Materials Science Forum Vol. 845

Abstract: Abrupt phase transitions in the structure of ferromagnet/ electroactive polymer/ non-magnetic metal have been found in the number of studies in magnetic field as huge magnetoresistive effects. The sign of magnetoresistance depends on the conductivity state that the polymer has possessed at the beginning of the experiment.The ways of managing of metal/ dielectric phase transition in the polymer layer with the aid of weak external magnetic field are considered. The model of local conversion of charge mobility in the conductive channels is suggested.

Abstract: In the last decade, considerable attention has been focused on the search of new multiferroic materials and the ways of improvement of their magnetoelectric properties. In this short review, we survey the progress in study of multiferroics focusing the high temperature multiferroic bismuth ferrite and rare earth iron garnets. We discuss the recent results of investigation of domain walls in multiferroics, concentrating the most important magnetoelectric manifestations (electric polarization and magnetization), and the pinning effect appearing as clamping of ferroelectric and magnetic domain walls.

Abstract: The magnetocaloric effect (MCE) of the Tm₂Fe_17-xMn_x (x = 0, 0.2, 0.5, 1, 1.5) and Tm₂Fe₁₆, Tm₂Fe₁₇, Tm₂Fe₁₈, Tm₂Fe₁₉ compounds has been investigated by means of magnetic measurements. The MCE exhibits two peaks near the temperatures of ferromagnetic ordering Θ_T and T_N for Tm₂Fe₁₇, Tm₂Fe₁₈, Tm₂Fe₁₉ compositions or one peak near T_C for the Tm₂Fe₁₆ and Tm₂Fe_17-xMn_x (x ≥ 0.2) ferromagnets. The peak entropy change -ΔS_M varies non-monotonically in the systems, with the maximal values-3.25 J/kg K at T = 310 for Tm₂Fe_16.8Mn_0.2 and-3.5 J/kg K at T = 290 K for Tm₂Fe₁₇ in a field of 5 T. On the contrary, the relative cooling power decreases monotonically with decrease of Fe content because of contraction of the -ΔS_M peaks.

Abstract: It was investigated the influence of Ca substitution by Zn on the fluctuations of specific resistivity, thermal power and exsess conductivity of Bi₂Sr₂Ca_0.6Zn_0.4Cu₂O_X and Bi₂Sr₂Ca₁Cu₂O_X in the 70-300К temperature interval. Experimental results explained taking into account strong anisotropy and multiband character of superconducting state in these compounds. The microscopic parameters such as dimensional crossover temperature (T_cr), interlayer coupling strength (J), and zero temperature coherence length along c axis (ξ₀) are estimated.

Abstract: The polymer thin film with non-linear electrophysical properties was used as detector for phase transition in metals (Wood's alloy and indium). A voltage has been applied to the metal-polymer-metal structure and the current through structure has been measured as function of temperature. Two independent series of experiments were carried out when the polymer film is heated together with the metal and when the metal is heated only. They revealed sharp change in the current through experimental structure at the melting-crystallization point of metals. This effect is related to a change in electrochemical potential when there is a phase transition in the metal and it results in a change of the temperature dependence of current. This method can be successfully used to detect the critical temperature without the need for direct contact of the polymer detector with the test material.

Abstract: Ceramic samples of tungstoantimonic acid (TAA) were obtained by ion exchange of sintered potassium tungstoantimonate, KWSbO₆, in acid solution. The ac conductivity of these samples is by 2 orders of magnitude higher than the conductivity of pressed powder of tungstoantimonic acid. The conduction mechanism in this material seems similar to the one in polyantimonic acid (PAA), though the conductivity of TAA is by 1.5 orders of magnitude lower than that of PAA. Based on these results and literature data on the conductivity of PAA and its derivatives, the directions for the search for materials with improved proton-conducting properties in this family of compounds are discussed.

Abstract: Changing LED performance characteristics, depending on Indium atoms concentration and at different temperatures were simulated. It was suggested that a heterostructure having p-n junction area S₀ can be considered as a sum of “SmallLEDs (SLEDs)” electrically connected in parallel. Good correlation in simulation and experimental results has been obtained. It was determined that the best AlGaInP heterostructure contains 5 wide QW (width 10 nm each) in active region. The main thing is that in central ones there is maximum radiation and at the both ends of active region are “barriers” which help to concentrate electrons/holes in active region and additionally “protect” QW from different defects.

Abstract: The paper presents the results of the study of phase transitions in the system Sr-Sb-Si-O by means of X-ray diffraction, thermal analysis, dielectric spectroscopy. Four effects are observed in the interval from room temperature to 800°C. The first and last are chemical transformations due to dehydration and loss of oxygen, respectively. The second is a transition from tetragonal to cubic perovskite structure, and the third is disordering transition in oxygen sublattice possibly due to the desorption of structural water molecules

Abstract: Neutron diffraction has been carried out to study temperature evolution of crystal and magnetic structure parameters of the multiferroic (0.9)BiFeO₃ + (0.1)BaTiO₃ over region (300 – 1000) K. Crystal structure is rhombohedral over whole temperature region and it is described by the R3c space group. The lattice parameters increase with temperature. The Ba ions are placed in the Bi sublattice and the Ti ions partly occupy the Fe sublattice. Assuming that the sample has a modulated magnetic structure with the propagation vector k = [0.0045, 0.0045, 0], we obtained a temperature dependence of the Fe-ion magnetic moment. The value of the moment is equaled to be μ = (3.46 ± 0.05) μ_B at 300 K and becomes zero at 600 K.