Arrays of ultrathin monocrystalline submicrometer-sized Fe dots: Néel-Brown relaxation and activation volume

Magnetic after-effect measurements were performed on an array of etched monocrystalline disk-shaped Fe dots of thickness 1 nm and diameter 200 nm. From room temperature down to 5 K, the dependence on applied field and temperature of the relaxation rate was well described on the basis of the Néel-Brown model of magnetization reversal by a rotational Brownian motion. The distribution of anisotropy fields of the dots was deduced from the nonlogarithmic relaxation rate S_H,T(t) at room temperature by using a scaling procedure. From the analysis of the field dependence of S_H,T(t), the activation volume was estimated at approximately 1500 nm³, with a tendency to increase slightly for temperatures below 100 K. In terms of micromagnetic reversal mechanisms, such a small value indicates that a noncoherent reversal mode occurs in the dots.