Physicists have expanded their understanding of magnetic vortices

Physicists have expanded their understanding of magnetic vortices

An international team of physicists studied the energy structure of the spiral antiferromagnet GdRu2Si2. New features have been discovered that will allow to improve devices using magnetic memory. The work is published in the journal Nanoscale Advances.

Every year, hundreds of petabytes of data are created and collected on the planet, which must be stored somewhere. The devices used today, such as hdd and ssd types, have disadvantages in the form of relative fragility and limited data storage capacity. One of the next stages of the development of this industry may be the transition to magnetic drives that use small “vortices”. These magnetic vortices, called skyrmions, form in some substances and can be billionths of a meter in size.

As research shows, skyrmions have proven to be extremely resistant to external influences. Another important feature of them is that scientists can control their behavior by changing the temperature or applying an electric current. However, this area remains rather poorly studied, and research aimed at improving the understanding of the properties and devices of such substances is needed.

Serhiy Yeremeev, a leading researcher at the IFPM of the SB RAS, explains: “The centrosymmetric antiferromagnet GdRu2Si2 has been well known since the early 1980s. Recently, it has returned to the field of research projects with the discovery of square magnetic skyrmion lattices without geometrically broken symmetry. This skyrmion phase appears in an external magnetic field of 2–2.5 T at a temperature below 20 K. Although the magnetic properties of the material have been studied in great detail for many years, the appearance of the skyrmion phase has renewed and intensified the discussions, especially concerning the features of the appearance of skyrmions.

Figure 1. The Fermi surface of GdRu2Si2 in the paramagnetic phase, presented in the first Brillouin zone (a) and in the basis of the reverse lattice (b). (c) Calculated and experimental projections of the Fermi surface onto the (001) face. Source: Nanoscale Advances

The task of the scientists was to study the properties of this material and to predict possible candidates that could reveal the unusual properties of magnetic skyrmions, as well as to obtain detailed information about the surface and volume electronic structures and, most importantly, about how the electronic structure is modified when the temperature changes.

GdRu2Si2 single crystals of high purity and structural quality were grown. The samples were scaled in ultra-high vacuum and their electronic energy structure was studied at different temperatures using photoelectron spectroscopy. The use of synchrotron radiation made it possible to obtain high-quality data. The experimental results were compared with calculations of the electronic structure performed within the framework of the density functional theory.

Thus, the authors studied the bulk and surface electronic structure of the GdRu2Si2 material. The good agreement between experimental and theoretical results made it possible to characterize in detail the properties and orbital composition of the Fermi surface of GdRu2Si2. It was found that the spiral magnetic structure of the material underlying the formation of the skyrmion lattice is due to the special geometry of the Fermi surface. In particular, the main role is played by the regions of the Fermi surface marked by the red arrow in Figure 1c. They are responsible for the unusual magnetic interaction that leads to the formation of magnetic vortices. Although in GdRu2Si2 the skyrmion phase appears at a fairly low temperature, a deeper understanding of the underlying skyrmion firmness in centrosymmetric systems may help to predict new materials in which nanoscale skyrmions can appear at significantly higher temperatures, possibly even at room temperature.

Vasyl Stolyarov, director of the Center for Promising Methods of Mesophysics and Nanotechnologies of the Moscow Technical University, adds: “Recently, square lattices of skyrmions were discovered in this material. The grating has a period of 1.9 nm and the smallest skyrmion size observed to date. Thus, the material is attractive for the development of new generation magnetic memory devices with high recording density and low power consumption. In the future, we plan to apply the spin-resolved scanning tunneling microscopy developed at our center to visualize the magnetic texture of the surface in direct space.”

The work was carried out by an international team of scientists from the Institute of Strength Physics and Materials Science of the Siberian Branch of the Russian Academy of Sciences (Tomsk), St. Petersburg State University, Moscow State University of Applied Sciences, MISIS, VNDIA named after N. L. Dukhov, from Germany: Technical University of Dresden, Frankfurt University named after Goethe – and Spain: the University of the Basque Country, the Materials Physics Center of San Sebastian, the International Physics Center of Donostia, the Ikerbask Foundation, as well as Johann Kepler University (Austria) and Chalmers University of Technology (Sweden).

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