Brillouin light scattering from spin waves in magnetic layers and multilayers

We give an overview on our experimental and theoretical investigations of Brillouin light scattering in magnetic thin films, layered magnetic structures and superlattices. For epitaxial Fe(1 10) layers on W(1 10) the in-plane and out-of-plane magnetic surface anisotropy constants are determined, and the influence of Pd overlayers on the surface anisotropies is studied. For Fe/Pd superlattices a magnetic polarization of the Pd at the interfaces is established and the interface anisotropy constant is determined. For second order Fe/Pd superlattices, formed by alternating two Fe/Pd bilayers with different repeat periods, the Brillouin spectrum is obtained and compared to calculations. In the case of magnetic/nonmagnetic multilayered structures we investigate theoretically the crossing regime between dipolar and exchange-dominated modes. For small spacer-layer thicknesses, interlayer exchange coupling shifts the spin-wave frequencies of all but the highest-frequency dipolar mode into the exchange-mode regime. In case of all-magnetic multilayered structures, such as Fe/Ni multilayers, a new type of propagating collective excitations arising from coupled exchange modes is predicted.     

Structural analysis of ion irradiated polycrystalline NiFe/FeMn exchange bias systems

Structural investigations of the ion irradiated polycrystalline NiFe/FeMn exchange bias bilayer system were carried out by means of transmission electron microscopy. Key structural parameters like average grain size, lattice constant, and texture, as well as their dependence upon ion irradiation were determined. This information was extracted from a detailed analysis of a series of bright field images, dark field images, and diffraction patterns. Furthermore a previously established model was tested which ascribes changes in the magnetic properties upon irradiation with 5 keV He+ ions to the creation of point defects within the antiferromagnetic layer and to the intermixing between the ferromagnetic and antiferromagnetic layers. The obtained results indirectly support this model by excluding a change of the aforementioned structural parameters as a possible source of the observed modifications of the magnetic properties.     

Control of interlayer exchange coupling in Fe/Cr/Fe trilayers by ion beam irradiation

The manipulation of the antiferromagnetic interlayer coupling in epitaxial Fe/Cr/Fe(001) trilayers by 5 keV He ion beam irradiation has been investigated. It is shown that even for irradiation with low fluences a drastic change in strength of the coupling appears. For thin Cr spacers (below 0.6-0.7 nm) it decreases with fluence, becoming ferromagnetic for fluences above 2x10(14) ions/cm(2). The effect is connected with the creation of magnetic bridges in the layered system due to atomic exchange events caused by the bombardment. For thicker Cr spacers an enhancement of the antiferromagnetic coupling strength is found. A possible explanation of the enhancement effect is given.     

Radiation of caustic beams from a collapsing bullet

We show both theoretically and experimentally that a collapsing (2+1)-dimensional wave packet in a medium with cubic nonlinearity and a two-dimensional dispersion of an order higher than parabolic irradiates untrapped dispersive waves. The studies are performed for a spin-wave bullet propagating in an in-plane magnetized ferrimagnetic film. An induced uniaxial anisotropy in such a medium leads to the formation of narrow spin-wave caustic beams whose angles to the bullet's propagation direction are modified by the motion of the source.     

Optical conductivity of CePd3, YPd3 and PrPd3

The optical conductivity of intermediate valence CePd₃ at 300 K exhibits a maximum near 16 meV, which is absent in the reference materials YPd₃ and PrPd₃. Using a modified Drude model with a memory function ansatz this anomaly has been identified as a resonant electron-electron scattering process of conduction electrons at the localized 4f states near the Fermi level E_F. The model fit gives estimates of the width of the 4f states, of their separation from E_F and of the f-d hybridization energy. Intra-4f transitions of CePd₃ are stronger compared to those of PrPd₃ due to the stronger f-d hybirdization. 4d→4f transition energies of CePd₃ are reduced due to an electron-hole Coulomb binding energy.     

Observation of coherence and partial decoherence of quantized spin waves in nanoscaled magnetic ring structures

Experiments and simulations are reported, which demonstrate the influence of partial decoherence of spin-wave modes on the dynamics in small magnetic structures. Microfocus Brillouin light scattering spectroscopy was performed on 15 nm thick Ni81Fe19 rings with diameters from 1 to 3 microm. For the so-called "onion" magnetization state several effects were identified. First, in the pole regions of the rings spin-wave wells are created due to the inhomogeneous internal field leading to spin-wave confinement. Second, in the regions in between, modes are observed which show a well pronounced quantization in radial direction but a transition from partial to full coherency in azimuthal direction as a function of decreasing ring size. In particular for larger rings a continuous frequency variation with position is observed which is well reproduced by spin-wave calculations and micromagnetic simulations.     

Oscillatory energy exchange between waves coupled by a dynamic artificial crystal

We describe a general mechanism of controllable energy exchange between waves propagating in a dynamic artificial crystal. We show that if a spatial periodicity is temporarily imposed on the transmission properties of a wave-carrying medium while a wave is inside, this wave is coupled to a secondary counterpropagating wave and energy oscillates between the two. The oscillation frequency is determined by the width of the spectral band gap created by the periodicity and the frequency difference between the coupled waves. The effect is demonstrated with spin waves in a dynamic magnonic crystal.