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Nano-Scale Magnonic Circuits for Novel Computing Systems

Periodic Reporting for period 4 - MagnonCircuits (Nano-Scale Magnonic Circuits for Novel Computing Systems)

Okres sprawozdawczy: 2020-12-01 do 2021-11-30

A disturbance in the local magnetic order of a solid body can propagate across a material just like a wave. This wave is named spin wave, and its quanta are known as magnons. Recently, physicists proposed the usage of magnons to carry and process information instead of electrons which are used in electronics. This technology opens access to a new generation of computers in which data are processed without motion of any real particles like electrons. This leads to a sizable decrease in the accompanying heating losses and, consequently, to lower energy consumption, which is crucial due to the ever increasing demand for computing devices. Moreover, unique properties of spin waves allow for the utilisation of unconventional computing concepts, giving the vision of a significantly faster and more powerful next-generation of information processing systems. The strategic goal of the MagnonCircuits ERC Starting Grant project is to make a transformative change in the data processing paradigm from traditional electronics to magnon-based circuits. The research field of magnonics, which addresses magnon-based data processing, is at the initial stage of its evolution and, to this point, it is located within the physics rather than within the engineering domain. Thus, physical concepts as well as a technological basis should be established on the current stage to achieve the ambitioned goals of all-magnon data processing in the future.
In general, ERC StG MagnonCircuits is a very ambitious and high-risk project that became an apparent success.

First, the success is declared by the high quantity and quality of scientific publications. In total, 44 peer-reviewed scientific articles were published in the frames of MagnonCircuits (including 5 arXiv preprints that are currently under review). Team members intensively presented the results at international conferences. Only PI had 25 invited talks and 10 invited seminars during the MagnonCircuits period. Eight press releases were published and multiple times re-published by different media.

The main highlights results are:
• The generation of coherent SWs by the Spin Hall Effect (SHE) and Spin Transfer Torque (STT) in YIG/Pt nanostructures was evidenced using BLS spectroscopy. Furthermore, the temporal evolution of the auto-oscillations was investigated [1] (references are numbered according to the tab "Publications").
• A first spin-wave majority gate is demonstrated experimentally on the macro-scale [2] (Applied Physics Letters, 116 citations since 2017). This research was performed jointly with imec, Belgium – an organization that builds bridges between academia and industry (e.g. Intel is one of their main customers).
• A nanoscaled SW directional coupler was successfully investigated numerically [6] (Science Advances, 95 citations since 2019) and realized experimentally [21] (Nature Electronics, 35 citations since 2020). As opposed to the originally-planned two-dimensional structures, the directional coupler exhibits practically no parasitic reflections. It can serve as a spin-wave power splitter, combiner, frequency separator, multiplexer, and (reconfigurable) interconnect. The directional coupler was used to propose and investigate the first integrated magnonic circuit numerically in the form of magnonic half-added [21]. This breakthrough – a simple magnonic structure that substitutes 14 transistors in electronics - goes far beyond the originally-proposed ERC research plan.
• Yttrium Iron Garnet (YIG) thin films were structured, and the corresponding patterning technology was developed. The smallest lateral size of the nanostructures is 50 nm and is well below the planned 100 nm [11] (Physical Review Letters, 56 citations since 2019). The phenomenon of exchange unpinning was discovered. In cooperation with the external collaborator R. Verba a quasi-analytic theory was developed and experimentally confirmed.
• The fundamentally new phenomenon of Bose-Einstein Condensation by rapid cooling was discovered in YIG/Pt nanostructures [24] (Nature Nanotechnology, 28 citations since 2020). This phenomenon paves the way for the usage of macroscopic quantum magnon states in conventional spintronics. In the follow-up papers, the control of the BEC state by spin-orbit torque (spin Hall effect in combination with spin transfer torque) was demonstrated [43] (Physical Review Letters 2021). Moreover, the generation of the nonlinear magnonic bullet mode using spin-orbit torque and investigation of its interaction with magnon BEC was studied for the first time [42].
• We presented an experimental demonstration of the parallel parametric generation of spin waves in YIG nano-waveguides [23]. In the follow-up paper [49], the parametric amplification of spin waves in nano-waveguides with vanished ellipticity is investigated and demonstrated experimentally for the first time.
• The propagation of coherent spin waves in waveguides of sub-100 nm sizes was demonstrated first [26] (Nano Letters, 30 citations since 2020). These waveguides operate with a single spin-wave mode and prohibit undesired elastic magnon scattering events. In the follow-up paper [37], the propagation length of spin waves in 50 nm wide waveguide was increased up to 8 µm via the transverse magnetization of the waveguide. An interesting phenomenon of hybridizing the waveguide edge modes into one uniform mode was discovered and investigated.
• The fast long-wavelength exchange spin waves were demonstrated in partially-compensated Ga:YIG thin films [48]. This result allows fast, isotropic spin-wave transport for all wavelengths in nanoscale systems independently of dipolar effects.
All the reported results are significantly beyond state of the art. Below are the main highlights which went beyond the originally-planned ERC StG MagnonCircuits tasks.

• Using the developed directional coupler [21], one of the first complex magnonic circuits (32-bit magnonic ripple carry adder) was developed and tested theoretically jointly with the group of F. Riente [46]. This is one of the first engineering (rather than physical) articles on spin-wave computing and potentially constructs a bridge between academic investigations and industrial devices.
• An array of Abrikosov vortices in a hybrid structure consisting of superconducting and magnetic layers was used to experimentally realize a reconfigurable magnonic crystal [9] (Nature Physics, 54 citations since 2019). The further investigations of Abrikosov vortex dynamics [22] (Nature Communications, 25 citations since 2020) allowed for the increase of the vortex speed (a press release was published). Consequently, ultra-fast vortex motion allowed for the first observation of spin-wave excitation by Cherenkov scattering [44]. This mechanism was successfully used for the excitation of exchange spin waves of wavelengths below 50 nm.
• Nanoscale spin-wave wake-up receiver was proposed and investigated numerically [12]. Due to its passive construction, this device may be used to activate the primary receiver circuit in power-critical internet of things applications.
• The conceptually new approach for developing magnon-based devices using inverse design was proposed and investigated numerically [39] (Nature Communications, 6 citations since 2021).
An example of future magnon circuit: A spin-wave XOR logic gate based on two magnon transistors