Seth Kurfman1 Yuanhua Zheng2 Andrew Franson1 Brandi Wooten2 Michael Chilcote1 Kristen Buchanan3 Joseph Heremans2 1 Ezekiel Johnston-Halperin1

1, Department of Physics, The Ohio State University, Columbus, Ohio, United States
2, Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, Ohio, United States
3, Department of Physics, Colorado State University, Fort Collins, Colorado, United States

The study of quantum 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 the nearly universal reliance on yttrium iron garnet (YIG), which for half a century has reigned as the unchallenged leader in high-Q, low-loss magnetic resonance, and more recently in the exploration of coherent quantum coupling between magnonic and spin or superconducting degrees of freedom. Surprisingly, the organic-based, ferrimagnetic coordination compound vanadium tetracyanoethylene (V[TCNE]x~2) (Tc > 600 K) has recently emerged as a compelling alternative to YIG. Since V[TCNE]x~2 is deposited via chemical vapor deposition (CVD) at 50° C, it can be conformally deposited on a wide variety of substrates with Q rivaling the very best YIG films, which must be grown epitaxially on GGG substrates at temperatures over 800° C. Recent studies on V[TCNE]x~2 reveal ultra-low damping (α = 7.96 × 10-5) in thin films, as well as the ability to pattern microstructures (via standard lithographic techniques) exhibiting high-Q resonances (Q ~ 8000) with narrow peak-to-peak FMR linewidths (0.5 Oe at 9.86 GHz), making V[TCNE]x~2 an exciting candidate material for studying quantum coherent magnonic interactions. In that context, a deeper understanding of the magnonic properties of V[TCNE]x~2 has the potential to substantially impact key applications in coherent magnonics. Here, we present experimental measurements of the longitudinal spin Seebeck effect (LSSE) in V[TCNE]x~2 thin films, with spin Seebeck resistance (SSR) comparable to the very best YIG/Pt heterostructures. Further, these spin-thermal measurements provide insight into magnonic properties of V[TCNE]x~2, such as the magnon density of states, magnon spin diffusion length, and magnon-phonon coupling. These properties are central to the design and fabrication of future generations of quantum coherent devices, and will help to guide the development of this emerging class of highly coherent magnetic materials.