  When an ordered spin system of a given dimensionality undergoes a second
order phase transition the dependence of the order parameter i.e. magnetization
on temperature can be well-described by thermal excitations of elementary
collective spin excitations (magnons). However, the behavior of magnons
themselves, as a function of temperature and across the transition temperature
TC, is an unknown issue. Utilizing spin-polarized high resolution electron
energy loss spectroscopy we monitor the high-energy (terahertz) magnons,
excited in an ultrathin ferromagnet, as a function of temperature. We show that
the magnons' energy and lifetime decrease with temperature. The
temperature-induced renormalization of the magnons' energy and lifetime depends
on the wave vector. We provide quantitative results on the temperature-induced
damping and discuss the possible mechanism e.g., multi-magnon scattering. A
careful investigation of physical quantities determining the magnons'
propagation indicates that terahertz magnons sustain their propagating
character even at temperatures far above TC.
