Foldover of nonlinear eigenmodes in magnetic thin film based feedback rings

The foldover effect of nonlinear eigenmodes in feedback ring systems was observed. Experiments made use of a system that consisted of a magnetic thin film strip, which supported the propagation of spin waves, and a microwave amplifier, which amplified the signal from the output of the magnetic strip and then fed it back to the input of the magnetic strip. The signal amplitude vs frequency response in this ring system showed resonant peaks which resulted from ring eigenmodes. With an increase in the resonance amplitude, those resonant peaks evolved from symmetric peaks to asymmetric ones and then folded over to higher frequencies. The experimental observations were reproduced by theoretical calculations that took into account the nonlinearity-produced frequency shift of the traveling spin waves.

Figure 1

Schematic of a magnetic thin film strip based active feedback ring system.

Figure 2

(a) Transmission profile (left) and spin-wave dispersion curve (right, red curve) for a YIG strip. (b) Amplitude-frequency response (left) and spin-wave dispersion curve (right) for a YIG strip based ring. (c) The same data as in (b) shown on smaller frequency scales. The green curve in (a) is a theoretical fit. The vertical dashed lines in (c) indicate the wave numbers of the ring eigenmodes.

Figure 3

Foldover of ring eigenmodes. (a) Ring amplitude-frequency responses measured at different ring overall net loss ($L$) levels. (b) Amplitude-frequency responses for the $n=21$ eigenmode measured at three $L$ levels.

Figure 4

Comparison of experiments and calculations. (a) Experimental (blue curve) and theoretical (red curve) ring amplitude-frequency responses for three different ring net loss ($L$) levels. (b) Calculated amplitude-frequency response for $L=1.0\mathrm{dB}$.