Three-magnon instability in a cavity

空洞共振器における3マグノン不安定性

山本 慧   ; 紅林 秀和*

Yamamoto, Kei; Kurebayashi, Hidekazu*

Magnetic materials are good model systems for studying non-linear dynamics. Since its discovery in 1950s, the instability of ferromagnetic resonance (FMR) under high-power microwave driving has led to the seminal work by Suhl that identified its mechanism [1], and the subsequent development revealed a rich array of phenomena including auto-oscillations and chaos [2]. The first-order Suhl instability, also known as three-magnon instability, has the lowest threshold power among all the known non-linear phenomena in ferromagnetic dynamics. Nevertheless, it has attracted comparatively little attention so far, perhaps because it requires a pair of spin wave modes whose eigen frequency is half of the FMR frequency. This condition is almost automatically satisfied, however, in film geometry with a low static magnetic field, a setup that is prevalent in spintronics. In this work, we study the three-magnon instability when the ferromagnetic film is placed in a microwave cavity. In spin cavitronics, one of the aims is to couple the FMR mode and cavity photons strongly so as to enhance the coherence of the former for potential quantum magnonic applications. Understanding the effect of non-linearity is important to control such devices since a strong coupling tends to induce a large FMR mode amplitude, which in turn may set off the spin wave instability. We show, both theoretically and experimentally, that the three-magnon instability generally blurs the anti-crossing of FMR mode and cavity photon. It does not completely wipe out the magnon-photon hybridization, however. Sufficiently strong input power sees a coalescence of the split resonance peaks into one, which still maintains the resonant Lorentzian shape (Fig. 1). We discuss the dependence of line shape on temperature and phase detuning between the magnetic film and the cavity.