Structure, Magnetocaloric Effect, and Critical Behavior of Phase Transition of La 0.65 Ca 0.35-x Gd x MnO 3 ( x = 0.00, 0.05, 0.15) Prepared by High-Pressure Heat Treatment

A series of La 0.65 Ca 0.35-x Gd x MnO 3 (x = 0.00, 0.05, 0.15) samples were synthesized via a two-step route combining conventional solid-state reaction and high-pressure heat treatment. The effects of high-pressure processing and Gd 3+ substitution on the magnetocaloric properties and critical behavior of the phase transition were systematically investigated. X-ray diffraction analysis confirmed that all samples crystallize as a single-phase orthorhombic structure with the space group Pbnm. With increasing Gd 3+ content, the Curie temperature (T C ) gradually decreased to 132 K. Under an applied magnetic field of 7 T, the maximum magnetic entropy change (∣–ΔS M max ∣) decreased from 5.74 J·kg -1 ·K -1 (x = 0) to 3.89 J·kg -1 ·K -1 (x = 0.15). Despite the reduction in entropy change, the full width at half maximum of the magnetic refrigeration temperature span (ΔT FWHM ) broadened significantly from 70.76 K (x = 0) to 100.14 K (x = 0.15), corresponding to an enhancement of 41.5%. The nature of the magnetic phase transition was determined using Arrott plots, universal scaling curves, and the field exponent. The results reveal that the x = 0.00 and x = 0.05 samples undergo first-order magnetic transitions, whereas the x = 0.15 sample exhibits a second-order transition. Critical behavior analysis further demonstrates that the phase transition of the x = 0.15 sample can be well described by the mean-field model, yielding a critical exponent β = 0.485. Compared with literature data for samples synthesized under ambient pressure, high-pressure heat treatment effectively enhanced the T C of samples with identical compositions, providing a promising synthetic strategy for designing low-temperature magnetocaloric materials with broadened operating temperature windows.