Structural and magnetic properties of N+-irradiated Co/Pt multilayers with perpendicular bimodal behaviour

We report on microstructural and magnetic investigations of the modification of (Co 0.5 nm/Pt 6 nm)15 multilayer (ML) films by 150 keV nitrogen ion irradiation. X-ray reflectivity and high angle diffraction experiments clearly show the progressive disruption of the ML structure with increasing the ion dose, from local atomic displacements at the Co/Pt interfaces up to complete intermixing of Pt and Co layers. Magneto-optical measurements display the associated decrease of perpendicular magnetic anisotropy of such a system leading to a reorientation of the anisotropy from out-of-plane to in-plane of the film before reaching the paramagnetic limit. A bimodal behaviour, i.e. two-step reversals appearing in the perpendicular hysteresis loop of the as-grown ML, is discussed facing to its evolution under ion irradiation process.