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#### Fachbereich / Organisatorische Einheit

- Fachbereich Physik (262) (entfernen)

An experimental study of spin wave quantization in arrays of micron size magnetic Ni80Fe20 wires by means of Brillouin light scattering spectroscopy is reported. Dipolar-dominated Damon-Eshbach spin wave modes laterally quantized in a single wire with quantized wavevector values determined by the width of the wire are studied. The frequency splitting between quantized modes, which decreases with increasing mode number, depends on the wire sizes and is up to 1.5 GHz. The transferred wavevector interval, where each mode is observed, is calculated using a light scattering theory for confined geometries. The frequen-cies of the modes are calculated, taking into account finite size effects. The results of the calculations are in a good agreement with the experimental data.

Collisions of Spin Wave Envelope Solitons and Self-Focused Spin Wave Packets in Magnetic Films
(1999)

Head-on collisions between two-dimensional self-focused spin wave packets and between quasi-one-dimensional spin wave envelope solitons have been directly observed for the first time in yttrium-iron garnet (YIG) films by means of a space- and time-resolved Brillouin light scattering technique. We show that quasi-one-dimensional envelope solitons formed in narrow film strips ("waveguides") retain their shapes after collision, while the two-dimensional self-focused spin wave packets formed in wide YIG films are destroyed in collision.

High frequency switching of single domain, uniaxial magnetic particles is discussed in terms of transition rates controlled by a small transverse bias field. It is shown that fast switching times can be achieved using bias fields an order of magnitude smaller than the effective anisotropy field. Analytical expressions for the switching time are derived in special cases and general configurations of practical interest are examined using numerical simulations.

We present detailed studies of the enhanced coercivity of exchange-bias bilayer Fe/MnPd, both experimentally and theoretically. We have demonstrated that the existence of large higher-order anisotropies due to exchange coupling between different Fe and MnPd layers can account for the large increase of coercivity in Fe/MnPd system. The linear dependence of coercivity on inverse Fe thickness are well explained by a phenomenological model by introducing higher-order anisotropy terms into the total free energy of the system.

Static and dynamic properties of patterned magnetic permalloy films are investigated. In square lattices of circular shaped permalloy dots an anisotropic coupling mechanism has been found, which is identified as being due to intrinsically unsaturated parts of the dots caused by spatial variations of demagnetizing field. In arrays of magnetic wires a quantization of the surface spin wave mode in several dispersionless modes is observed and quantitatively described. For large wavevectors the frequency separation between the modes becomes smaller and the frequencies converge to the dispersion of the dipole-exchange surface mode of a continuous film.

Annual Report 1997
(1998)

Annual Report 1998
(1999)

We report on the exchange bias effect as a function of the in-plane direction of the applied field in two-fold symmetric, epitaxial Ni80Fe20/Fe50Mn50 bilayers grown on Cu(110) single crystal substrates. An enhancement of the exchange bias field, Heb, up to a factor of two is observed if the external field is nearly, but not fully aligned perpendicular to the symmetry direction of the exchange bias field. From the measurement of the ex-change bias field as a function of the in-plane angle of the applied field, the unidirectional, uniaxial and four-fold anisotropy contributions are determined with high precision. The symmetry direction of the unidirec-tional anisotropy switches with increasing NiFe thickness from [110] to [001].

Epitaxial growth of metastable Pd(001) at high deposition temperatures up to a critical thickness of 6 monolayers on bcc-Fe(001) is reported, the critical thickness being depending dramatically on the deposition temperature. For larger thicknesses the Pd film undergoes a roughening transition with strain relaxation by forming a top polycrystalline layer. These results allow to make a correlation between previ-ously reported unusual magnetic properties of Fe/Pd double layers and the crystallographic structure of the Pd overlayer.

An overview of the current status of the study of spin wave excitations in arrays of magnetic dots and wires is given. We describe both the status of theory and recent inelastic light scattering experiments addressing the three most important issues: the modification of magnetic properties by patterning due to shape aniso-tropies, anisotropic coupling between magnetic islands, and the quantization of spin waves due to the in-plane confinement of spin waves in islands.