Enhancing Low Field Magnetoresistance in La0.7Ca0.25Sr0.05MnO3 /Mn3O4 Composite Nanoparticles: Unveiling its Transport Mechanism

The low-field magnetoresistance of La0.7Ca0.25Sr0.05MnO3/Mn3O4 composite nanoparticles with an average particle size range of 28 to 32 nm is currently being investigated. These nanoparticles were synthesized using the sol-gel method, with sintering temperatures varying from 700°C to 900°C in 100°C intervals. The crystal structure, surface morphology, and electrical transport properties were analyzed. Rietveld refinement confirms the presence of a distorted monoclinic structure [ S.G. P21/c, COD:1525829] and a spinel structure in Mn3O4 [S.G. I41/amd, COD: 1514115]. As the sintering temperature increases, the crystallite and particle sizes of La0.7Ca0.25Sr0.05MnO3/Mn3O4 composite increase, with a corresponding increase in the Mn3O4 fraction. The temperature-dependent resistivity shows a magnetic transition from ferromagnetic-metallic to paramagnetic-insulator behavior with a transition temperature 170 K. The lowest resistivity, observed in 30 nm particles with 19% of Mn3O4, is attributed to the lowest grain boundary contributions and the smallest activation energy required for electron to hop between localized states. The low-field magnetoresistance of La0.7Ca0.25Sr0.05MnO3/Mn3O4 composite nanoparticles reaches up to 30% when the particle size is reduced to 28 nm with 17% of Mn3O4 by applying a magnetic field of 5 kOe at 5 K. This LFMR value is higher than that of previous La0.7Ca0.3MnO3/Mn3O4 composite nanocrystallites and is comparable to pure La0.7Ca0.3MnO3. The role of the insulating Mn3O4 phase and nanoparticle size in enhancing LFMR is discussed herein.