Influences of the aspect ratio and film thickness on switching properties of elliptical Permalloy elements

The size dependence on the switching properties of microstructured Permalloy (Ni₈₀Fe₂₀) ellipses were investigated by magnetoresistance measurements and magnetic force microscopy. Elements with fixed short axes of 1 μm, long axes varying from 2 to 10 μm, and film thickness varying from 8 to 55 nm were fabricated by electron beam lithography through a lift-off technique. A single-domain configuration was observed in the elements with the range of aspect ratios (long/short axis) from 5 to 10. More complex domain structures appear in the lower aspect ratio and thicker samples. The switching properties show a strong dependence on the film thickness as well as the aspect ratio. The switching fields of uniform magnetization reversal increase with increasing thickness up to a critical value (24c<40 nm), whereas they decrease with increasing thickness above t_c. Nevertheless, the switching fields only show weak dependency on aspect ratio.