Three-dimensional numerical simulations of stripe chopping in a magnetic perpendicular material

A three-dimensional (3-D) dynamic micromagnetic model, with resolutions up to 128×128×16, for computing the magnetization dynamics in a ferromagnetic material is presented. Its originality lies in the expression of the demagnetizing field calculation, which is surprisingly simple in the frame of the distribution theory. The model was designed to investigate the 3D stripe chopping process in a plate of constant thickness with perpendicular anisotropy. The plate contains two flat walls delimiting a stripe domain. The magnetization is assumed to be periodic in the two directions of the film plane. We applied uniform perpendicular chopping fields on the initial state submitted to a 220-oersteds (oe) bias field. The damping constant is α=0.5. When the walls are structure-free, the stripe domain delimited by an unwinding pair of walls chops at 20 oe. It chops at 145 oe when the walls are winding. The chopping margin is therefore equal to 125 oe. When one of the walls contains a 2π vertical Bloch line (VBL), there is almost no margin. Both states chop at 20 oe. But the chopping process takes place later in the winding case: at about 19 ns instead of about 7 ns in the unwinding case. It happens in the regions of the walls between pairs of VBL. After it is completed, a magnetic bubble is formed. It collapses gradually. When α is reduced to 0.1, the modulus of the lower chopping field is several Oe lower in all cases but one: when the walls are structure free and winding. When the stripe chops, the process happens sooner than for α=0.5