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Disperzní relace magnonických krystalů s netriviální prostorovou distribucí magnetické anizotropie
Wojewoda, Ondřej ; Hamrle,, Jaroslav (oponent) ; Flajšman, Lukáš (vedoucí práce)
Magnonika je poměrně novým vědním oborem zabývajícím se spinovými vlnami, což jsou kolektivní excitace magnetizace. Základními stavebními prvky magnonických obvodů, které umožňují kontrolu nad disperzí spinových vln jsou magnonické krystaly. Jejich periodická struktura zapříčiní vznik komplexní pásové struktury s pásem zakázaných frekvencí. Periodické struktury lze klasicky dosáhnout modulací tloušťky materiálu nebo skokovou změnou saturační magnetizace. Předložená práce se zabývá teoretickým popisem disperzních relací magnonických krystalů, kde je periodicity systému dosahováno modulací směru uniaxiální magnetické anizotropie a kontinuální změnou saturační magnetizace. Pro lepší vhled do chování spinových vln v prostředí se změnou magnetických vlastností je uvedena teorie popisující lom a odraz spinových vln na rozhraní, která byla dále ověřena numerickými simulacemi.
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Spin wave excitation and propagation in magnonic crystals prepared by focused ion beam direct writing
Křižáková, Viola ; Olejník,, Kamil (oponent) ; Urbánek, Michal (vedoucí práce)
Paramagnetic Ni-stabilized fcc Fe thin films epitaxially grown on Cu(100) are known for their capability to undergo ion-beam-induced phase transformation into ferromagnetic bcc phase. To bring these metastable films closer to the application, a Cu(100) substrate can be further substituted by Si(100) with a Cu(100) buffer layer. With the use of a focused ion beam, magnetic properties of the films can be locally tailored and modulated. Moreover, this alternative approach to the preparation of media suitable for spin-wave guidance provides patterning possibilities unattainable by conventional lithography techniques. Magnetic structures prepared in this way are studied by all-electrical spin-wave spectroscopy. This thesis covers the entire process from the metastable thin film growth, through the patterning, to structural studies and static and dynamic magnetic characterization. A broadband ferromagnetic resonance and propagating spin wave spectroscopy experiments are performed on focused-ion-beam-transformed continuous layers and microstructures. Microscale coplanar waveguides are used for inductive excitation and detection of spin waves with defined wavevectors. Magnetic properties such as saturation magnetization and damping are extracted from the ferromagnetic resonance measurements and characteristics of the propagating modes such as spin-wave decay length or group velocity are studied and compared with common ferromagnetic materials.
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Study of magnonic crystals in a frequency domain
Turčan, Igor ; Hrabec, Aleš (oponent) ; Urbánek, Michal (vedoucí práce)
Characterization of magnetodynamic properties of nanomagnets and nanostructured magnetic materials requires methods appropriate for probing the typical timescales of these systems, i.e. in the sub-nanosecond range. The lack of appropriate time-domain characterization techniques is linked to the limits of current electronics. Other possible approach is to use the frequency domain characterization in GHz range. The most common frequency domain characterization technique is the ferromagnetic resonance (FMR) measurement. From FMR spectra it is possible to extract valuable information about the magnetic system: the damping parameter, saturation magnetization etc. The method we utilize for detection of spin-wave excitations aims for the simplification of the characterization experiment. We employ the thermoelectric detection of spin waves in magnetic strips via anomalous Nernst effect. The method is based on the heat generation inside a magnetic film due to the relaxation of spin waves to the lattice. The dissipation of spin-wave energy heats the magnetic strip and creates a temperature gradient towards the substrate (perpendicular to the surface). This leads to generation of an electric field perpendicular to both the temperature gradient and the magnetization direction. The voltage is usually in the V range, hence it can be measured with common laboratory equipment. Despite its simplicity, this method yields very interesting results and can be used for characterization of magnonic waveguides, magnonic metamaterials, spin-wave emitters and other spin-wave devices.
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