Simulation of magnetization reversal in polycrystalline patterned Co elements

The influence of the polycrystalline microstructure on the switching mechan isms of acicular shaped Co elements was investigated using finite element m icromagnetics. The Gilbert equation of motion with a Gilbert damping consta nt alpha = 1 was solved using a semi-implicit time integration scheme. The elements were 200X40 nm(2) and 25 nm thick. The grain size is approximately 8 nm, leading to edge irregularities of the same size. The competitive eff ects 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 ri pple becomes more pronounced. At an applied field of 95 kA/m a vortex, orig inally formed near sharp edge irregularities, moves into the width of the e lement. The vortex reaches the opposite edge after about one ns. Then a tra nsverse domain structure of a head to head domain wall forms and the revers ed domain expands through the entire particle. The magnitude of magnetostat ic 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 magnet ostatic interaction field agree well with in situ magnetization experiments in a transmission electron microscope. (C) 1999 American Institute of Phys ics. [S0021-8979(99)69508-1].