Long-range order in a copper borate Cu3B2O6

We measured muon spin relaxation for a copper borate Cu3B2O6, which has a t wo-dimensional network of Cu2+ spins. It was previously reported that the g round state of this compound was a spin singlet state [G. A. Petrakovskii e t al.: Phys. Solid State 412 (1999) 610]. We, however, observed an oscillat ion of muon spins at low temperatures, which indicates that the ground stat e is a long-range-ordered state. A critical slowing down of the fluctuation s of electronic spins is not observed near the critical temperature, simila r to 11 K, suggesting that the magnetic transition is not all ordinary seco nd-order phase transition. In addition, the coexistence of paramagnetic dom ains and ordered domains is observed at 6 K < T < 12 K.