Enhancement of the Magnetotransport Behavior in a Phase-Separated LaAgCaMnO₃ Polycrystalline: Unraveling the Role of a Multi-Double-Exchange Mechanism

Manganese-based perovskite oxides have attracted much attention since the phenomenon of colossal magnetoresistance (CMR) was revived, where the magnetoresistance (MR) can attend higher orders of magnitude larger than that typically found in other types of materials. While trying to understand the mechanism behind CMR, morphostructural, magnetoelectrical, and theoretical studies have been performed on the La_0.4Ag_0.2Ca_0.4MnO₃ (LACMO) sample. An orthorhombic-shaped single-phase LACMO sample with size distribution D_SEM = 2.5 μm was synthesized using a conventional sol–gel chemical method. The mixed valence states of different chemical elements were revealed by X-ray photoemission spectroscopy. The results of thermal dependence of magnetization were obtained according to the zero field cooled–field cooled cooling–field cooled warming protocols using a superconducting quantum interference device magnetometer. The phase separation (PS) phenomenon was observed because of the competition between the superexchange and the double-exchange (DE) mechanisms within the same material. Field dependence of magnetization at varying temperatures and in cooling–warming processes has also been reported and discussed in this paper. The MR value was found to be 99% under an applied field of 2 T. In essence, the reported CMR arises from the metal–insulator phase transition T_ρ = 64 K, which accompanies the transition from an insulator state to a metallic state. Not only the large CMR value was noted in our sample, but also the high obtained value of the temperature coefficient of resistivity of 69% at 123 K in zero field has also been reported. The obtained CMR values are the result of PS and multi-DE phenomenon. The latter proved to be directly correlated with the improved magnetotransport properties because an additional hopping mechanism is provided by an interstitially filled Mn²⁺ ion, which is electronically involved in the conduction mechanism, showing an unconventional behavior in the transport properties of manganite.