Orientadores: Roberto Luzzi, Áurea Rosas Vasconcellos / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-10-31T13:25:44Z (GMT). No. of bitstreams: 1 Vannucchi_FabioStucchi_D.pdf: 4583166 bytes, checksum: 8617f375a632ae1e33b0c986862ea535 (MD5) Previous issue date: 2011 / Resumo: Este trabalho tem por objetivo desenvolver uma teoria sobre a termodinâmica de não-equilíbrio de mágnons excitados por fonte externa e em contato com um banho térmico. A teoria é aplicada ao recém observado acúmulo de mágnons nos estados de mínima freqüência em experimentos com filmes finos magnetizados de granada de ferro-ítrio afastados do equilíbrio via fonte de radiação de microondas. Seguindo um formalismo de ensembles estatísticos de não-equilíbrio, a partir do hamiltoniano do sistema magnético são obtidas as equações de evolução para as variáveis termodinâ­micas. Entre estas equações, as que descrevem a evolução das populações médias de mágnons são estudadas em detalhe e atenção especial é dada às contribuições não- lineares. Mostra-se que, por um lado, o termo não-linear proveniente da interação do sistema magnético com a rede cristalina transfere energia das populações dos modos de alta freqüência para os de baixa, gerando um acúmulo nos modos de mínima fre­qüência - o Efeito Fröhlich. Por outro lado, a interação mágnon-mágnon origina uma contribuição à evolução das populações que tende a termalizar o sistema em termos de uma temperatura de não-equilíbrio. É ainda utilizada uma ¿modelagem de dois flui­dos¿, em que as equações cinéticas das populações de mágnons associadas a todos os modos são contraídas em apenas duas, que representa os modos de mínima freqüência e aqueles alimentados. Esta modelagem permite estudar quantitativamente a evolução temporal do sistema via integração numérica. Constata-se que, para um determinado intervalo de taxas de alimentação, forma-se o condensado devido ao Efeito Fröhlich. Para valores mais altos da potência da fonte de alimentação, a contribuição devido à interação entre mágnons torna-se dominante e a formação do condensado é inibida. Por fim, os diversos processos de relaxação do condensado para o equilíbrio são investigados em função do valor da fonte externa do sistema / Abstract: The purpose of this work is to develop a nonequilibrium thermodynamic theory on rnagnons excited by an external pumping source embedded in a thermal bath. This the­ory is applied to the recently observed experiments with magnetic thin films of yttrium iron garnet driven out of equilibrium through microwave radiation pumping. Resorting to a Non-Equilibrium Statistical Ensemble Formalism, the evolution equations for the magnon populations are obtained and studied. The nonlinear contribution arising out of the spin-lattice interaction transfers the energy in excess of equilibrium from the pumped frequency modes to the lower frequency ones, and, in a cascade down process, occurs a large grow in the magnon populations of the lowest in frequency modes - a phenomenon we call Frohlich Effect. In opposition to this contribution, there is anot her one, generated by the magnon-magnon interaction, that tends to lead the system to a state of internal nonequilibrium thermalization. We introduce a modeling consisting in a kind of ¿two-fluid model", in which the kinetic equations for the magnon populations are contracted in only two, representing the modes lowest in frequency and the ones that are pumped by the external source. A quantitative study is performed through numerical integration of the two related evolution equations. The emergence of the condensate, due to the Frohlich Effect, is evidenced for a range of source power. For higher feeding taxes, the contribution originated by the magnon-magnon interaction becomes dominant and the emergence of the condensate is inhibited. Finally, the sev­eral relaxation processes in the condensate leading to equilibrium are analyzed in terms of the source power / Doutorado / Física Estatistica e Termodinamica / Doutor em Ciências

Ce travail de doctorat porte sur la croissance par ablation laser pulsée de films ultrafins de Grenat de Fer et d’Yttrium dopés au Bismuth (BiYIG). Ces films d’épaisseur nanométriques sont caractérisés puis utilisés pour des applications magnon-spintroniques. Cette thèse englobe deux thématiques différentes de la physique : la science des matériaux et les applications magnon-spintroniques.La motivation de cette thèse repose sur le besoin, venant de la communauté magnon-spintronique, d’un nouveau matériau magnétique ultrafin à anisotropie ajustable. En effet, au court des dernières années, une avancée majeure dans le domaine a été l’obtention d’auto-oscillations magnétiques induites par un courant de charge dans un isolant magnétique. Ce résultat a été rendu possible grâce à l’utilisation d’un film ultrafin (20 nm) de Grenat de Fer et d’Yttrium (YIG) possédant de très faibles pertes magnétiques. Ces films ultrafins de YIG sont également intéressants pour la magnonique puisqu’il est aussi possible d’y propager et de manipuler des ondes de spin sur de grandes distances. Cependant, la direction facile d’aimantation dans ces films est fixée par l’anisotropie de forme et n’est pas un paramètre ajustable. Pour pousser plus loin les possibilités dans le domaine de la magnon-spintronique un matériau ultrafin, présentant des pertes magnétiques similaires au YIG, dans lequel il serait possible de stabiliser une anisotropie perpendiculaire serait désirable.La croissance par épitaxie en phase liquide de films de YIG substitués de plusieurs microns d’épaisseur a permis de mettre en évidence que l’anisotropie magnétique pouvait être modifiée par dopage. Notamment que la substitution des atomes d’Yttrium par les atomes de Bismuth sur les sites atomiques dodécaédriques permet d’obtenir une direction facile d’aimantation hors du plan, le BiYIG est également reconnu pour sa forte activité magnéto-optique. Cette thèse présente la croissance par ablation laser pulsée de films ultrafins (7 à 50 nm d’épaisseur) de BiYIG. Dans ces films l’anisotropie magnétique a deux origines : l’anisotropie de croissance et l’anisotropie de contrainte. Dans ces films grâce à la contrainte les deux types anisotropies magnétique (planaire ou perpendiculaire) peuvent être obtenues. La caractérisation dynamique des films montre que la substitution d’Yttrium par le Bismuth n’augmente pas les pertes magnétiques et que l’amortissement de Gilbert dans le BiYIG est comparable à celui du YIG. De plus l’augmentation de l’activité magnéto optique du BiYIG par rapport à celle du YIG rend ce nouveau matériau très intéressant pour des techniques expérimentales impliquant l’interaction lumière/ moment magnétique (BLS, Kerr microscope…).Pour observer des phénomènes spintronique nous avons déposé une couche de Pt. Des mesures de transport comme la magnetoresistance Hall de spin, l’effet Hall de spin inverse ou l’effet Hall anormal témoignent d’un transfert de courant de spin a l’interface BiYIG-Pt. Grâce à l’anisotropie perpendiculaire, il est également possible d’observer de nouveaux phénomènes comme la génération d’onde de spin cohérent à partir d’auto-oscillations. Ce nouvel isolant magnétique combinant une faible épaisseur, un faible amortissement magnétique et une anisotropie magnétique modifiable est donc un matériau prometteur pour des applications magnon-spintroniques et ouvre de nouvelles possibilités pour le domaine. / This PhD work focuses on the Pulsed Laser Deposition (PLD) growth of Bismuth doped Iron Garnet nanometer thick films. Those films are charcterised and used for magnon-spintronics applications. This PhD has two main focuses : material science and magnon-spintronics applications.The aim of this PhD is to fill up the need in the magnon-spintronics community of an ultrathin magnetic material combining low magnetic losses and tunable magnetic anisotropy. Indeed the recent breakthrough in the domain was the ability of generating magnetic auto-oscillations from a charge current in a magnetic insulator. This result has been obtained by using an 20 nm thick film of Yttrium Iron Garnet (YIG) with low magnetic losses (α=2⋅〖10〗^(-4) ). Those ultrafin films of YIG can also be used for spin waves propagation over micrometeter distances. However the easy magnetic axis in those films is set to in plane due to the shape anisotropy and it is not a tunable parameter. To go further in terms of magnon-spintronics applications a perpendicularly easy magnetized low losses ultra-thin magnetic material would be desirable. Liquid Phase Epitaxy growth of micrometer thick doped YIG during the 70’s evidenced that the magnetic anisotropy could be modified by doping or substitution. Especially the substitution of Yttrium atoms by Bismuth ones on the dodecaedric atomic sites allows to stabilise out of plane magnetic anisotropy. Morevover the BiYIG is also known to posses high magneto optical activity.This PhD presents the growth by Pulsed Laser Deposition of ultrathin BiYIG films (7 to 50 nm thick). In those films the uniaxial magnetic anisotropy has two main origins : the magneto elastic and the growth induced anisotropy. Using the strain in those films it is possible to obtain both out of plane and in plane magnetic anisotropy. The dynamical characterisation shows that magnetic losses in the perpendicular easy magnetized films are comparable to the one of YIG ultrathin films. The high magneto optical activity in those films makes the BiYIG ultrathin films suitable for ligth based detection technics involving ligth/magnetism interaction. By sputtering a Pt sublayer on the top of BiYIG ultra thin films we could observ different spintronic phenomena evidencing the transfer of spin current from the metal to the insulator. Low losses and nanometer thickness in perpendicularly easy magnetized BiYIG films allow to observ current induced magnetic auto oscillation in the same fashion as what was previously done with ultrathin YIG. The perpendicular magnetic anisotropy allows however to couple those auto oscillation to spin waves, which was not possible for in plane magnetized YIG fims. This new phenomena is related to the unique properties of the ultrathin BiYIG.BiYIG ultrathin films are thus opening new perspectives in the magnon spintronic commnutiy due to their low thickness and tunable magnetic anisotropy.

In this thesis the effect of interfacial Dzyaloshinskii-Moriya-interaction (iDMI) is studied. This effect, which induces a frequency shift of spin waves with fixed wave vector, is investigated for different multilayers. Only spin waves with its wave vector perpendicular to the magnetization are detected. These spin waves are called surface waves. Brillouin light scattering (BLS) technique is used for the wave vector dependent detection of spin wave frequencies. Different setups were build and compared including a new setup, which does not use a focusing lens in front of the sample. This setup exhibits an increased wave vector resolution and simplifies changing between different samples. However, the signal intensity obtained by this setup is too low to measure spin waves under the given circumstances in reasonable time.

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Spin-torque transfer (STT) effects have brought spintronics ever closer to practical electronic applications, such as MRAM and active broadband microwave spin-torque oscillator (STO), and have emerged as an increasingly attractive field of research in spin dynamics. Utilizing materials with perpendicular magnetic anisotropy (PMA) in such applications offers several great advantages such as low-current, low-field operation combined with high thermal stability. The exchange coupling that a PMA thin film exerts on an adjacent in-plane magnetic anisotropy (IMA) layer can tilt the IMA magnetization direction out of plane, thus creating a stack with an effective tilted magnetic anisotropy. The tilt angle can be engineered via both intrinsic material parameters, such as the PMA and the saturation magnetization, and extrinsic parameters, such as the layer thicknesses.       STOs can be fabricated in one of a number of forms—as a nanocontact opening on a mesa from a deposited pseudospin-valve (PSV) structure, or as a nanopillar etching from magnetic tunneling junction (MTJ)—composed of highly reproducible PMA or predetermined tilted magnetic anisotropy layers.       All-perpendicular CoFeB MTJ STOs showed high-frequency microwave generation with extremely high current tunability, all achieved at low applied biases. Spin-torque ferromagnetic resonance (ST-FMR) measurements and analysis revealed the bias dependence of spin-torque components, thus promise great potential for direct gate-voltage controlled STOs.       In all-perpendicular PSV STOs, magnetic droplets were observed underneath the nanocontact area at a low drive current and low applied field. Furthermore, preliminary results for microwave auto-oscillation and droplet solitons were obtained from tilted-polarizer PSV STOs. These are promising and would be worth investigating in further studies of STT driven spin dynamics. / Effekter av spinnvridmoment (STT) har fört spinntroniken allt närmare praktiska elektroniska tillämpningar, såsom MRAM och den spinntroniska mikrovågsoscillatorn (STO), och har blivit ett allt mer attraktivt forskningsområde inom spinndynamik. Användning av material med vinkelrät magnetisk anisotropi (PMA) i sådana tillämpningar erbjuder flera stora fördelar, såsom låg strömförbrukning och funktion vid låga fält i kombination med hög termisk stabilitet. Den utbyteskoppling (”exchange bias”) en PMA-tunnfilm utövar på ett intilliggande skikt med magnetisk anisotropi i planet (IMA) kan få IMA-magnetiseringsriktningen att vridas ut ur planet, vilket ger en materialstack med en effektivt sett lutande magnetisk anisotropi. Lutningsvinkeln kan manipuleras med både inre materialparametrar, såsom PMA och mättningsmagnetisering, och yttre parametrar, såsom skikttjocklekarna. STO:er kan tillverkas som flera olika typer - som en nanokontaktsöppning på en s.k. mesa av en deponerad pseudospinnventilstruktur (PSV) eller som en nanotråd etsad ur en magnetisk tunnlingsövergång (MTJ) –och bestå av mycket reproducerbar PMA eller av skikt med på förhand bestämt lutning av dess magnetiska anisotropi. MTJ-STO:er av CoFeB med helt vinkelrät anisotropi visar högfrekvent mikrovågsgenerering med extremt stort frekvensomfång hos strömstyrningen, detta vid låg biasering. Mätning och analys av spinnvridmoments-ferromagnetisk resonans (ST-FMR) avslöjade ett biasberoende hos spinnvridmomentskomponenter, vilket indikerar en stor potential för direkt gate-spänningsstyrda STO:er. I helt vinkelräta PSV-STO:er observerades magnetiska droppar under nanokontaktområdet vid låg drivström och lågt pålagt fält. Dessutom erhölls preliminära resultat av mikrovågssjälvsvängning och av s.k. ”droplet solitons” hos PSV-STO:er med lutande polarisator. Dessa är lovande och skulle vara värda att undersökas i ytterligare studier av STT-driven spinndynamik. / <p>QC 20160829</p>

Paramagnetické niklem stabilizované tenké vrstvy plošně centrovaného kubického Fe, epitaxně narostené na monokrystalickém substrátu Cu(100) jsou známy svou schopností strukturní a magnetické fázové přeměny při ozáření iontovým svazkem, a to do prostorově centrované kubické struktury charakteristické feromagnetickými vlastnostmi. Monokrystalický Cu(100) substrát je možné také nahradit Si(100) s mezivrstvou Cu(100). Pomocí fokusovaného iontového svazku lze dále snadno lokálně modifikovat magnetické vlastnosti ozařované vrstvy. Tato metoda přímého zápisu magnetických struktur je alternativou k běžným litografickým technikám, nabízející nové jimi nedosažitelné možnosti. Připravené magnetické struktury následně využíváme k propagaci spinových vln. V práci je představen celý proces od růstu vrstev, přes přípravu mikrostruktur, až po studium jejich struktury a statických i dynamických magnetických vlastností. S využitím vektorového síťového analyzátoru studujeme ve vrstvách a v mikrostrukturách připravených fokusovaným iontovým svazkem feromagnetickou rezonanci a propagující se spinové vlny. Zdrojem spinových vln o definovaných vlnových vektorech jsou litograficky připravené koplanární vlnovody, sloužící také k induktivní detekci vln. Pomocí feromagnetické rezonance kvantitativně určujeme materiálové charakteristiky jako jsou saturační magnetizace a parametr útlumu a ze spekter propagujících módů následně určujeme charakteristiky spinových vln, které porovnáváme s dalšími feromagnetickými materiály.

Magnetické nanostruktury mají zajímavé vlastnosti, které umožňují jejich aplikace v základním výzkumu i průmyslu. Jednou z těchto vědeckých disciplín je i magnonika - výzkumný obor, který se zabývá fyzikou spinových vln, které lze použít v nediskrétních výpočtech s nízkými ztrátami energie. Výroba magnetických struktur fokusovaným iontovým svazkem (FIB) je alternativní metoda k běžně používaným litografickým metodám. Materiál použitý v této práci - metastabilní železo - je schopen při ozáření iontovým svazkem podstoupit fázovou transformaci z paramagnetické plošně centrované kubické krystalové struktury na feromagnetickou fázi s prostorově centrovanou kubickou krystalovou strukturou. Jednou z vlastností, která ovlivňuje šíření spinových vlny, je magnetická anizotropie. Tato práce představuje vliv depozičních podmínek v ultra vysokém vakuu během přípravy metastabilní železné vrstvy na magnetickou anizotropii struktur vytvořených pomocí FIB do tohoto filmu. Dále prezentujeme souvislosti mezi parametry FIB, krystalografickými vlastnostmi výsledných struktur a jejich magnetickou anizotropií.

<p>In recent years, correlated magnetic phenomena have emerged as a unique resource for enabling alternative computing, memory, and sensing applications. This has led to the exploration of novel magnetic hybrid platforms with the promise of improved figures of merit over the state-of-the-art. In this dissertation, we delve into several example platforms where magnets interact with various other degrees of freedom, resulting in enhanced figures of merit and/or the emergence of novel functionalities.</p><p>First, we investigate the possibility of utilizing the collective resonant mode of nanomagnets to enhance the electric field sensitivity of quantum spin defects. While quantum systems have garnered significant attention in recent years for their extraordinary potential in information processing, their potential in the field of quantum sensing remains yet to be fully explored. Quantum systems, with their inherent fragility to external signals, can be harnessed as powerful tools to develop highly efficient sensors. In this dissertation, we explore the potential of a specific type of quantum sensor, namely the quantum spin defects as an electric field sensor, when integrated with a nanomagnet/piezoelectric composite multiferroic. This integration yields at least an order of magnitude enhancement in sensitivity, presenting a promising avenue for quantum sensing applications.</p><p>Next, we shift our focus towards harnessing magnetic correlation in the emerging class of atomically thin magnets known as van der Waals magnets. These magnets provide distinctive opportunities for controlling and exploiting magnetic correlations. Specifically, these platforms allow for tunable magnetic interactions by twisting two vertically adjacent layers of the magnet, features that are unique to van der Waals materials. By capitalizing on such twist degrees of freedom, we demonstrate the creation of twist-tunable nanoscale magnetic ground states. This capability opens up avenues for applications such as high-density memories and magnon crystals.</p><p>Interestingly, the same material platform also allows for exploiting magnetic correlation by controlling the local electrical environment. We uncover the symmetry-allowed spin-charge coupling mechanisms in the heterostructures of such magnets, a prediction that has received experimental support. Utilizing such an understanding, we propose a setup for the electrical generation of magnons. Magnons—the elementary excitation of spin waves—have garnered a lot of attention these days due to their potential to couple various diverse physical systems and in the field of low dissipation computing. Our findings offer a potential pathway towards the realization of magnon-based spintronic devices.</p>

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