Ultrafast magnetization Dynamics of Antiferromagntic NiO

The dynamics of antiferromagnetic (AFM) compounds is substantially different from that of ferromagnets: Magnetic switching is facilitated by the absence of a macroscopic magnetic moment, so that spin reversal within 10-100~fs was predicted, and the discreet states of AFM dielectrics can lead to long coherence times. We investigated the magnetization dynamics of the AFM exchange-bias compound NiO by optical pump/probe experiments using second harmonic generation as probe of the AFM order. A photoinduced ultrafast reorientation of the Ni$^{2+}$ spins within 100~fs was observed and detected by quantum beating between the hard- and easy-axis state of the crystal. The lifetime of the photoinduced state is 1~ns and limited by spin-lattice relaxation. However, a second laser pulse can trigger an ultrafast return of the system into the ground state so that controlled switching of an AFM order parameter was demonstrated for the first time. [Phys.\ Rev.\ Lett.\ {\bf 93}, 117402 (2004)]