Magnetization loops of submicron ferromagnetic permalloy dot arrays

Polycrystalline Permalloy films were deposited onto silicon substrates by rf sputtering. Arrays of cylindrical dots with diameter d between 150 and 550 nm were patterned by optical holographic lithography. The dots were arranged on a square lattice with period p, ranging from 310 to 1030 nm. The height h of dots, given by the thickness of the permalloy films, ranged between 10 and 80 nm. The shape of the magnetization loops of the dot arrays depends strongly on the aspect ratio r=d/h. The in-plane saturation field is given by the intrinsic demagnetization field of single, isolated dots. The dipolar interaction between the dots is negligible. For cylinders with elliptical base plane, the magnetization loop depends on the field direction. The measured magnetization loops of the arrays are the superposition of the magnetization loops of the single dots without dipolar interaction. A model of magnetization reversal in applied fields is given.