Giant Magnetocaloric Effect and Magnetic Critical Behavior in Polycrystalline GdPO₄

Magnetocaloric effect (MCE) and critical behavior of polycrystalline GdPO₄ prepared by chemical precipitation and solid-state sintering method were investigated. X-ray diffraction (XRD) patterns indicate that GdPO₄ is crystalized in monazite phase with monoclinic crystal structure. Magnetization measurements confirm the presence of ferromagnetic-paramagnetic phase transition, as well as the presence of ferromagnetic and antiferromagnetic interactions between Gd³⁺ ions in the low-temperature region. Arrott curves indicate that the sample exhibits second-order magnetic phase transition behavior. The maximum value of magnetic entropy change (−∆S_M^max) and relative cooling power (RCP) are 50.1 J/(kg·K) (11.3 J/(kg·K)) and 416.3 J/kg (56.4 J/kg) at 5 T (1 T) magnetic field, respectively. The maximum value of adiabatic temperature change (∆T_ad^max) reaches 36.5 K under an applied magnetic field of 5 T, indicating that GdPO₄ is a good candidate for low-temperature magnetic refrigeration. Critical behavior is analyzed and the existence of short-range ordering (SRO) magnetic exchange interactions in the long-range ordering (LRO) of GdPO₄ is suggested to be responsible for the large magnetocaloric effect.