Realization of a spin-12spatially anisotropic square lattice in a quasi-two-dimensional quantum antiferromagnetCu(en)(H2O)2SO4

Abstract

Theoretical and experimental studies of a quasi-two-dimensional quantum antiferromagnet $\text{Cu}(\mathrm{en}){({\mathrm{H}}_{2}\mathrm{O})}_{2}{\mathrm{SO}}_{4}$ $(\mathrm{en}={\mathrm{C}}_{2}{\mathrm{H}}_{8}{\mathrm{N}}_{2})$ were performed. ab initio calculations of exchange interactions confirmed that the system represents a realization of a spatially anisotropic zigzag square lattice. Corresponding quantum Monte Carlo calculations of thermodynamic quantities were realized and the results were applied in the analysis of experimental susceptibility, magnetization, and specific heat studied at temperatures ranging from nominally 300 mK up to 8 K and magnetic fields up to 14 T. The analysis of experimental results provided the estimates of intralayer exchange couplings, $J/{k}_{B}=3.5\ifmmode\pm\else\textpm\fi{}0.2$ K and ${J}^{\ensuremath{'}}=0.35J$. Theoretical analysis of spin symmetries in $\text{Cu}(\mathrm{en}){({\mathrm{H}}_{2}\mathrm{O})}_{2}{\mathrm{SO}}_{4}$ structure predicted the presence of symmetric exchange anisotropies (out-of plane and in-plane spin anisotropy) and a spin-flop transition within the easy plane induced by the magnetic field applied along the easy axis. Isothermal magnetization measurements indicated the expected transition in the field 200 mT applied along the $b$ axis which was finally identified as the easy axis lying within the easy plane $bc$. Magnetic phase diagrams with saturation fields about 6.5 T show nearly identical behavior in all studied directions. Differences appear only in weak magnetic fields as a result of the presence of weak exchange anisotropies $\ensuremath{\approx}{10}^{\ensuremath{-}3}J$. The present analysis suggests that $\text{Cu}(\mathrm{en}){({\mathrm{H}}_{2}\mathrm{O})}_{2}{\mathrm{SO}}_{4}$ can be a model system for exploring the interplay of quantum fluctuations, exchange anisotropies, and magnetic field in the two-dimensional lattice space.