Enhanced magnetoresistance and evolution of Griffiths-like phase in La1-xCaxMnO3 (x=0.4, 0.5) nanoparticles

Abstract

Temperature and magnetic field-dependent electrical and magnetic properties of La1-xCaxMnO3 (x = 0.4, 0.5) polycrystalline materials prepared by the sol-gel method are studied. An apically compressed/elongated-type distorted orthorhombic pnma-O'-phase crystallization occurs in the sample with the addition of Ca. The crystallized phases are ensured by the Rietveld refinement using the Fullprof package. Dangling bonds on the surfaces of the nanosized particles affect the vibrational features. Field emission scanning electron microscopy (FESEM) images depict the agglomeration of uniformly sized grains. With increase of Ca concentration, super-exchange (SE) interactions overcome the dominant double-exchange (DE) interactions and shift the Curie-temperature to a lower value (T-C = 267 - 234K). Based on the Banerjee's criterion, the Arrott plot confirms a second-order magnetic phase transition in the samples. Temperature-dependent evolution of the Griffiths-like phase (GP) is observed in the samples and GP% increases with Ca content. The various transitions in the different temperature ranges and magnetic field-dependent electrical transport behaviours are explained using different theoretical models. The dopant concentration influences the Mn3+/Mn4+ ratio, leading to changes in the conductivity, which is mediated by ferromagnetically (FM) ordered conduction channels, altering the metal to insulator (M-I) as well as the ferromagnetic to paramagnetic (FM-PM) as well as the ferromagnetic to antiferromagnetic (FM-AFM) transition temperatures. The electrical transport in the high temperature region is explained using variable range and small polaron hopping (VRH and SPH) models. Using Holstein's relation, it is evident that non-adiabatic SPH (NASPH) model is the most adequate method to explain the high-temperature electrical conductivity. The half-doped samples show a higher value (similar to 95%) of magnetoresistance (MR). The present study shows an increase in the T-c and TM-I towards room temperature and in the MR percentage, which may be good for different applications.