The muon spin rotation technique is used to study magnetic ordering in ultrapure samples of ${\mathrm{SrCu}}_{1-x}{\mathrm{Ni}}_x{\mathrm{O}}_2$, an archetypical $S=1/2$ antiferromagnetic Heisenberg chain system with a small number of $S=1$ defects. The ordered state in the parent compound is shown to be highly homogeneous, contrary to a previous report [M. Matsuda et al., Phys. Rev. B 55, R11953 (1997)]. Even a minute number of Ni impurities results in inhomogeneous order and a decrease of the transition temperature. At as little as $0.5$% Ni concentration, magnetic ordering is entirely suppressed. The results are compared to previous theoretical studies of weakly coupled spin chains with site defects.

• Received 13 March 2016
• Revised 1 June 2016

DOI:https://doi.org/10.1103/PhysRevB.93.214430