\muSR-dtected Soft mode toward a Possible Phase Transition in disordered   spin gap system (CH3)2CHNH3-Cu(ClxBr1-x)3
  Measurements of macroscopic properties have indicated that the bond-disordered spin-gap system (CH3)2CHNH3-Cu(ClxBr1-x)3 is gapless when x is between 0.44 and 0.87. Using muon spin relaxation to investigate microscopic properties of sample with x=0.35, we observed a dynamical spin fluctuation, whose characteristic frequency decreases with decreasing temperature, indicating a magnetic ground state. 
T. Goto, T. Suzuki, I. Watanabe, K. Kanada, T. Saito, A. Oosawa, H.   Manaka