We report on transient reflectivity and magneto-optical Kerr measurements from the colossal magnetoresistance compounds La0.67Ca0.33MnO3 (LCMO), La0.67Sr0.33MnO3 (LSMO), and Sr2FeMoO6 (SFMO) as a function of temperature and magnetic field. In LCMO and LSMO, an unusually slow (∼1 μs) carrier relaxation component is revealed in the transient reflectivity traces. The component disappears as the transition temperature is approached from below. This slow decay process is attributed to spin-lattice relaxation of carriers in localized states and shows a close relationship with the spectral weight near the Fermi surface. The attribution is further supported by our pump-probe magneto-optical Kerr measurements. In addition to the clear observation of magnetic precessions, a long-lived exponentially decaying background reflects the spin-related relaxation of photoexcited electrons. In contrast to manganites, the temperature dependence of transient reflectivity is negligible, although there is a significant change in transient Kerr rotation in SFMO. Our results show that the dynamics of charge, spin, and lattice are strongly correlated with each other in the manganites, but the spin degree of freedom is thermally insulated from the electron and lattice systems.

Photoinduced spin waves are investigated for Fe films on AlGaAs (001) using the time resolved magneto‐optical Kerr effect. We observe first and second order standing spin waves as well as uniform magnetization precession. The anisotropy fields and exchange stiffness constant are obtained from the field and azimuthal dependence of the spin wave frequencies using the Landau‐Lifshitz‐Gilbert equation. A comparison with spin wave resonance measurements reveals a more pronounced interface effect on the low order standing spin waves in thin films. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoinduced magnetization dynamics is investigated in chemically ordered (LaMnO3)2n/(SrMnO3)n superlattices using the time-resolved magneto-optic Kerr effect. A monotonic frequency-field dependence is observed for the n=1 superlattice, indicating a single spin population consistent with a homogeneous hole distribution. In contrast, for n≥2 superlattices, a large precession frequency is observed at low fields indicating the presence of an exchange torque in the dynamic regime. We attribute the emergence of exchange torque to the coupling between two spin populations—viscous and fast spins.

We report on observation of coherent longitudinal acoustic phonons in Fe/AlGaAs (001) heterostructure by time-resolved second-harmonic generation (TRSHG). The phonon induces not only a fast and highly damped oscillatory signal but also a long-lived one that exhibits an oscillating period identical to that obtained from the reflectivity measurement. The long-lived oscillatory second-harmonic generation is generated by an electric field modulated at 42.5 GHz because of the interference of the incident beam and the reflected light by the propagating phonon. TRSHG lets us probe the internal field modulated by phonons and differentiate their impacts at the interface and in the bulk.

Pronounced spin precessions are observed in a geometry with negligible canting of the magnetization in ferromagnetic La0.67Ca0.33MnO3 thin films using the time-resolved magneto-optical Kerr effect. The precession amplitude monotonically decreases with increasing field, indicating that the coherent spin rotation may be triggered by a transient exchange field and not by demagnetization and/or anisotropy field modulation. We attribute the transient exchange field to emergent antiferromagnetic interactions due to charge transfer and modification of the kinetic energy of eg electrons under optical excitation.