Thermal transport of the frustrated spin-chain mineral linarite: Magnetic heat transport and strong spin-phonon scattering*

The mineral linarite is a prototype frustrated spin-1/2 chain compound with competing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions with magnetic ordering below T_N=2.8 K in a mutliferroic elliptical spin-spiral ground state. Upon the application of a magnetic field along the spin-chain direction, distinct magnetically ordered phases can be induced. We report the thermal conductivity κ of this material across the magnetic phase diagram as well as in the paramagnetic regime. We found that in linarite the heat is carried mainly by phonons but shows a peculiar non-monotonic behavior in field. In particular, κ is highly suppressed at the magnetic phase boundaries, indicative of strong scattering of the phonons off critical magnetic fluctuations. Even at temperatures far above the magnetically ordered phases, this leads to a reduction of the phononic thermal conductivity. The mean free path due to spin-phonon scattering (l_spin-phonon) was determined as function of temperature. A power law behavior was observed mainly above 0.5 K indicating the thermal activation of spin fluctuations. In the critical regime close to the saturation field, l_spin-phonon shows a 1/T dependence. Furthermore, a magnon thermal transport channel was verified in the helical magnetic phase. We estimate a magnon mean free path which corresponds to about 1000 lattice spacings.