The technology of magnetic storage of digital data has progressed enormously in the last two decades thanks to the scientific advances in nanomagnetism. At present, magnetic bits have dimensions of few tens of nm and magnetic devices are based in stacks of ultrathin (few nm) films of different magnetic characteristics. The fundamental understanding of the magnetic interactions between the layers, the practical realization and the detailed characterization of the magnetic properties are necessary steps to achieve magnetic devices with unprecedented dimensions and performances. Magnetic domain imaging is a major field of activity to visualize magnetic structures from few tens of nm down to atomic scale, not only at the material surface but also at buried layers.
A team of researchers from Universidad de Oviedo and Centro de Investigación en Nanomateriales y Nanotecnología (CSIC-Universidad de Oviedo) in collaboration with scientists from Universidad de Oporto (Portugal), ALBA Synchrotron (Barcelona) and Centro Nacional de Microelectrónica (Barcelona) has succeeded in analyzing in detail the magnetic characteristics of domains at a nanometric scale of both simple and buried layers of NdCo alloys with perpendicular anisotropy. The experimental method used is Magnetic Transmission X-ray microscopy with lateral resolution of 40 nm. Depending on the specific X-ray energies used it is possible to identify the magnetic response of each chemical element and obtain the image of buried layers with different chemical composition from the surface.
The studied NdCo layers present characteristic magnetic stripe domain patterns in which the magnetization points alternatively up and down, giving rise to an image of dark/bright bands that can be seen in the figure (panel a). In this experiment, the films were mounted on a goniometer which allowed to vary the angle of incidence of the photon beam relative to the surface of the films. Thus, contrast changes as a function of the incident X-ray direction could be measured and analyzed in a quantitative manner. By acquiring series of images at different angles at exactly the same location on the films it was possible to determine the angle of the magnetization with the surface both in Ndco single layers and in NdCo/Permalloy bilayers.
Magnetic singularities were observed, localized at some of the dislocation cores within the magnetic stripe pattern (see Green ellipse in panel a). They have been identified as meron-like topological defects (i.e. ½ skyrmions) with a magnetic structure as sketched in panel b. In these regions, the magnetization performs a single turn helix with a well defined chirality which is the smallest possible reversed domain in this system. Micromagnetic calculations confirmed the above findings which provide an additional insight of the magnetization reversal in magnetic heterostructures.
Panel a: Magnetic transmission x-ray microscopy image of stripe domains in a 55 nm thick NdCo film buried under a 40 nm thick permaloy overlayer, obtained at ALBA synchrotron. Green ellipse contains a dislocation with a meron-like magnetic structure localized at its core. Panel b: Sketch of the micromagnetic structure of a meron-like topological defect (1/2 skyrmion).
Article and authors
“Nanoscale Imaging of Buried Topological Defects with Quantitative X-Ray Magnetic Microscopy” por C.Blanco-Roldán, C. Quirós, A. Sorrentino, A. Hierro-Rodríguez, L. M. Álvarez-Prado, R. Valcárcel, M. Duch, N. Torras, J. Esteve, J. I. Martín, M. Vélez, J. M. Alameda, E. Pereiro, and S. Ferrer. Nature Communications, September 4th, 2015.