To study the magnetic dynamics of superparamagnetic nanoparticles we use scanning probe relaxometry and dephasing of the nitrogen-vacancy (NV) center in diamond, characterizing the spin-noise of a single 10-nm magnetite particle. Additionally, we show the anisotropy of the NV sensitivitys dependence on the applied decoherence measurement method. By comparing the change in relaxation (T 1 ) and dephasing (T 2 ) time in the NV center when scanning a nanoparticle over it, we are able to extract the nanoparticles diameter and distance from the NV center using an Ornstein-Uhlenbeck model for the nanoparticles fluctuations. This scanning-probe technique can be used in the future to characterize different spin label substitutes for both medical applications and basic magnetic nanoparticle behavior.
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