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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We present an overview of selected copper-based quasi-2D square-lattice spin-1/2 materials with an easy-plane anisotropy, providing the possibility to study emergent Berezinskii-Kosterlitz-Thouless (BKT) correlations. In particular, in those materials with a comparatively small exchange coupling, the effective XY anisotropy of the low-temperature spin correlations can be controlled by an applied magnetic field, yielding a systematic evolution of the BKT correlations. In cases where the residual interlayer correlations are small enough, dynamical BKT correlations in the critical regime may be observed experimentally, whereas the completion of the genuine BKT transition is preempted by the onset of long-range order.