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

Magnetic after-effect measurements were performed on an array of etched mon ocrystalline disk-shaped Fe dots of thickness 1 nm and diameter 200 nm. Fro m room temperature down to 5 K, the dependence on applied field and tempera ture of the relaxation rate was well described on the basis of the Neel-Bro wn model of magnetization reversal by a rotational Brownian motion. The dis tribution of anisotropy fields of the dots was deduced from the nonlogarith mic relaxation rate S-H,S-T(t) at room temperature by using a scaling proce dure. From the analysis of the field dependence of S-H,S-T(t), the activati on volume was estimated at approximately 1500 nm(3), with a tendency to inc rease slightly for temperatures below 100 K. In terms of micromagnetic reve rsal mechanisms, such a small value indicates that a noncoherent reversal m ode occurs in the dots. (C) 1999 American Institute of Physics. [S0021-8979 (99)00514-9].