Minority magnons and mode branching in monolayer Fe3GeTe2

We predict the presence of minority magnons in monolayer Fe3GeTe2 using first principles calculations.Minority magnons constitute a new type of collective magnetic excitation which increase the magnetic moment-contrary to ordinary (majority) magnons which lower it-giving rise to spinraising poles in the dynamic susceptibility χ -+ (ω).The presence of such quasi-particles is made possible by the nontrivial ferromagnetic band structure of Fe3GeTe2.We calculate the susceptibility using time-dependent density functional theory and perform a detailed mode analysis, which allows us to identify and investigate individual magnon modes as well as the Stoner excitations that constitute the many-body spectrum.The analysis reveals a plethora of both majority and minority excitations, which in addition to the main magnon branches include both satellite, valley and spininversion magnons thanks to the electron itinerancy of the system.Crucially, the analysis allows us to separate peaks of a coherent collective nature from those of mixed magnon/Stoner nature.To this end, we predict that the lowest energy minority magnon mode of monolayer Fe3GeTe2 indeed constitutes a coherent collective quasi-particle at long wavelengths and introduce a simplistic gaussian model for the observed minority magnon enhancement.The underlying physics is in no way restricted to Fe3GeTe2, and minority magnons are thus expected to be observable in other complex ferromagnetic materials as well.