Exchange constant variation effects in magnetocaloric and barocaloric isothermal potentials

Abstract

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.

Effects of the temperature dependence of the bulk modulus on magnetic exchange-entropy

“…From relation (12), we observe the M 2 dependence together with an explicit temperature contribution for the deformation through the bulk modulus.…”

“…This last is obtained using g ¼ g 0 ð1 þ bxÞ and Eq. (12). Now, with the calculated MðH; TÞ and gðH; TÞ we use Eqs.…”

“…In a previous paper, some of the authors of the current work have reported a contribution to magnetic entropy for elastic ferromagnets called additional exchange-entropy [12]. When it is added to the conventional second-order field-induced entropy change, we recover the total entropy change obtained from the Maxwell's relation that involves derivatives of magnetization with respect to temperature.…”

“…This is also the situation in the case that there are more contributions to the free energy. In this respect, for any gðH; TÞ dependence, it has been derived a field-induced additional exchange-entropy change [12] given by:…”

Origin of Hysteresis in La0.67Ca0.33MnO3

“…For systems that exhibit co-incident magnetic and structural transitions the interplay between a first order structural change and a second order magnetic transition can be complicated, depending on the relationship between the two transitions, see Jia et al, (2006) [25], Bean and Rodbell (1962) [26]. Due to the sensitivity of a structural transition on pressure this has led to the investigation of the barocaloric properties of materials with respect to their magnetocaloric effects (de Medeiros (2008) [27]) and the suggestion of corrections to the mean field approach to account for such behavior (see Amaral et al, 2007 [28], Plaza and Campoy (2007) [29]). figure 3(b).…”