We characterize supramolecular magnetic structures, consisting of two weakly coupled antiferromagnetic rings, by low-temperature specific heat, susceptibility, magnetization and electron paramagnetic resonance measurements. Intra- and inter-ring interactions are modeled through a microscopic spin-Hamiltonian approach that reproduces all the experimental data quantitatively and legitimates the use of an effective two-qubit picture. Spin entanglement between the rings is experimentally demonstrated through magnetic susceptibility below 50 mK and theoretically quantified by the concurrence.
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.
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