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

The giant anisotropic magnetocaloric effect in DyAl(2)

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)We report on calculations of the anisotropic magnetocaloric effect in DyAl(2) using a model Hamiltonian including crystalline electrical field effects. The anisotropic effect is produced by the rotation of a constant magnetic field from the easy to a hard magnetic direction in the crystal and is enhanced by the first order nature of the field induced spin reorientation transition. The calculated results indicate that for a field with modulus of 2 T rotating from a hard to the easy direction, the isothermal magnetic entropy (Delta S(iso)) and adiabatic temperature (Delta T(ad)) changes present peak values higher than 60% the ones observed in the usual process, in which the field direction is kept constant and the modulus of the field is varied. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.3009974]1049Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)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

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

Investigation on the magnetocaloric effect in (Gd,Pr)Al-2 solid solutions

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)A theoretical and experimental investigation on the magnetocaloric properties of the rare earth pseudobinary compounds Gd1-nPrnAl2 is presented. The calculated isothermal entropy and adiabatic temperature changes under magnetic field variations from 0 to 2 T and from 0 to 5 T are in good agreement with the experimental data. For the Pr-concentrations n = 0.25, 0.5 and 0.75 the experimental data present an inverse magnetocaloric effect which was theoretically predicted and associated with the competition between the opposite magnetizations of the Gd and Pr sublattices. The two-sublattice Hamiltonian used in the calculations takes into account the crystal field, exchange and Zeeman interactions. (C) 2010 Elsevier B.V. All rights reserved.3236794798Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq