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978-3-8439-2539-6, Reihe Physik
Mapping and control of magnetic excitations in artificial lattices
155 Seiten, Dissertation Universität Hamburg (2016), Softcover, A5
The investigations in this thesis both in the time and the frequency domain provide an evaluation of spin dynamics on their typical sub-nanosecond time scale as well as the controllability of their excitation spectra in different types of magnonic crystals.
In the first part, ferromagnetic resonance in a thin magnetic film is probed by means of nuclear resonant scattering of synchrotron radiation at the 14.4 keV resonance of isotopic 57Fe. The precessional motion of the magnetic moments results in a reduction of the hyperfine field acting on the isotopic nuclei which can be observed in time spectra that describe the temporal evolution of the nuclear decay. Fits to the time spectra on the basis of a stochastic relaxation model allow for the precise mapping of the precession trajectory and the determination of the opening angle of the excited magnetic moments. The controllability of suchlike spin dynamics is studied in deterministic fractals by means of broadband-ferromagnetic transmission measurements and micromagnetic simulations. The strictly self-similar hierarchical modulation of deterministic fractals makes them to auspicious candidates for magnonic applications. Their spin-wave mode spectrum is found to be sensitive to the connectivity of the fractal sub-elements. Sierpinski carpets with a topology that guarantees a maximum connectivity of the sub-elements feature a distinct mode spectrum which can be tuned both by geometrical structuring as well as the homogeneity of the magnetization pattern via an external field. On the other hand, Sierpinski triangles with a vanishing connectivity feature a practically degenerate mode spectrum.
The last part of the thesis aims at the control of vortex-core polarizations by means of perpendicularly aligned bias fields. The bias fields are generated by dipolar stray fields of two different types of perpendicularly magnetized ferromagnetic disks. Magneto-optical Kerr microscopy shows that the bimodal switching distribution of the disks depends both on the field increment rate as well as the interdisk distance. Besides a homogeneous pattern, a stripe and a checkerboard pattern of antiparallel magnetized disks can be reliably adjusted and provide access to magnonic vortex crystals with a reprogrammable band structure via an external magnetic field. First measurements of the vortex-core polarizations by means of magnetic force microscopy as well as suggestions for an improved sample design are provided.