Mapping Nanomagnetic Fields Using a Radical Pair Reaction

We visualized the magnetic field around ferromagnetic nanostructures using a combination of a standard epifluorescence microscope and a fluorescence chemical indicator of magnetic field (H. Lee et al., Nano Lett. DOI: 10.1021/nl202950h). The indicator was a chain-linked electron donor-acceptor molecule, phenanthrene-(CH$_{2})_{12}$-O-(CH$_{2})_{2}$-dimethylaniline, that forms spin-correlated radical pairs upon photoexcitation. The magnetic field altered the coherence spin dynamics, yielding an 80{\%} increase in exciplex fluorescence in a 0.1 T magnetic field. The magnetic field distributions were quantified to precision of 1.8 $\times $ 10$^{-4}$ T by image analysis and agreed with finite-element nonmagnetic simulations.