Influence of heat sintering on the physical properties of bulk La_0.67Ca_0.33MnO₃ perovskite manganite: role of oxygen in tuning the magnetocaloric response

The effect of heat treatments on bulk poly-crystalline La_0.67Ca_0.33MnO₃ perovskite manganite is presented, to explore the possible enhancement in magnetocaloric performance. Samples were prepared via conventional solid-state reaction route with annealing and sintering at various temperatures. Detailed measurements of temperature-dependent and field-dependent magnetization were carried out to estimate the Curie point and order of magnetic transition. The increased sintering temperature results in a steep transition near the T_C, and establishes the magnetic sensitivity as well as the active zone for substantial magnetocaloric performance, at about 168.2% for the LCM9 (sintered at 900 °C) sample. The cause for the significant improvement in the magnetic and magnetocaloric response is brought to light using detailed X-ray photoelectron spectroscopy (XPS) analysis, highlighting the role of oxygen in modifying the Mn³⁺/Mn⁴⁺ charge ratio. The maximum value of the isothermal magnetic entropy change for the optimized sample is found to be 6.4 J kg⁻¹ K⁻¹, achieved at 269 K, while temperature-averaged entropy change (TEC) values, TEC(ΔT_H-C = 3 K) and TEC(ΔT_H-C = 5 K), of 6 J kg⁻¹ K⁻¹ and 5.2 J kg⁻¹ K⁻¹, respectively, were obtained with a low magnetic field change of 20 kOe. The obtained isothermal entropy change at low field for the optimized La_0.67Ca_0.33MnO₃ sample is higher than that of pure Gd and most oxide-based materials. The relative cooling power (RCP) value is around 93 J kg⁻¹ (ΔH = 20 kOe). The order of the phase transition is examined with universal scaling; the scaled entropy change curves confirm the collapse onto a single curve for LCM9, asserting second-order character, whereas the breakdown of the curve with a dispersion relation (d) of 101.1% at Θ = −5 confirms the onset of intrinsic first-order nature in the case of the high-temperature-sintered samples. Calorimetry measurements show thermal hysteresis of 2.4 K and 7.1 K for LCM11 (sintered at 1100 °C) at ramp rates of 5 K min⁻¹ and 10 K min⁻¹, respectively, confirming the first-order nature of the magnetic transition.