Influence of Al${}_2$O${}_3$ on the low-field magnetoresistance of sol–gel grown La${}_{0.67}$Ca${}_{0.33}$MnO${}_3$:Al${}_2$O${}_3$ nanocomposites

Low-field magnetoresistance (LFMR) is required by the high-efficiency magnetic devices and it can be achieved by introducing a secondary insulating phase which acts as the artificial barrier layer in manganite composite. However, this will induce a higher resistivity in the composites. In this work, (1 - x) La${}_{0.67}$Ca${}_{0.33}$MnO${}_3$ (LCMO):x Al${}_2$O${}_3$ (x = 0.00, 0.05, 0.10, 0.15, and 0.20) composites have been prepared by the Pechini sol–gel method. Their structural, magnetic, electrical, and magneto-transport properties were systematically studied to determine the relationship between the LFMR and the concentration of Al${}_2$O${}_3$. X-ray diffraction (XRD) pattern indicates that LCMO and Al${}_2$O${}_3$ coexisted in the composites, indicating that Al${}_2$O${}_3$ is segregated outside the LCMO grains. Pure LCMO and its composites behaved as a paramagnetic state in room temperature. The T${}_{MI}$ value has been suppressed in composites resulting from the weak connectivity between grains. The enhancement of LFMR can be observed in the broad range of temperatures (80–240 K) for all samples. More interestingly, a slight reduction of LFMR has been demonstrated by the composite samples. The spin-polarised tunnelling that is responsible for LFMR in composites of this work might be dominated by the nanosized LCMO or restricted by the thick boundary layers of Al${}_2$O${}_3$ phase. This is a significant finding to the existing works of LCMO:Al${}_2$O${}_3$ composites, as Al${}_2$O${}_3$ has improved the LFMR in composites from the available literature.