Graphene Induced Colossal Magnetoresistance and Critical Behavior in Phase Segregated Iron Oxide

Interfacial magnetic and electronic properties in two-dimensional systems placed in proximity with magnetic substrates have been shown to exhibit exotic interfacial phenomena (Nanoscale 5, 1902-1909 (2013)). Further, bi-phase iron oxide (BPIO) consisting of a majority phase Fe₃O₄ and minority phase α-Fe₂O₃ has been shown to be a unique platform for observing the coexistence of competing magnetic phases. Here, the change in the bulk magnetic and electronic properties of 100 nm thick BPIO due to the inclusion of monolayer graphene (Gr) is explored. Magnetometry measurements indicate strong antiferromagnetic (AFM) coupling between BPIO and Gr as compared to bare BPIO via a 50% reduction in saturation magnetization. Further, magnetoresistance (MR) measurements were performed on both BPIO/Gr/Pt and BPIO/Pt. While BPIO/Pt shows negative MR throughout the temperature range, BPIO/Gr/Pt exhibits a change in the sign of MR from positive to negative below T_p = 240 K. Furthermore, unlike BPIO/Pt, BPIO/Gr/Pt shows a peak in the temperature dependent resistivity measurement which coincides with changes in the sign of MR. These results can be understood via a percolation model and competing localization and delocalization of carriers, induced by strong AFM coupling in Gr and the phase segregated nature of BPIO.