  One of the main goals of spintronics is to improve transport of information
carriers and to achieve new functionalities with ultra-low dissipation. A most
promising strategy for this holy grail is to use pure magnon currents created
and transported in insulating magnets, in the complete absence of any
conducting metallic elements. Here we propose a realistic solution to this
fundamental challenge by analyzing magnon and heat transport in insulating
ferromagnetic junctions. We calculate all transport coefficients for magnon
transport and establish Onsager relations between them. We theoretically
discover that magnon transport in junctions has a universal behavior, i.e. is
independent of material parameters, and establish a magnon analog of the
celebrated Wiedemann-Franz law which governs charge transport at low
temperatures. We calculate the Seebeck and Peltier coefficients which are
crucial quantities for spin caloritronics and demonstrate that they assume
universal values in the low temperature limit. Finally, we show that our
predictions are within experimental reach with current device and measurement
technologies.
