  Understanding the profound impact of correlation effects and crystal
imperfections is essential for an accurate description of solids. Here we study
the role of correlation, disorder, and multi-magnon processes in THz magnons.
Our findings reveal that a significant part of the electron self-energy, which
goes beyond the adiabatic local spin density approximation, arises from the
interaction between electrons and a virtual magnon gas. This interaction leads
to a substantial modification of the exchange splitting and a renormalization
of magnon energies, in agreement with the experimental data. We establish a
quantitative hierarchy of magnon relaxation processes based on first
principles.
