Magnetic entropy table-like shape and enhancement of refrigerant capacity in La1.4Ca1.6Mn2O7-La1.3Eu0.1Ca1.6Mn2O7 composite

摘要

In this work, we have investigated the structural, magnetic and magnetocaloric properties of La1.4Ca1.6Mn2O7 (A) and La1.3Eu0.1Ca1.6Mn2O7 (B) oxides. These compounds are synthesized by a solid-state reaction route and indexed with respect to Sr3Ti2O7-type perovskite with the I4/mmm space group. The substitution of La by 10% Eu enhances the value of magnetization and reduces the Curie temperature (T-C). It is also shown that these compounds undergo a first-order ferromagnetic-paramagnetic phase transition around their respective T-C. The investigated samples show large magnetic entropy change (S-M) produced by the sharp change of magnetization at their Curie temperatures. An asymmetric broadening of the maximum of S-M with increasing field is observed in both samples. This behaviour is due to the presence of metamagnetic transition. The S-M(T) is calculated for A(x)/B1-x composites with 0 x 1. The optimum S-M(T) of the composite with x = 0.48 approaches a nearly constant value showing a table-like behaviour under 5 T. To test these calculations experimentally, the composite with nominal composition A(0.48)/B-0.52 is prepared by mixing both individual samples A and B. Magnetic measurements show that the composite exhibits two successive magnetic transitions and possesses a large MCE characterized by two S-M(T) peaks. A table-like magnetocaloric effect is observed and the result is found to be in good agreement with the calculations. The obtained S-M(T) is approximate to 4.07 J kg(-1) K-1 in a field change of 0-5 T in a wide temperature span over T-FWHM approximate to 68.17 K, resulting in a large refrigerant capacity value of approximate to 232.85 J kg(-1). The MCE in the A(0.48)/B-0.52 has demonstrated that the use of composite increases the efficiency of magnetic cooling with H-0 = 5 T by 23.16%. The large T-FWHM and RC values together with the table-like (-S-M)(max) feature suggest that the A(0.48)/B-0.52 composite can meet the requirements of several magnetic cooling composites based on the Ericsson-cycle. In addition, we show that the magnetic field dependence of MCE enables a clear analysis of the order of phase transition. The exponent N presents a maximum of N > 2 for A, B and A(0.48)/B-0.52 samples confirming a first-order paramagnetic-ferromagnetic transition according to the quantitative criterion. The negative slope observed in the Arrott plots of the three compounds corroborates this criterion.