  Ultrafast strengthening or quenching of the ferromagnetic order of
semiconducting Eu1-xGdxO was achieved by resonant photoexcitation. The
modification of the magnetic order is established within 3 ps as revealed by
optical second harmonic generation. A theoretical analysis shows that the
response is determined by the interplay of chemically and optically generated
carriers in a nonequilibrium scenario beyond the three-temperature model.
General criteria for the design of spintronics materials with tunable ultrafast
spin dynamics are given.
