Charge transfer induced ferromagnetism and anomalous temperature increment of coercivity in ultrathin α-Fe₂O₃ decorated graphene 2D nanostructures

To overcome the detrimental effect of charge transfer from a transition metal to 2D substrates like graphene, we have grown ultrathin antiferromagnetic α-Fe₂O₃ layers on both sides of the graphene surface. Anomalous magnetic behavior, viz., coercivity and exchange bias, increases with increasing temperature with strong ferromagnetic ordering. The highest values of coercivity and large exchange bias are obtained as 3335 Oe and 2361 Oe, respectively. Large enhancement (646%) in exchange bias is observed with an increase in temperature from 2 K to 70 K. Interlayer exchange coupling between the ferromagnetic layers becomes strongest at 300 K to achieve an ultralow coercivity of 22 Oe by growing an α-Fe₂O₃ phase on both sides of the graphene surface. A 32% negative magnetoresistance is observed as a result of exchange bias which changes with temperature. All these results are explained on the basis of the charge transfer effect at the interface of the graphene/α-Fe₂O₃ nanostructure at the low temperature region and the spin canting effect of surface states at the higher temperature region. Theoretical Density Functional Theory calculation is also done to understand the interface interaction, quantitative evaluation of charge transfer, and density of states.