Abstract: Wave interactions are responsible for the various aspects of the behavior of different systems in nature, including processes in the oceans and atmosphere, star hydrodynamics, and even the evolution of the Universe. Spin waves (and their quanta–magnons) in magnetically ordered materials are highly nonlinear compared to, for example, phonons or photons in solids. One of the most suitable systems for studying magnons is a single-crystalline ferrimagnetic Yttrium Iron Garnet (YIG, Y3Fe5O12) produced in the form of a thin film. The strong nonlinearity, combined with the high quality factor of magnons in YIG, facilitates the observation of various types of nonlinear interaction processes. For example, the phenomenon of Bose-Einstein condensation of magnons at the bottom of their frequency spectrum, observed in YIG films [1,2], provides significant assistance in these studies. In the currently presented experiments, condensation was achieved by the parametric pumping of magnons with microwave radiation.
In this presentation, I will show the construction of a 2D wavevector-resolved Brillouin light scattering spectroscopy. Using this setup, we were able to not only directly measure for the first time the full spin wave dispersion of thin YIG film in 2D, but also to register the results of several novel nonlinear processes that involve not only “real” quasiparticles (the eigenmodes of the medium) but produce as a final product virtual quasiparticles (out-of-spectrum waves) caused by various types of nonlinear interactions. The most nontrivial and intriguing process involves a pair of parametrically excited magnons and a partially coherent Bose-Einstein condensate (BEC). This process is enhanced by full-phase correlations in parametric magnon pairs with opposite wavevectors [3].
Nonlinear effects in magnetic systems are not only limited to magnetic nano-elements. Artificial spin ices (ASIs) are ensembles of geometrically structured, interacting magnetic nano-elements that exhibit frustration [4]. In this talk, I will present a macroscopic ASI where 1-inch magnets mounted on low-friction rotors are arranged in a square lattice. The system is driven into a nonlinear regime by an external coil with a field of 2-20 Hz range. Mixing and coupling between high-symmetry modes is observed. The dynamics are experimentally captured by a high-frame-rate camera and then digitized to recover the spectrum for each magnet. Modeling based on the torque equation for each elongated magnet and their coupling using a monopole-charge approximation [5] reproduces the dynamics with remarkable accuracy. While at completely different spatial and temporal scales, these dynamics are similar to those observed at the nanoscale when ASIs are driven into a nonlinear regime [6]. Our macroscopic ASI can be considered as a testbed for nonlinear phenomena at a scale appropriate for dissemination to the general public.
[1] S. O. Demokritov et al., Bose-Einstein Condensation of Quasi-Equilibrium Magnons at Room Temperature under Pumping, Nature 443, 430 (2006).
[2] A. A. Serga et al., Bose-Einstein Condensation in an Ultra-Hot Gas of Pumped Magnons,
Nat. Commun. 5, 3452 (2014).
[3] V. S. L’vov et al., Correlation-enhanced interaction of a Bose-Einstein condensate with parametric magnon pairs and virtual magnons, Phys. Rev. Lett. 131, 156705 (2023).
[4] S. H. Skjærvø et al., Advances in artificial spin ice, Nat. Rev. Phys. 2, 13 (2020)
[5] P. Mellado et al., Macroscopic Magnetic Frustration, Phys. Rev. Lett. 109, 257203 (2012)
[6] S. Lendinez et al., Nonlinear multi-magnon scattering in artificial spin ice, Nat. Commun. 14, 3419 (2023)