  We observe that an rf microwave field strongly influences the transport of
incoherent thermal magnons in yttrium iron garnet. Ferromagnetic resonance in
the nonlinear regime suppresses thermal magnon transport by 95%. The transport
is also modulated at non-resonant conditions in two cases, both related to the
magnon band minimum. Firstly, a strong enhancement of the nonlocal signal
appears at a static magnetic field below the resonance condition. This increase
only occurs at one field polarity and can be as large as 800%. We attribute
this effect to magnon kinetic processes, which give rise to band-minimum
magnons and high-energy chiral surface modes. Secondly, the signal increases at
a static field above the resonance condition, where the rf frequency coincides
with the magnon band minimum. Our study gives insight into the interplay
between coherent and incoherent spin dynamics: The rf field modifies the
occupation of relevant magnon states and, via kinetic processes, the magnon
spin transport.
