order to study the mechanisms of formation of atomic collective excitations in the Zr47Cu46Al7 system. It was found that a linear combination of three Gaussian functions is sufficient to reproduce the (C?L(k, ω)) spectra, whereas at least four Gaussian contributions are necessary to exactly describe the (C?T(k, ω)) spectra of the supercooled melt and the amorphous metallic alloy. It is shown that the collective atomic excitations in the equilibrium melt at T = 3000 K and in the amorphous metallic alloy at T = 250 K are characterized by two dispersion acoustic-like branches related with longitudinal and transverse polarizations. 相似文献
The temperature dependences of the intense magnetocaloric effect ΔTAD(T, H) and the heat capacity Cp(T) of the (La0.4Eu0.6)0.7Pb0.3MnO3 manganite are directly measured using adiabatic calorimetry. The experimental dependences ΔTAD(T) are in satisfactory agreement with those calculated from the data on the behavior of the magnetization. The factors responsible for the absence of an anomaly in the experimental temperature dependence of the heat capacity Cp(T) in the range of the magnetic phase transition are discussed. 相似文献
Temperature dependences of specific heat Cp(T) and coefficient of thermal expansion ;(T) for Na0.95Li0.05NbO3 sodium-lithium niobate ceramic samples are investigated in the temperature range of 100–800 K. The Cp(T) and α(T) anomalies at T3 = 310 ± 3 K, T2 = 630 ± 8 K, and T1 = 710 ± 10 K are observed, which correspond to the sequence of phase transitions N ? Q ? S(R) ? T2(S). The effect of heat treatment of the samples on the sequence of structural distortions was established. It is demonstrated that annealing of the samples at 603 K leads to splitting of the anomaly corresponding to the phase transition Q → R/S in two anomalies. After sample heating to 800 K, the only anomaly is observed in both the Cp(T) and ;(T) dependence. Possible mechanisms of the observed phenomena are discussed. 相似文献
The results of x-ray structural studies of the [N(C2H5)4]2CdBr4 crystal at low temperatures are presented. The unit cell parameters and the thermal expansion coefficients along the main crystallographic directions are measured at temperatures in the range from 90 to 320 K. The integrated intensities of the diffraction reflections are investigated as a function of the temperature. It is shown that the curves a = f(T), c = f(T), I500 = f(T), and I006 = f(T) at temperatures T1 ≈ 174 K and T2 ≈ 226 K exhibit anomalies in the form of abrupt changes in the lattice parameters and the diffraction reflection intensities. This indicates that the [N(C2H5)4]2CdBr4 crystal undergo phase transitions at these temperatures. Moreover, there is an anomaly in the form of a small maximum at the temperature T3 = 293 K. 相似文献
The magnetization M(H) in the superconducting state, dc magnetic susceptibility χ(T) in the normal state, and specific heat C(T) near the superconducting transition temperature Tc have been measured for a series of fine-crystalline YBa2Cu3Oy samples having nearly optimum values of y = 6.93 ± 0.3 and Tc = (91.5 ± 0.5) K. The samples differ only in the degree of nanoscale structural inhomogeneity. The characteristic parameters of superconductors (the London penetration depth and the Ginzburg–Landau parameter) and the thermodynamic critical field Hc are determined by the analysis of the magnetization curves M(H). It is found that the increase in the degree of nanoscale structural inhomogeneity leads to an increase in the characteristic parameters of superconductors and a decrease in Hc(T) and the jump of the specific heat ΔC/Tc. It is shown that the changes in the physical characteristics are caused by the suppression of the density of states near the Fermi level. The pseudogap is estimated by analyzing χ(T). It is found that the nanoscale structural inhomogeneity significantly enhances and probably even creates the pseudogap regime in the optimally doped high-Tc superconductors. 相似文献
The temperature dependence of the residual polarization of the nonergodic relaxation state (NERS) obtained from the measurements of pyroelectric current during zero-field heating (ZFH) in the temperature interval from 10 to 295 K is investigated for the Cd2Nb2O7 relaxation system in two cases: (1) after sample cooling in a constant electric field E (FC) from T = 295 K to a preset temperature, which is much lower than the “freezing” temperature of the relaxation state (Tf ≈ 182 K), field removal, and subsequent cooling in zero field (ZFC) to T = 10 K and (2) after ZFC from T = 295 K to the same temperature below Tf, application of the same field, and FC to T = 10 K. The behavior of the PrFC (T) and PrZFC (T) dependences is analyzed. In the field E < 2 kV/cm, the PrZFC curves as functions of 1/T have a broad low-intensity peak in the region T ≈ Tf, which vanishes in stronger fields, when the PrFC (1/T) curves intersect at T ≈ Tf and have no anomalies. The difference in the behavior of PrZFC (T) and PrFC (T) indicates the difference in the nature of NERS formed during ZFC and FC of the system upon a transition through Tf. In the ZFC mode, NERS exhibits glasslike behavior; in the FC regime, features of the ferroelectric behavior even in the weak field. Analogous variations of PrZFC (T) and PrFC (T) in a classical ferroelectric KDP are also given for comparison. 相似文献
The temperature-dependent field cooling (FC) and zero-field cooling (ZFC) magnetizations, i.e., MFC and MZFC, measured under different magnetic fields from 500 Oe to 20 kOe have been investigated on two exchange–spring CoFe2O4/CoFe2 composites with different relative content of CoFe2. Two samples exhibit different magnetization reversal behaviors. With decreasing temperature, a progressive freezing of the moments in two composites occurs at a field-dependent irreversible temperature Tirr. For the sample with less CoFe2, the curves of ?d(MFC ? MZFC)/dT versus temperature T exhibit a broad peak at an intermediate temperature T2 below Tirr, and the moments are suggested not to fully freeze till the lowest measuring temperature 10 K. However, for the ?d(MFC ? MZFC)/dT curves of the sample with more CoFe2, besides a broad peat at an intermediate temperature T2, a rapid rise around the low temperature T1~15 K is observed, below which the moments are suggested to fully freeze. Increase of magnetic field from 2 kOe leads to the shift of T2 and Tirr towards a lower temperature, and the shift of T2 is attributable to the moment reversal of CoFe2O4.
Graphical abstract CoFe2O4/CoFe2 composites with different relative content of CoFe2 were prepared by reducing CoFe2O4 in H2 for 4 h (S4H) and 8 h (S8H). The temperature-dependent FC and ZFC magnetizations, i.e., MFC and MZFC, under different magnetic fields from 500 Oe to 20 kOe have been investigated. Two samples exhibit different magnetization reversal behaviors. With decreasing temperature, a progressive freezing of the moments in two composites occurs at field-dependent irreversible temperature Tirr. For the S4H sample, the curves of ?d(MFC ? MZFC)/dT versus temperature T exhibit a broad and field-dependent relaxing peak at T2 below Tirr (figure a), and the moments were suggested not to fully freeze till the lowest measuring temperature 10 K. However, for the S8H sample, it exhibits the reentrant spin-glass state around 50 K, as evidenced by a peak in the MFC curve (inset in figure b) and as a result of the cooperative effects of the random anisotropy of CoFe2O4, exchange–spring occurring at the interface of CoFe2O4 and CoFe2 together with the inter-particle dipolar interaction (figure c); in ?d(MFC ? MZFC)/dT curves, besides a broad relaxing peat at T2, a rapid rise around the low-temperature T1~15 K is observed, below which the moments are suggested to fully freeze. Increase of magnetic field from 2 kOe leads to the shift of T2 and Tirr towards a lower temperature, and the shift of T2 is attributable to the moment reversal of CoFe2O4.
The temperature dependences of the velocity of longitudinal sound waves and the internal friction in a La0.82Ca0.18MnO3 single crystal with the Curie temperature TC = 181 K have been studied. As temperature decreases, the single crystal is shown to undergo the transition from the pseudocubic O* to the Jahn–Teller O’ phase at T ~ 254 K and the reverse transition from O’ to O* phase at T ~ 84 K. The velocity of sound and the internal friction in the O’ phase are found to be significantly smaller than those in the O* phase. 相似文献
The crystal and magnetic structures of La0.75Ca0.25MnO3 manganite are studied under high pressures up to 4.5 GPa in the temperature range 12–300 K by the neutron diffraction method. At normal pressure and temperature TC = 240 K, a ferromagnetic state is formed in La0.75Ca0.25MnO3. At high pressures P ≥ 1.5 GPa and at temperatures T < TN ≈ 150 K, a new A-type antiferromagnetic state appears. A further increase in pressure leads to an increase in the volume fraction of the antiferromagnetic phase, which coexists with the initial ferromagnetic phase. The effect of high pressure causes a considerable increase in TC with the slope dTC/dP ≈ 12 K/GPa. Calculations performed in the framework of the double exchange model with allowance for the electron-phonon interaction make it possible to explain this pressure dependence of TC on the basis of experimental data. 相似文献
We report a quantitative investigation of the magnetic field-temperature phase diagram by taking into account a simple phenomenological model arising out of the interplay of kinetic arrest and thermodynamic transitions in a magnetic glass Pr0.5Ca0.5Mn0.975Al0.025O3, through magnetization measurements. Such studies are necessary as kinetic arrest plays an important role in the formation of “magnetic glasses”, which has been observed in systems undergoing first order magnetic phase transitions. It has been shown that disorder in a system results in the formation kinetic arrest (HK,TK) band, like supercooling (H*,T*) and superheating (H**,T**) band. Quantitative proofs are given to show that (HK,TK) band is anticorrelated with (H*,T*) and (H**,T**) bands, while the later two are correlated among themselves. Analysis of time dependence of magnetization at different temperatures is carried out to establish the fact that the kinetic arrested state is different from the supercooled state. 相似文献
The thermopower, S, magnetothermopower, ΔS/S, resistivity, ρ, and magnetoresistivity, Δρ/ρ, depending on the temperature T and magnetic field H, have been studied in an Nd0.5Sr0.5MnO3 single crystal consisting of three types of clusters: an antiferromagnetic CE-type with charge-orbital ordering (below the Neel temperature TNCE ~ 145 K) and an A-type with TNA ~ 220 K; a ferromagnetic at 234 ≤ T ≤ 252 K, and a ferromagnetic metal phase below the Curie temperature TC = 248 K. The thermopower was found to be negative, indicating the dominance of the electronic type of conductivity. In the S(T) curves, a sharp minimum is observed in the temperature range of 100 K ≤ T ≤ 133 K, close to TNCE, where the absolute S value attains 53 μV/K. With a further increase in temperature, the absolute S value decreases rapidly; at 200 K it is equal to 7 μV/K. It then slightly increases, reaching its maximum value of 15 μV/K at a temperature of 254 K, which is close to TC. The absolute thermopower decreased under the influence of the magnetic field; i.e., a negative magnetothermopower occurs. In {ΔS/S}(T) curves, a sharp minimum is observed at T = 130 K close to TNCE, where the magnetothermopower reaches a huge value of ~45% at H = 13.23 kOe. A broad minimum in the {ΔS/S}(T) curves is observed near the Curie temperature and its value is also high, viz., ~15% in the maximum measuring magnetic field of 13.23 kOe. The extremely high magnetothermopower values mean that the charge-orbital ordered nanoclusters or ferron type make the main contribution to the thermopower of the entire sample. The behavior of the ρ(T) and {Δρ/ρ}(T) curves is similar to that of the S(T) and {ΔS/S}(T) dependencies, which is in agreement with this conclusion. 相似文献
The low-temperature specific heat Cp of La(Fe0.873Co0.007Al0.12)13 compound has been measured in two states: (i) antiferromagnetic (AFM) with a Néel temperature of TN = 192 K and (ii) ferromagnetic (FM). The FM order appears at T = 4.2 K in a sample exposed to an external magnetic field with induction BC ≥ 2.5 T and is retained for a long time in a zero field at temperatures up to T*C = 23 K. The coefficient γFM in the low-temperature specific heat C = γT + βT3 in the FM state differs quite insignificantly from that (γAFM) in the AFM state. Contributions to the low-temperature specific heat, which are related to a change in the elastic and magnetoelastic energy caused by magnetostrictive deformations, are considered. 相似文献
Polarization-optical study of twinning and measurements of the Raman spectra and birefringence in oxyfluoride (NH4)3Ti(O2)F5 were carried out over the temperature range 90–350 K. Phase transitions were detected at temperatures T01 = 266 K (second-order transition) and T02 = 225 K (first order). It is assumed that the crystal symmetry is changed as follows: Fm3m ? I4/mmm ? I4/m. Anomalies of the spectral parameters are established in the frequency range of internal vibrations of ammonium ions and Ti(O2)F5 complexes. An analysis of the results shows that the transition at T01 is likely due to small shifts of the tetrahedral groups from their position on the triad axis and that the transition at T02 is due to fluorine-oxygen ordering of Ti(O2)F5 complexes. 相似文献
Magnetization M(H,T) in magnetic fields H up to 90 kOe and at temperatures 2 K ≤ T < Tc (where Tc is the superconducting transition temperature), along with magnetic susceptibility χ(T) in the normal state Tc < T < 400 K for optimally oxygen-doped samples of YBa2Cu3O6.92 with varying degrees of defects in the crystal structure, are studied to determine the influence of structural inhomogeneity on the electron systems characteristics of cuprate superconductors. It is shown that the existence of structural inhomogeneity of samples leads to the manifestation of peculiarities appropriate to pseudogap regime in their properties. 相似文献
(NH4)3ZrF7 single crystals were grown, and polarization-optical and x-ray diffraction studies were performed on powders and crystalline plates of various cuts over a wide temperature range. Phase transitions are revealed at temperatures T1↑ = 280 K, T2↑ = 279.6 K, T3↑ = 260–265 K, and T4↑ = 238 K on heating and at T1↓ = 280 K, T2↓ = 269–270 K, T3↓ = 246 K, and T4↓ = 235 K on cooling. The sequence of changes in symmetry is established to be as follows: Oh5(Z = 4) ? D2h25(Z = 2) ? C2h3(Z = 2) ? Ci1(Z = 108) ? monoclinic2(Z = 216). 相似文献
New data on the specific heat, thermal expansion, and magnetization of the CaMn7O12 phase require a revision of the current concepts of the sequence of phase transitions in this compound. It is found that a spin-glass phase transition occurs in CaMn7O12 at TM = 49 K, whereas the transition at TS = 89 K exhibits the features of a first-order phase transition and thereby is apparently of structural origin. In the range TM < T< TS, the CaMn7O12 compound exhibits negative thermal expansion, which is also indicative of structural changes. 相似文献
The spins of Ru5+ ions in Sr2YRuO6 form a face-centered cubic lattice with antiferromagnetic nearest neighbor interaction J≈25 meV. The antiferromagnetic structure of the first type experimentally observed below the Néel temperature TN=26 K corresponds to four frustrated spins of 12 nearest neighbors. In the Heisenberg model in the spin-wave approximation, the frustrations already cause instability of the antiferromagnetic state at T=0 K. This state is stabilized by weak anisotropy D or exchange interaction I with the next-nearest neighbors. Low D/J~I/J~10?3 values correspond to the experimental TN and sublattice magnetic moment values. 相似文献
Magnetic, elastic, magnetoelastic, transport, and magnetotransport properties of the Eu0.55Sr0.45MnO3 ceramics have been studied. A break was detected in the temperature dependence of electrical resistivity ρ(T) near the temperature of the magnetic phase transformation (41 K), with the material remaining an insulator down to the lowest measurement temperature reached (ρ=106 Ω cm at 4.2 K). In the interval 4.2≤T≤50 K, the isotherms of the magnetization, volume magnetostriction, and ρ were observed to undergo jumps at the critical field HC1, which decreases with increasing T. For 50≤T≤120 K, the jumps in the above curves persist, but the pattern of the curves changes and HC1 grows with increasing T. The magnetoresistance Δρ/ρ = (ρH?ρH=0)/ρH is positive for H<HC1 and passes through a maximum at 41 K, where Δρ/ρ = 6%. For H>HC1, the magnetoresistance is negative, passes through a minimum near 41 K, and reaches a colossal value of 3×105 % at H=45 kOe. The volume magnetostriction is negative and attains a giant value of 4.5×10?4atH=45 kOe. The observed properties are assigned to the existence of three phases in Eu0.55Sr0.45MnO3, namely, a ferromagnetic (FM) phase, in which carriers are concentrated because of the gain in s-d exchange energy, and two antiferromagnetic (AFM) phases of the A and CE types. Their fractional volumes at low temperatures were estimated to be as follows: ~3% of the sample volume is occupied by the FM phase; ~67%, by the CE-type AFM phase; and ~30%, by the A-type AFM phase. 相似文献