Awari, Nilesh

English

This Ph.D. thesis focuses on studying magnetization dynamics of magnetic materials on sub-picosecond timescales using terahertz (THz) radiation either as excitation or as a sensitive probe. At the beginning of the thesis, coherent excitations of spin precession are studied in thin films of ferrimagnetic Mn3-xGa Heusler alloys using THz emission spectroscopy. In this study, precise tunability of the THz emission frequency from these films is shown, which makes them highly promising candidates for future wireless communication technologies. Furthermore, the frequency dependence of THz driven ultra-fast demagnetization in conducting ferromagnetic CoFeB thin films is investigated. The ultra-fast demagnetization shows a peak behavior around 0.5 THz which is explained as a competition between the efficiency of Elliot-Yafet type spin scattering and the electron scattering in a Drude like model. This study will help to understand the microscopic mechanisms responsible for demagnetization taking place on sub-picosecond timescales. At the end of the thesis, magnon modes in insulating NiO have been studied with resonant excitation utilizing the magnetic field of the THz transients. This study reveals two magnon modes which are distinguished by their different characteristic magnetic field dependencies. The field dependence of these magnon modes is explained using an eight sub-lattice model. In general, the results presented in this thesis shed light on the field of ultra-fast magnetization dynamics at THz frequencies and are believed to provide a pathway to deepen our knowledge of magnetization dynamics at speeds which are technologically relevant for efficient spintronics devices

Dissertations of the University of Groningen

   University of GroningenA terahertz view on magnetization dynamicsAwari, NileshIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2019Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Awari, N. (2019). A terahertz view on magnetization dynamics. [Thesis fully internal (DIV), University ofGroningen]. University of Groningen.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 21-01-2023SummaryThe advances in spintronics have seen a great revolution in technology in the last fewdecades. In order to develop spintronics further into high-frequency regime (at sub-picosecond timescale or at THz frequencies), it is important to study spin-dependentphysical properties of magnetic materials at such high-frequency regime. The workdescribed in this thesis is a contribution towards the understanding of the physics behindthe high-speed ultra-fast spintronics.Laser-driven tunable, narrow-band THz emission from ferrimagnetic Heusleralloys. First, we study Mn3-XGa ferrimagnetic thin films for laser-driven THz emission.We observe the tunable, narrow -band in the range of 0.15 THz - 0.5 THz as a function ofMn content, temperature, and applied magnetic field. In this work we emphasis on theTHz emission spectroscopy technique for characterization of magnetization dynamicsin the sub-THz frequency regime. We observed that THz emission spectroscopy is anefficient technique to study ferromagnetic modes as compared to time-resolved magneto-optical probe techniques in the sub-THz frequency range. This project also resulted inTHz emission spectroscopy end-station which allowed us to do interesting scientific andtechnologically relevant experiments.This work shows that Mn3-XGa thin films can be used as a free layer in spin transfertorque devices which will allow using these devices in the THz frequency range. Thefrequency of such devices can be further increased by changing the magnetic propertiesof these films or with atomic substitutions. One can also use these materials for makingan on-chip narrow-band, tunable THz source.THz induced ultra-fast demagnetization in amorphous CoFeB. This experi-ment allows us to study the frequency dependence of ultra-fast demagnetization in THzregime. The time scales involved in this experiment are very similar to fundamen-tal timescales of scattering processes, which allows to study spin-dependent scatteringevents and its effect on magnetization dynamics in sub-picosecond timescale. The non-monotonic dependence of ultra-fast demagnetization on the THz pump frequency is99100 Summaryexplained using the Elliot-Yafet type spin-flip scattering mechanism and Drude conduc-tivity model. This type of experiments allows one to study the fundamental physics atvery short timescales.Such experiments will allow us to study electron, spin, and phonon dynamics at sub-picosecond timescale which is very important to understand spin transport at such ultra-short timescales. Another importance of this experiment would be to study the effectof spin-orbit coupling on spin transport at a sub-picosecond timescale to design efficientdata storage devices.THz control of antiferromangetic mode in NiO. In this experiment, we use anintense THz pump to resonantly excite the antiferromagnetic (AFM) mode in NiO. Theresonant excitation allows us to study the AFM mode as a function of temperatureand externally applied magnetic field. A new magnetic mode was observed and it wasshown that the two sub-lattice model is not enough to explain the observed results. Weused more detailed eight sub-lattice model to explain our results. We also establishedthat dipolar-interactions and magneto-crystalline anisotropy are of great importance todescribe spin dynamics accurately.Experiments of these kinds are essential to gain control over the magnetic order of thematerial. The ultra-fast switching of magnetic order at THz frequencies will be helpfulfor spintronics memory devices.To summarize, this work discusses the magnetization dynamics at THz frequency rangewhich will enable to understand fundamental physics at play at sub-picosecond timescales.This work may provide a pathway to deepen our knowledge of spin dynamics at speedswhich are technologically relevant for efficient spintronics devices.

A terahertz view on magnetization dynamics

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A terahertz view on magnetization dynamics

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