Instability of insulator state towards nanocrystallinity in ( La 0.5 Y 0.5 ) 0.7 Ca 0.3 MnO 3 compound: Enhancement of low field magnetoresistance

The present study shows the modification of the insulator state to metallic state with reduction of particle size in ${\left({\mathrm{La}}_{0.5}{Y}_{0.5}\right)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ compound by magnetotransport measurements. The decrease in the activation energy as well as increase in the effective density of states near Fermi level is observed with particle size reduction. This modification leads to the decrease in the resistivity in the nanoparticle in the low-temperature regime ($T<150\text{K}$). On the other hand, the temperature dependent dc susceptibility data shows the evolution of the non-Griffiths phase to Griffiths phase with reduction of particle size in the temperature range $100\text{K}<T⩽200\text{K}$. This evolution from non-Griffiths phase to Griffiths phase leads to the formation of large ferromagnetic clusters at the expense of antiferromagnetic interactions which is responsible for the non-Griffiths phase. The presence of the large ferromagnetic clusters helps to create percolation path for electronic transport and is the probable reason for the enhancement of magnetoresistance in this temperature range. On the other hand, enhancement of low field magnetoresistance (LFMR) observed below 100 K has been attributed to the increased spin polarized tunneling (SPT) component due to the increase in the size of the ferromagnetic clusters in the nanoparticle compares with that of bulk.