The magnetic Hamiltonian of the Heisenberg quantum antiferromagnet $\mathrm{Sr}\mathrm{Cu}{\mathrm{Te}}_2{\mathrm{O}}_6$ is studied by inelastic neutron scattering technique on powder and single-crystalline samples above and below the magnetic transition temperatures at 8 and 2 K. The high-temperature spectra reveal a characteristic diffuse scattering corresponding to a multispinon continuum, confirming the dominant quantum spin chain behavior due to the third neighbor interaction $J_{\text{intra}}=4.22$ meV (49 K). The low-temperature spectra exhibit sharper excitations at energies <1.25 meV, which can be explained by considering a combination of weak antiferromagnetic first nearest neighbor interchain coupling $J_1=0.17$ meV ($1.9$ K) and even weaker ferromagnetic second nearest neighbor $J_2=-0.037$ meV ($-0.4$ K) or a weak ferromagnetic $J_2=-0.11$ meV ($-1.3$ K) and antiferromagnetic $J_6=0.16$ meV ($1.85$ K), giving rise to the long-range magnetic order and spin-wave excitations at low energies. These results suggest that $\mathrm{Sr}\mathrm{Cu}{\mathrm{Te}}_2{\mathrm{O}}_6$ is a highly one-dimensional Heisenberg system with three mutually perpendicular spin chains coupled by a weak ferromagnetic $J_2$ in addition to the antiferromagnetic $J_1$ or $J_6$, presenting a contrasting scenario from the highly frustrated hyper-hyperkagome lattice (equally strong antiferromagnetic $J_1$ and $J_2$) found in the isostructural quantum spin liquid candidate $\mathrm{Pb}\mathrm{Cu}{\mathrm{Te}}_2{\mathrm{O}}_6$ .

• Received 22 February 2021
• Revised 12 July 2021
• Accepted 23 September 2021

DOI:https://doi.org/10.1103/PhysRevB.104.144402