R&D at University of Basel: Concept for New Storage Medium

Physicists from Switzerland, Germany and Ukraine have proposed a new data storage medium.

In an antiferromagnetic single crystal, regions with different orientations of the antiferromagnetic order have been created (blue and red regions), separated by a domain wall. Their course can be controlled by structuring the surface. This is the basis for a new storage medium concept.
(Image: Department of Physics, University of Basel).

The technique is based on specific properties of antiferromagnetic materials that had previously resisted experimental examination.

Using nanoscale quantum sensors, an international research team has succeeded in exploring certain previously uncharted physical properties of an antiferromagnetic material. Based on their results, the researchers developed a concept for a new storage medium published in the journal Nature Physics. The project was coordinated by researchers from the Department of Physics and the Swiss Nanoscience Institute at the University of Basel.

Antiferromagnets make up 90% of all magnetically ordered materials. Unlike ferromagnets such as iron, in which the magnetic moments of the atoms are oriented parallel to each other, the orientation of the magnetic moments in antiferromagnets alternates between neighboring atoms. As a result of the cancelation of the alternating magnetic moments, antiferromagnetic materials appear non-magnetic and do not generate an external magnetic field.

Antiferromagnets hold great promise for exciting applications in data processing, as the orientation of their magnetic moment – in contrast to the ferromagnets used in conventional storage media – cannot be accidentally overwritten by magnetic fields. In recent years, this potential has given rise to the budding research field of antiferromagnetic spintronics, which is the focus of numerous research groups around the world.

Quantum sensors provide new insights
In collaboration with the research groups under Dr. Denys Makarov (Helmholtz-Zentrum in Dresden, Germany) and Professor Denis D. Sheka (Taras Sevchenko National University of Kyiv, Ukraine), the team led by Professor Patrick Maletinsky in Basel examined a single crystal of chromium(III) oxide (Cr₂O₃). This single crystal is an almost perfectly ordered system, in which the atoms are arranged in a regular crystal lattice with very few defects.

“We can alter the single crystal in such a way as to create two areas (domains) in which the antiferromagnetic order has different orientations,” explains Natascha Hedrich, lead author of the study.

These two domains are separated by a domain wall. To date, experimental examinations of domain walls of this sort in antiferromagnets have only succeeded in isolated cases and with limited detail.

“Thanks to the high sensitivity and excellent resolution of our quantum sensors, we were able to experimentally demonstrate that the domain wall exhibits behavior similar to that of a soap bubble,” Maletinsky explains.

Like a soap bubble, the domain wall is elastic and has a tendency to minimize its surface energy. Accordingly, its trajectory reflects the crystal’s antiferromagnetic material properties and can be predicted with a high degree of precision, as confirmed by simulations performed by the researchers in Dresden.

Surface architecture determines trajectory
The researchers exploit this fact to manipulate the trajectory of the domain wall in a process that holds the key to the proposed new storage medium. To this end, Maletinsky’s team selectively structures the surface of the crystal at the nanoscale, leaving behind tiny raised squares. These squares then alter the trajectory of the domain wall in the crystal in a controlled manner.

The researchers can use the orientation of the raised squares to direct the domain wall to one side of the square or the other. This is the fundamental principle behind the new data storage concept: if the domain wall runs to the ‘right’ of a raised square, this could represent a value of 1, while having the domain wall to the ‘left’ could represent a value of 0. Through localized heating with a laser, the trajectory of the domain wall can be repeatedly altered, making the storage medium reusable.

“Next, we plan to look at whether the domain walls can also be moved by means of electrical fields,” Maletinsky explains. “This would make antiferromagnets suitable as a storage medium that is faster than conventional ferromagnetic systems, while consuming substantially less energy.”

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Original publication: Nanoscale mechanics of antiferromagnetic domain walls

Nature Physics has published an article written by Natascha Hedrich, Kai Wagner, Department of Physics, University of Basel, Basel, Switzerland, Oleksandr V. Pylypovskyi, Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Ion Beam Physics and Materials Research, Dresden, Germany, Brendan J. Shields, Department of Physics, University of Basel, Basel, Switzerland, Tobias Kosub, Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Ion Beam Physics and Materials Research, Dresden, Germany, Denis D. Sheka, Taras Shevchenko National University of Kyiv, Kyiv, Ukraine, Denys Makarov, Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Ion Beam Physics and Materials Research, Dresden, Germany, and Patrick Maletinsky, Department of Physics, University of Basel, Basel, Switzerland.

Abstract: “Antiferromagnets can encode information in their ordered magnetic structure, providing the basis for future spintronic devices. The control and understanding of antiferromagnetic domain walls, which are the interfaces between domains with differing order parameter orientations, are key ingredients for advancing antiferromagnetic spintronic technologies. However, studies of the intrinsic mechanics of individual antiferromagnetic domain walls are difficult because they require sufficiently pure materials and suitable experimental approaches to address domain walls on the nanoscale. Here we nucleate isolated 180° domain walls in a single crystal of Cr₂O₃, a prototypical collinear magnetoelectric antiferromagnet, and study their interaction with topographic features fabricated on the sample. We demonstrate domain wall manipulation through the resulting engineered energy landscape and show that the observed interaction is governed by the surface energy of the domain wall. We propose a topographically defined memory architecture based on antiferromagnetic domain walls. Our results advance the understanding of domain wall mechanics in antiferromagnets.“