Magnetic structure and magnetocalorics of GdPO4

The magnetic ordering structure of GdPO4 is determined at T=60 mK by the diffraction of hot neutrons with wavelength ?=0.4696 Å. It corresponds to a noncollinear antiferromagnetic arrangement of the Gd moments with propagation vector k=(1/2,0,1/2). This arrangement is found to minimize the dipole-dipole interaction and the crystal-field anisotropy energy, the magnetic superexchange being much smaller. The intensity of the magnetic reflections decreases with increasing temperature and vanishes at T?0.8 K, in agreement with the magnetic ordering temperature TN=0.77 K, as reported in previous works based on heat capacity and magnetic susceptibility measurements. The magnetocaloric parameters have been determined from heat capacity data at constant applied fields up to 7 T, as well as from isothermal magnetization data. The magnetocaloric effect, for a field change ?B=0-7T, reaches -?ST=375.8mJ/cm3K-1 at T=2.1 K, largely exceeding the maximum values reported to date for Gd-based magnetic refrigerants.