Large reversible magnetocaloric effect in GdPO4 nano-spherical particles at ultralow temperatures

This study reports the synthesis of gadolinium phosphate (GdPO 4 ) with a monoclinic crystal structure ( P2 1 / n ) using a high-temperature solid-state reaction method. Rietveld refinement of the X-ray diffraction (XRD) data revealed lattice constants of a = 0.66497(5) nm, b = 0.68457(5) nm, and c = 0.63327(4) nm. The magnetic and magnetocaloric properties of GdPO 4 were investigated in detail. The ferromagnetic (FM) to paramagnetic (PM) phase transition temperature was determined to be around 1.8 K. The transition was strongly supported by Arrott plots and a universal phenomenological curve describing the magnetocaloric effect (MCE). GdPO 4 exhibits significant magnetic entropy changes (−ΔS M ). The maximum values of - Δ S M max are 20.43 J/(kg·K) and 44.79 J/(kg·K) for a magnetic field change (μ 0 ΔH) of 2 T and 9 T, respectively. Our analysis indicates that the GdPO 4 nano-spherical particles exhibit short-range magnetic ordering, characterized by the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) interactions.