  Effects of disorder and external field on the competing spin-Peierls and
antiferromagnetic states are studied theoretically in terms of the numerical
transfer matrix method applied to a quasi one-dimensional spin 1/2 Heisenberg
model coupled to the lattice degree of freedom. We show that, at temperatures
above the impurity-induced antiferromagnetic phase, inhomogeneous spin-Peierls
lattice distortions remain to exist showing a re-entrant behavior. This feature
can be drastically altered by very weak perturbations, e.g., the staggered
magnetic field or the change in interchain exchange coupling $J_\perp$, leading
to a huge response, which is analogous to the colossal magnetoresistance
phenomenon in perovskite manganese oxides.
