Simulation of magnetization reversal in polycrystalline patterned Co elements

The influence of the polycrystalline microstructure on the switching mechanisms of acicular shaped Co elements was investigated using finite element micromagnetics. The Gilbert equation of motion with a Gilbert damping constant α=1 was solved using a semi-implicit time integration scheme. The elements were 200×40 nm2 and 25 nm thick. The grain size is approximately 8 nm, leading to edge irregularities of the same size. The competitive effects of the shape anisotropy and the random, magnetocrystalline anisotropy lead to a magnetization ripple structure with a wavelength of about 100 nm for zero applied field. With increasing applied field, the magnetization ripple becomes more pronounced. At an applied field of 95 kA/m a vortex, originally formed near sharp edge irregularities, moves into the width of the element. The vortex reaches the opposite edge after about one ns. Then a transverse domain structure of a head to head domain wall forms and the reversed domain expands through the entire particle. The magnitude of magnetostatic interaction field for an array of elements with 50 nm gaps was found to be in the range of 8–20 kA/m. The calculated switching field and the magnetostatic interaction field agree well with in situ magnetization experiments in a transmission electron microscope.