Abstract: The study of coherent magnonic interactions relies implicitly on the ability to excite and exploit long-lived spin wave excitations in a magnetic material. That requirement has led to a reliance on yittrium iron garnet (YIG), which for half a century has reigned as the unchallenged leader in low-loss magnetic resonance despite extensive efforts to identify alternative materials. Surprisingly, the organic-based ferrimagnet vanadium tetracyanoethylene (V[TCNE]x; x ≈ 2) has recently emerged as a compelling alternative to YIG. In contrast to other organic-based materials, V[TCNE]x exhibits robust magnetism, has a single-peaked narrow magnetic resonance feature (less than 1 G at 10 GHz), and has a Curie temperature over 600 K with sharp hysteresis switching to full saturation at room temperature.
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 in direct contrast to the isotropic in-plane response of typical thin films. When different magnon modes excited in these structures are brought into resonance by varying the orientation of an in-plane magnetic field, we observe anticrossing behavior, indicating strong coherent coupling between excitations. Furthermore, micromagnetic simulations of the nanostructures faithfully reproduce the experimentally measured spectra, including spin-wave and other higher-order modes. We also study films of the V[TCNE]x analogs vanadium methyl tricyanoethylenecarboxylate (V[MeTCEC]x) and vanadium ethyl tricyanoethylene-carboxylate (V[ETCEC]x), which show a temperature-dependent switch in the easy axis from in-plane to out-of-plane, suggesting underlying effects beyond shape anisotropy and pointing toward the presence of an additional contribution to the uniaxial anisotropy.
These results introduce a new degree of freedom for organic-based magnetism and spintronics, allowing for the engineering of magnetic anisotropy in a material that exhibits both robust room-temperature magnetic order and the benefits of low cost, mechanical flexibility, and facile synthesis found in other organic materials.