We explore the effects of noncommuting applied fields on the ground-state ordering of the quasione-dimensional spin-1/2 XY-like antiferromagnet Cs2CoCl4 using single-crystal neutron diffraction. In zero field interchain couplings cause long-range order below TN =217(5) mK with chains ordered antiferromagnetically along their length and moments confined to the (b, c) plane. Magnetic fields applied at an angle to the XY planes are found to initially stabilize the order by promoting a spin-flop phase with an increased perpendicular antiferromagnetic moment. In higher fields the antiferromagnetic order becomes unstable and a transition occurs to a phase with no long-range order in the (b, c) plane, proposed to be a spin liquid phase that arises when the quantum fluctuations induced by the noncommuting field become strong enough to overcome ordering tendencies. Magnetization measurements confirm that saturation occurs at much higher fields and that the proposed spin-liquid state exists in the region 2.10< HSL <2.52 T a. The observed phase diagram is discussed in terms of known results on XY-like chains in coexisting longitudinal and transverse fields.

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The magnetic ordering structure of GdPO 4 is determined at T = 60 mK by the diffraction of hot neutrons with wavelength λ = 0.4696Å. 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 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 B = 0 − 7 T, reaches − 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.