Mapping nanomagnetic fields using a radical pair reaction.

We used a fluorescent chemical indicator of magnetic field to visualize the magnetic field around ferromagnetic nanostructures. The indicator was a chain-linked electron donor-acceptor molecule, phenanthrene-(CH2)12-O-(CH2)2-dimethylaniline, that forms spin-correlated radical pairs upon photoexcitation. The magnetic field altered the coherent 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 1.8×10(-4) T by image analysis and agreed with finite-element nanomagnetic simulations.