From German Researchers, Multilayer Metamaterials with Ferromagnetic Domains Separated by Antiferromagnetic Domain Walls for Data Highway

From Helmholtz-Zentrum Dresden-Rossendorf (HZDR)

Researchers from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), TU Chemnitz, TU Dresden and Forschungszentrum Jülich have been the 1st to demonstrate that not just individual bits, but entire bit sequences can be stored in cylindrical domains: tiny, cylindrical areas measuring just around 100 nanometers.

Information is stored by the depth-dependent direction of magnetization in domain walls, which are located between the cylinder domains and their surroundings. This magnetization of domain walls can point clockwise or counterclockwise in individual blocks, which are separated vertically by ruthenium layers. By systematically varying these directions, researchers can encode different bit sequences within cylinder domain, as is shown here with some artistic interpretation.
(Source: B. Schröder, HZDR)

As the team reports in the journal Advanced Electronic Materials, these findings could pave the way for novel types of storage and sensors, including even magnetic variants of neural networks.

“A cylindrical domain, which we physicists also call a bubble domain, is a tiny, cylindrical area in a thin magnetic layer. Its spins, the electrons’ intrinsic angular momentum that generates the magnetic moment in the material, point in a specific direction. This creates a magnetization that differs from the rest of the environment. Imagine a small, cylinder-shaped magnetic bubble floating in a sea of opposite magnetization,” says professor Olav Hellwig, Institute of Ion Beam Physics and Materials Research,, HZDR, describing the subject of his research. He and his team are confident that such magnetic structures possess a great potential for spintronic applications.

Domain walls form at the edges of this cylindrical domain, fringe areas in which the direction of magnetization changes. In the magnetic storage technology, which Hellwig’s team is trying to achieve, it will be crucial to precisely control the spin structure in the domain wall, since its clockwise or counterclockwise direction can be used directly to encode bits.

The researchers are also focusing on another aspect:

“Our current hard disks, with their track widths of 30 to 40 nanometers and bit lengths of 15 to 20 nanometers, accommodate approximately 1TB on a surface the size of a postage stamp. We are working to overcome this data-density limitation by extending storage into the 3rd dimension,” Hellwig explains.

Solution: Metamaterials in 3D
Magnetic multilayer structures are an appealing way of controlling the internal spin structure of domain walls because the magnetic energies involved can be adjusted by combining different materials and layer thicknesses. Hellwig’s team used blocks of alternating layers of cobalt and platinum, separated by layers of ruthenium, and deposited them on silicon wafers. The resulting metamaterial is a synthetic antiferromagnet. Its special feature is a vertical magnetization structure in which adjacent layer blocks have opposite directions of magnetization, resulting in a net neutral magnetization overall.

“This is where the concept of the ‘racetrack’ memory comes in. The system is like a racetrack, along which the bits are arranged like a string of pearls. The ingenious thing about our system is that we can specifically control the thickness of the layers and thus, their magnetic properties. This allows us to adapt the magnetic behavior of the synthetic antiferromagnet to enable the storage not only of individual bits, but entire bit sequences, in the form of a depth-dependent magnetization direction of the domain walls,” explains Hellwig.

This opens up the prospect of transporting such multi-bit cylinder domains along these magnetic data highways in a controlled, fast, and energy-efficient manner.

There is also potential for other applications in magnetoelectronics. For instance, they can be used in magnetoresistive sensors or in spintronic components. In addition, such complex magnetic nano-objects have great potential for magnetic implementations in neural networks that could process data in the same way as the human brain.

Article: Multilayer Metamaterials with Ferromagnetic Domains Separated by Antiferromagnetic Domain Walls

Advanced Electronic Materials has published an article written by Ruslan Salikhov, Fabian Samad,Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany, Institute of Physics, Chemnitz University of Technology, Reichenhainer Straße 70, 09107 Chemnitz, Germany, and Center for Materials, Architectures and Integration of Nanomembranes (MAIN), Chemnitz University of Technology, Rosenbergstraße 6, 09107 Chemnitz, Germany, Sebastian Schneider, Dresden Center for Nanoanalysis, cfaed, Technische Universität Dresden, 01069 Dresden, Germany, Darius Pohl, Bernd Rellinghaus, Dresden Center for Nanoanalysis, cfaed, Technische Universität Dresden, 01069 Dresden, Germany, Benny Böhm, Institute of Physics, Chemnitz University of Technology, Reichenhainer Straße 70, 09107 Chemnitz, Germany, and Center for Materials, Architectures and Integration of Nanomembranes (MAIN), Chemnitz University of Technology, Rosenbergstraße 6, 09107 Chemnitz, Germany, Rico Ehrler, Institute of Physics, Chemnitz University of Technology, Reichenhainer Straße 70, 09107 Chemnitz, Germany, and Center for Materials, Architectures and Integration of Nanomembranes (MAIN), Chemnitz University of Technology, Rosenbergstraße 6, 09107 Chemnitz, Germany, Jürgen Lindner, Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany, Nikolai S. Kiselev, Peter Grünberg Institute, Forschungszentrum Jülich and JARA, 52425 Jülich, Germany, and Olav Hellwig, Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany, and Institute of Physics, Chemnitz University of Technology, Reichenhainer Straße 70, 09107 Chemnitz, Germany, and Center for Materials, Architectures and Integration of Nanomembranes (MAIN), Chemnitz University of Technology, Rosenbergstraße 6, 09107 Chemnitz, Germany.

Abstract: “Magnetic nano-objects possess great potential for more efficient data processing, storage, and neuromorphic-type applications. Using high perpendicular magnetic anisotropy synthetic antiferromagnets in the form of multilayer-based metamaterials, the antiferromagnetic interlayer exchange energy is purposefully reduced below the out-of-plane demagnetization energy, which controls magnetic domain formation. In this unusual magnetic energy regime, as demonstrated via macroscopic magnetometry and microscopic Lorentz transmission electron microscopy, it becomes possible to stabilize nanometer-scale stripe and bubble textures consisting of ferromagnetic out-of-plane domain cores separated by antiferromagnetic in-plane Bloch-type domain walls. This unique coexistence of mixed ferromagnetic/antiferromagnetic order on the nanometer scale opens so far unexplored perspectives in the architecture of magnetic domain landscapes as well as the design and functionality of individual magnetic textures, such as bubble domains with depth-wise alternating chirality.“