D. Andreica, N. Cavadini, H. U. Güdel, F. N. Gygax, K. Krämer, M. Pinkpank and A. Schenck
Muon-induced break up of spin-singlet pairs in the double-chain compound KCuCl₃
Physica B 289-290, 176-180 (2000)                 

Abstract: Monoclinic KCuCl₃ is a spin-1/2 ladder compound with a dimerized nonmagnetic spin-singlet ground state at low temperatures. Yet a mSR study by Higemoto et al. showed a pronounced m⁺-spin relaxation below 20 K, signaling the onset of a broad static field distribution, reminiscent of a frustrated spin system. With the aim to gain further insight into this phenomenon we have performed ZF-, LF- and TF-measurements on high-quality single crystals. The TF-measurements with B_ext || b-axis, || [2 0 1] and || [1 0 -2] (B_ext = 0.6 T) yielded a split signal consisting of up to seven resolvable components. The frequency shifts follow a Curie-behavior from 6 up to at least 40 K and do not scale at all with the bulk susceptibility. 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 the 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 [1]. We conclude that the presence of the m⁺ breaks the near-neighbor dimerized spin pairs and also 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 remaining volume.