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We have studied the effect of metastable, irreversibility induced by repeatedthermal cycles on the electric transport and magnetization of polycrystallinesamples of La1-xCaxMnO3 (0.48\leq x \leq 0.55) close to charge ordering. Withtime and thermal cycling (T<300 K) there is an irreversible transformation ofthe low-temperature phase from a partially ferromagnetic and metallic to onethat is less ferromagnetic and highly resistive for the composition close tocharge ordering (x=050 and 0.52). Irrespective of the actual ground state ofthe compound, the effect of thermal cycling is towards an increase of theamount of the insulating phase. We have observed the magnetic relaxation in themetastable state and also the revival of the metastable state (in a relaxedsample) due to high temperature thermal treatment. We observed changes in theresistivity and magnetization as the revived metastable state is cycled. Thetime changes in the magnetization are logarithmic in general and activationenergies are consistent with those expected for electron transfer between Mnions. Changes induced by thermal cycling can be inhibited by applying magneticfield. These results suggest that oxygen non-stoichiometry results inmechanical strains in this two-phase system, leading to the development offrustrated metastable states which relax towards the more stable charge-orderedand antiferromagnetic microdomains. Our results also suggest that the growthand coexistence of phases gives rise to microstructural tracks and strainaccommodation, producing the observed irreversibility.

Frustrated Metastable Behavior of Magnetic and Transport Properties in Charge Ordered La1-xCaxMnO3+d Manganites