Nondiffusive Transport and Anisotropic Thermal Conductivity in High-Density Pt/Co Superlattices

Despite the numerous reports over the last two decades dedicated to the study of interfacial thermal transport, physics of thermal transport across nanoscale metallic multilayers is less explored. This is in part due to the relatively high conductance characteristic of these interfaces, which renders them difficult to characterize. Interfacial transport in these systems has so far appeared to be diffusive, a surprising behavior when the interface density increases and the layer thicknesses become comparable with the mean free path of electrons. To address the limit of diffusive theories describing heat transport across high-density metallic interfaces, we systematically investigate heat transport in and across Pt/Co multilayers via frequency domain thermoreflectance. Sensitivity gained from offsetting the laser beam and reducing the laser spot size allows for the measurement of anisotropic thermal conductivity of the multilayers. By changing the number of interfaces while keeping the overall thickness of Pt and Co in the multilayer structure constant, the effect of interface density on the multilayers' effective thermal conductivity is studied. The extracted Pt/Co interface thermal boundary conductance is then compared to the calculations from the electronic diffuse mismatch model and experimental data available in the literature. We show that as the multilayer period thickness becomes much smaller than the electron mean free path, measurements markedly deviate from the diffusive transport theory. We attribute this deviation to the nondiffusive nature of heat transport in subnanometric scales at interface densities above 1/nm.