Quenched charge disorder in CuO2 spin chains: Experimental and numerical studies

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We report on measurements of the magnetic response of the anisotropic CuO₂ spin chains in lightly hole-doped Laₓ (Ca,Sr)₁4-x Cu₂4 O₄1, x>=5. The experimental data suggest that in magnetic fields B > 4T (applied along the easy axis) the system is characterized by short-range spin order and quasi-static (quenched) charge disorder. The magnetic susceptibility chi(B) 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 quasi-static holes. Both the numerically calculated susceptibility curves chi(B) and the temperature dependence of the position of the anomaly are in qualitative agreement with the experimental data.