Restriction lift date: 2026-05-31
Nanostructured magnetic materials for integrated magnonic devices
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Date
2024
Authors
Samanta, Arindam
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Publisher
University College Cork
Published Version
Abstract
This PhD thesis explores the fabrication, characterization, and application of advanced exchange spring (ES)/exchange coupled (EC) nanoheterostructures and their magnetic properties. Current challenges in the field of magnetics include the development of materials that demonstrate tunable magnetic properties, particularly in terms of controlling anisotropy and spin dynamics at the nanoscale. This thesis addresses these challenges by utilizing an electrodeposition technique, we have been able to develop for the first time in situ amorphous/nanocrystalline cobalt-phosphorus (CoP) thin films at room temperature. These films exhibit a unique transverse exchange spring structure due to the interplay between in-plane (IP) and out-of-plane (OOP) anisotropies. The inherent IP anisotropy of the amorphous phase competes with the OOP anisotropy of the nanocrystalline structure, producing characteristic stripe domain structures that evolve into novel corrugated stripe domain shapes.
Systematic investigations reveal the evolution of hysteresis loops in these thin films, showing a transition from low coercivity non-ES loops to staircase-ES loops with multiple coercivities in thicker films. The First Order Reversal Curve (FORC) distributions demonstrate various reversal mechanisms within the samples, confirming the transition between non-ES and ES states based on the prevalent interfacial exchange coupling. Additionally, the field-dependent Brillouin Light Scattering (BLS) spectra unveil distinct spin wave modes, with ES films showing well-resolved bulk and Damon-Eshbach surface spin wave modes, while non-ES films exhibit mode doublets below a certain applied field threshold. These findings indicate a linear dependence of mode frequencies on magnetic field intensity and enhanced exchange coupling in thicker ES films.
On the other hand, ultrafast magnetization dynamics studies highlight the in-plane magnetic orientation (φ) dependent ultrafast demagnetization and precessional dynamics of electrodeposited non-exchange spring nanostructured CoP alloys. The precession frequency shows dominant two-fold anisotropy superposed with moderate four-fold anisotropy, while the Gilbert damping coefficient exhibits four-fold anisotropy. The ultrafast demagnetization remains nearly isotropic with φ, suggesting a significant role of spin-orbit coupling (SOC) in anisotropic precessional dynamics and isotropic spin-fiip scattering processes. These detailed studies of ultrafast spin dynamics reveal crucial dynamical properties for potential applications in high-frequency integrated magnetic passives for future monolithic on-chip power supplies.
Furthermore, this thesis introduces novel "Magnon Microwave Antennas" (MMAs) for generating tunable microwave frequencies without external bias magnetic fields. The MMAs, comprising patterned arrays of magnetostrictive nanomagnets embedded in piezoelectric heterostructures, generate multimode microwave frequencies through the phonon-magnon coupling. Static magnetic studies elucidate various magnetization reversal processes within the nanowire and nanodot arrays, unveiling the critical role of demagnetization energy distribution in tuning the domain configuration and power-phase distributions of these MMAs. Functional tuneability has been proposed to be achieved through amplitude-dependent training using different combinations of nanowire and nanodot dimensions, topologies, material properties, and array configurations. The non-volatile nature of the spin textures generated in MMAs under bias-free conditions holds promise for energy-efficient logic and low-power computing applications.
Thus, the comprehensive research presented in this thesis paves the way for the development and exploitation of next-generation nano-heterostructures for various cutting-edge magnetic vis-à-vis magnonic applications, including on-chip reservoir computing, leveraging the unique magnetic vis-à-vis magnonic properties and their tuneability in these advanced materials/devices.
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Keywords
Magnon microwave antenna , Nanoheterostructured magnetic materials , Integrated magnonic devices , Spin wave , Magnon , Micromagnetic simulation , PyFORC , Electrodeposition , MPMS3 SQUID magnetometer , Phono-magnon coupling , Brillouin light scattering (BLS) , Time-resolved magneto-optical Kerr effect (TRMOKE) , Reservoir computing , First order reversal curve analysis
Citation
Samanta, A. 2024. Nanostructured magnetic materials for integrated magnonic devices. PhD Thesis, University College Cork.