High-Q spin wave excitations in the organic-based ferrimagnet vanadium tetracyanoethylene*

The development of quantum magnonics relies implicitly on the ability to excite and exploit long lived spin wave excitations in a magnetic material. That requirement has led to the nearly universal reliance on yittrium iron garnet (YIG), which for half a century has reigned as the unchallenged leader in high-Q, low loss magnetic resonance and spin wave excitation despite extensive efforts to identify alternative materials. Surprisingly, the organic-based ferrimagnet vanadium tetracyanoethylene (V[TCNE]₂) has recently emerged as a compelling alternative to YIG. In contrast to other organic-based materials V[TCNE]₂ exhibits a Curie temperature of over 600 K with robust room temperature hysteresis with sharp switching to full saturation. Further, since V[TCNE]₂ is grown via chemical vapor deposition (CVD) at 50 C it can be conformally deposited as a thin film on a wide variety of substrates. Our recent work has exploited this potential to construct a microwave waveguide in which V[TCNE]₂ is deposited as a bridge across two coplanar waveguides, exhibiting standing wave spin-wave resonances with Q of over 3,200 under ambient conditions. This Q rivals the very best thin-film YIG devices, which must be grown epitaxially on GGG substrates at temperatures over 800 C. Work in preparation shows that this Q can be further enhanced by moving to the thick film geometry, which is well known to reduce surface scattering, yielding Qs that are competitive with polished YIG spheres. When added to the ease of patterning and integration afforded by the low temperature CVD deposition process, these results clearly demonstrate the potential for V[TCNE]₂ to play a major role in the development of long-lived coherent spin wave excitations in quantum magnonic devices.