  Recent experiments showed that the robust charge-ordering in manganites can
be weakened by reducing the grain size down to nanoscale. Weak ferromagnetism
was evidenced in both nanoparticles and nanowires of charge-ordered manganites.
To explain these observations, a phenomenological model based on surface phase
separation is proposed. The relaxation of superexchange interaction on the
surface layer allows formation of a ferromagnetic shell, whose thickness
increases with decreasing grain size. Possible exchange bias and softening of
the ferromagnetic transition in nanosized charge-ordered manganites are
predicted.
