  The spin-gap system with a bond-disorder (CH3)2CHNH3-Cu(ClxBr1-x)3 is
reported by experiments on macroscopic quantities of the uniform susceptibility
and the specific heat to show a magnetic order at low temperatures when the
value of x is within the limited region between 0.44 and 0.87, and otherwise a
gapped behavior. We have investigated microscopically the ground state of the
sample with x=0.35 by muon spin relaxation (mSR) to observe that the frequency
spectrum of the spin fluctuation tends to concentrate around zero energy as
lowering temperatures. This behavior was interpreted as the soft-mode toward
the possible magnetic phase transition at absolute zero.
