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Magnetometry of single ferromagnetic nanoparticles using magneto-optical indicator films with spatial amplification



Andrew L. Balk, Carlos M. Hangarter, Samuel M. Stavis, John Unguris


We present a magneto-optical imaging technique to rapidly measure two dimensional, sub-micrometer maps of fringe fields from single magnetic nanorods with microtesla sensitivity. Our technique implements an ultrathin, low-coercivity, perpendicularly-magnetized film as an indicator of the stray field of the nanorods. The stray field is manifested as a shift of down-to-up and up-to-down ferromagnetic transitions in the thin film near the nanorods. This measurement is insensitive to many common noise sources, requires no cryogen, and is easily adaptable to different test environments. We first discuss our method to fabricate the high sensitivity indicator film with low energy argon ion irradiation. We then present a high resolution fringe field map from a single nanorod as measured by the indicator film, and discuss the noise limitations. Our data determines a minimum measureable particle diameter of several tens of nanometers for typical transition metals. We then use our method to obtain hysteresis loops from multiple, single nanorods in parallel, and demonstrate that the nominally identical nanorods show polydispersity in coercive field. We discuss future improvements and applications for our indicator film technique.
Applied Physics Letters


Magnetic nanoparticles, magnetic anisotropy, single particle measurements, nanomagnetism
Created March 16, 2015, Updated November 10, 2018