Rapidly driven nanoparticles: Mean first-passage times and relaxation of the magnetic moment

TL;DR

This paper develops an analytical approach to calculate mean first-passage times for magnetic nanoparticles under rapidly rotating fields, revealing how such fields significantly accelerate relaxation and induce strong magnetization.

Contribution

It introduces a novel analytical method for MFPT calculation in driven nanoparticles and verifies it with numerical simulations in the high-frequency limit.

Findings

Rotating magnetic fields drastically reduce relaxation times.

The system exhibits strong stationary magnetization along the easy axis.

Analytical results agree with numerical simulations.

Abstract

We present an analytical method of calculating the mean first-passage times (MFPTs) for the magnetic moment of a uniaxial nanoparticle which is driven by a rapidly rotating, circularly polarized magnetic field and interacts with a heat bath. The method is based on the solution of the equation for the MFPT derived from the two-dimensional backward Fokker-Planck equation in the rotating frame. We solve these equations in the high-frequency limit and perform precise, numerical simulations which verify the analytical findings. The results are used for the description of the rates of escape from the metastable domains which in turn determine the magnetic relaxation dynamics. A main finding is that the presence of a rotating field can cause a drastic decrease of the relaxation time and a strong magnetization of the nanoparticle system. The resulting stationary magnetization along the direction of the easy axis is compared with the mean magnetization following from the stationary solution of the Fokker-Planck equation.