Extremely well isolated 2D spin-$1/2$ antiferromagnetic Heisenberg layers with small exchange coupling in the molecular-based magnet CuPOF

Abstract

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, $\hat{I}{¼}^{+}$SR, ESR, and NMR spectroscopy, this material is determined as an excellent realization of the 2D square-lattice $S=1/2$ 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 $\hat{I}{¼}^{+}$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 $T_{\mathrm{N}}=2.8$ K and 7 T.

14 pages, 8 figures, as well as 10 pages and 18 figures of supplemental material