Competing Interactions in Doped Manganites

The magnetic and transport properties of the doped manganites colossal magnetoresistive (CMR) compounds with the perovskite structure are mainly characterized by a competition between ferromagnetism and antiferromagnetism, and between a metallic and insulator behavior. The effects of the magnetic and transport measurements of the CMR compound have been investigated by different DC and AC techniques. The magnetic and transport behavior of La1-xCaxMnO3+ (x=0.48, 0.50, 0.52 and 0.55 and =0.015) compositions close to charge ordering, was studied through resistivity DC magnetization and AC susceptibility measurements. With time and thermal cycling (T>300 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. For instance, an increase of resistivity can be observed by thermal cycling, where no effect is obtained for lower Ca concentration. The time changes in the magnetization are logarithmic in general and activation energies are consistent with those expected for electron transfer between Mn ions.