Photoinduced Femtosecond Formation of Ferromagnetism in a Strongly Correlated Antiferromagentic Manganite

There has been strong current interest to manipulate collective spins and even induce magnetic phase transitions in their highly \textit{non-equilibrium, non-thermal }states at \textit{femtosecond} time scales. Such processes offer opportunities to exceed the upper limit of the magnetic switching speed (0.1-10 GHz) in modern magneto-optical recording industry and magnetic storage/logic devices. One prominent system to explore such femtosecond magnetism is strongly correlated manganites, which are truly ``responsive'' near the phase boundary, exhibiting extreme sensitivity to external stimuli, such as light, electric and magnetic fields. Using ultrafast two-color magnetic circular dichroism spectroscopy, we have observed a substantial photoinduced magnetization enhancement in Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ within 180 fs above a threshold pump fluence and at low temperature. Such a photoinduced critical behavior vanishes at elevated temperature. These results clearly show a photoinduced ultrafast antiferromagnetic to ferromagnetic phase transition, demonstrating particularly, that one can reveal a hidden, thermally inaccessible ground state at fs time scales.