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 lambda = 0.4696 angstrom. 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 approximate to 0.8 K, in agreement with the magnetic ordering temperature T-N = 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 Delta B = 0 – 7 T, reaches -Delta S-T = 375.8 mJ/cm(3)K(-1) at T = 2.1 K, largely exceeding the maximum values reported to date for Gd-based magnetic refrigerants.