Non-thermal electrons open the non-equilibrium pathway of the phase transition in FeRh

The optical excitation of metals initially creates short-lived non-Fermi distributions of the electrons. The electrons and holes excited far above and below the Fermi level quickly relax to hot Fermi-distributions that subsequently cool via electron-phonon scattering. Here, we show that such non-thermal charge carriers beyond the Fermi-distribution speed up the prototypical first-order antiferromagnetic-to-ferromagnetic phase transition in FeRh. In ultrafast x-ray diffraction experiments, we vary the maximum electron temperature by increasing the pump pulse duration up to 10 ps. For direct optical excitation of FeRh, ferromagnetic domains nucleate within 8 ps as soon as the successively deposited energy surpasses the site-specific threshold energy. In contrast, suppressing the direct optical excitation by an optically opaque Pt layer leads to a nucleation on a 50 ps timescale driven by the near-equilibrium heat transport. These findings unambiguously identify the photo-excitation of non-thermal electrons and not electron-phonon non-equilibria to enable the rapid phase transition in FeRh.