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.