Dynamical criticality of single magnetic nanoparticles near phase transition*

Understanding the critical phenomena of magnetic nanoparticles (MNPs) is crucial to both fundamental physics and the state-of-the-art technology. NV centers in diamond simultaneously provide nanoscale spatial resolution, high magnetometry sensitivity and large temporal dynamic range under ambient conditions, and therefore are ideal quantum sensors for studying the dynamical criticality of MNPs. We developed a broadband sensing protocol with NV center spins to study the magnetic fluctuations of single magnetic nanoparticles near the critical point. We observed paramagnetic and superparamagnetic fluctuations of the MNP and extracted the correlation time and amplitude of the magnetic fluctuations.