MICROMAGNETICS AT A FINITE-TEMPERATURE USING THE RIDGE OPTIMIZATION METHOD

The thermally activated transitions of the magnetization are studied f or a chain of three identical interacting ferromagnetic particles. The saddle points are located by implementation of the ridge method with an initial direction of search chosen at random using the statistical properties of small collective thermal excitations. The solution is re fined using a technique that combines aspects of the conjugate gradien t and a Newton-type optimization. The methodology used identifies the transitions of relatively high probability of occurrence and deduces t he energy barrier of least height. From the study of the topology of t he energy surface, it is shown that the irreversible response of the s ystem to an external field is modified at finite temperature. The mode l predicts a reduction in the symmetry of the ''fanning'' mode of magn etic reversal and a finite probability for transitions to states other than the state of negative saturation that is attributed to the local uniaxial anisotropy of the particles. The field dependence of the ene rgy barrier exhibits a discontinuity when the final state following th e activation process is no longer stable.