Exchange constant variation effects in magnetocaloric and barocaloric isothermal potentials

Frequently, magnetothermal studies of many ferromagnetic materials are focused considering the system such as a paramagnet subject to a molecular field being characterized by an exchange constant. However, magnetostructural transitions can produce abrupt changes in the exchange constant triggering magnetic transitions of first order at transformation temperatures, which can be shifted by applying magnetic field or pressure. On thermodynamic bases we show how an abrupt but continuous variation of the exchange constant at the transition can play an important role in the magnetocaloric and barocaloric isothermal potentials in systems showing structural transformation.751

Magnetoresistivity as a probe to the field-induced change of magnetic entropy in RAl2 compounds (R=Pr,Nd,Tb,Dy,Ho,Er)

The heat capacity C-P(T) of the ferromagnetic compounds RAl2 (R=Pr,Nd,Tb,Dy,Ho,Er) was measured at zero and applied magnetic field of 5 T in the temperature interval from 2 to 200 K. From these results are calculated the magnetic component of the entropy change, -Delta S-mag(T)=S(0,T)-S(H,T). From resistivity measurements, rho(H,T), from 2 to 300 K in the same compounds, we calculated the resistivity change due to the applied magnetic field, -Delta rho(mag)(T)=[rho(mag)(0,T)-rho(mag)(H,T)]. The results are compared and we observed a similar dependence between -Delta rho(mag)(T) and (T/T-C)(m)Delta S-mag(T) with m=0 for T >= T-C and m=1 for T <= T-C. A simple model using a Hamiltonian considering molecular and crystalline electric fields, in a mean field approximation, is adopted for the calculus. Our results show that theory and experiment are in good agreement showing that the magnetoresistivity is a probe to the field-induced change of magnetic entropy in these compounds and can be extended to other materials. A model for the factor connecting both quantities, -Delta S-mag(T) and -Delta rho(mag)(T), is developed. This factor contains mainly the effective exchange integral which is related to Fermi energy that in turn is related to the electron effective mass.741

Staircase-type magnetic-field dependence of the activation energy of Josephson interlayer vortices in Bi2Sr2CaCu2O8

The anomalous depinning of Josephson interlayer vortices in Bi2Sr2CaCu2O8 high-T-c superconductor occurring at nearly field-independent temperature T-x=20-40 K has been studied by means of ac susceptibility measurements with magnetic field applied parallel to CuO2 planes. From the frequency dependence T-x(v(m)) we define the flux-creep activation energy for Josephson vortices U-JV(H) which increases with field and shows well-defined plateaus. In contrast, the activation energy of in-plane pancake vortices U-PV(H) decreases with field, i.e., demonstrates a qualitatively different behavior.63

Magnetocaloric effect: Overcoming the magnetic limit

We have studied anomalous peaks observed in magnetocaloric -Delta S(T) curves for systems that undergo first-order magnetostructural transitions. The origin of those peaks, which can exceed the conventional magnetic limit, Rln(2J+1), is discussed on thermodynamic bases by introducing an additional-exchange contribution (due to exchange constant variation arising from magnetostructural transition). We also applied a semiphenomenological model to include this additional-exchange contribution in Gd(5)Si(2)Ge(2) and MnAs-based systems, obtaining excellent results for the observed magnetocaloric effect. (c) 2008 Elsevier B. V. All rights reserved.321544644

Experimental and theoretical analyses of PrAl2 and NdAl2 composite for use as an active magnetic regenerator

We report the theoretical and experimental investigations on the magnetocaloric effect in the PrAl2 and NdAl2 compounds and a composite of these compounds for use as an active magnetic regenerator. The theoretical calculations were performed considering the crystalline electrical field anisotropy and the magnetocaloric potentials were calculated in the three main crystallographic directions. The experimental data, obtained for the polycrystalline samples, are in good agreement with the theoretical results. Also, an optimum molar fraction of the PrAl2 and NdAl2 composite was determined theoretically and experimentally and discussed in the framework of the optimum regeneration Ericsson cycle. (C) 2005 American Institute of Physics.97

Synthesis and magnetic characterization of Pb(1-x)Mn(x)S nanocrystals in glass matrix

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)The synthesis of manganese-doped PbS (Pb(1-x)Mn(x)S) nanocrystal (NC) dots within a borosilicate glass matrix has been investigated by atomic force microscopy, electron paramagnetic resonance and magnetic measurements. The fusion method was employed in the preparation of the magnetic semiconductor NC dots whereas the measurements performed showed changes in the physical properties of the manganese-doped dots as a result of the Mn(2+)-incorporation into the hosting PbS crystal structure. Nevertheless, the data indicated that only a small fraction of the nominal Mn-doping was incorporated into the PbS NC dot, in both lower (0.5%) and higher (40%) nominal doping ends. For the lower nominal Mn-doping end (0.5%) we found only about 0.05% actually incorporated into the PbS NC dot whereas about 0.45% appeared dispersed throughout the glass template as isolated paramagnetic centres.4116Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG)FINATECConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Exposição ambiental a interferentes endócrinos com atividade estrogênica e sua associação com distúrbios puberais em crianças Environmental exposure to endocrine disruptors with estrogenic activity and the association with pubertal disorders in children

A substância exógena que causa efeitos adversos na saúde de um organismo ou sua descendência, como resultado de distúrbios na função hormonal, é denominada interferente endócrino. Nos últimos anos, produtos ambientais com atividades hormonais têm sido documentados como causadores de anormalidades puberais ou reprodutivas em animais. Os poucos casos comprovados em humanos foram aqueles relacionados a exposições acidentais. Apesar disso, pediatras e pais recomendam a suspensão de todos os alimentos potencialmente contaminados, em especial carne (aves, gado) e derivados da soja quando a criança apresenta alguma alteração puberal. Estas recomendações, se não embasadas cientificamente, podem ter conseqüências deletérias, não apenas pela eliminação de fontes protéicas da dieta, como também por retardar a investigação de causas tratáveis. Por outro lado, a não investigação dos efeitos adversos destes produtos é da mesma forma danosa. Esta revisão descreve os principais interferentes endócrinos responsáveis por alterações puberais em humanos e conclui que, excetuando exposições acidentais a altas quantidades destes produtos, mais estudos são necessários para responsabilizar a ação crônica e em baixas doses destas substâncias na alteração do tempo de desenvolvimento puberal em nossa espécie.<br>Endocrine disruptors are exogenous substances with adverse health effects in intact organisms or their progeny, secondary to changes in endocrine function. Recent years have witnessed constant reports of environmental factors with hormone-like effects causing pubertal or reproductive abnormalities in animals. The few cases proven to be associated with pubertal disorders in humans have been related to accidental exposure. Nevertheless, pediatricians and parents recommend suspending all possible estrogen-contaminated food, especially meat (poultry, beef) and soy products, when the child presents with a pubertal disorder. These recommendations, if not scientifically sound, may have deleterious consequences by eliminating sources of dietary protein and possibly delaying the investigation of other potential and treatable causes. On the other hand, not investigating potential side effects of these products could have similar harmful effects. The current article describes the main endocrine disruptors associated with pubertal disorders in humans and concludes that except for accidental exposure to high doses, more research is needed on the effects of chronic and low-dose exposures in altering human pubertal development