Muon-induced break up of spin-singlet pairs in the double-chain compound KCuCl3

Monoclinic KCuCl3 is a spin-1/2 ladder compound with a dimerized nonmagneti c spin-singlet ground state at low temperatures. Yet a mu SR study by Higem oto et al. [1] showed a pronounced mu(+)-spin relaxation below 20 K;, signa ling the onset of a broad static held distribution, reminiscent of a frustr ated spin system. With the aim to gain further insight into this phenomenon we have performed ZF-, LF- and TF-measurements on high-quality single crys tals. The TF-measurements with B-ext parallel to b-axis, parallel to [201] and parallel to[102] (B-ext = 0.6 T) yielded a split signal consisting of u p to seven resolvable components. The frequency shifts follow a Curie-behav ior from 6 up to at least 40 E; and do not scale at all with the bulk susce ptibility. This result implies the presence of free electron spins down to 6 K;. Below 6 K; the lines broaden severely and it is difficult to follow t he shifts to lower temperatures. However ZF- and LF-measurements display a slowing down of the spin dynamics and the transition into a spin-glass-like state below 5 K in qualitative agreement with Ref [1]. We conclude that th e presence of the mu(+) breaks the near-neighbor dimerized spin pairs and a lso that those few liberated spins develop a quasi-static behavior below 5 K in part of the volume (35%). Relative slow fluctuations persist in the re maining volume. (C) 2000 Elsevier Science B.V. All rights reserved.