Ballistic propagation of magnons in MnF2

We present evidence of ballistic transport of energy by high energy, large wave vector magnons in the antiferromagnet MnF2. Non-equilibrium magnons are created in broad frequency bands by heat pulses or in narrow frequency bands, throughout the Brillouin zone, by pumping the far-infrared (FIR) two magnon absorption bands. The ballistic magnon signal is extracted from the total observed signal and used to calculate the magnon velocity as a function of magnon energy. The results compare favorably with theory. Non-ballistic signals are interpreted with respect to magnon decay processes.