The case of study consists in a weak Perpendicular Magnetic Anisotropy (PMA) material where the specific properties lead to the formation of an inhomogeneous and purely three-dimensional magnetization configuration called weak magnetic stripe domains. This configuration can be understood as a self-assembled two-dimensional magnetic crystal presenting a great variety of magnetic topological defects (figure 1). Magnetic topological textures are currently at the scope of Nanomagnetism and Spintronics research communities due to their potential to be used in high density and low-energy consumption storage systems where the information could not only be preserved but also processed. Skyrmions, anti-skyrmions, bobbers, and more recently, hopfions are some examples of these non-trivial topology textures. In the studied system, the interesting magnetic textures formed at the stripe pattern defects have been analysed and studied with unprecedented detail, showing for instance the first experimental three-dimensional magnetic image of a Meron texture (up to our knowledge) (figure 2). This achievement must be emphasized as previous explanations and understanding of these textures were based on indirect interpretation from the combination of conventional two-dimensional magnetic experimental data and three-dimensional micromagnetic simulations. In our case, the Merons are formed in the system via the nucleation of a magnetic singularity called Bloch point which acts a zip running along the weak stripe domains. This guided movement and its interaction with other confined textures present in the system (vortex and anti-vortex pairs for instance), is key for controlled guiding of topological defects through weak PMA materials to be exploited as information carriers. In our system, we have observed both Meron-like and Bloch points experimentally confirming the aforementioned proposed mechanism based on micromagnetic simulations, and we are currently preparing more experiments for the study of the movement of the topological textures driven by current pulses.
The great variety of magnetic non-trivial topology textures which can be supported by weak PMA materials has been studied using the unique capabilities of the MagTEM facility available at the University of Glasgow. This Transmission Electron Microscope (TEM) possesses a probe aberration correction system, and most important, is specifically designed to study magnetism with lateral resolutions up to 1 nm in an almost magnetic field-free environment. Briefly, the magnetic induction of the sample can be measured via the deflection of the super-sharp electron-beam probe after passing through the sample. This is the Differential Phase Contrast (DPC) method based on Scanning TEM (figure 3). As it is an in-focus technique, its resolution is far better than standard de-focus Lorentz imaging and electron-phase retrieval via the Transport of Intensity Equation method. The developed algorithm for Soft X-rays has been adapted, following previous works on vector tomography applied to TEM, for tomographic reconstruction of DPC datasets. We expect to achieve a resolution in the reconstruction of a continuous magnetic thin film of around 5 nm in a total area of approximately 1.65 um2. This will show with unprecedented detail the magnetic configuration of the topological textures hosted in weak PMA system helping for instance to understand in detail the coupling in between Bloch points and radial vortices.
