Long-range propagating paramagnon-polaritons in organic free radicals

Abstract

Materials are commonly distinguished by their magnetic response into diamagnetic, paramagnetic, and magnetically ordered (ferro-, ferri-, and antiferromagnetic) phases. Diamagnets and paramagnets lack spontaneous long-range order, whereas ordered magnets develop such order below their Curie or N\'eel temperature and support single spin-wave excitations (magnons). Magnons have found applications in radio-frequency technologies and computation, magneto-optics, and foundational quantum experiments. Above the Curie/N\'eel temperature, long-range order is lost and the material transitions to a paramagnetic phase, with localised spin alignment in small patches, producing paramagnons with only short-range propagation. Here we show that long-range coherence is preserved in the organic free radical 2,2,6,6-tetramethylpiperidin-1-oxyl above the N\'eel temperature using all-electrical propagating spin-wave spectroscopy in external magnetic fields. We observe coherently excited low-energy paramagnon-polaritons up to $\mathbf{23\,\mathrm{GHz}}$ , propagating over $\mathbf{8\,\mathrm{mm}}$ at supersonic group velocities exceeding $\mathbf{100\,\mathrm{km\,s^{-1}}}$. Using free radicals as magnon carriers integrates organic materials with spintronics and opens the way to organic electronics, dense information storage, and quantum technologies.