Spin waves and their quanta, magnons, are the wave-like excitations of a magnetically ordered medium. The technological prospect of utilizing them as low-loss information carriers has driven various research efforts in the field of magnonics. Spin waves arise further interest due to their inherently strong nonlinear behavior which results from their interaction with the surrounding magnetic texture. Hence, magnons are subject to a variety of nonlinear effects and allow for extensive studies of such phenomena. In this work, the propagating spin-waves in micro structured Co25Fe75 conduits have been investigated by means of micro focused Brillouin light scattering spectroscopy. Due to the low intrinsic damping of this metallic compound, spin-wave decay lengths in the order of 20 μm can be observed which have not been reported elsewhere for other ferromagnet thin film materials. Furthermore, nonlinear four-magnon scattering processes can be observed when increasing the spin-wave amplitudes applying a sufficiently strong microwave excitation. This phenomenon introduces additional losses for propagating waves as it diverts energy into the parametric generation of secondary states. It is shown that the reduction of the spin-wave decay lengths reaches up to 50 %. In the second part, a novel approach for the utilization of four-magnon scattering is presented. It is shown that an additional driving signal at a secondary driving frequency can steer the nonlinear process in such a way, that a set of secondary parametric states with a well-defined frequency spacing is populated. This process is referred to as stimulated four-magnon scattering, as it enhances specific nonlinear scattering events. As a result, frequency combs with multiple equidistant modes are observed, which exhibit frequency spacings of 400 MHz up to 2 GHz. These complex spin-wave spectra can actively be tuned in various ways using external parameters such as the driving signals. These results advance the understanding of nonlinear spin waves in general and expands the range of possible technological applications of magnons.:List of Figures
List of Tables
List of Abbreviations and Acronyms
List of Symbols
1 Introduction
2 Theoretical background
2.1 Interactions in microstructured thin film ferromagnets
2.1.1 Exchange interaction
2.1.2 Dipolar interaction and demagnetizing fields
2.2 Magnetization dynamics in ferromagnetic thin films
2.2.1 The Landau-Lifshitz and Gilbert equation
2.2.2 Spin waves
2.3 Nonlinear phenomena
2.3.1 Four-magnon scattering
3 Materials and Methods
3.1 Materials and sample fabrication
3.1.1 The low damping alloy Co25Fe75
3.1.2 Patterning: electron beam and optical lithography
3.1.3 Microwave antenna structures
3.2 Brillouin light scattering
3.2.1 Magnon-photon interaction
3.2.2 The Tandem Fabry Pérot interferometer
3.2.3 BLS microscopy (μBLS)
3.2.4 Phase-resolved BLS (PR-μBLS)
3.2.5 Temporal resolution (TR-μBLS)
3.3 Micromagnetic simulations in MuMax3
3.3.1 Mesh and material parameters
3.3.2 Simulation of magnetization dynamics
4 Results
4.1 Magnon transport in Co25Fe75 micro-conduits
4.1.1 Low external fields and magnetic groundstate
4.1.2 Magnon transport at low driving powers
4.1.3 Impact of nonlinear four-magnon scattering on magnon transport
4.2 Magnon frequency combs
4.2.1 Introduction on stimulated four-magnon scattering
4.2.2 Experimental realization
4.2.3 Amplitude-dependent observations
4.2.4 Tunability of spin-wave frequency combs
4.2.5 Variations of the excitation geometry
5 Summary and outlook
Own publications
Bibliography 109 Acknowledgement
A Appendix
A.1 Fabrication of Co25Fe75 microstructures
A.2 Atomic Force Microscopy measurement on a 5 μm wide conduit
A.3 BLS measurement of spin-wave decay lengths in a 5 μm wide conduit
A.4 Calculations: Temporal profile of stimulated four-magnon scattering
A.5 Power dependent frequency comb formation measured at positions II & III
A.6 Averaged frequency comb mode numbers at reversed magnetic field polarity
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:92082 |
| Date | 07 August 2024 |
| Creators | Hula, Tobias |
| Contributors | Hellwig, Olav, Fassbender, Jürgen, Schultheiss, Helmut, Technische Universität Chemnitz, Helmholtz-Zentrum Dresden-Rossendorf e.V. |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/acceptedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | 10.14278/rodare.2178 |
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