Demonstrating robust quantum integration of molecule-based ferrimagnets via atomic layer deposition

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 standard magnonic material: yttrium iron garnet (YIG). However, V[TCNE]_x’s rapid degradation in ambient conditions has bottlenecked its implementation in practical applications. Existing methods of encapsulation with epoxy provide protection at room temperature but obscure intrinsic properties, induce strain at cryogenic temperatures, and introduce challenges to microscale device integration. Here, we demonstrate that ultrathin alumina films deposited via low-temperature atomic layer deposition effectively protect V[TCNE]_x by preserving its magnetic and magnonic properties following prolonged ambient exposure. Additionally, these sub-100 nm transparent films allow advanced spectroscopy, magnetometry, and cavity magnonics to provide further unprecedented insight into the intrinsic properties of V[TCNE]_x. This approach advances material characterization efforts as well as molecule-based quantum information science by paving the way for scalable, monolithic integration in hybrid quantum technologies.