Photoexcitation of antiferromagnetic NiO leads to ultrafast reorientation of Ni2+ spins due to change of the magnetic anisotropy. Recovery of the magnetic ground state occurs as coherent oscillation of the antiferromagnetic order parameter between hard- and easy-axis states manifesting itself as quantum beating. The coherence time is approximately 1 ns with the beating frequency being determined by the anisotropy energy.

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 Pr1−xCaxMnO3 optical pumping triggers a transition from an insulating AFM to a conducting metallic state within 230 fs.

The spin and lattice dynamics of antiferromagnetic bulk Cr 2 O 3 after excitation with an intense laser pulse were investigated using amplitude and phase resolved optical second-harmonic generation as probe of the magnetic and crystallographic subsystems. Demagnetization occurs on three time scales ranging from Ӷ1 to ϳ 7 ps. In comparison to ferromagnetic compounds, this is a two orders of magnitude faster response and an additional spin-lattice interaction channel is observed.

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Coherent control of the antiferromagnetic ͑AFM͒ order parameter of NiO is achieved by sequences of optical pump pulses modifying the magnetic anisotropy. The modification allows reversible switching of the AFM order parameter between hard-and easy-axis states. The switching frequency is determined by the magnetic anisotropy only and can therefore be higher than in the case of ferromagnetic spin precession. For NiO frequencies up to 100 GHz are achieved. Possibilities for device application are discussed.