New types of magnetic data storage are energy-efficient and robust. Ferromagnets with operating frequencies in the gigahertz range are used here. This could be further increased with antiferromagnets, but these cannot be excited efficiently. Researchers from Kaiserslautern and Mainz have shown that magnetic heterostructures - each consisting of a thin layer of antiferromagnet and ferromagnet - can combine the advantages of both material classes: They have found a high operating frequency with efficient excitation. The work has been published in the renowned journal Physical Review Letters and has been awarded an "Editor's suggestion".

Magnetic materials play a central role in information processing and transmission in electronic devices. "We distinguish between different classes of magnets," says Professor Dr. Mathias Weiler, who heads the Applied Spin Phenomena working group in the Department of Physics at the Rhineland-Palatinate University of Technology (RPTU) in Kaiserslautern. "Ferromagnets have a net magnetization and are also known as permanent magnets, which can be seen to be magnetized from the outside. They are easy to excite. Their dynamics are in the gigahertz range."

A second class of magnetic materials behaves quite differently: antiferromagnets. "You can't tell from the outside that they are magnetized. They show no magnetic moment that can be interacted with. This makes them difficult to excite," explains PhD student Hassan Al-Hamdo, first author of the current study. Once they are excited, however, they exhibit much faster dynamics in the terahertz range. This fact makes them interesting for various fields of application, such as communication technologies and magnetic memories, as the processing speed could be significantly accelerated. "However, as antiferromagnets cannot be excited efficiently, their potential applications are limited," continues Weiler.

The team led by the two Kaiserslautern physicists, together with research colleagues from Mainz, has now shown how the faster dynamics of antiferromagnets can still be used. For their experiments, they reached into their bag of tricks and used a hybrid material. "It consists of two thin layers, one ferromagnetic and one antiferromagnetic," explains Weiler. The ferromagnetic layer consists of a common nickel-iron compound, which is also found in transformers, for example. The antiferromagnetic layer consists of a manganese-gold compound.

The special feature of the heterostructure is the arrangement of the spins directly at the antiferromagnetic-ferromagnetic interface. Al-Hamdo: "The spin describes the intrinsic angular momentum of a quantum particle and is the basis of all magnetic phenomena. At the interface, we find a well-defined order of the spins. This leads to an unusually strong coupling of the antiferromagnetic and ferromagnetic spins. The coupling is so high that the spins of the antiferromagnet align themselves with the magnetization in the ferromagnet. This property is unique."

The heterostructure was developed by colleagues at Johannes Gutenberg University Mainz. Colleagues from Mainz also developed the theoretical model to explain the experimental results from Kaiserslautern.

"By using the unique properties of our heterostructure, we have succeeded in transferring a magnetic excitation from the ferromagnet to the antiferromagnet. In doing so, we have achieved a higher frequency than is the case with the pure ferromagnet. The frequency lies between that of the antiferro and the ferromagnet," summarizes Weiler.

These results are interesting for future applications. "Newer mobile applications will require higher frequencies," Weiler cites as an example. "This coupling brings us into these areas." Areas of application could also include storage technologies such as magnetic random-access memory or microwave generation using spin-torque oscillators, where higher frequencies would increase performance.

The work was made possible by the transregional collaborative research center "SFB/TRR 173 Spin + X - Spin in its collective environment", which has been funded by the German Research Foundation since 2016 and in which the Kaiserslautern research teams work closely with physicists from Mainz. In Kaiserslautern, this research is also supported by the state-funded Center for Optics and Materials Science (OPTIMAS).

The study has been published in the journal Physical Review Letters: "Coupling of Ferromagnetic and Antiferromagnetic Spin Dynamics in Mn2Au/NiFe Thin Film Bilayers". Hassan Al-Hamdo, Tobias Wagner, Yaryna Lytvynenko, Gutenberg Kendzo, Sonka Reimers, Moritz Ruhwedel, Misbah Yaqoob, Vitaliy I. Vasyuchka, Philipp Pirro, Jairo Sinova, Mathias Kläui, Martin Jourdan, Olena Gomonay, and Mathias Weiler

Physical Review Letters 131, 046701 (2023)
DOI: 10.1103/PhysRevLett.131.046701

Questions answered:
Prof. Dr. Mathias Weiler
AG Applied Spin Phenomena
Department of Physics / RPTU in Kaiserslautern
Phone: 0631 205-4099
E-Mail: mathias.weiler(at)rptu.de