Effect of a Magnetic Field on the Superparamagnetic Relaxation Time

The relaxation time for the fluctuations in the direction of the magnetization vector is calculated for very fine, single-domain ferromagnetic particles which have a uniaxial (shape or crystalline) anisotropy. A magnetic field is assumed to be applied parallel to the easy magnetization axis, and the relaxation time is computed for various magnitudes of this field. It is found that the commonly used approximation for high-energy barriers is about as justified as in the case of zero magnetic field, down to barriers of the order of $\mathrm{kT}$.