Studies in sol-gel glasses with incorporation of Cu and Fe, using PAS, X-ray diffraction and EPR

SiO2 Glasses with incorporation of either Cu or Fe were prepared by the sol-gel process and annealed in air at temperatures within the range of 100 to 500 degreesC. The samples were characterized using Visible Photoacoustic Absorption Spectroscopy (PAS), X-ray powder diffraction and Electron Paramagnetic Resonance (EPR). Under the annealing both systems show significant changes in composition, producing an evolution in the optical absorption spectra. In both systems, the formation of the metal oxide particles is observed after annealing at or above 300 degreesC. The optical and EPR spectra are interpreted in terms of the coordination of the metal ions using the ligand field theory. It is observed that the color of the samples has strong change with both the type of metal incorporated and with the annealing temperature.18320721

Determination of the entropy change using the acoustic detection technique in the investigation of the magnetocaloric effect

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)In this paper we demonstrate the use of the acoustic detection as an alternative way to determine the entropy variation, Delta S(T), a parameter normally used to characterize the magnetocaloric effect. The measurements were performed for a Gd sample in the 252-316 K temperature range for magnetic fields from zero up to 50 kOe. The reversible adiabatic curves were built in a T versus H diagram, and specific heat data obtained at zero-magnetic field were employed to assign the entropy values of each curve. Subsequently, the entropy was plotted as a function of temperature for fixed magnetic fields, and therefore the isothermal entropy variation, Delta S(T), was found as a function of the temperature for several magnetic field steps.4344Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)FAEPEX-UnicampFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Magnetic coupling between Gd and Pr ions and magnetocaloric effect in Gd0.5Pr0.5Al2 compound

In this work, we report the theoretical and experimental investigations on the magnetic and magnetocaloric properties for Gd0.5Pr0.5Al2 compound in different magnetic fields. The magnetization features Gd0.5Pr0.5Al2 is ferrimagnetic at low temperatures. We also present data from X-ray magnetic circular dichroism (XMCD) experiments for this compound, with which we have confirmed that the magnetic moments of the Pr ions are antiparallel to the magnetic moments of the Gd ions. The magnetocaloric parameters, Delta T-S and Delta S-T, were obtained from calorimetric data and both curves present normal and inverse magnetocaloric effect. A theoretical model for ferrimagnetic coupling, including the crystalline electrical field anisotropy, was used to describe the Delta T-S and Delta S-T experimental results. (C) 2009 Elsevier B.V. All rights reserved.321193014301

Study on the observation of Eu2+ and Eu3+ valence states in low silica calcium aluminosilicate glasses

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)The optical, magnetic and structural properties of Eu doped low silica calcium aluminosilicate glasses were investigated. The optical absorption coefficient presented two bands at 39 246 and 29 416 cm(-1), which were assigned respectively to the 4f(7) (S-8(7/2)) -> 4f(6) (4F(J)) 5d (T-2g), and 4f(7) (S-8(7/2)) -> 4f(6) (4F(J)) 5d (E-g) transitions of Eu2+. The fluorescence measured at 300 K on a sample doped with 0.5 wt% of Eu2O3 exhibited a broad band centered at 17 350 cm(-1), which is attributed to the 4f(6)5d -> 4f(7) transition of Eu2+, whereas the additional peaks are due to the D-5(0) -> F-7(J) (J = 1, 2, 4) transitions of Eu3+. From magnetization and XANES data it was possible to evaluate the fractions of Eu2+ and Eu3+ for the sample doped with 0.5 and 5.0 wt% of Eu2O3, the values of which were approximately 30 and 70%, respectively.225Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Ambient pressure colossal magnetocaloric effect tuned by composition in Mn1-xFexAs

The magnetocaloric effect (MCE) is the basis for magnetic refrigeration, and can replace conventional gas compression technology due to its superior efficiency and environment friendliness(1-3). MCE materials must exhibit a large temperature variation in response to an adiabatic magnetic-field variation and a large isothermal entropic effect is also expected. In this respect, MnAs shows the colossal MCE, but the effect appears under high pressures(4). In this work, we report on the properties of Mn1-xFexAs that exhibit the colossal effect at ambient pressure. The MCE peak varies from 285K to 310K depending on the Fe concentration. Although a large thermal hysteresis is observed, the colossal effect at ambient pressure brings layered magnetic regenerators with huge refrigerating power closer to practical applications around room temperature.51080280

A General Approach to First Order Phase Transitions and the Anomalous Behavior of Coexisting Phases in the Magnetic Case

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)First order phase transitions for materials with exotic properties are usually believed to happen at fixed values of the intensive parameters (such as pressure, temperature, etc.) characterizing their properties. It is also considered that the extensive properties of the phases (such as entropy, volume, etc.) have discontinuities at the transition point, but that for each phase the intensive parameters remain constant during the transition. These features are a hallmark for systems described by two thermodynamic degrees of freedom. In this work it is shown that first order phase transitions must be understood in the broader framework of thermodynamic systems described by three or more degrees of freedom. This means that the transitions occur along intervals of the intensive parameters, that the properties of the phases coexisting during the transition may show peculiar behaviors characteristic of each system, and that a generalized Clausius-Clapeyron equation must be obeyed. These features for the magnetic case are confirmed, and it is shown that experimental calorimetric data agree well with the magnetic Clausius-Clapeyron equation for MnAs. An estimate for the point in the temperature-field plane where the first order magnetic transition turns to a second order one is obtained (the critical parameters) for MnAs and Gd(5)Ge(2)Si(2) compounds. Anomalous behavior of the volumes of the coexisting phases during the magnetic first order transition is measured, and it is shown that the anomalies for the individual phases are hidden in the behavior of the global properties as the volume.196942949Fundacao de Amparo Pesquisa do Estado deConselho Nacional de Desenvolvimento CientificoCoordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)U.S. Department of EnergyOffice of ScienceOffice of Basic Energy Sciences [DE-AC02-06CH11357]Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Office of Basic Energy Sciences [DE-AC02-06CH11357