  The effect of exchange bond randomness on the ground state and the
field-induced magnetic ordering was investigated through magnetization
measurements in the spin-1/2 mixed quantum spin system
(Tl$_{1-x}$K$_{x}$)CuCl$_3$ for $x<0.36$. Both parent compounds TlCuCl$_3$ and
KCuCl$_3$ are coupled spin dimer systems, which have the singlet ground state
with excitation gaps ${\Delta}/k_{\rm B}=7.7$ K and 31 K, respectively. Due to
bond randomness, the singlet ground state turns into the magnetic state with
finite susceptibility, nevertheless, the excitation gap remains. Field-induced
magnetic ordering, which can be described by the Bose condensation of excited
triplets, magnons, was observed as in the parent systems. The phase transition
temperature is suppressed by the bond randomness. This behavior may be
attributed to the localization effect.
