We report on measurements of the magnetic response of the anisotropic $\mathrm{Cu}{\mathrm{O}}_2$ spin chains in lightly hole-doped ${\mathrm{La}}_x{(\mathrm{Ca},\mathrm{Sr})}_{14-x}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$, $x⩾5$. The experimental data suggest that in magnetic fields $B\gtrsim 4\mathrm{T}$ (applied along the easy axis) the system is characterized by short-range spin order and quasistatic (quenched) charge disorder. The magnetic susceptibility $\chi \left(B\right)$ shows a broad anomaly, which we interpret as the remnant of a spin-flop transition. To corroborate this idea, we present Monte Carlo simulations of a classical, anisotropic Heisenberg model with randomly distributed, static holes. Our numerical results clearly show that the spin-flop transition of the pure model (without holes) is destroyed and smeared out due to the disorder introduced by the quasistatic holes. Both the numerically calculated susceptibility curves $\chi \left(B\right)$ and the temperature dependence of the position of the anomaly are in qualitative agreement with the experimental data.

• Received 10 March 2006

DOI:https://doi.org/10.1103/PhysRevB.73.224415