  We investigated slow relaxations of the magnetostriction and residual
magnetostriction of the phase-separated system
(Eu$_{1-x}$Gd$_{x}$)$_{0.6}$Sr$_{0.4}$MnO$_3$, in which the metamagnetic
transition from a paramagnetic insulating state to a ferromagnetic metallic
state is accompanied by a lattice shrinkage. The relaxations are well fitted by
a stretched exponential function, suggesting the strong frustraction between
the double exchange interaction and Jahn-Teller effect. We have revealed that
the Gd substitution suppresses the frozen phase-separated phase at low
temperatures and stabilizes the paramagnetic insulating state in the dynamic
phase-separated phase at intermediate temperatures. The former origin would be
the randomness effect and the latter would be the suppression of the double
exchange interaction.
