Thermal activation in Permalloy nanorectangles at room temperature

The effect of thermal activation in planar Permalloy nanorectangles of thickness Lz 1⁄4 2:5 nm, width Ly 1⁄4 20nm and different in plane aspect ratios (r 1⁄4 Lx=Ly) has been analyzed assuming uniform magnetization in the sample at room temperature. The relaxation times were computed by integrating the Langevin equation in presence of external fields that reduce the energy barrier separating the two energy minima. From these data we obtain the pre-exponential factor in the Arrhenius formula for each aspect ratio. That allows us to estimate the relaxation time in the absence of external field. A minimum aspect ratio of r 1⁄4 3:25 is required for a lifetime larger than 100 years, which is a commonly accepted stability criterion. r 2005 Elsevier B.V. All rights reserved. PACS: 75.40.Mg; 75.75.+1; 75.20. g