Distance Dependance of Proximity Exchange in a Molecular Quantum Magnetic Sensor

Molecular color centers, like Cr(o-tolyl)_4, show promise as a flexible platform for quantum magnetic sensing and share many similarities to conventional sensors, like diamond NV centers. In 2D magnetic materials, such as CrI₃, large discrepancies in magnetic fields have been measured by different methods. We propose that molecular color centers can be used to address this discrepancy by allowing for a single sensing modality to be used over a wider range of distances and orientations. Prior studies have suggested proximity exchange, between the magnetic substrate and sensor, may be responsible for the discrepancies. To that end, we used density functional theory calculations and magneto-static modeling to understand how proximity exchange and dipolar interactions impact the excited states in the Cr(o-tolyl)₄ molecule, which would be probed using optically detected magnetic resonance. We show proximity exchange dominates at short distances due to molecule-substrate interactions, but at further distances the molecule behaves as a typical magnetic sensor, with magneto-static effects dominating changes to the energy of the excited state. Our models effectively demonstrate how a molecular color center could be used to measure the magnetic field of a 2D magnet and the role of important distance-dependent interactions that contribute to the field.