Magnon Heat Conduction and Magnon Scattering Processes in Fe-Ni Alloys

The thermal and electrical conductivities of 70-wt%-Ni-30-wt%-Fe, 81-wt%-Ni-19-wt%-Fe, and 67-wt%-Ni-33-wt%-Cu have been measured between 1.5 and 4.5 K, in external fields up to 6 T. Both nickel-iron alloys show a small magnon contribution κₘ to the thermal conduction, equal to about 3% of the total conductivity at 4 K. In 70-wt%-Ni-30-wt%-Fe κₘ∝T1.9 is found. Using a simple kinetic-theory model, the magnon lifetime τₘ is derived equal to 1.7×{}10^-10 sec at 4 K and varying as T^-0.6. This implies τₘ∝ω^-0.6. In the zero-magnetostriction alloy 81-wt%-Ni-19-wt%-Fe, κₘ∝T1.5 and τₘ∝T^-1∝ω^-1 is found to hold. Our lifetime values agree with those derived from magnetic resonance linewidths. The nickel-copper alloy shows no magnon heat transport, implying a magnon lifetime of less than 2×{}10^-11 sec at 3.55 K. Shorter magnon lifetime in Ni-Cu than in Ni-Fe may reflect faster magnon-electron relaxation, corresponding to increased smearing of the electron momentum gap by alloy disorder. The latter correlates, in turn, with larger electrical resistivity and smaller magnetization.