We investigate the heat transport through a rare earth multilayer system composed of Yttrium (Y), Dysprosium (Dy) and Niobium (Nb) by ultrafast X-ray diffraction. This is an example of a complex heat flow problem on the nanoscale, where several different quasi-particles carry the heat. The Bragg peak positions of each layer represent layer-specific thermometers that measure the energy flow through the sample after excitation of the Y top-layer with fs-laser pulses. In an experiment-based analytic solution to the nonequilibrium heat transport problem, we derive the individual contributions of the spins and the coupled electron-lattice system to the heat conduction. The full characterization of the spatiotemporal energy flow at different starting temperatures reveals that the spin excitations of antiferromagnetic Dy speed up the heat transport into the Dy layer at low temperatures, whereas the heat transport through this layer and further into the Y and Nb layers underneath is slowed down. The experimental findings are compared to the solution of the heat equation using macroscopic temperature-dependent material parameters without separation of spin-and phonon contributions to the heat. We explain, why the simulated energy density matches our experiment-based derivation of the heat transport, although the simulated thermoelastic strain in this simulation is not even in qualitative agreement.Heat transport at the nanoscale has become an important problem of contemporary physics.[1-3] The field is driven largely by the need to improve heat transport characteristics in integrated circuits operating at high clock rates.[4] The design length scales approach the physical limits, where wave fundamental properties of phonon-heat conduction play an important role. [5,6] Research on the functionality of interfaces in nanoelectronics is prevalent, and the heat transport characteristics of interfaces depend strongly e.g. on the roughness of the interface, which is often hard to control in the fabrication process [1,2,7]. In many insulators and semiconductors the heat capacity is dominated by phonons, whereas electrons only contribute significantly at high temperatures. The heat transport in metals in contrast is dominated by the conduction band electrons and the excitation of phonons typically reduces the heat transport, because they act as scatterers for electrons [8]. In some magnetic materials with strong exchange interactions and large magnetic moments the spin-correlations can contribute more than half of the specific heat over large temperature ranges [9][10][11]. One classical example is the rare earth Dysprosium, which we are investigating in this article. Similar to phonon excitations, the magnetic excitations are known to reduce the heat conductivity when it is dominated by the electrons. [12] On the other hand heat conduction by magnons may dominate in antiferromagnets.[13] The transport of heat across interfaces in nanostructures with magnetic and nonmagnetic layers is far beyond what can be safely simulated on an...