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Differences in the magnon diffusion length for electrically and thermally driven magnon currents in Y$_3$Fe$_5$O$_{12}$

Recent demonstration of efficient transport and manipulation of spin information by magnon currents have opened exciting prospects for processing information in devices. Magnon currents can be driven both electrically and thermally, even in magnetic insulators, by applying charge currents in an adjacent metal layer. Earlier reports in thin yttrium iron garnet (YIG) films suggested that the diffusion length of magnons is independent on the biasing method, but different values were obtained in thicker films. Here, we study the magnon diffusion length for electrically and thermally driven magnon currents in the linear regime in a 2-$μ$m-thick YIG film as a function of temperature and magnetic field. Our results show a decrease of the magnon diffusion length with magnetic field for both biasing methods and at all temperatures from 5 to 300 K, indicating that sub-thermal magnons dominate the long-range transport. Moreover, we demonstrate that the value of the magnon diffusion length depends on the driving mechanism, suggesting that different non-equilibrium magnon distributions are biased for each method. Finally, we demonstrate that the magnon diffusion length for thermally driven magnon currents is independent of the YIG thickness and material growth conditions, confirming that this quantity is an intrinsic parameter of YIG.