Organic Free Radicals Demonstrate Long-Range Propagating Paramagnon-Polaritons Beyond Curie Temperatures

The behaviour of magnetism in materials typically falls into distinct categories, but scientists are now challenging these boundaries, revealing unexpected long-range coherence in organic free radicals. Sebastian Knauer, Roman Verba, and Rostyslav O. Serha, all from the University of Vienna, alongside colleagues including Denys Slobodianiuk and Andreas Ney, demonstrate that the organic free radical 2,2,6,6-tetramethylpiperidin-1-oxyl maintains this coherence even above the temperature at which long-range magnetic order is usually lost. Their all-electrical propagating spin-wave spectroscopy reveals coherently excited low-energy paramagnon-polaritons travelling at supersonic speeds exceeding 3,000 metres per second, and propagating over significant distances. This discovery integrates organic materials with the field of spintronics, potentially paving the way for advances in organic electronics and high-density information storage technologies.

Materials are commonly distinguished by their magnetic response into diamagnetic, paramagnetic, and magnetically ordered phases. Diamagnets and paramagnets lack long-range magnetic order, whereas ordered magnets develop this order below a specific temperature and support spin-wave excitations called magnons. Magnons are attracting attention for applications in radio-frequency technologies, computation, magneto-optics, and fundamental quantum physics.

TEMPO Paramagnon-Polariton Propagation at Millikelvin Temperatures

Researchers have investigated 2,2,6,6-Tetramethylpiperidin-1-oxyl (TEMPO), a stable free radical, as a material for propagating hybrid excitations called paramagnon-polaritons at extremely low temperatures, down to 16 millikelvin. This work combines detailed experimental measurements with theoretical modelling to understand the underlying physics and demonstrates the propagation of these excitations, potentially paving the way for novel spintronic devices. The research focuses on TEMPO, an unusual choice as most studies utilise traditional magnetic materials. Experiments conducted at extremely low temperatures revealed that paramagnon-polaritons, arising from the coupling of spin waves with electromagnetic waves in a paramagnetic state, can propagate over measurable distances within the TEMPO sample.

The team characterised the relationship between frequency and wavevector, observing how this relationship changes with temperature, suggesting TEMPO could be a viable material for developing new spintronic devices offering advantages in miniaturisation and energy efficiency. The team prepared TEMPO by melting it and filling it into a capillary tube, which was then mounted on a circuit board with microwave antennas. They used Electron Paramagnetic Resonance and Vibrating Sample Magnetometry to characterise the magnetic properties and magnetisation of TEMPO. Cryogenic Propagating Spin Wave Spectroscopy was used to observe and characterise the propagation of paramagnon-polaritons, involving cooling the sample with a dilution refrigerator and measuring microwave signal transmission and reflection with a vector network analyser.

Supersonic Spin-Wave Propagation in Organic Radical

Scientists have demonstrated coherent spin-wave propagation in the organic free radical 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO), even at temperatures above its Néel temperature. This achievement integrates organic materials with spintronics, revealing that long-range coherence is preserved in TEMPO, enabling the observation of coherently excited low-energy paramagnon-polaritons propagating at supersonic velocities exceeding 100 kilometres per second. Measurements at 85 millikelvin demonstrate a shift with increasing external magnetic field, confirming the magnetic nature of the signal and following the expected Zeeman relation. The amplitude of the signal approximately doubles with increasing field, attributable to the larger effective magnetisation. Below 400 millitesla, the signal becomes undetectable, consistent with insufficient polarisation for coherent transport. Time-resolved measurements reveal a propagation time of approximately 40 nanoseconds over a 4-millimeter distance, confirming the supersonic velocity of the spin waves.

Supersonic Coherence in Paramagnetic Organic Radicals

This research demonstrates the preservation of long-range coherence in organic free radicals, specifically 2,2,6,6-tetramethylpiperidin-1-oxyl, even at temperatures above its Néel temperature. By employing all-electrical propagating spin-wave spectroscopy, scientists observed coherently excited low-energy paramagnon-polaritons propagating at supersonic velocities exceeding 100 kilometres per second. These findings challenge the conventional understanding that long-range magnetic order is lost above the Néel temperature, revealing a previously unobserved state of coherent spin propagation in a paramagnetic material. The team confirmed these observations through both continuous-frequency measurements and time-resolved experiments, establishing a clear link between the observed signal and actual wave propagation.

Analysis of the data, including inverse fast Fourier transforms and pulse measurements, consistently indicated group velocities in the range of 100 to 440 kilometres per second. Theoretical calculations of paramagnon dispersion support these experimental findings, suggesting such high velocities are indeed possible within the material. This work integrates organic materials with spintronics, opening avenues for advancements in organic electronics and dense information storage technologies, and establishes a new understanding of coherent spin dynamics in paramagnetic systems.