Superparamagnetic reversal of single magnetic nanoparticles near phase transition

The magnetic properties of single magnetic nanoparticles, especially the superparamagnetic (SPM) reversal near the phase transition point, are relevant to information storage, biomedicine imaging, and paleology. However, it is challenging to study the SPM reversal of single nanoparticles due to the weak magnetic moment and the sharp temperature dependence of the reversal rate. Here, we utilize nitrogen-vacancy centers in diamond as a quantum probe to study the SPM reversal of individual magnetic particles. Such a configuration enables the verification of the Neel-Arrhenius law and the Stoner-Wohlfarth model at the single-particle level. We observe that the reversal rate changes from 10^-2 Hz to 10⁴ Hz in approximate 10 K temperature range. The SPM reversal rate as a function of temperature indicates that the Neel-Arrhenius law is still valid when the temperature is 1/20 Tc close to the Curie temperature of the magnetic nanoparticle.