Size-dependent magnon thermal transport in a nanostructured quantum magnet

Magnons are fundamental excitations of spin waves in magnetic materials, which are able to carry both heat and spin information. Examining magnon thermal transport is essential for advancing energy-efficient spintronics and novel energy conversion technologies. Unlike the well-established thermal properties of phonons and electrons, the size effect on magnon thermal transport remains elusive. Here, we quantitatively measure the role of grain size on magnon thermal transport in the nanostructured quantum magnet La2CuO4. Samples with controlled grain sizes are synthesized using a bottom-up method. The measured magnon thermal conductivity decreases as the grain size is reduced. Analysis using a two-dimensional magnon transport kinetic model, combined with optical and electrical measurements, suggests that magnon-boundary scattering effectively restricts the magnon mean free path, suppressing magnon thermal conductivity. These results offer valuable insights into the prediction and control of magnon transport at the nanoscale, especially for applications in quantum information processing and energy conversion.