Effect of nanostructuration on the magnetic properties of CoPt films

We have nanostructured CoPt alloy layers prepared by molecular beam epitaxy, deposited directly on a MgO(0 0 1) substrate. The initial layer had the L10 tetragonal structure, ordered in the growth direction with an easy magnetization direction perpendicular to the layer plane. We realized by electron beam lithography and ion etching a network of dots spaced of 1 (mu)m and lateral sizes of 1 (mu)m, 500 nm and 300 nm, respectively. Whereas the continuous layers had a labyrinthine magnetic structure after perpendicular demagnetization, all the dots are monodomains with randomly distributed up and down magnetization. This is due to the fact that during the demagnetization the magnetic field is no longer sufficient to reach the nucleation field. Indeed, a single-dot hysteresis loop measured by field-dependent magnetic force microscopy shows a weak nucleation field distribution. However, the 3D micromagnetic calculations show a multidomain state for these dot sizes. This magnetic structure is similar to that observed on patterned samples, heated a short time (60 s) above the Curie temperature.