We demonstrate 27 ±11 nm precision using stray magnetic fields from hard disk drive media to assemble 13 nm Fe_3O_4 magnetic nanoparticles in de-ionized water. By aligning the substrate polishing striations found in atomic force microscopy (AFM) images of the media topography, we determine the variance of multiple nanoparticle coatings. The widths of the assembled features are ~200 nm, and features are always distinct when separated by a median distance of 500 nm. As these nanoparticles predictably follow the field gradient, trapped nanoparticles offer a nanoscale precision metrology for mapping the stray magnetic fields of high density media using AFM. Thus our approach offers a means to verify magnetic force microscopy images, as well as a method for nanomanufacturing complex nanoparticle assemblies.
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