The interplay of the magnetic field and interlayer correlations was experimentally investigated in a quasi-two-dimensional quantum magnet $\mathrm{Cu}\left(en\right){\mathrm{Cl}}_2$. For this purpose, extensive quantum Monte Carlo (QMC) studies of the finite-temperature properties of the spin-1/2 Heisenberg antiferromagnet (HAF) on the rectangular and the spatially anisotropic zig-zag square lattices were performed. The QMC studies revealed the equivalency of both models for any values of magnetic fields and spatial anisotropies $R=J_{\mathrm{interchain}}/J_{\mathrm{intrachain}}$. The analysis based on the decomposition of both lattices into local Hamiltonians confirmed the equivalence. Despite the large influence of the interlayer coupling and rather complicated distribution of the intralayer exchange pathways in $\mathrm{Cu}\left(en\right){\mathrm{Cl}}_2$, considering several constraints in the data analysis enabled the extraction of the main features of the low-dimensional magnetic subsystem from the specific heat and magnetization. It was found that $\mathrm{Cu}\left(en\right){\mathrm{Cl}}_2$ can be treated as the realization of the spin-1/2 HAF on the rectangular/zig-zag square lattice with the intralayer spatial anisotropy $R\approx 0.2$, the intrachain exchange coupling 2.20 ± 0.15 K, the saturation field 3.7 ± 0.1 T, the spin-flop field about 0.1 T, and the spin anisotropy of the orthorhombic symmetry. The possibility to investigate magnon instabilities in the strong-field regime of $\mathrm{Cu}\left(en\right){\mathrm{Cl}}_2$ is discussed.

• Received 11 July 2019

DOI:https://doi.org/10.1103/PhysRevB.100.134416