  Femtosecond light-induced phase transitions between different macroscopic
orders provide the possibility to tune the functional properties of condensed
matter on ultrafast timescales. In first-order phase transitions, transient
non-equilibrium phases and inherent phase coexistence often preclude
non-ambiguous detection of transition precursors and their temporal onset.
Here, we present a study combining time-resolved photoelectron spectroscopy and
ab-initio electron dynamics calculations elucidating the transient
subpicosecond processes governing the photoinduced generation of ferromagnetic
order in antiferromagnetic FeRh. The transient photoemission spectra are
accounted for by assuming that not only the occupation of electronic states is
modified during the photoexcitation process. Instead, the photo-generated
non-thermal distribution of electrons modifies the electronic band structure.
The ferromagnetic phase of FeRh, characterized by a minority band near the
Fermi energy, is established 350+- 30 fs after the laser excitation. Ab-initio
calculations indicate that the phase transition is initiated by a photoinduced
Rh-to-Fe charge transfer.
