Nonlinear dynamics in the ultra-low loss ferrimagnet vanadium tetracyanoethylene (V[TCNE]_x, x~2)

The molecule-based ferrimagnet (FiM) vanadium tetracyanoethylene (V[TCNE]_x, x~2) has gained attention for applications in magnon-based quantum information systems due to its extremely low-loss magnetic excitations (α~4×10^-5), rivaling those of the current field standard material yttrium iron garnet, YIG. Its molecular origin yields a highly porous lattice with very low saturation magnetization as well as a fully spin-polarized band gap. This collection of properties provides a unique platform to explore nonlinearities in the dynamic excitations of both the ground state ferromagnetic resonance (FMR) and quantized magnon modes. In this work we explore these nonlinearities using both high-fluence microwave excitation and coupling to paramagnetic spins. We report phenomenology beyond the linear saturation regime, including the emergence of additional resonances in FMR with features that shift resonance frequency with magnon occupation number. Additionally, we consider coupling to reservoirs of paramagnetic spins in both low and high-power regimes. These studies lay a foundation for the integration of magnetic and paramagnetic functionality into quantum devices and circuits, leveraging the ability to directly pattern V[TCNE]_x into high fidelity superconducting microwave devices.