Role of Grain Size on Magnon and Phonon Thermal Transport in the Spin Ladder Compound Ca9La5Cu24O41

Abstract

The antiferromagnetic spin ladder compound Ca9La5Cu24O41 is a promising material for thermal management applications due to its largest magnon thermal conductivity near room temperature among all magnetic materials. It remains elusive how boundary and defect scattering processes affect the magnon and phonon thermal transport in this material. Here, we report the thermal transport investigation of Ca9La5Cu24O41 polycrystals with different grain sizes as compared with single crystals. The thermal conductivity measurements reveal the contrasting role of size effects on magnon and phonon transport. As the average grain size decreases from 4.2 to 0.83 μm, the lattice thermal conductivity of polycrystalline Ca9La5Cu24O41 below about 100 K is significantly suppressed, due to the enhanced phonon-boundary scattering. In comparison, the magnon thermal transport is less affected by grain size. According to kinetic model analysis, the magnon mean free paths of two polycrystals are found to be much smaller than the single-crystal values, which is attributed to the magnon-defect scattering instead of magnon-boundary scattering. The obtained magnon mean free path of about 65 Å near 100 K is comparable to the lattice constant of the unit cell along the spin ladder direction, suggesting possible magnon localization in the disordered polycrystalline samples. These results offer useful insights into the development of magnetic materials for spin caloritronic and thermal management applications.