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

Abstract

In this work, we have investigated the structural, magnetic and magnetocaloric properties of La_1.4Ca_1.6Mn₂O₇ (A) and La_1.3Eu_0.1Ca_1.6Mn₂O₇ (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 (ΔS_M) produced by the sharp change of magnetization at their Curie temperatures. An asymmetric broadening of the maximum of ΔS_M with increasing field is observed in both samples. This behaviour is due to the presence of metamagnetic transition. The ΔS_M(T) is calculated for A_x/B_1−x composites with 0 ≤ x ≤ 1. The optimum ΔS_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 ΔS_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 ΔS_M(T) is ≈4.07 J kg⁻¹ K⁻¹ in a field change of 0–5 T in a wide temperature span over ΔT_FWHM ∼ 68.17 K, resulting in a large refrigerant capacity value of ≈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 μ₀H = 5 T by 23.16%. The large ΔT_FWHM and RC values together with the table-like (−ΔS_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.