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MagIC – Magnonics, Interactions and Complexity: a multifunctional aspects of spin wave dynamics

Periodic Reporting for period 2 - MagIC (MagIC – Magnonics, Interactions and Complexity: a multifunctional aspects of spin wave dynamics)

Reporting period: 2017-02-01 to 2019-01-31

The artificial patterning of nanoscale structures provides an excellent opportunity for modifying spectra of their excitations, and therefore for designing novel devices and metamaterials, with unforeseen properties useful for practical utilization. Magnetic materials with modulated properties enable control of the spin wave excitations and facilitate tunable spectra at microwave frequency range controlled via magnetic field. These excitations are at the core of magnonics, a research and technology field studying and utilizing spin waves.

The international research effort enabled by the MagIC project was devoted to magnonics and its cross-disciplinary opportunities with photonics, phononics, superconductivity and electronics. The most prominent directions of research within MagIC were the exploration of nonlinear effects, control of the magnetic loss, development of theoretical models of the spin wave scattering at nanoscale, investigation of the effect of the broken periodicity and fractal structures on magnonic spectra, and exploitation of the interaction between elementary excitations of different nature in magnetic nanostructures. In accord with its main objective, the MagIC project was successfully conducted and the target was achieved. We formulated theoretical grounds for exploitation in magnonics of boundary conditions at the interfaces, established principal properties of magnonic quasicrystals, created new innovative interdisciplinary directions in research and technology, ranging from concepts of multifunctional photonic-magnonic, magneto-phononic and magnon-fluxonic elements, all tunable via magnetic field, to applications.

The MagIC is a part of RISE program, thus main part of the activity was realized through the exchange visits of researchers between 4 EU and 5 Ukrainian research and academic institutions. We successfully fulfilled all secondments in the total of 168 person/months, realized by 53 researchers. The large number of exchange visits allowed to achieve research objectives, to transfer and exchange knowledge, and to establish collaborations that will last long after the end of the project.
The advanced theoretical and experimental studies of spin wave phenomena were conducted in three areas: periodic magnonic structures, nonperiodic magnonic structures and on the interplay between magnonic and nonmagnetic phenomena in nanostructures.

In the first area, studies on damping of high frequency spin waves in nanostructures were conducted. We explored ways for minimization and optimization of the wave attenuation. We have developed theory of the exchange spin wave damping, which proved to be vital for high frequency spin waves in the curved waveguides and chiral textures. We designed the microwave response with nanopatterned magnonic structures with and without Dzyaloshinskii-Moriya interactions. We fabricated and performed extensive ferromagnetic resonance measurements of magnetic nanodots and antidots arranged in periodic lattices with the complex unit cells.

In the second area, we provided comprehensive theory of various forms of magnetic boundary conditions and their influence on the spin wave reflection, transmission and emission. The developed theory is crucial for experimental data analysis and design of the magnonic devices. We demonstrated theoretically influence the conditions at the film edge on the Goos-Haenchen shift of the spin waves. Further on, we showed the control of the transmitted wave phase by variation the properties at the interface. The property used for demonstration of the ultra-narrow metasurface for spin waves.

We achieved significant progress in developing models of the local spin wave excitation. We showed that any nonuniformity in a magnetic system, i.e. edge, magnetization texture, inhomogeneity of the field, can be a source of the spin waves. We demonstrated that propagation of the spin waves can be effectively controlled and enhanced by the gradual change of the magnetic parameters; with this we started new direction of research, called graded index magnonics.

We established fabrication procedure of the ultra-thin YIG films and demonstrated the large propagation length of GHz frequency spin waves, proving YIG being promising material for nanoscale magnonics. Based on the metallic ferromagnetic thin films we have demonstrated the principal properties of spin waves in quasicrystal structures, including multiple band gaps, localization and re-programmability.

In the area of the cross-disciplinary opportunities, we proposed and theoretically investigated 1D magnon-phononic (magphonic) crystals. We have developed a theory of longitudinal spin oscillations in ferromagnetic materials excited by ultrashort pulses of the laser light. The interesting feature is that the acoustic and exchange longitudinal spin oscillations have frequency higher than the transversal spin waves at the same wave vector.
The results obtained in the framework of the MagIC have been published in 117 scientific papers and presented in 210 presentations at international conferences. To strength the project visibility, we organized and co-organized 2nd and 3rd International Advanced School on Magnonics 2016 in Exeter, UK and 2018 in Kyiv, Ukraine, respectively, MagIC workshop in July 2017 near Poznan, and a special session devoted to magnonics during the Sol-Skymag conference in 2018 in San Sebastian, Spain.
Most of the research performed goes beyond the state-of-the-art in both theory and technology. In particular, we proposed novel method for generation of plane spin waves and spin wave beams in thin ferromagnetic films using microwaves. The proposed development is important for magnonic applications in processing and transferring information. We have established technology for fabrication FeMnGa alloys with controlled stoichiometry and have shown coexistence of two phases in such alloys. With this transformation, the significant changes in the magnetic properties have been discovered.

Exploitation of the magnonic crystals in bi-layered structures allowed us to show the controlled transmission of spin waves between layers. We numerically showed co-directional and contra-directional transmission with three and two times reduction of the spin wave wavelength. We developed novel ideas for controlling spin wave propagation in thin films, which allowed to design the lenses for spin waves based on the metasurface and the Luneburg lens concepts. Those achievements are of big importance for further development of magnonic devices.

We developed the theory of the longitudinal spin oscillations in the ferromagnetic materials under the ultrashort pulses of the laser light. The interesting feature is that the acoustic and exchange longitudinal spin oscillations have frequency higher than the transversal spin waves at the same wave vector. The obtained results are interesting not only from the physical point of view but also with respect of their potential application when using ultrafast magnetism.

For the first time, we have demonstrated a coexistence and mutual influence of the spin wave excitations in the ferromagnetic film and lattice of Abrikosov vortexes in superconducting films, opening new field of magnon-fluxonics. The achievement is interesting from the physical point of view and of the potential application involving quantum computing.