The magnetization dynamics of single-crystalline Fe͑001͒ thin films with Cr cap layers has been studied by an all-optical time-resolved pump-probe technique. The system is characterized by a fourfold in-plane magnetic anisotropy. We observed long-lived ͑ϳ1 ns͒ magnetization oscillations caused by the ultrafast ͑ϳ0.15 ps͒ optical pulse excitation. The oscillations are associated with the temporal variation of the magnetization component M z normal to the film surface. The phase of the oscillations is independent of the polarization state of the pump beam giving evidence for a predominantly thermal origin of the excitation. The amplitude of the oscillations considerably depends on the in-plane orientation and magnitude of the magnetic field. The azimuthal variation of the oscillation frequency at constant magnetic field follows the fourfold in-plane magnetic anisotropy. Angle and field variations of the frequency are well described by a uniform precession mode known from the theory of ferromagnetic resonance. Our analysis indicates that the precession amplitude is determined by the frequency of the uniform mode and an in-plane tilting of the effective magnetic field directly caused by the pumping light beam.

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We have studied the magnetic field dependences of magnetic optical second harmonic generation ͑SHG͒ in MBE-grown Fe/ Cr/ Fe/ Ag/ GaAs͑100͒ heterostructures displaying both bilinear and biquadratic interlayer exchange coupling. The magnetic field H was applied in the ͑100͒ surface plane along both easy ͓͑001͔͒ and hard ͓͑110͔͒ axes of the in-plane fourfold magnetic anisotropy. The SHG has been measured in reflection at near normal incidence for different polarization combinations ͑pp , ps , ss , sp͒ of the fundamental and second harmonic light in longitudinal and transversal geometries. The magnetic field variation of the SHG signal clearly reflects the field-induced transformations of the magnetic state at the interfaces in the trilayer. It strongly depends on the configuration of light polarization, experimental geometry ͑longitudinal or transversal͒, and orientation of the magnetic field H relative to the crystal axes. In contrast to linear magneto-optical Kerr effect, which is odd in magnetic field, magnetic SHG is either even in H or does not display a definite parity at all, depending on the polarization configuration. We interpret the data based on a model accounting for nonmagnetic and magnetic contributions to SHG from the surface and interfaces described by C 4v point symmetry. Taking into account the changes of the mutual orientation of interfacial magnetizations allows us to describe the general features of the measured field dependences of SHG.

The surface magnetic and structural properties of magnetite thin films have been probed by nonlinear second-harmonic generation optical method in high- (centrosymmetric, Oh) and low- (noncentrosymmetric, C1) temperature phases. A model taking into account the cubic magnetocrystalline anisotropy and magnetic symmetry reduction due to a magnetically modified surface layer is supposed to describe the azimuth variations of the nonlinear response. The metal-isolator transition (Verwey) manifests itself in an increase of nonlinear response (∼25%) and a decrease in magnetic contrast (∼50%).