Micromagnetics of imperfect permalloy particles (abstract)

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Abstract
In previous micromagnetic modeling1,2 of permalloy particles made by nanolithography,3 each individual particle is assumed to be a rectangular plate, but scanning electron micrographs show that the particles are better described by thin plates with round ends. Using our finite element code, we have investigated the effects of corner shapes of the permalloy particles. These particles are all 1.47 mm long, 0.18 mm wide, and 50 nm thick. The particle with semicircular ends has a higher coercivity and shows better agreement with the experiments than the particle with square ends. Qualitatively distinct behaviors are also found among interacting pairs of particles with different shapes at the ends. We have also studied the effects of voids. When the dimension of a void is much less than the exchange length of the material, the micromagnetics of the particle is not affected. For larger voids, particles with one, two, and three voids have been studied. Their easy axis hysteresis loops are all compared to the hysteresis loop of a voidless particle. It has been found that voids near the ends of the particle can serve as pinning sites and tend to increase the coercivity. On the other hand, a void in the middle of the particle reduces the coercivity because of an increase of the self demagnetizing field.

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