Strong photon-magnon coupling using a lithographically defined, low damping molecular ferrimagnet

We report on strong coupling between microwave resonator photons and magnons hosted by the molecular ferrimagnet vanadium tetracyanoethylene V[TCNE]_x. Our work is motivated by the challenge of scalably integrating an lithographically patterned, low-damping magnetic material with planar superconducting circuits, thus enabling a host of new quantum circuit designs that leverage the properties of magnetism to create new function. We take advantage of the properties of V[TCNE]_x, which has ultra-low intrinsic damping, can be grown at low processing temperatures on arbitrary substrates, and patterned via electron beam lithography. We demonstrate the scalable, lithographically integrated fabrication of hybrid quantum magnonic devices consisting of a thin-film superconducting resonator coupled to low-damping, thin-film V[TCNE]_x. Our devices exhibit a cooperativity as high as 1181 at T <!--[if gte msEquation 12]>~ 0.4 K, which is deep in the strong coupling regime and suitable for quantum circuit integration. Owing to the unique properties of V[TCNE]_x, our samples reveal a spectrum of high-order magnon modes that couple to the resonator. This work paves the way for the flexible exploration of high-cooperativity hybrid magnonic quantum devices in which magnonic circuits may be scalably integrated with superconducting circuits.