MAgnon Excitation in the STRong Out-of-equilibrium regime – MAESTRO

To reach the ambitious goals of Maestro, its consortium (CEA-SPEC, CEA-SPINTEC, CNRS-UMf, UBO-LabSTICC) gathers expertise in magnetization dynamics, growth of ultra-thin garnet films with tunable properties, state-of-the-art nanofabrication, unique high frequency experimental methods to probe the spatio-temporal patterns of SWs, and micromagnetic simulations required to conceive, fabricate, characterize, model and optimize magnon spintronic devices.

Three different means to excite magnons in the strong out-of-equilibrium regime shall be compared:
(i) standard harmonic (RF) excitation, where selection rules impose the frequency and spatial symmetry of the primarily excited SWs.
(ii) static (dc) SOT excitation, which enhances/suppresses all SW modes (no selection rule).
(iii) the combination of the former two, i.e., stimulated amplification by dc SOT of RF excited SWs.

- We have demonstrated the filtering of a radio frequency signal by a micron-sized magnonic crystal based on a 20nm thick YIG film. By mapping the intensity and phase of the spin waves propagating in this magnonic crystal by µ-BLS spectroscopy, we obtained the dispersion of the spin waves and their characteristic attenuation. Efficient filtering is achieved with selectivity of 20 MHz at an operating frequency of 4.9 GHz.

- We have demonstrated that in a YIG nanodisk, where geometric confinement drastically suppresses nonlinear interactions between spin waves, the dynamics of magnetization can maintain spatial coherence up to very large precession angles by exciting it with a high amplitude microwave field. To study the stability of this regime when it is disturbed, we applied a second microwave excitation, of lower amplitude than the first one. A new resonance is then detected in the rotating frame of the magnetization: the two absorption peaks observed are the signature of a coherent nutation motion of the magnetization, similar to that of a spinning top around its axis of precession. The resonant excitation of the nutation modes also makes it possible to control the bistability of the nonlinear dynamics of the magnetization.

- We have demonstrated the long-distance transport of angular momentum via the phonons generated by magneto-elastic coupling in the YIG. Its experimental signature are the interferences observed by ferromagnetic resonance in a sample composed of 2 layers of YIG separated by a non-magnetic substrate of GGG 0.5 mm thick, which are due to the strong coupling of the dynamics of the two layers of YIG via the transverse sound waves in the GGG. Using a Pt electrode deposited on one of the YIG layers, the angular momentum transmitted by the other layer via phonons was also detected electrically.

The µ-BLS measurements carried out in 2020 on the propagation of spin waves in 1D Bi: YIG / Pt guides and their amplification by SOT couple are very promising. They demonstrate that the effects of non-linear saturation that we observed on YIG / Pt guides of similar geometry are largely postponed.

- B. Divinskiy et al., Appl. Phys. Lett. 116, 062401 (2020)
- V. E. Demidov et al., J. Appl. Phys. 127, 170901 (2020)
- K. An et al., Phys. Rev. B 101, 060407(R) (2020)
- Y. Li et al., Phys. Rev. X 9, 041036 (2019)

Magnon spintronics is a novel information technology paradigm exploiting the spin degree of freedom, whose ambition is to develop a vectorial processing platform with higher energy efficiency while remaining adaptive and miniaturized, which will offer new functionalities that go beyond the potential of CMOS based electronics. It relies in particular on spin-orbit related effects (spin torque, spin-charge conversion, etc), which are used to excite, detect and control spin-waves (SWs), or their quanta magnons, with characteristic frequencies ranging from GHz to THz and wavelengths from µm to nm.

Since the members of the project have recently demonstrated the possibility to modulate the relaxation time of SWs propagating in thin films of YIG, a magnetic insulator considered as the ideal material for magnonics due to its unmatched dynamical quality factor, the novel concept of active magnon-based media is now foreseen as a reachable goal. Importantly, this first step towards such a functional technology also revealed undesirable saturation effects due to nonlinear coupling between magnon modes. The goal of the Maestro project is thus to master SW instabilities and turbulences occurring in low loss magnetic materials, which will allow to exploit the operation of SW devices at large spin currents. This will be achieved:
- by exploiting magnonics concepts through periodic nanostructuration to engineer the magnons' propagation spectrum and to lift the degeneracy between SW modes, which are the most likely to participate to nonlinear processes.
- by exploiting spintronics concepts through the regenerative spin-orbit torque (SOT) associated to interfacial spin transfer process in order to decrease/increase the relaxation of SW modes depending on the polarity of the electrical current injected in an adjacent metallic layer of an heavy material.

The main objective is to demonstrate the manipulation of high amplitude coherent SWs in magnonic devices that could therefore deliver large output signals. For this, three different means to excite magnons in the strong out-of-equilibrium regime shall be compared:
i) standard harmonic (RF) excitation, where selection rules impose the frequency and spatial symmetry of the primarily excited SWs.
ii) static (dc) SOT excitation, which enhances/suppresses all SW modes (no selection rule).
iii) the combination of the former two, i.e., stimulated amplification by dc SOT of RF excited SWs.

With (i), the aim is to reach very large amplitude magnetization dynamics in YIG nanostructures and to study its stability. Based on this knowledge, the goal is to be able to propagate a high amplitude SW in a properly designed magnonic crystal. With (ii), one objective is to stabilize large power auto-oscillations under dc pumping, by engineering the nonlinear coupling between SW modes. Another goal is to study how dc pumping alters the energy distribution of magnons, from the bottom of the band (GHz) up to thermal magnons (THz), and the opportunity to induce solitons and even a Bose-Einstein condensate, which has recently been predicted to appear under specific conditions. With (iii), the main objective is to mitigate or even suppress the nonlinear saturation of the SW amplification achieved through SOT, and to benchmark the performance of an electrically controlled delay line based on this idea. Ultimately this effort will allow to conceive novel smart and compact novel microwave components capable to prepare for the long term evolution of wireless telecommunications.

To reach the ambitious goals of Maestro, its consortium (CEA-SPEC, CEA-SPINTEC, CNRS-UMf, UBO-LabSTICC) gathers expertise in magnetization dynamics, growth of ultra-thin garnet films with tunable properties, state-of-the-art nanofabrication, unique high frequency experimental methods to probe the spatio-temporal patterns of SWs, and micromagnetic simulations required to conceive, fabricate, characterize, model and optimize magnon spintronic devices.