EFFECT OF AN OBLIQUE MAGNETIC FIELD ON THE SUPERPARAMAGNETIC RELAXATION-TIME - 2 - INFLUENCE OF THE GYROMAGNETIC TERM

The effect of a uniform magnetic field applied at an oblique angle to the easy axis of magnetization on the superparamagnetic (longitudinal or Neel) re:laxation time is investigated by numerically solving the F okker-Planck equation for the smallest nonvanishing eigenvalue. It is demonstrated that the reciprocal of the asymptotic formula for the Kra mers escape rate in the intermediate to high damping limit for general nonaxially symmetric potentials when applied to the present problem, yields an acceptable asymptotic approximation to the Neel time for mod erate to high values of the damping. Alternatively the corresponding K ramers low dissipation formula (energy controlled diffusion) provides an acceptable approximation for very small values of the damping. The effect of the gyromagnetic term which gives rise to coupling between t he longitudinal and transverse modes of motion generally corresponds t o an increase of the smallest nonvanishing eigenvalue and so to a decr ease of the Neel relaxation time. The integral relaxation time or area under the slope of the curve of the decay of the magnetization is als o evaluated. It is demonstrated that for sufficiently high values-of t he uniform field (much less, however, than that required to destroy th e bistable nature of the potential) the reciprocal of the lowest nonva nishing eigenvalue (proportional to the Neel time, or the time of reve rsal of the magnetization) and the integral relaxation time may differ exponentially from one another signifying the contributions of modes other than that associated with the overbarrier (Neel) relaxation proc ess to the overall relaxation process. The overall behavior is qualita tively similar (apart from the azimuthal dependence) to that of the ax ially symmetric case which arises due to the depletion of the shallowe r of the two potential wells by the uniform field, so that the fast pr ocesses in the deeper of the two wells may come to dominate the relaxa tion process at sufficiently high values of the uniform field.