  Bulk magnetic and thermodynamic measurements, along with mean-field
calculations, were conducted on the ferromagnetic K3Gd5(PO4)6 powders. No
magnetic ordering was observed until 2 K, while the application of an external
field B > 1 T resulted in the splitting of the Gd3+ ground state multiplet and
induced a non-cooperative Schottky effect. The average nearest-neighbor
exchange strength |J1/kB| is determined to be 0.017 K, which leads to a
remarkably large low field magnetic entropy change {\Delta}Sm = 36.2 J kg-1 K-1
under applied field change B = 2 T at temperature T = 2 K, as well as a maximum
adiabatic temperature change Tad = 10.9 K. We contend that ferromagnetic
gadolinium orthophosphates serve as a promising reservoir for exploring
advanced magnetic refrigerants applicable under low magnetic fields.
