Large four-fold magnetic anisotropy in two-dimensional modulated Ni80Fe20 films

2-D modulated structures consisting of square arrays of 60 nm thick Ni80Fe20 circular dots underneath a continuous Ni80Fe20 film were fabricated using deep ultraviolet lithography. The array pitch for all the samples was fixed at 620 nm, while the dot diameter varied from 300 nm to 550 nm. Four-fold in-plane magnetic anisotropy (FFA) was detected in the samples using ferromagnetic resonance technique. It was found that the deposition of continuous film on top of the array drastically increases the FFA of the system, with a maximum anisotropy field of 120 Oe being observed for the sample with intermediate diameter. This increase is due to the appearance of a field well in the film underneath the dots with a shape that is dependent on magnetic field orientation and to the coupling of magnetization oscillations in dots and film.