Spin-wave excitations, confinement, and coupling in high-quality organic-based magnetic structures*

The study of coherent magnonic interactions relies implicitly on the ability to excite and exploit long lived spin wave excitations in a magnetic material. Surprisingly, the organic-based ferrimagnet vanadium tetracyanoethylene (V[TCNE]_x; x ≈ 2) has recently emerged as a low-loss material and offers a compelling alternative to yittrium iron garnet (YIG). Here, we present the synthesis of a new class of organic-based magnetic nanostructures consisting of nanowires of V[TCNE]_x that assemble along the ridges of a grooved substrate. These nanowires exhibit uniaxial magnetic anisotropy with an in-plane easy axis perpendicular to the nanowires, which is in direct contrast to the isotropic in-plane response of typical thin-films. These nanostructures support the excitation of multiple modes, and when these different magnon modes are brought into resonance by varying the orientation of an in-plane magnetic field, we observe anticrossing behavior, indicating strong coherent coupling between the excitations. Furthermore, micromagnetic simulations using real nanowire profiles extracted from cross-sectional scanning electron microscopy faithfully reproduce the experimentally measured spectra without any free parameters, including spin-wave and other higher-order modes. Additionally, we use this data to explore the origin of the induced anisotropy in this materials system and note that these results offer insight into a whole class of organic-based magnetic materials of the form M[Acceptor]_x (M = transition metal; x ≈ 2). These results also introduce a new degree of freedom for organic-based magnetism and spintronics, and together with recent demonstration of encapsulation technologies and demonstrated functional microwave devices that exhibit high quality factors across a frequency range, suggest future promising applications in microwave electronics and quantum magnonics.