Ultrafast magnetization dynamics of antiferromagnetic compounds

In three different antiferromagnetic (AFM) materials we show that the magnetization dynamics of AFM compounds differs noticeably from that of ferromagnetic compounds. Optical second harmonic generation and linear reflection were used to monitor with a temporal resolution of <1 ps the evolution of the AFM order parameter subsequent to an intense optical excitation. Comparison of the dynamical properties of the model antiferromagnet Cr₂O₃ with model ferro- and ferrimagnets reveals that spin-lattice relaxation as the temporally limiting thermalization process of the magnetic subsystem is more complex and inherently faster than in compounds displaying a spontaneous magnetization. The exchange-bias compound NiO exhibits an ultrafast photoinduced AFM phase transition in the course of which the AFM order parameter is switched by 90°. Properly timed sequences of pump pulses can repeatedly reorient the AFM order parameter within 10 ps, thus demonstrating ultrafast AFM switching. In the colossal-magnetoresistance compound Pr _1-xCa_xMnO₃ optical pumping triggers a transition from an insulating AFM to a conducting metallic state within 230 fs.