Controlling the Nonlinear Relaxation of Quantized Propagating Magnons in Nanodevices

Relaxation of linear magnetization dynamics is well described by the viscous Gilbert damping processes. However, for strong excitations, nonlinear damping processes such as the decay via magnon-magnon interactions emerge and trigger additional relaxation channels. Here, we use space- and time-resolved microfocused Brillouin light scattering spectroscopy and micromagnetic simulations to investigate the nonlinear relaxation of strongly driven propagating spin waves in yttrium iron garnet nanoconduits. We show that the nonlinear magnon relaxation in this highly quantized system possesses intermodal features, i.e., magnons scatter to higher-order quantized modes through a cascade of scattering events. We further show how to control such intermodal dissipation processes by quantization of the magnon band in single-mode devices, where this phenomenon approaches its fundamental limit. Our study extends the knowledge about nonlinear propagating spin waves in nanostructures which is essential for the construction of advanced spin-wave elements as well as the realization of Bose-Einstein condensates in scaled systems.

Figure 1

(a),(b) SEM images of the $w=400\mathrm{nm}$ (multimode) and $w=100\mathrm{nm}$ (single-mode) wide conduits (shaded in orange), respectively. (c),(d) Magnon band structures of the multimode and single-mode conduits, respectively. Color plots are obtained by micromagnetic simulations and dashed lines from analytical calculations. Note the different scales of the frequencies. (e),(f) Measured spin-wave spectra of the multimode and single-mode conduits in the presence of different powers, respectively. The excited modes are represented by the yellow dots in (c) and (d).

Figure 2

Spin-wave amplitude in the multimode conduit as a function of microwave excitation power when $f=3.85\mathrm{GHz}$. The secondary magnons created by the second-order Suhl instability are denoted as $f^{+}$ and $f^{-}$. The back and gray arrows indicate the onset of instability and the rise of the secondary magnons, respectively.

Figure 3

Results of the micromagnetic simulations in the multimode conduit. (a) spin-wave frequency spectra when the microwave current varies. (b)–(d) Magnon band structures (linear scale) of the driven system correspond to the black, red and blue curves in (a), respectively. The scaling of (b)–(d) is independent from each other.

Figure 4

Time-resolved spin-wave amplitude measured by $\mu \mathrm{BLS}$ spectroscopy. (a) Beginning of the pulse. (b) End of the pulse. Black arrows indicate the onset and the decay of the instability, respectively. Note that the decay rates correspond to the intensity of the magnons.