Magneto-optic measurement of Brownian relaxation of magnetic nanoparticles

We demonstrate a magneto-optic technique to measure Brownian relaxation of magnetic nanoparticles suspended in liquids. We used AC susceptibility data as a function of frequency of the applied AC magnetic field to verify that the results agree with those obtained via a conventional inductive detection technique. However, compared with a commercial AC susceptometer using the conventional detection scheme, our magneto-optic setup is able to detect a density of nanoparticles at least three orders smaller. This technique has the potential of being used as a sensor for magnetic nanoparticles such as in local temperature, viscoelasticity, or molecular-binding measurements.