R&D: Magnetic Whirls in Future Data Storage Devices From Martin Luther University Halle-Wittenberg

From: Martin Luther University Halle-Wittenberg

Magnetic (anti)skyrmions are microscopically small whirls that are found in special classes of magnetic materials.

Schematic representation of the magnetization in an advanced racetrack memory data storage. Skyrmions (blue) and antiskyrmions (red) constitute the ‘1’ and ‘0’ bits, respectively.
(Foto: Börge Göbel/MLU )

These nano-objects could be used to host digital data by their presence or absence in a sequence along a magnetic stripe. A team of scientists from the Max Planck Institutes (MPI), Microstructure Physics (Halle, Germany) and Chemical Physics of Solids (Dresden, Germany) and the Martin Luther University Halle-Wittenberg (MLU) has now made the observation that skyrmions and antiskyrmions can coexist bringing about the possibility to expand their capabilities in storage devices. The results were published in the scientific journal Nature Communications (and see below).

With the ever-increasing volumes of digital data from the growing numbers of devices, the demand for data storage capacity has been enhanced dramatically over the past few years. Conventional storage technologies are struggling to keep up. At the same time, the ever-increasing energy consumption of these devices – hard disk drives (HDD) and random-access memories (RAM) – is at odds with a ‘green’ energy landscape. Required are entirely new devices that have greater performance at a drastically reduced energy consumption.

A promising proposal is the magnetic racetrack memory-storage device. It consists of nanoscopic magnetic stripes (the racetracks) in which data is encoded in magnetic nano-objects, typically by their presence or absence at specified positions. One possible nano-object is a magnetic (anti)skyrmion: this is an extremely stable whirl of magnetization with a size that can be varied from micrometers to nanometers. These objects can be written and deleted, read and, most importantly, moved by currents, therefore allowing the racetrack to be operated without any moving parts.

“By stacking several racetracks, one on top of each other, to create an innately three-dimensional memory-storage device, the storage capacity can be drastically increased compared to solid state drives and even hard disk drives. Moreover, such a racetrack memory device would operate at a fraction of the energy consumption of conventional storage devices. It would be much faster, and would be much more compact and reliable“, explains Prof Stuart Parkin, director, MPI, Microstructure Physics (Halle) and Alexander von Humbold, professor, MLU.

“Skyrmions and antiskyrmions are ‘opposite’ magnetic whirls. However, until recently, it was believed that these two distinct objects can only exist in different classes of materials.” explains Prof Ingrid Mertig, institute of physics, MLU.

The research team from Max Planck institutes in Halle and Dresden and the MLU has now discovered that antiskyrmions and skyrmions can coexist under certain conditions in the same material. Dr Börge Göbel, a member of Mertig’s research group, provided the theoretical explanation for the unexpected experimental observations that were carried out by Jagannath Jena in Parkin’s group. The measured single crystal materials, Heusler compounds, were prepared by Dr Vivek Kumar in the group of Prof Claudia Felser, MPI (Dresden).

Skyrmions and antiskyrmions are stabilized in different materials by a magnetic interaction that is directly tied to the structure of the host material. In some materials only skyrmions can form, while in other materials, antiskyrmions are energetically preferred by this interaction. However, what was previously overlooked is that the individual magnets in each material (the ‘magnetic dipoles’) also significantly interact with each other via their dipole-dipole interaction. This interaction always prefers skyrmions. For this reason, even ‘antiskyrmion materials’ can exhibit skyrmions (but not vice versa). This happens preferably as the temperature is lowered. At a critical transition temperature, the two distinct objects coexist.

Besides its fundamental relevance, this finding allows for an advanced version of the racetrack memory data storage, where a bit sequence could, for example, be encoded by a sequence of skyrmions (‘1’ bit) and antiskyrmions (‘0’ bit). This concept would be more reliable than conventional racetracks

About the study: Jena, Jagannath et al. Elliptical Bloch skyrmion chiral twins in an antiskyrmion system. Nature Communications (2020). doi: 10.1038/s41467-020-14925-6

Article: Elliptical Bloch skyrmion chiral twins in an antiskyrmion system

Nature Communications has published an article written by Jagannath Jena, Max Planck Institute of Microstructure Physics, Weinberg 2, 06120, Halle (Saale), Germany, Börge Göbel, Max Planck Institute of Microstructure Physics, Weinberg 2, 06120, Halle (Saale), Germany, and Institute of Physics, Martin Luther University Halle-Wittenberg, 06120, Halle (Saale), Germany, Tianping Ma, Max Planck Institute of Microstructure Physics, Weinberg 2, 06120, Halle (Saale), Germany, Vivek Kumar, Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Strasse 40, 01187, Dresden, Germany, Rana Saha, Max Planck Institute of Microstructure Physics, Weinberg 2, 06120, Halle (Saale), Germany, Ingrid Mertig, Institute of Physics, Martin Luther University Halle-Wittenberg, 06120, Halle (Saale), Germany, and Max Planck Institute of Microstructure Physics, Weinberg 2, 06120, Halle (Saale), Germany,Claudia Felser, Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Strasse 40, 01187, Dresden, Germany, and Stuart S. P. Parkin, Max Planck Institute of Microstructure Physics, Weinberg 2, 06120, Halle (Saale), Germany.

Field-induced stabilization of antiskyrmions in D_2d material
Click to enlarge
The top row (a–f) shows the observation under the decreasing field mode at 300 K. A magnetic field is applied and decreased coming from the field-polarized phase. The texture exhibits helices with decreasing period length. In the bottom row (g–l) we apply the modified protocol (see schematic figures m, n) where we tilt the field B by 35°–40° in the [1 1 0] direction and come back to [0 0 1] orientation (as indicated) at high fields until a dense array of antiskyrmions has formed in h. We observe two different types of features: round ones indicating conventional antiskyrmions and square ones, which are topologically identical to antiskyrmions. A highly periodic lattice of the square objects is metastable even at vanishing field (k) and transforms into the helical phase only at a negative field of −128 mT. The scale bar corresponds to 500 nm. In m we show the tilting of the sample and n illustrates the procedure that was used for the measurement of the LTEM images shown in g–l.

Abstract: “Skyrmions and antiskyrmions are distinct topological chiral spin textures that have been observed in various material systems depending on the symmetry of the crystal structure. Here we show, using Lorentz transmission electron microscopy, that arrays of skyrmions can be stabilized in a tetragonal inverse Heusler with D_2d symmetry whose Dzyaloshinskii-Moriya interaction (DMI) otherwise supports antiskyrmions. These skyrmions can be distinguished from those previously found in several B20 systems which have only one chirality and are circular in shape. We find Bloch-type elliptical skyrmions with opposite chiralities whose major axis is oriented along two specific crystal directions: [010] and [100]. These structures are metastable over a wide temperature range and we show that they are stabilized by long-range dipole-dipole interactions. The possibility of forming two distinct chiral spin textures with opposite topological charges of ±1 in one material makes the family of D_2d materials exceptional.“