Magnon heat conduction and magnon lifetimes in the metallic ferromagnetFe68Co32at low temperatures

The thermal and electrical conductivities of the alloy ${\mathrm{Fe}}_{68}$ ${\mathrm{Co}}_{32}$ have been measured with high accuracy between 1.2 and 4.5 K, in external fields up to 5.2 T. The results show that magnons contribute about 10% of the total heat conduction at 4 K. The magnon contribution ${\kappa }_m$ varies roughly like $T^{1.3}$ implying for the magnon lifetime ${\tau }_m\propto T^{-1.2\mathrm{}}\propto {\omega }^{-1.2\mathrm{}}$. This is not far from the prediction ${\tau }_m\propto {\omega }^{-1\mathrm{}}$ of the $s$-$d$ exchange theory of magnonelectron scattering, in the dirty limit ${\Lambda }_{e}q\ll 1$ where ${\Lambda }_e$ is the electronic mean free path and $q$ is the magnon wave number. The magnon lifetime value is ${\tau }_m\simeq 7\times {10}^{-10\mathrm{}}$ sec at 4 K, which is four times as large as for the nickel-rich Ni-Fe alloys investigated before. This large ${\tau }_m$ value is probably caused by the very small electronic density of states at the Fermi level. The effect of dipole-dipole interaction on the magnon dispersion curve is unusually important in ${\mathrm{Fe}}_{68}$ ${\mathrm{Co}}_{32}$, and is taken into account exactly.