  Significant decrease of spontaneous magnetization in frustrated
one-dimensional ferro- and ferrimagnets due to non-magnetic impurities is
predicted. Using the density-matrix renormalization group method and the exact
diagonalization method, we confirm that the total spin can vanish due to a
single impurity in finite chains. Introducing the picture of magnetic domain
inversion, we numerically investigate the impurity-density dependence of
magnetization. In particular, we show that even with an infinitesimal density
of impurities the magnetization in the ground state is reduced by about 40%
from that of the corresponding pure system. Conditions for the materials which
may show this anomalous impurity effect are formulated.
