Vortex motion in amorphous ferrimagnetic thin film elements

Amorphous Fe64Gd36 thin film square elements are investigated by imaging in the Fresnel mode of a transmission electron microscope (TEM). The equilibrium state without an applied field shows the well-known four-domain flux closure pattern with in-plane magnetization. However, the vortex is displaced from the center of the square element and the domain walls are curved. In a reference measurement of a thin Ni81Fe19 element, the vortex core is perfectly centered and the domain walls straight. When an increasing external field is applied in-plane, the vortex core can be moved. While this motion of the vortex core is linear in NiFe elements, in the ferrimagnetic FeGd squares the vortex core moves by sudden jumps. Micromagnetic simulations show that the asymmetry of the domain patterns as well as the vortex core pinning and depinning can be attributed to random anisotropy and a patchy microstructure in amorphous films.