Magnetic domain wall propagation in nanostructures of alloys with perpendicular magnetic anisotropy

Abstract

Among the new challenges of the domain walls (DW) propagation, the control of DW movements induced by spin injection is extensively studied. Magnetic Transmission Electron Microscopy is a powerful tool for imaging the magnetic domain and analyse in situ their movement. It offers a nanoscale resolution and is very sensitive to magnetic induction inside the sample. Most of the investigations on DW propagation using Lorentz Microscopy (LTEM) are done on planar configuration [1–2]. Recently, the first observations of current-excited DW propagation, the well known Spin-Torque effect, were achieved [2]. The work presented in this paper concerns an original approach of the study of DW propagation in CoPt multilayers with perpendicular magnetic anisotropy. The Lorentz Microscopy is used to reveal domain walls by tilting the sample, giving rise to in-plane magnetic components, essential for magnetic contrast. The multi-layers were sputtered on silicon nitride windows. They consist of a stack of [Co/Pt]n multilayer (the Co thickness being <1 nm to keep the perpendicular anisotropy). The number of layers n in the stacking determines the total Co volume in the sample. This value is an important parameter for the contrast in the LTEM images and impacts the Signal-To-Noise Ratio. Indeed, for low value of n, high defocus is needed to get Fresnel contrast on the domain walls. Moreover, for thick film the magnetic distribution inside the layers becomes complex. Figure 1 presents a magnetization process of such a film observed by Lorentz microscopy with a huge defocalisation (more than 400 µm).