We present measurements of thermal and electrical conductivity of polycrystalline permalloy (Ni-Fe), aluminum, copper, cobalt, and nickel thin films with thickness $<200$ nm. A micromachined silicon-nitride membrane thermal-isolation platform allows measurements of both transport properties on a single film and an accurate probe of the Wiedemann–Franz (WF) law expected to relate the two. Through careful elimination of possible effects of surface scattering of phonons in the supporting membrane, we find excellent agreement with WF in a thin Ni-Fe film over nearly the entire temperature range from 77 to 325 K. All other materials studied here deviate somewhat from the WF prediction of electronic thermal conductivity with a Lorenz number, $L$, suppressed from the free-electron value by $10\%\text{to}20\%$. For Al and Cu we compare the results to predictions of the theoretical expression for the Lorenz number as a function of $T$. This comparison indicates two different types of deviation from expected behavior. In the Cu film, a higher than expected $L$ at lower $T$ indicates an additional thermal conduction mechanism, while at higher $T$ lower than expected values suggests an additional inelastic scattering mechanism for electrons. We suggest the additional low-$TL$ indicates a phonon contribution to thermal conductivity and consider increased electron-phonon scattering at grain boundaries or surfaces to explain the high-$T$ reduction in $L$.

• Received 13 October 2014
• Revised 10 September 2015

DOI:https://doi.org/10.1103/PhysRevB.92.214410