Influence of Al₂O₃ on the low-field magnetoresistance of sol–gel grown La_0.67Ca_0.33MnO₃:Al₂O₃ nanocomposites

Abstract

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_0.67Ca_0.33MnO₃ (LCMO):x Al₂O₃ (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 Al₂O₃. X-ray diffraction (XRD) pattern indicates that LCMO and Al₂O₃ coexisted in the composites, indicating that Al₂O₃ 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 Al₂O₃ phase. This is a significant finding to the existing works of LCMO:Al₂O₃ composites, as Al₂O₃ has improved the LFMR in composites from the available literature.