Low-temperature Relaxation Process Research Articles

Effects of vanadium doping on the charge ordering and low-temperature spin-glass phase in Pr0.45Ca0.55MnO3

We report structural, magnetic, and dielectric properties of the Pr0.45Ca0.55Mn1-xVxO3 (x = 0.05 and 0.1) polycrystalline compounds, prepared by solid state reaction method. Pnma space group and orthorhombic crystal structure of the compounds are confirmed by the Rietveld refinement of X-ray diffraction data. Magnetic study reveals a multiple-phase-separated metastable magnetic behavior with charge ordered phase followed by a spin-glass state at the low-temperature region. For a single change of vanadium content from x = 0.05–0.1, the charge ordering temperature decreases from 245 K to 239 K, the antiferromagnetic ordering temperature decreases from 131 K to 127 K, and the ferromagnetic cluster-glass ordering temperature increases from 41 K to 44 K. This indicates the weakening of charge ordering strength and development of ferromagnetic components in the antiferromagnetic background, which is also confirmed by isothermal hysteresis. Colossal dielectric constant is observed in both samples. In the measured temperature range, an implication of a ferroelectric phase transition around 253 K is observed in Pr0.45Ca0.55Mn0.90V0.10O3 compound. The calculated results indicate that the low-temperature relaxation follows variable range hopping behavior, whereas the high-temperature one takes place according to the Arrhenius model. The fitting parameters ascribe the low-temperature relaxation process to the hopping of polaron charge carriers at localized sites, whereas the high-temperature relaxation is related to Maxwell-Wagner relaxation, caused by blocking of charge carriers at grain boundaries.

Low-energy excitations and magnetic anisotropy of the layered van der Waals antiferromagnet Ni2P2S6

The quasi-two-dimensional antiferromagnet Ni$_2$P$_2$S$_6$ belongs to the family of magnetic van der Waals compounds that provides a rich material base for the realization of fundamental models of quantum magnetism in low dimensions. Here, we report high-frequency/high-magnetic field electron spin resonance measurements on single crystals of Ni$_2$P$_2$S$_6$. The results enable to reliably determine the positive, 'easy'-plane type of the single ion anisotropy of the Ni$^{2+}$ ions with the upper limit of its magnitude $\lesssim 0.7$ meV. The resonance response reveals strongly anisotropic spin fluctuations setting in shortly above the N\'eel temperature $T_{\rm N} = 158$ K and extending in the antiferromagnetically ordered state down to low temperatures. There, a new low-energy magnon excitation gapped from the ground state by only $\sim 1$ meV was found. This magnon mode may explain unusual low temperature relaxation processes observed in a latest nuclear magnetic resonance experiments and together with the estimate of the single ion anisotropy they should enable setting up a realistic spin model for an accurate description of magnetic properties of Ni$_2$P$_2$S$_6$.

Microscopic dynamics of highly permeable super glassy polynorbornenes revealed by quasielastic neutron scattering

The molecular dynamics of addition-type poly(tricyclononenes) with Si-substituted bulky side groups has been investigated by a combination of neutron time-of-flight and neutron backscattering spectroscopy on a time scale from 0.1 ps to ca. 3 ns. The investigated poly(tricyclononenes) PTCNSi1 and PTCNSi2g both bear a high microporosity which makes them promising candidates for active separation layers for gas separation membranes. At least for larger gas molecules it is assumed that the pathways for diffusion require an enlargement of pre-existing micropores in terms of an activated zone. A low temperature relaxation process was found for both polymers by the performed neutron scattering experiments. This process was assigned to the methyl group rotation. It was analysed in terms of a jump diffusion in a three-fold potential. The analysis of the dependence of the elastic incoherent structure factor on the scattering vector yields the number of methyl groups which might be immobilized. For PTCNSi1 (3 methyl groups in the monomeric unit) it was found that all methyl groups take part in the methyl group rotation whereas for PTCNSi2g (6 methyl groups in monomeric unit) a considerable number of methyl groups are blocked in their rotation. This immobilization of methyl groups is due to the sterically demanding arrangement of the methyl groups in PTCNSi2g. This conclusion is further supported by the result that the activation energy for the methyl group rotation is three times higher for PTCNSi2g than that of PTCNSi1.

Dielectric Relaxation Mechanism in High-Pressure Synthesized BiCr0.5Mn0.5O3

The compound BiCr0.5Mn0.5O3, synthesized at high pressure and high temperature, shows a giant dielectric constant over a wide range of temperatures. Two relaxation processes are observed commencing around 200 K and 300 K. The low-temperature relaxation process is attributed to Maxwell–Wagner polarization at the grain boundary, whereas the second relaxation is attributed to the electrode polarization effect. Impedance spectroscopy reveals that the oxide is electrically inhomogeneous and dominant contribution arises from semiconducting grains and insulating grain boundary below room temperature. Above room temperature, the electrode polarization effect also contributes to the observed giant dielectric constant.

Current Relaxation in TlGa1 – xDyxSe2 (x = 0.01, 0.03) Single Crystals

The low-temperature relaxation processes in TlGa1 – xDyxSe2 (x = 0.01, 0.03) single crystals have been studied experimentally. The physical parameters which characterize the electron processes in Ag–TlGa1 – xDyxSe2–Ag samples have been determined using the estafette transfer mechanism of the charge formed at deep traps due to the carrier injection from a contact: the effective mobility of the charge transferred due to deep centers, the sample contact capacity, the region of accumulation of the charge in the samples, the contact charging constant, and the flight time of charge carriers through the sample.

Origins of dielectric relaxations in AgNb7O18 ceramic

The origins of the relaxor behavior and low-temperature dielectric relaxation processes in AgNb7O18 ceramic were studied and a new relaxation mechanism was proposed through combined bond valence energy landscape (BVEL) calculation and equivalent circuit modeling (ECM). The BVEL calculation suggests that the Ag1 displacement subjects to a double-well potential that originates from the anisotropic coordination symmetry and low coordination number at Ag1 site. The relaxor ferroelectric behavior, observed at 200 K ≤ T ≤ 270 K, is sensitive to thermal history and is ascribed to the Ag1 hopping in double-well potential. The temperature dependence of the matrix capacitance (obtained by ECM) is very similar with that of b axis (obtained by in situ XRD) at 123 K < T < 174 K. Therefore, a phase transition being responsible for the low-temperature dielectric relaxation is assumed. The combined BVEL and ECM techniques performed in this work are therefore quite helpful to understand the complicated relaxor phenomena and explore new relaxors.

Preparation of Ba2FeNbO6 double perovskite ceramics from molten-salt synthesized powders and their dielectric and magnetic properties

Double perovskite Ba2FeNbO6 (BFN) ceramics were prepared from the molten-salt synthesized BFN powders. The BFN powders have a cubic structure with $$ Fm\bar{3}m $$ space group, confirmed by X-ray diffraction patterns. Their average particle sizes increased with the annealing temperature and dwell time whereas decreased with the molten-salt content. X-ray photoemission spectra revealed both Fe2+ and Fe3+ ions present in the BFN powders and oxygen in the forms of lattice oxygen and oxygen vacancies, while niobium was present as Nb5+. The BFN ceramics exhibited significant frequency-dependent dielectric behavior, and two kinds of dielectric relaxor processes were observed. Their activation energies were 0.16 eV and 0.47 eV, respectively. The electron hopping process (between Fe2+ and Fe3+ ions) with the activation energy of 0.16 eV contributes the low-temperature dielectric relaxation process, and an activation energy of 0.47 eV associated with the high-temperature dielectric relaxation is originally from the barrier energy of the thermal motion of the oxygen vacancies. The dielectric relaxation observed at high-temperature region can be explained by Maxwell–Wagner interfacial polarization in the BFN ceramics. M–H plots of the BFN ceramics at different temperatures revealed their antiferromagnetic nature and a weak ferromagnetic behavior at 2 K. The remanent magnetization (Mr) of the BFN ceramics was measured to be 1.75 emu/g at 2 K and coercive field (Hc) as 1.26 kOe. Magnetization splitting was observed at ~ 290 K between the temperature-dependent magnetizations of the BFN ceramics, which were measured under zero field-cooled (ZFC) and field-cooled modes with an external field of 1 kOe. In the ZFC curve, a maximum peak was observed at 14 K, corresponding to the Neel temperature (TN). The TN of the present BFN ceramics shifts toward low temperature as compared with those prepared by conventional solid-state reaction methods. Such a reduction of the Neel temperature is ascribed to the size effect.

Релаксация тока в монокристаллах TlGa-=SUB=-1-x-=/SUB=-Dy-=SUB=-x-=/SUB=-Se-=SUB=-2-=/SUB=- (x=0.01; 0.03)

Abstract The low-temperature relaxation processes in TlGa_1 – _ x Dy_ x Se_2 ( x = 0.01, 0.03) single crystals have been studied experimentally. The physical parameters which characterize the electron processes in Ag–TlGa_1 – _ x Dy_ x Se_2–Ag samples have been determined using the estafette transfer mechanism of the charge formed at deep traps due to the carrier injection from a contact: the effective mobility of the charge transferred due to deep centers, the sample contact capacity, the region of accumulation of the charge in the samples, the contact charging constant, and the flight time of charge carriers through the sample.

Magnetic after-effects in Ni ferrite nanoparticles

Magnetic after-effects in ferrite nanoparticles have been measured with magnetic disaccommodation technique. For higher annealing temperature multidomain formation is detected with low temperature relaxation process similar to single crystal and polycrystalline NiFe2O4. As expected, no room temperature processes are observed due to low vacancy content. On the other hand, logarithmic relaxations are observed at lower annealing temperatures, revealing monodomain character.

Internal structural changes in keratin fibres resulting from combined hair waving and stress relaxation treatments: a Raman spectroscopic investigation.

The objective of our research was to investigate the influence of chemical treatments (reduction, stress relaxation and oxidation) on hair keratin fibres. The structure of cross-sections at various depths of virgin white human hair resulting from permanent waving treatments with stress relaxation process was directly analysed at a molecular level using Raman spectroscopy. In particular, the three disulphide (-SS-) conformations in human hair were compared by S-S band analysis. The gauche-gauche-gauche (GGG) and gauche-gauche-trans (GGT) contents of -SS- groups remarkably decreased, while the trans-gauche-trans (TGT) content was not changed by performing the reduction process with thioglycolic acid. In addition, the high-temperature stress relaxation process after reduction accelerated the disconnection of -SS- (GGG and GGT) groups in the human hair, while the low-temperature stress relaxation process after reduction accelerated the reconnection of -SS- (GGG and GGT) groups. Moreover, the S-O band intensity at 1042 cm(-1) , assigned to cysteic acid, existing in the cuticle region and the surface of the cortex region increased, while the GGG content significantly decreased by performing the oxidation process after the reduction and the high-temperature stress relaxation processes. The author concluded that the high-temperature relaxation process after reduction accelerated the disconnection of -SS- (GGG and GGT) groups, thereby leading to the remarkable local molecular disorganization (an increase in the cysteic acid content and a decrease in the GGG content) on the cuticle and cortex cells during the oxidation process.

Dielectric properties of liquid-crystal azomethine polymer with alkyl-substituted side chain

Relaxation processes in side-chain liquid-crystal azomethine polymer have been studied using dielectric spectroscopy in wide ranges of temperatures and frequencies. Two low-temperature relaxation processes (β1 and β2) caused by low-amplitude noncooperative motion of kinetic units in side groups and a high-temperature α-process caused by the large-scale cooperative motion of kinetic units in the main polymer chain have been revealed.

Low-temperature relaxation process and memory effect in a nonstoichiometric magnetite of Fe3−δO4 with δ=0.03

The low-temperature relaxation process has been investigated in a nonstoichiometric magnetite Fe3−δO4 with δ=0.03. Far below the Verwey transition at TV=90K, the measurements of AC susceptibility χac display a frequency-dependent anomaly of the shoulder in χ' accompanied with the peak in χ″ and their weak thermal hysteresis. These low-temperature anomalies are related to a thermal relaxation process owing to the domain-wall mobility and extra electron exchange inside the walls. Moreover, the low-temperature relaxation process is revealed to exhibit strong memory effect via field-cooling magnetization measurements. Interesting discrete sudden jumps are observed during the logarithmic decay of magnetization in zero field with aging time, indicating the spontaneous magnetization reversals via adjustment of domain configuration.

High‐Temperature Dielectric Relaxation in Pb(Mg1/3Nb2/3)O3–PbTiO3 Single Crystals

We reported the dielectric properties of Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystal in the temperature range of 300–1073 K and the frequency range of 100 Hz–10 MHz. Our results showed the coexistence of both true‐ and pseudo‐relaxor behaviors in the crystal. The true relaxor behavior related to the paraelectric‐ferroelectric phase transition occurs at~423 K. The pseudo‐relaxor behavior appearing at~773 K was found to be related to oxygen vacancies. Further investigation reveals that the pseudo‐relaxor behavior has fine structure: it contains two oxygen‐vacancy‐related relaxation processes. The low‐temperature relaxation process is a dipolar relaxation created by the hopping motions of the oxygen vacancies, and the high‐temperature relaxation process is a Maxwell‐Wagner relaxation caused by the sample/electrode contacts.

Small polaron migration associated multiple dielectric responses of multiferroic DyMnO3 polycrystal in low temperature region

Utilizing temperature dependent dielectric/impedance spectroscopy, multi-dielectric responses involving two dielectric relaxations (DRs) and two magnetic-order-associated dielectric anomalies were observed in polycrystalline DyMnO3. It is elucidated that both DRs’ dynamics, established in terms of equivalent circuit model and small polaron (SP) theories, are closely linked with localized SP migration features. Namely, low-temperature relaxation process can be attributed to short range polaronic variable-range-hopping induced dipolar-type relaxation in grains, whereas the higher-temperature one is due to Maxwell-Wagner relaxation at grain/grain boundary interfaces, which are governed by SP nearest-neighbor-hopping conduction. Additionally, magnetic-orders-associated dielectric anomalies may be assigned to strong spin-lattice couplings by magnetoelasticity-aroused lattice deformation.

A study of gamma-irradiated polyethylenes by temperature modulated differential scanning calorimetry

Various polyethylenes (PEs) and the effects of high-energy radiation on their structures were widely studied in the past using conventional Differential Scanning Calorimetry (DSC) measurements. In this work, we used the Temperature Modulated Differential Scanning Calorimetry (TMDSC) technique in order to obtain more information about the influence of the initial structural differences and gamma radiation on the evolution in structure and thermal properties of different polyethylenes. For this reason, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) samples were exposed to gamma radiation, in air, to a wide range of absorbed doses (up to 2400kGy). The separation of the total heat flow TMDSC signal into a reversing and non-reversing part enabled us to observe the low-temperature enthalpy relaxation (related to the existence of the “rigid amorphous phase”) and recrystallisation processes, as well as to follow their radiation-induced evolution and/or that of melting in a more revealing manner compared to the case of the conventional DSC. Consequently, our results indicate that TMDSC could improve the understanding of radiation-induced effects in polymers.

Site-Dependent Segmental Dynamics Revealed Using Broadband Dielectric Spectroscopy on Well-Defined Functionalized Polystyrenes

The site-dependent segmental dynamics of polystyrene has been investigated by using a combination of broadband dielectric spectroscopy (BDS) and well-defined, functionalized polystyrenes (Mn ≈ 2200 and 4200 g/mol) with strongly polar cyano (CN) groups precisely located either at the end of the chain (chain-end functionalization) or in the middle of the chain (in-chain functionalization). Since broadband dielectric spectroscopy (BDS) is sensitive to the local dipole moment fluctuations, it is possible to selectively probe the polymer motions responsible for the dipole moment fluctuations using these cyano-functionalized polymers. By means of rheology and differential scanning calorimetry (DSC), it was confirmed that neither chain-end nor in-chain CN-functionalization affected the overall polymer matrix properties as compared with an equivalent H-functionalization analogue. This was confirmed for important properties such as the glass transition temperature (Tg) and viscosity (η). Results obtained for the two different molecular weights show that the dielectric signal associated with the chain-end fluctuations exhibits an accelerated and more heterogeneous dynamics than the average segmental α-relaxation mainly at high temperature. Closer to the Tg, the time scale of the chain-end fluctuations and the overall dynamics approach each other, and the apparent decoupling disappears. In contrast, for the segments located in the middle of the chain, the dynamics display rather similar features as the mean response although it is distinctly slower approaching Tg. This has been interpreted as combination of the intrinsic dynamic segmental heterogeneity of the polymer chain combined with the increasing cooperativity length close to Tg. The functionalization with the CN groups also allows the observation of a rather weak low-temperature relaxation process that is interpreted as a specific, more localized movement of the functional groups. The activation energies obtained are typical for motions of single molecules or segments. Interestingly, the time scale associated with this relaxation seems to decrease with molecular weight, suggesting poorer local packing for larger chains. Furthermore, the time scale depends also on the functionalization site, indicating the relevance of intermolecular interactions.

The electret properties of tetrafluoroethylene-hexafluoropropylene copolymer films modified in glow discharge

The electret properties of tetrafluoroethylene-hexafluoropropylene copolymer films modified in direct-current discharge were studied. It was shown that the treatment of the films mounted on the anode had an insignificant effect on the relaxation of negative homocharge despite substantial changes in the chemistry of the surface. However, the low-temperature relaxation process responsible for the instability of the electret state in the untreated film is completely suppressed in the case of decay of positive homocharge. Deep traps associated with high-temperature relaxation processes in the polymer are simultaneously eliminated from the spectrum of surface states.

Low-temperature relaxation processes in a Cu–Nb nanostructured fiber composite

The elastic and dissipative properties of nanostructured superconducting fiber composites Cu–32 vol.% Nb at frequencies of the order of 70kHz are investigated in the temperature range 2–320K. The composites, prepared by the method of intensive plastic deformation, consist of a fragmented copper matrix uniformly filled with niobium fibers having diameters of 200–500nm. It is found that the acoustic properties of the composite are mainly determined by the properties of the copper matrix. Near 90K a relaxation peak of the internal friction is observed, with activational parameters close to those of the Bordoni peak in copper. Below 12K the temperature dependence of the dynamic Young’s modulus is described by the thermally activated relaxation of two-level systems in a highly disordered medium. The influence of high-temperature annealing on the observed effects is investigated.

Bismuth-induced dielectric relaxation in the (1−x)La(Mg1∕2Ti1∕2)O3–xBi(Mg1∕2Ti1∕2)O3 perovskite system

The temperature variation of the dielectric permittivity and loss of the solid solutions (1−x)La(Mg1∕2Ti1∕2)O3–xBi(Mg1∕2Ti1∕2)O3 [(1−x)LMT–xBMT] (0⩽x⩽0.3) measured at radio, microwave, and far infrared frequency ranges has been analyzed in comparison with that observed in other bismuth-containing and bismuth-free perovskite ceramics based on LMT. It has been found that the low temperature dielectric response of the (1−x)LMT–xBMT compositions with x⩾0 is frequency dependent over a wide range from radio to microwave frequencies. The considerable compositional growth of the dielectric permittivity and loss associated with the amount of bismuth in the system was revealed to be not contributed by the lattice polar phonon modes. The effect was suggested to be related to the low-temperature dielectric relaxation process due to a hopping movement of charge carriers in crystallographic A-sites of the perovskite lattice. Particular role of local lattice distortions caused by the anisotropic chemical bonds involving bismuth 6s2 electrons in a localization of hopping charge carriers in perovskites and other oxygen-octahedral compositions is considered. The characteristic features of the Bi-induced dielectric relaxation and those typical of the ferroelectric relaxors are compared and discussed.

Ternary Thiol−Ene/Acrylate Photopolymers: Effect of Acrylate Structure on Mechanical Properties

Model ternary thiol−ene/acrylate photopolymerization involving acrylate homopolymerization and copolymerization of thiol−ene and thiol−acrylate monomers were monitored by real-time FTIR. In all ternary mixtures, including those prepared with different acrylate concentrations, acrylate conversion was 100%. However, thiol−ene conversions were found to be controlled by their initial concentrations. The influence of acrylate monomer chemical structure on the thermophysical properties of ternary thiol−ene/acrylate systems was studied with DMA, DSC, and the absorbance of a nondestructive impact energy. The addition of acrylate to the thiol−ene system increased the rubbery modulus while the tan δmax shifted to higher temperatures. Densely cross-linked, heterogeneous matrix formation was observed with the broadening of tan δ peaks at high acrylate concentrations. The high impact absorption of these ternary thermoset photopolymers was correlated with the dynamic mechanical damping ability of the networks. Acrylates with higher functionality and low molecular weight per double bond are more effective at increasing the glass transition temperature of the thiol−ene polymer network. Fracture behavior of ternary thiol−ene/acrylate networks under impact shows a dependence on the chemical structure of the acrylate, component concentrations, and low-temperature relaxation processes. The ternary matrix formed with a bisphenol A based difunctional acrylate monomer exhibited improved impact energy absorption at room temperature. Finally, tensile properties of polymer networks formed with thiol−ene/acrylate and thiol−acrylate mixtures are given for comparison purposes.