Thermal Magnetization Reversal in Arrays of Nanoparticles

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  • 12 October 2000
Abstract
The results of large-scale simulations investigating the dynamics of magnetization reversal in arrays of single-domain nanomagnets after a rapid 180-degree reorientation of the applied field at nonzero temperature are presented. The numerical micromagnetic approach uses the Landau-Lifshitz-Gilbert equation including contributions from thermal fluctuations and long-range dipole-dipole demagnetizing effects implemented using a fast-multipole expansion. The individual model nanomagnets are 9 nm x 9 nm x 150 nm iron pillars similar to those fabricated on a surface with STM-assisted chemical vapor deposition [S. Wirth, et al., J. Appl. Phys 85, 5249 (1999)]. Nanomagnets oriented perpendicular to the surface and spaced 300 nm apart in linear arrays are considered. The applied field is always oriented perpendicular to the surface. When the magnitude of the applied field is less than the coercive value, about 2000 Oe for an individual nanomagnet, magnetization reversal in the nanomagnets can only occur by thermally activated processes. Even though the interaction from the dipole moment of neighboring magnets in this geometry is only about 1 Oe, this can be a significant fraction of the difference between the applied and coercive fields. The magnetic orientations of the neighbors are seen to change the behavior of the nanomagnets in the array significantly. One effect is that the mean switching time of a nanomagnet is found to depend on its position within the array.

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