Time-resolved Kerr microscopy is used to study the excitations of individual micron-scale ferromagnetic thin film elements in their remnant state. Thin (18 nm) square elements with edge dimensions between 1 and 10 µm form closure domain structures with 90 degree Néel walls between domains. We identify two classes of excitations in these systems.The first corresponds to precession of the magnetization about the local demagnetizing field in each quadrant, while the second excitation is localized in the domain walls. Two modes are also identified in ferromagnetic disks with thicknesses of 60 nm and diameters from 2 µm down to 500 nm. The equilibrium state of each disk is a vortex with a singularity at the center. As in the squares, the higher frequency mode is due to precession about the internal field, but in this case the lower frequency mode corresponds to gyrotropic motion of the entire vortex. These results demonstrate clearly the existence of well-defined excitations in inhomogeneously magnetized microstructures.

We have observed localized spin-wave modes in individual thin-film ferromagnetic wires using time-resolved Kerr microscopy as a micron-scale spectroscopic probe. The localization is due to the partial demagnetization of a wire when an external field is applied in the plane of the film and perpendicular to the long axis of the wire. Spatially-resolved spectra demonstrate the existence of distinct modes at the edges of a rectangular wire.Spectral images clearly show the crossover of the two edge modes into a single mode in low applied fields, in agreement with the results of micromagnetic simulations.

Time-resolved ferromagnetic resonance is used to study magnetic relaxation in epitaxial Fe1−xCox films grown on vicinal GaAs(100) surfaces. The magnetic free energy of these films is determined by a combination of a four-fold volume anisotropy and a two-fold anisotropy due to surface bonding and morphology. The response following a high bandwidth (∼10 GHz) magnetic field pulse is measured using the polar Kerr effect. Samples with transverse dimensions much larger than the spatial extent of the field pulse show simple free induction decays. The measured precession frequencies are in good agreement with a coherent rotation model in the case of films grown on (100) surfaces with little or no (<2°) miscut. The magnetic response of 20 μm diameter disks is more complicated, particularly for static fields along the hard direction. Long-lived features appear in the response, and single precession frequencies are no longer observed. Micromagnetic simulations indicate the critical role played by the combination of the anisotropy and demagnetizing fields in the patterned structures.

We have explored the dynamics of magnetization reversal asymmetry in exchange biased FeF 2 / Fe bilayers using subnanosecond time-resolved Kerr magnetometry. The data reveal an increase in the characteristic precession frequency with decreasing temperature, even above the Néel temperature of the antiferromagnet, which we interpret in terms of the previously observed anisotropy enhancement due to antiferromagnetic spin fluctuations. Below the Néel point the magnetization precession is strongly suppressed due to the damping provided by exchange coupling to the antiferromagnetic layer. Dynamic hysteresis loops measured at a fixed delay between the magnetic field pulse and the optical probe pulse reveal distinct reversal asymmetry that is not observed in the corresponding static loops. The asymmetry takes the form of a suppression of the Kerr rotation signal in the part of the hysteresis loop where nucleation of reverse domains is energetically favorable. The formation of reverse domains prevents the magnetization from rotating coherently on nanosecond time scales. The temperature dependence of this dynamic asymmetry is found to be nonmonotonic and appears to be correlated with the coercivity.

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