Proximity Exchange in 2D Magnets Deduced from a Cr(o-tolyl)₄ Magnetic Sensor

Molecular color centers, like Cr(o-tolyl)_4, show promise as a more flexible platform for magnetic sensing with many similarities to more frequently studied quantum magnetic sensors, like diamond NV centers. Using these molecules in thin film geometries would allow for the sensing of magnetic fields over a wide range of distances, angstroms to micrometers, from the analyte. This is particularly important for 2D magnetic materials, such as CrI₃, that have shown large discrepancies in magnetic fields measured by different methods. Past studies have suggested proximity exchange, that occurs between the magnetic substrate and a very close sensor, may be responsible for the discrepancies. To that end, we used density functional theory calculations to understand how the molecular color center, Cr(o-tolyl)₄, adsorbs on a monolayer CrI₃ substrate and how proximity exchange, or wavefunction overlap, impacts the excited states in the Cr(o-tolyl)₄ molecule at different distances from the substrate. Magneto-static modeling is used to predict dipolar interactions between the magnetic sensor and the substrate. By combining these models, we show that at short distances proximity exchange dominates in molecule-substrate interaction, but at further distances the molecule would act as a typical magnetic sensor, with magneto-static effects dominating changes to the energies 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.<br/><br/>This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under award DE-SC0019356.