We report the measurements of resistivity and ac susceptibility in two colossal magnetoresistanace compositions $({\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3,x=0.50,0.52)$ close to charge ordering, which have been prepared in air. With time and thermal cycling $(T<\mathrm{}300\mathrm{}\hspace{0.5em}\mathrm{K})$ there is an irreversible transformation of the low-temperature phase from a partially ferromagnetic and metallic to one that is less ferromagnetic and highly resistive. The time changes in the magnetization are logarithmic in general and activation energies are consistent with those expected for electron transfer between Mn ions. The data suggest that oxygen nonstochiometry results in mechanical strains in this two-phase system, leading to the development of metastable states which relax towards the more stable charge-ordered and antiferromagnetic microdomains.

• Received 30 July 2001

DOI:https://doi.org/10.1103/PhysRevB.65.144442