We report on a comprehensive characterization of the newly synthesized Cu 2+-based molecular magnet [Cu(pz) 2 (2-HOpy) 2 ](PF 6) 2 (CuPOF), where pz = C 4 H 4 N 2 and 2-HOpy = C 5 H 4 NHO. From a comparison of theoretical modeling to results of bulk magnetometry, specific heat, μ + SR, ESR, and NMR spectroscopy, this material is determined as an excellent realization of the two dimensional square-lattice S = 1 2 antiferromagnetic Heisenberg model with a moderate intraplane nearest-neighbor exchange coupling of J/k B = 6.80(5) K, and an extremely small interlayer interaction of about 1 mK. At zero field, the bulk magnetometry reveals a temperature-driven crossover of spin correlations from isotropic to XY type, caused by the presence of a weak intrinsic easy-plane anisotropy. A transition to long-range order, driven by the low-temperature XY anisotropy under the influence of the interlayer coupling, occurs at T N = 1.38(2) K, as revealed by μ + SR. In applied magnetic fields, our 1 H-NMR data reveal a strong increase of the magnetic anisotropy, manifested by a pronounced enhancement of the transition temperature to commensurate long-range order at T N = 2.8 K and 7 T.

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