The magnetic responses of a spin-1/2 ladder doped with non-magnetic
impurities are studied using various methods and including the regime where
frustration induces incommensurability. Several improvements are made on the
results of the seminal work of Sigrist and Furusaki [J. Phys. Soc. Jpn. 65,
2385 (1996)]. Deviations from the Brillouin magnetic curve due to interactions
are also analyzed. First, the magnetic profile around a single impurity and
effective interactions between impurities are analyzed within the bond-operator
mean-field theory and compared to density-matrix renormalization group
calculations. Then, the temperature behavior of the Curie constant is studied
in details. At zero-temperature, we give doping-dependent corrections to the
results of Sigrist and Furusaki on general bipartite lattice and compute
exactly the distribution of ladder cluster due to chain breaking effects. Using
exact diagonalization and quantum Monte-Carlo methods on the effective model,
the temperature dependence of the Curie constant is compared to a random dimer
model and a real-space renormalization group scenario. Next, the low-part of
the magnetic curve corresponding to the contribution of impurities is computed
using exact diagonalization. The random dimer model is shown to capture the
bulk of the curve, accounting for the deviation from the Brillouin response. At
zero-temperature, the effective model prediction agrees relatively well with
density-matrix renormalization group calculations. Finite-temperature effects
are displayed within the effective model and for large depleted ladder models
using quantum Monte-Carlo simulations. In all, the effect of incommensurability
does not display a strong qualitative effect on both the magnetic
susceptibility and the magnetic curve. Consequences for experiments on the
BiCu2PO6 compound and other spin-gapped materials are briefly discussed.Comment: 24 pages, 20 figure