Broadband Optical Detection of Ferromagnetic Resonance From the Organic-Based Ferrimagnet V[TCNE]x Using N-V Centers in Diamond

Nitrogen-vacancy (N-V) center defects in diamond enable local, optical detection of magnons, owing to their sensitivity to gigahertz frequency magnetic field noise. Applied to ferromagnetic dynamics sensing, N-Vs can be used to sense an increase in the population of N-V-resonant magnons in a nearby ferromagnetic film, which arise from microwave drive of the uniform-mode ferromagnetic resonance and the subsequent scattering of uniform-mode magnon population throughout the magnon states. In this work we utilize N-V sensing to detect ferromagnetic resonance in films of the low magnetization, ferrimagnetic, coordination compound vanadium tetracyanoethylene (V[TCNE]ₓ). From an applications standpoint V[TCNE]ₓ has excellent magnetic resonance properties rivalling the best magnetic oxides and it can be deposited on a wide range of substrates, unlike most oxides. The remarkably low magnetization of V[TCNE]ₓ offers the chance to probe N-V-ferromagnet coupling in a yet-unexplored regime. We simultaneously detect broadband FMR spectroscopy of V[TCNE]ₓ films by both conventional microwave absorption and N-V techniques, finding few-gauss magnetic resonance linewidth and a magnetization of 49 G by both methods. Using spin-wave theory, we calculate the N-V-resonant spin-wave wavelengths to be on the order of tens of nanometers, with wavevectors on the order of 10⁸0.1em0exm^-1. Our measurements highlight that the low magnetization of V[TNCE]ₓ results in very short wavelength spin waves at few-GHz frequencies, which can have significant advantage for future nanoscale magnonic applications.