  We present the temperature dependence of the thermal magnon density in a thin
ferromagnetic layer. By employing Brillouin light scattering and varying the
temperature, an increase of the magnon density accompanied by a lowering of the
spin-wave frequency is observed with increasing temperature. The magnon density
follows the temperature according to the Bose-Einstein distribution function
which leads to an approximately linear dependency. In addition, the influence
of this effect in spin-transfer-torque-driven systems is presented. In
particular, the increase in the magnon density with temperature sets the limit
for a suppression of magnons in charge current-driven systems. Hence, the
maximum possible suppression of thermal magnons occurs at a finite current.
