  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-$\mu$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.
