Extremely well isolated two-dimensional spin-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mfrac><mml:mn>1</mml:mn><mml:mn>2</mml:mn></mml:mfrac></mml:math> antiferromagnetic Heisenberg layers with a small exchange coupling in the molecular-based magnet CuPOF

We report on a comprehensive characterization of the newly synthesized ${\mathrm{Cu}}^{2+}$-based molecular magnet $[\mathrm{Cu}{(\mathrm{pz})}_{2}{(2\text{\ensuremath{-}}\mathrm{HOpy})}_{2}]{({\mathrm{PF}}_{6})}_{2}$ (CuPOF), where $\mathrm{pz}={\mathrm{C}}_{4}{\mathrm{H}}_{4}{\mathrm{N}}_{2}$ and $2\text{\ensuremath{-}}\mathrm{HOpy}={\mathrm{C}}_{5}{\mathrm{H}}_{4}\mathrm{NHO}$. From a comparison of theoretical modeling to results of bulk magnetometry, specific heat, ${\ensuremath{\mu}}^{+}\mathrm{SR}$, ESR, and NMR spectroscopy, this material is determined as an excellent realization of the two dimensional square-lattice $S=\frac{1}{2}$ antiferromagnetic Heisenberg model with a moderate intraplane nearest-neighbor exchange coupling of $J/{k}_{\mathrm{B}}=6.80(5)$ 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 ${T}_{\mathrm{N}}=1.38(2)$ K, as revealed by ${\ensuremath{\mu}}^{+}\mathrm{SR}$. In applied magnetic fields, our $^{1}\mathrm{H}$-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 ${T}_{\mathrm{N}}=2.8$ K and 7 T.