  The strong spin-spin exchange interaction in some low-dimensional magnetic
materials can give rise to a high group velocity and thermal conductivity
contribution from magnons. One example is the incommensurate layered compounds
(Sr,Ca,La)14Cu24O41. The effects of grain boundaries and defects on
quasi-one-dimensional magnon transport in these compounds are not well
understood. Here we report the microstructures and anisotropic thermal
transport properties of textured Sr14Cu24O41, which are prepared by solid-state
reaction followed by spark plasma sintering. Transmission electron microscopy
clearly reveals nano-layered grains and the presence of dislocations and planar
defects. The thermal conductivity contribution and mean free paths of magnons
in the textured samples are evaluated with the use of a kinetic model for
one-dimensional magnon transport, and found to be suppressed significantly as
compared to single crystals at low temperatures. The experimental results can
be explained by a one-dimensional magnon-defect scattering model, provided that
the magnon-grain boundary scattering mean free path in the anisotropic magnetic
structure is smaller than the average length of these nano-layers along the c
axis. The finding suggests low transmission coefficients for magnons across
grain boundaries.
