Micromagnetic computation for wall coercivity caused by magnetic nonuniformity

The wall coercivity caused by the magnetic nonuniformity has been studied numerically. The Landau-Lifshits-Gilbert (LLG) equation is integrated by an explicit scheme of the modified Dufort-Frankel method. The computation was carried out for a two-dimensional grid system representing the magnetization direction in a magnetic film plane. Typical magnetic parameters for magneto-optic recording media were assumed. Spatial variations of uniaxial anisotropy were treated as the magnetic nonuniformity. The validity of our numerical approach was demonstrated with preliminary one-dimensional computations, compared to analytical solutions. A wall coercive field of 2.5 kOe was observed for the wall coupling with a pinning site (30-Å width and 360-Å spacing along the wall) of 10 times larger anisotropy compared to the ordinary region. A two-dimensional anisotropy variation (K=10 ⁶-10⁷ erg/cm³) with a wavelength larger than 60 Å also caused a wall coercivity on the order of 1 kOe, compared to those in magneto-optic recording media. It was also found that a fine pinning site on the order of 100 Å caused a notable coercivity for the bubble domain wall surrounding it.