Ferromagnetic resonance observation of a phase transition in magnetic field-aligned Fe2O3 nanoparticles

Fe₂O₃ hematite (alpha) nanoparticles suspended in the liquid phase of the liquid crystal 4,4-azoxyanlsole (PAA) are cooled below the freezing temperature (397 K) in a 4000 G dc magnetic field. The in field solidification locks the direction of maximum magnetization of the particles parallel to the direction of the applied dc magnetic field removing the effects of dynamical fluctuations of the nanoparticles on the magnetic properties allowing a study of the intrinsic magnetic properties of the nanoparticles as well as the anisotropic behavior of the ferromagnetic resonance (FMR) signal. Freezing in PAA allows temperature-dependent measurements to be made at much higher temperature than previous measurements. The field position, line width and intensity of the FMR signal as a function of temperature as well as the magnetization show anomalies in the vicinity of 200 K indicative of a magnetic transition, likely the previously observed Morin transition shifted to lower temperature due to the small particle size. Weak ferromagnetism is observed below T_c in contrast to the bulk material where it is antiferromagnetic below T_c. The Raman spectrum above and below 200 K shows no evidence of a change in lattice symmetry associated with the magnetic transition.     

Cure Behavior and Thermal Properties of Poly (Amide-Amidic Acid) Modified Diglycidyl Ether of Bisphenol-A/4,4′-Diaminodiphenylsulfone

Poly (amide-amidic acid) (PAA) was selected to modify diglycidyl ether of bisphenol-A (DGEBA)/4,4′-diaminodiphenylsulfone (DDS). The cure behavior was studied by means of nonisothermal differential scanning calorimeter (DSC) analysis, indicating that PAA played a role of catalyst during the process of the curing reaction. Results of Fourier transform infrared spectroscopy (FT-IR) analysis showed that the PAA acted as a co-curing agent when the PAA content was 3.2–38.4 phr and also as a modifier when the PAA content was 12.8–38.4 phr. The glass transition temperature (T_g ) decreased with the increase of PAA content. The thermal stability improved when the PAA content was 3.2–6.4 phr because of the catalytic effect of PAA. The flexural strength improved for the varying PAA content studied in this work, with the highest flexural strength being obtained when the PAA content was 6.4 phr. The fracture surface morphology was observed using scanning electron microscopy (SEM); the morphologies varied with changing content of PAA.     

PAA-assisted synthesis of CaCO3 microcrystals and affecting factors under microwave irradiation

In the present paper, we described a polyacrylic acid (PAA)-assisted microwave irradiation route for synthesis of Calcium carbonate (CaCO₃) microcrystals. CaCl₂·2H₂O and NaHCO₃ were used as the starting reactants. Researches showed that the presence of PAA could strongly affect the phase and morphology of CaCO₃ crystals. X-ray powder diffraction (XRD) analyses showed that the product prepared from the system with/without PAA corresponded to Vaterite/Calcite, respectively. Scanning electron microscopy (SEM) observations showed that the hierarchical CaCO₃ microcrystals were obtained in the presence of PAA. Some factors influencing the morphology of the as-synthesized CaCO₃ crystals were systematically investigated.     

Successive phase transitions in tetra(isopropylamine)decachlorotricadmate (II) [(CH3)2CH NH3]4Cd3Cl10] crystal - Dielectric and dilatometric studies

Single crystal of a new [(CH₃)₂CHNH₃]₄Cd₃Cl₁₀ compound was grown and its properties have been characterized by dielectric and dilatometric investigations. Dielectric measurement evidenced the phase transitions at T₁ = 352.8 K, T₂ = 293.5 K and T₃ = 261.5 K on cooling run. Dilatometric measurement of thermal expansion showed clearly two successive phase transitions at T₁ and at T₂. No temperature hysteresis was observed for phase transitions at T₁ and T₂. Large temperature hysteresis was observed at T₃ in dielectric studies. Transitions at T₁ and T₃ are classified as a first order and at T₂ as a continuous one. Anomalies of electric permittivity and expansion connected with the transitions are observed at practically the same temperatures and close to those observed earlier in DSC (Differential Scanning Calorimetry) studies. Results of dilatometric studies were applied to estimate critical coefficient β for continuous phase transition at T₁ which is equal to 0.40 ± 0.01.     

F-Zentren und Kolloide in der amorphen,hexagonalen und kubischen Phase von LiJ

By simultaneous evaporation of LiI and Li onto a cooled substrate F centers can be produced in the hexagonal (78 K<T _K <200 K) and amorphous (T _K <78 K) phase of one and the same salt. In both modifications there exist two types of centers F and F^*. The F^* center differs from the cubic F center (T _d -symmetry) by a nearby Frenkel defect. In hexagonal films the normal F band peaks at 2.58 eV, whereas the transitions of the F^* center appear at 2.92 and 2.58 eV too. Polarized irradiation at 20 K causes a dichroic behaviour of the F^* centers. Both types of centers can be transformed into one another photochemically. In the amorphous phase all transitions are shifted to lower energies by about 0.1 eV. After the phase change amorphous→hexagonal the absorption bands shift back by the same amount of energy. AboveT _K =230 K the excess metal forms colloids. The absorption bands are due to colloidal centers embedded in the crystalline material (2.25 eV) and films adsorbed to the crystallites (3.1 eV), respectively. By annealing a particle growth can be observed. After electrolytic colouration cubic single crystals of LiI exhibit an absorption band peaking at 2.36 eV. However, it is not yet sure, if this band is allowed to be ascribed to F centers.     

Effect of γ-irradiation on the heat capacity of an [NH2(CH3)2]2 · CuCl4 crystal in the temperature range 80–300 K

The heat capacity of [NH₂(CH₃)₂]₂ · CuCl₄ crystals prior to and after γ-irradiation with doses of 1, 5, 10, and 50 MR is measured by the calorimetric method in the temperature range 80–300 K. It is found that, as the temperature decreases, the temperature dependence C _p (T) exhibits two anomalies which correspond to phase transitions from the incommensurate to the ferroelectric phase at T _c =281 K and from the ferroelectric to the ferroelastic phase at T ₁=255 K. The nature of the anomalies is typical of a first-order phase transition. In addition, a smeared anomaly in the form of a small increase in the heat capacity of the ferroelectric phase is observed at T≈275 K. It is demonstrated that when the dose of γ-irradiation increases, the anomalies decrease in magnitude and the phase transition temperatures are displaced: T _c increases and T ₁ decreases.     

In situ observation of containerless solidification from undercooled Lu2O3 and Y2O3 melts

Containerless solidification processes in undercooled Lu₂O₃ and Y₂O₃ melts were investigated using an aerodynamic levitation furnace and a high-speed video. Double recalescence, indicative of two successive phase transitions, was observed for both oxides. The melting point following the first recalescence (T _M1) was higher than that of the second (T _M2) for Y₂O₃, which suggests that Y₂O₃ has a phase transition from a melt to a high temperature phase at T _M1, and then to a room temperature phase at T _M2. Two solid phases of Lu₂O₃ were also identified at T _M1 and T _M2. However, in contrast to Y₂O₃, T _M1 was lower than T _M2 for Lu₂O₃. This implies that the first crystallized phase from a melt is a metastable phase. The life duration of this metastable Lu₂O₃ was in the order of tens of milliseconds.     

Electron spin relaxation of copper(II) ions in diamagnetic crystals

The electron spin-lattice and spin-spin phase relaxation measurements of Cu²⁺ ions in various crystals are reviewed and discussed. Examples of the Debye temperature determination from a wide temperature range measurements of the spin-lattice relaxation time T₁ are shown. An influence of the Jahn-Teller dynamics on T₁ is presented. The phase relaxation described by the phase memory time T_M is affected by temperature due to the spin packet width modulation by molecular motions. The T_M is anisotropic in crystals and can be different for different hyperfine lines of an EPR spectrum.     

Electrical and thermal properties in CuV2S4

The successive phase transitions of the cubic spinel structure CuV₂S₄ have been studied by the electrical resistivity and the specific heat. In the resistivity and the specific heat two anomalies have been observed at T₁～91 K and T₃≈55 K. The specific heat indicates that the phase transition at T₁ is in the vicinity of a tricritical point, where the resistivity has a small sharp peak. On the other hand, the specific heat shows a step at T₃, where a hysteresis of the resistivity indicates the existence of the first-order phase transition.     

Dielectric properties of betaine phosphite-betaine phosphate solid-solution crystals in the improper ferroelastic phase

Temperature dependences of dielectric permittivity in the improper ferroelastic phase, including the region of the improper ferroelastic phase transition occurring at T=T_c1, were studied in the betaine phosphite-betaine phosphate solid-solution crystals. At a betaine phosphate (BP) concentration of 10%, the phase transition temperature T_c1 was found to shift toward higher temperatures by about 5 K compared to betaine phosphite (BPI) crystals, where T_c1=355 K. The phase transition remains in the vicinity of the tricritical point. As the BP concentration in BPI is increased, the dielectric anomaly at T=T_c1 weakens substantially compared to pure BPI. The nonlinear temperature dependence of reciprocal dielectric permittivity in the improper ferroelastic phase of BPI_xBP_1?x crystals is described in the concentration region 0.9≤x≤1 in terms of a thermodynamic model taking into account the biquadratic relation of the nonpolar order parameter of the improper ferroelastic phase transition to polarization. The decrease in the ferroelectric phase transition temperature T_c1 (or in the temperature of loss of improper ferroelastic phase stability) with increasing BP concentration in the above limits is due to the decreasing effect of the nonpolar mode on the polar instability, which is accompanied by a weakening of the dielectric anomaly at T=T_c1     

Ferroic phase transitions in β-LiNH4SO4

Abstract

The paper reviews the results of experimental and theoretical studies of ferroic phase transitions in β-LiNH₄SO₄ and its deuterated analogue. β-LiNH₄SO₄ undergoes succesive phase transitions: a paraelectric - ferroelectric phase transition at T₁ ? 462 K, a ferroelectric - ferroelastic phase transition at T₂ ? 283 K and a transition from one ferroelastic phase to the other at T₃ ? 28 K. Attention is focused on the influence of the order of phase transitions on the pattern of ferroelectric and ferroelastic domain structure, and also on the role played by the dynamics of molecular groups in the mechanism of transitions. The pre-transition effect connected with the ferroelectric-paraelectric transition: heterophase, capable of accounting for anomalies in different physical properties present 1-3 K below T₁ is shown. The anomalous temperature variation of spontaneous polarisation of the crystal is discussed within the framework of the phenomenological model of weak ferroelectrics.     

Ferroelectric phase transitions of the 0.32PIN-0.345PMN-0.335PT single crystals studied by temperature-dependent Raman spectroscopy,dielectric and ferroelectric performance

The ferroelectric phase transition characteristics of the 0.32Pb(In_1/2Nb_1/2)O₃-0.345Pb(Mg_1/3Nb_2/3)O₃-0.335PbTiO₃ (0.32PIN-0.345PMN-0.335PT) single crystals were studied by the temperature-dependent Raman spectroscopy and some electrical properties. Ferroelectric monoclinic phase was confirmed at room temperature by the numbers of the Raman modes. Successive ferroelectric phase transitions, i.e. ferroelectric monoclinic phase to ferroelectric tetragonal phase transition (FE_M-FE_T) and ferroelectric tetragonal phase to paraelectric cubic phase transition (FE_T-P_C), are evidenced by the anomalies of Raman modes line width, peaks intensity and their ratios around T_M-T and T_C/T_m temperatures. The temperature dependent permittivity derivative ξ = d?/dT not only provides further evidence of the successive ferroelectric phase transitions, but also demonstrates the second-order transition characteristic of the FE_M-FE_T phase transition and the first-order transition feature of the FE_T-P_C phase transition. The FE_M-FE_T phase transition is also confirmed by the abnormal narrowing of the P-E loops, decrease of the P_r and E_c values, and extremums of the pyroelectric performance.     

Phase Transformations in Nd–Fe–B-Based Alloys under High Pressure Torsion at Different Temperatures

In this work, we studied the behavior of the Nd–Dy–Fe–Co–Cu–B alloy for permanent magnets under high pressure torsion (HPT). In the initial state of the studied alloy, it mainly contained the crystalline phase τ₁ (Nd, Dy)₂(Fe, Co, Cu) ₁₄B. After HPT at room temperature (T_HPT = 30°C), a mixture of an amorphous phase with nanocrystalline inclusions of the τ₁ phase is observed in the alloy. In the equilibrium phase diagram, this state is equivalent to a mixture of the τ₁ phase with the melt at the temperature T_eff= ∼1100°C. The thus determined T_eff value is called the effective temperature. When the T_HPT temperature of the HPT treatment increases to 300 and 400°C, the amorphous phase disappears, and the Fe₂B and γ-Fe phases appear instead. In the equilibrium phase diagram, this state is equivalent to a mixture of phases τ₁+ Fe₂B + γ-Fe, which is observed in the temperature range from ∼950 to ∼1050°C. We explain this phenomenon by the fact that with an increase in the HPT temperature T_HPT, the rate of formation of defects during deformation remains constant, but the rate of their thermal relaxation (annihilation) increases. This is equivalent to decrease in the effective temperature T_eff in the equilibrium phase diagram. The previously predicted decrease in T_eff with an increase in T_HPT is observed for the first time.

     

Phase transition and ferroelastic property studied by using the Cs nuclear magnetic resonance in a CsHSO4 single crystal

The ¹³³Cs spin-lattice relaxation time in a CsHSO₄ single crystal was measured in the temperature range from 300 to 450 K. The changes in the ¹³³Cs spin-lattice relaxation rate near T_c1 (=333 K) and T_c2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin-lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T₁ around T_c2 (II-I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T₁⁻¹=A+BT. In addition, from the stress-strain hysteresis loop and the ¹³³Cs nuclear magnetic resonance, we know that the CsHSO₄ crystal has ferroelastic characteristics in phases II and III.     

Proton activity and spontaneous strain of Cs3H(SeO4)2 in the phase transition at 369 K

We have investigated the origin of the change in proton activity in the phase transition at T_II-III (=369 K) in Cs₃H(SeO₄)₂ from the viewpoint of its ferroelasticity by using ¹H NMR and X-ray measurements. It is found that the second moment of the ¹H NMR absorption line rapidly decreases at T_II-III with increasing temperature. From this result, we conclude that the hopping motion of a proton, which is the precursor motion in the superprotonic phase, becomes more active above T_II-III. This result is consistent with the fact that the electrical conductivity in phase II is larger than that in phase III. Furthermore, it is also found that the spontaneous strain decreases abruptly at T_II-III. From these results, it is deduced that the decrease in the spontaneous strain at T_II-III causes the increase in the proton activity at T_II-III. In addition, it is deduced that the increase in proton activity and the decrease in the spontaneous strain at T_II-III are closely related with the appearance of the superprotonic phase transition at T_I-II (=456 K).     

Cs3BiCl6 : Un compose ferroelastique presentant une polarisation remanente a temperature ambiante

X-ray diffraction, calorimetric, dielectric and thermocurrent measurements have been carried out on Cs₃BiCl₆ ceramic samples. Three phase transitions have been detected at T₁ = 395 K, T₂ = 684 K and T₃ = 727 K. The T₃ transition is of ferroelastic-paraelastic type. The low temperature phase is characterized by remanent polarization. The depolarization current results probably from charge traps in potential minima which disappear at rising temperature.     

Microstructure evolution of undercooled Fe-Co-Cu alloys

Rapid solidification of undercooled Fe-Co-Cu alloys was investigated by means of fluxing purification and cyclic superheating technique. A transition in microstructure from dendrites to phase-separation occurred above a phase-separation undercooling ΔT_sep. When ΔT<ΔT_sep, dendrite was observed, the trunks were rich in Fe and Co, while Cu was rich at inter-dendrites. However, the phase-separated microstructure was obtained once ΔT>ΔT_sep, with a large sphere of L1 phase located almost at the center of the sample and enwrapped by L2 phase. ΔT_sep was 222, 88 and 45 K for Fe₅₀Co₃₀Cu₂₀, Fe₂₅Co₂₅Cu₅₀ and Fe₁₅Co₁₅Cu₇₀ alloys in this work, respectively. It was investigated that L1 phase solidified before L2 phase after liquid separation and followed different ways.     

Messung der Lebensdauer angeregter Kernniveaus von Gd155

Using two different experimental arrangements, the halflife of the following excited nuclear states in Gd¹⁵⁵ have been measured by the method of delayed coincidences: 60.0 keV-level:T _1/2=(2.4±0.6) · 10^?10 sec, 86.5 keV-level:T _1/2=(6.35±0.09) · 10^?9 sec, 105.3 keV-level:T _1/2=(1.14±0.03) · 10^?9 sec. These results are discussed and compared with the predictions of the collective model.     

Zur Konzentrationsabhängigkeit der Elektronenspin-Relaxationszeiten von Diphenyl-Picryl-Hydrazyl in fester Phase

The influence of exchange and dipol interaction on the EPR-relaxation timesT ₁ andT ₂ of DPPH has been measured in theX-band between 295 and 2°K. The spinconcentration has been varied between 1.5·10²¹ spins/cm³ (single crystals) and 1.5·10¹⁸ spins/cm³ (solid solution in polystyrene). The EPR absorption was saturated stationarily. This gave the productT ₁ T ₂ from the saturation curve and the timeT ₂ from saturation broadening of the absorption spectrum.     

Pressure-induced phase transformation in In2Bi

We have made measurements of the pressure dependence of the superconducting transition temperature, T_c, for In₂Bi and related alloys. For In₂Bi- phase alloys, a large discontinuity in T_c is seen at 15–20 kbar, which we associate with a phase transformation first seen by Bridgman [1]. Our measurements suggest that this transformation is produced by the decomposition of In₂Bi into In₅Bi₃ and an In-rich phase. In the low pressure phase, T_c shows a minimum at 9–15 kbar whereas it depends linearly on pressure in the high pressure phase with ?T_c/?P equal to -4.9 × 10^-5 K bar^-1.