Disordered magnetic impurities in uniaxial critical quantum spin chains

We calculate exactly the thermodynamics of ensembles of magnetic impurities with randomly distributed host-impurity couplings in the critical region o f the uniaxial spin-1/2 quantum chain. We derive a finite set of integral e quations describing the complete temperature and magnetic field dependence of the system. Exact numerical results for arbitrary values of temperature and external magnetic field are obtained. We show that for strong disorder the quenching of the impurity moments is absent. For weak disorder the scre ening persists, but with a critical non-Fermi-liquid behaviour of the magne tic susceptibility and specific hear. We point out that the 'easy-plane' ma gnetic anisotropy does not qualitatively change the behaviour of the thermo dynamic characteristics of the disordered impurities in the antiferromagnet ic chain. On the other hand, anisotropy of ferromagnetic type can enhance t he tendency to quenching of the impurities for strong disorder.