Nonuniversal scaling of the magnetocaloric effect as an insight into spin-lattice interactions in manganites

Abstract

We measure the magnetocaloric effect of the manganite series La 0.67 Ca 0.33−x Sr x MnO 3 by determining the isothermal entropy change upon magnetization, using variable-field calorimetry. The results demonstrate that the field dependence of the magnetocaloric effect close to the critical temperature is not given uniquely by the critical exponents of the ferromagnetic-paramagnetic phase transition, i.e., the scaling is nonuniversal. A theoretical description based on the Bean-Rodbell model and taking into account compositional inhomogeneities is shown to be able to account for the observed field dependence. In this way the determination of the nonuniversal field dependence of the magnetocaloric effect close to a phase transition can be used as a method to gain insight into the strength of the spin-lattice interactions of magnetic materials. The approach is shown also to be applicable to first-order transitions.

Hysteresis and Phase Transition Kinetics in Magnetocaloric Materials

“…The main theoretical aspects concerning the description of MCE materials are reviewed in Section 3 by studying the thermodynamic properties of the Landau free energy of a first order transition and the consequent magnetic field induced entropy change, temperature change and hysteresis. In particular the derivation of the free energy from a microscopic theory has been the subject of several theoretical approaches . One of the main debated issues is the microscopic cause of the passage from the usual second order ferromagnet to a first order MCE material.…”

A detailed study of the hysteresis in La0.67Ca0.33MnO3

“…For weakly first-order materials even a small spread in critical temperatures may be enough to smooth out the transition and make the hysteresis disappear. Recently we showed, using detailed determination of the field dependence of Δs at T c in combination with a fit to the Bean-Rodbell model that it is possible to determine the order of the phase transition and the spread in critical temperature [24]. We find indeed that La 0.67 Ca 0.33 MnO 3 is weakly first order, with an η = 1.25.…”

Studies on the Arrott plots of inhomogeneous first order magnetic phase transitions

“…For first order magnetic systems, the Bean-Rodbell model is quite successful in describing the magnetic properties [3,4,17,34,35,[38][39][40][41][42][43][44]. Values of η slightly greater than 1 is sufficient in the case of weak first order magnetic systems [34,42,43] like La 2/3 Ca 1/3 MnO 3 and La(Fe,Mn,Si) 13 H (for example, η ≈ 1.2 − 1.3) while higher values of η describe systems with strong first order transition [38][39][40][41]. So, in the present study, a typical value of η = 1.5 is considered for representing the first order magnetic phase transition.…”

“…In-order to understand/appreciate the physical significance of the results, two well-known first order magnetic systems are considered and their Arrott plots are simulated with and without T C inhomogeneity. The magnetic systems La 2/3 Ca 1/3 MnO 3 and La(Fe,Mn,Si) 13 H are two well known magnetocaloric materials with first order magnetic phase transitions and both these systems are well described by the Bean-Rodbell model [42,43].…”

“…The material parameters used for the calculations are taken from references [42,43]. The parameters used are J = 1, η = 1.3, T 0 = 305 K and T C = 308 K for the La(Fe,Mn,Si) 13 H system [42] and J = 2, η = 1.25, T 0 = 265.9 K and T C = 268 K for the La 2/3 Ca 1/3 MnO 3 system [43]. Here the calculation details for the La 2/3 Ca 1/3 MnO 3 system are described.…”