Interacting Magnonic Currents and Chiral Spin Textures for Energy Efficient Spintronics
Final Report Abstract
In this project, we proposed to study the interaction between magnons and chiral spin textures for energyefficient spintronics. Magnons as collective excitations of the electrons’ spin precessional motion transport spin information without actual charge transport and its associated Ohmic losses. Chiral spin textures, especially magnetic skyrmions, hold great potential in the low-energy data storage industry due to their topological nature. Our proposal was motivated by the fact that both magnons and chiral spin textures share a common ground set by the interplay of dipolar, spin-orbit, and exchange energies rendering them perfect interaction partners. The propagation of magnons is fast and sensitive to the configuration of spin textures. Chiral spin textures are robust, non-volatile, and electrically reprogrammable on ultrashort timescales. Hence, the project’s goal was to shed light on the interactions between spin waves and topological spin textures. The core strategy of the project relied on iteratively optimizing magnetic systems and microstructured devices to host stable spin textures such as stripe domains or skyrmions and, at the same time, exhibit low magnetic damping to facilitate reasonable spin-wave propagation and their interaction with spin textures. Our goal was to design magnetic multilayers or magnetic films with asymmetric interfaces or with a compositional gradient along their thickness to induce significant Dzyaloshinskii-Moriya interaction (DMI) in the magnetic system. This DMI favors a fixed spin chirality such that magnetic skyrmions and chiral domain walls are stable at room temperature and without external magnetic fields. However, DMI also modifies the intrinsic magnetic damping such that one needs to test and optimize the parameters for the formation of spin textures and spin-wave transport in different material compositions. Unfortunately, the majority of the project’s run time was affected by the COVID-19 pandemic. After its start in China in December 2019 and its fast expansion to Germany, multiple extensive lockdowns and complete shutdowns of scientific activities at HZDR and in China obstructed the progress of the project. Laboratories were closed for extended periods, mostly shifted between China and Germany so an iterative optimization of materials and devices was almost impossible. In essence, the lack of operational sample fabrication facilities was a major obstacle to the project. Nevertheless, the project provided new insights into the physics of spin textures. Comparing the DMI values for Ta(2nm)/[Ta(3nm)/Co20Fe60B20(t)/MgO(1.5nm)]n/Ta(2nm) multilayers with different thicknesses t of the magnetic material and a varying number of stacks n = 3, 5, 10, 15, we conclude that for a given layer composition the strength of the DMI does not depend on the number of interfaces. In the future, this will allow for the geometrical design of spin textures without affecting their general stability. Studying Ta(3)/FMxNM1−x(t)/ Ta(3) multilayers with compositional changes along their thickness, where the ferromagnetic materials (FM) were Fe and Co and the nonmagnetic materials (NM) were Pt, Cu, and Ni, allowed us to quantify the DMI induced by the bulk magnetic asymmetry. In this study, which was published in Physical Review Letters17, we can conclude that the DMI arises from the mutual combination of both factors, large spin-orbit coupling and considerable bulk magnetic anisotropy induced by the composition gradient. Only one of the two ingredients does not suffice to design a magnetic system showing a sizable amplitude of DMI. Alongside the quantification of the DMI, samples for the generation and stabilization of magnetic spin textures such as stripe domains and skyrmions were designed and fabricated. Starting off with a well-known method to generate skyrmions electrically via inhomogeneous electric current distributions, we relied on the [Ta/Co20Fe60B20/MgO]n multilayers with n = 3, 5, 10, and 15. Even though these materials showed the expected stripe domains, spin-wave propagation was too poor due to the material’s large magnetic damping. Unfortunately, the COVID-19 pandemic did not allow for optimizing the multilayer stack such that spinwave propagation could be enhanced. Additionally, fabricating working antennas for the efficient excitation of spin waves was not possible at HZDR and has proven difficult to transfer to the Chinese partner.
Publications
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Magnetic texture based magnonics. Physics Reports, 905, 1-59.
Yu, Haiming; Xiao, Jiang & Schultheiss, Helmut
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Mapping the Stray Fields of a Micromagnet Using Spin Centers in SiC. IEEE Magnetics Letters, 12, 1-5.
Bejarano, Mauricio; Goncalves, Francisco Jose Trindade; Hollenbach, Michael; Hache, Toni; Hula, Tobias; Berencen, Yonder; Fassbender, Jurgen; Helm, Manfred; Astakhov, Georgy V. & Schultheiss, Helmut
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Quantifying the Dzyaloshinskii-Moriya Interaction Induced by the Bulk Magnetic Asymmetry. Physical Review Letters, 128(16).
Zhang, Qihan; Liang, Jinghua; Bi, Kaiqi; Zhao, Le; Bai, He; Cui, Qirui; Zhou, Heng-An; Bai, Hao; Feng, Hongmei; Song, Wenjie; Chai, Guozhi; Gladii, O.; Schultheiss, H.; Zhu, Tao; Zhang, Junwei; Peng, Yong; Yang, Hongxin & Jiang, Wanjun
