Signature of gate-tunable magnetism in graphene grafted with Pt-porphyrins

Abstract

Inducing magnetism in graphene holds great promises, such as controlling the exchange interaction with a gate electrode and generating exotic magnetic phases. Coating graphene with magnetic molecules or atoms has so far mostly lead to decreased graphene mobility. In the present work, we show that Pt-porphyrins adsorbed on graphene lead to an enhanced mobility and to gate-dependent magnetism. We report that porphyrins can be donor or acceptor, depending on graphene s initial doping. The porphyrins transfer charge and ionize around the charged impurities on graphene, decreasing the graphene doping and increasing its mobility. In addition, ionized porphyrins carry a magnetic moment. Using the sensitivity of mesoscopic transport to magnetism, in particular the superconducting proximity effect and conductance fluctuations, we explore the magnetic order induced in graphene by the interacting magnetic moments of the ionized porphyrins. Among the signatures of magnetism, we find two-terminal-magnetoresistance fluctuations with an odd component, a tell-tale sign of time reversal symmetry breaking at zero field, that does not exist in uncoated graphene sample. When graphene is connected to superconducting electrodes, the induced magnetism leads to a gate-voltage-dependent suppression of the supercurrent, modified magnetic interference patterns, and gate-voltage-dependent magnetic hysteresis. The magnetic signatures are greatest for long superconductor graphene superconductor junctions and for samples with the highest initial doping, compatible with a greater number of ionized and thus magnetic porphyrins. Our findings suggest that long-range magnetism is induced through graphene by the ionized porphyrins magnetic moment. This magnetic interaction is controlled by the density of carriers in graphene, a tunability that could be exploited in spintronic applications.