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

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.

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.