In dieser Arbeit werden magnetische Vortex-Dreifachlagen-Systeme untersucht. Mittels Magnetfeld, Strom und Röntgenzirkulardichroismus kann erstmals die magnetische Konfiguration der Vortexlagen mit dem simultan gemessenen Magnetowiderstand verglichen werden. Die senkrecht mit Strom durchflossenen Kobalt-Kupfer-Permalloy Scheiben werden in einem Mehrschrittprozess mittels Elektronenstrahllithographie auf einer Membran hergestellt, um mit Rastertransmissions-Röntgenmikroskopie untersuchbar zu sein. Die Auswertung der STXM-Bilder zeigt das gleiche Verhalten wie die Widerstandsmessungen und erlaubt eine eindeutige Zuordnung. Um auch die kleinsten scheibenförmigen Dreifachlagensysteme mittels ferromagnetischer Resonanz zu messen, wurde die Mikroresonator FMR optimiert. Damit können bereits etwa 2.3*10^7 Kobaltatome gemessen werden, wobei die Empfindlichkeit bis zu 4*10^6 Atomen ausreichend sein sollte. Durch 6-fache Mittelung lässt sich ein Kobaltwürfel mit einer Kantenlänge von 12,5nm detektieren. Dabei sind nicht nur die uniforme Mode, sondern auch lokal angeregte Moden sichtbar. Mittels mikromagnetischer Simulationen lassen sich den Resonanzen Modenbilder zuordnen. Die scheibenförmige Dreifachlage wird mit den FMR-Messungen sowohl mit verringertem Durchmesser, als auch mit reduzierter Zwischenschicht untersucht.

Made available in DSpace on 2014-07-29T15:07:10Z (GMT). No. of bitstreams: 1 Adisserta.pdf: 620282 bytes, checksum: 42737bbc61b6415b2b23fa4670060fd4 (MD5) Previous issue date: 2009-04-14 / The Ferromagnetic Resonance (FMR) technique was used to study the magnetic anisotropy of NiFe/Ru multilayers. The FMR spectra and absorption fields of multilayers with Ru layers thickness bellow 20 Å are the same as that of single monolayer films. Evidence of the coupling between the magnetic layers also is given by the fact that for the sample with Ru thickness equal to 7 Å, in normal configuration of the applied static field, spin-wave resonance modes also are excited by the microwave field. The interlayer coupling constant obtained is equal to 1, 1 £ 10¡7 erg/cm. For the set of samples with different NiFe thicknees, the analysis of the effective magnetic anisotropy in terms of volume and surface contributions, gives KV = 3, 4 £ 106 erg/cm3 and KS = 1, 2 erg/cm2. / A técnica de Ressonância Ferromágnética (RFM) foi aplicada para estudar a anisotropia magnética de multicamadas NiFe/Ru. Os espectros de RFM e os campos de absorção das multicamadas com espessura da camada de Ru abaixo de 20 Å são os mesmos daqueles obtidos para um filme monocamada. Uma evidência do acoplamento entre as camadas magnéticas destas amostras é dada pelo fato de que para a multicamada com espessura de Ru igual a 7 Å, na configuração perpendicular ao campo estático aplicado, modos de ressonância de ondas de spin são também excitados pelo campo de microondas. A constante de acoplamento inter-camadas obtida é igual a 1, 1 £ 10¡7 erg/cm. Para o conjunto de amostras com espessura variável da camada de NiFe, a análise da anisotropia magnética efetiva em termos das contribuições de volume e superfície resultou em KV = 3, 4 £ 106 erg/cm3 e KS = 1, 2 erg/cm2

Submitted by Erika Demachki (erikademachki@gmail.com) on 2015-01-16T17:15:31Z No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Tese - Márcio Solino Pessoa - 2013.pdf: 3016321 bytes, checksum: 0475321bc8fda0cf44f37f4a946b3725 (MD5) / Approved for entry into archive by Erika Demachki (erikademachki@gmail.com) on 2015-01-16T17:30:10Z (GMT) No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Tese - Márcio Solino Pessoa - 2013.pdf: 3016321 bytes, checksum: 0475321bc8fda0cf44f37f4a946b3725 (MD5) / Made available in DSpace on 2015-01-16T17:30:10Z (GMT). No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Tese - Márcio Solino Pessoa - 2013.pdf: 3016321 bytes, checksum: 0475321bc8fda0cf44f37f4a946b3725 (MD5) Previous issue date: 2013-06-07 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / This work reports the study of magnetic anisotropy of Fe/Mn bilayers, and Fe/Mn/Fe and Fe/Mn/Co trilayers, grown by molecular beam epitaxy onto monocrystalline MgO(001) substrates. Samples were produced with the Fe layer grown at a temperature of 175 ° C, with a thickness of 5 or 10 nm, the Co layer grown at 50 ° C, with a thickness of 2 or 5 nm, and the Mn layer grown at temperatures of 50 ° C, 150 ° C or 175 ° C, with thicknesses in the range from 0.80 to 2.20 nm. The angular variations of the in-plane absorption field, at Q-band (33.9 Ghz) microwave frequency, reveal the presence of a large four-fold magnetic anisotropy in all studied films. In samples with thicker Mn layers, two uniform resonance modes of bcc-Fe and bcc-Co layers are excited by the microwave field. The angular variations of the in-plane absorption field reveal for both layers the presence of fourth order cubic magnetic anisotropy. Films with the Mn layer thickness exceeding 1.16 nm exhibits, however, only a single absorption mode due to the overlapping of Fe-bcc and Co-bcc uniform resonance modes. Experiments at X-band (9.4 GHz) microwave frequency reveal the excitation of non-aligned and non-resonant modes, suggesting, for samples with the Mn layer grown at low temperature, a non-collinear coupling between the magnetic layers in the presence of low-intensity magnetic fields. Four-fold magnetic anisotropy constants in the range from 3.0 x 105 to 4.8 x 105 erg/cm3 for the Fe layer, and from 2.0 x 105 to 6.4 x 105 erg/cm3, for the Co layer, are given by the experiments. The magnetic parameters deduced from the in-plane measurements at Q-band microwave frequency are used to fit the in-plane and out-of-plane angular variations of resonance fields at X-band frequency. / Este trabalho relata o estudo da anisotropia magnética em bicamadas Fe/Mn, e tricamadas Fe/Mn/Fe e Fe/Mn/Co, crescidas por epitaxia de feixe molecular sobre substratos monocristalinos de MgO (001). As amostras foram produzidas com a camada de Fe crescida à temperatura de 175°C, com espessura de 5 ou 10 nm, a camada de Co crescida à temperatura de 50°C, com espessura de 2 ou 5 nm, e a camada de Mn crescida a temperaturas de 50°C, 150°C ou 175°C, com espessuras na faixa de 0.80 a 2.20 nm. As variações angulares do campo de ressonância no plano dos filmes, em banda-Q (33.9 GHz) de frequência de micro-ondas, revelam a presença de anisotropia magnética cúbica de quarta ordem em todas as amostras estudadas. Para as tricamadas Fe/Mn/Co com menores espessuras da camada de Mn, os modos uniformes de ressonância de Fe e de Co, são excitados pelo campo oscilante de micro-ondas. As variações angulares de campo de ressonância no plano dos filmes revelam a presença de anisotropia magnética de quarta ordem nas duas camadas. Filmes com camada de Mn com espessura superior a 1.16 nm apresentam, entretanto, apenas um modo uniforme de ressonância, atribuído à sobreposição dos modos uniformes de ressonância das camadas de Fe-bcc e de Co-bcc, como resultado de efeitos de interface que reduzem a diferença entre as magnetizações efetivas das duas camadas. Experimentos em banda-X (9.4 GHz) de frequência de micro-ondas revelam a presença de modos não alinhados e não ressonantes, sugerindo, para amostras com camada de Mn crescida em baixa temperatura, um acoplamento não colinear entre as camadas magnéticas na presença de campo magnético de baixa intensidade. Constantes de anisotropia cúbica de 3.0 x 105 a 4.8 x 105 erg/cm3 para a camada de Fe, e de 2.0 x 105 a 6.4 x 105 erg/cm3, para a camada de Co, são deduzidos dos experimentos. Os parâmetros magnéticos deduzidos das variações angulares do campo de ressonância no plano das amostras, em banda-Q de frequência de micro-ondas, são usados para simular em banda-X as variações angulares do campo de ressonância no plano e fora do plano dos filmes.

La mise en évidence d’effets de polarisation d’échange (“exchange bias”, EB) ouvre de nouvelles perspectives dans le domaine émergeant de la spintronique organique. Dans une première partie de la thèse, on étudie l’EB des systèmes Co/MPc et Py/MPc (M=Mn, Co, Fe, Zn) par magnétométrie. Pour tous ces systèmes, l’EB est observé avec des températures de blocage de 100K environ. Ces études sont complétées par des mesures de résonance ferromagnétique confirmant les valeurs du champ de polarisation. Dans une troisième partie, on étudie les propriétés magnétiques des tricouches Co/Pc/Co. Les cycles d’hystérèse présentent des marches indiquant un renversement séquentiel des couches de cobalt. A basse température, on observe de l’anisotropie unidirectionnelle pour les deux couches mais leurs champs de polarisation diffèrent. / Observation of exchange bias (EB) phenomenon by using molecular materials as a pinninglayer open the horizon for tremendous perspective in the field of organic spintronics. Thefirst part of the thesis is devoted to the study of EB of Co/MPc and Py/MPc (M=Mn, Co, Fe,Zn) by static magnetometry. The existence of EB is evidenced in all Pc molecules with block-ing temperature around 100K. The second part is devoted to the study of EB by dynamicFMR measurements. The values of EB measured by this method are compatible with staticmagnetometry measurements. The third part is devoted to study magnetic properties of thetrilayer Co/Pc/Co systems. Hysteresis loops exhibit a stepped shape indicative of successivereversal of each layer. Low temperature loops show that both Co layers experience unidi-rectional anisotropy after field cooling, with differing bias fields.

This Dissertation explores the interaction between the magnetisation and an alternating current in a uniform ferromagnetic system. Diluted magnetic semiconductors (Ga,Mn)As and (Ga,Mn)(As,P) have been studied. Due to their strong spin-orbit coupling and well-understood band-structure, these materials are well-suited to this investigation. The combined effect of spinorbit coupling and exchange interaction permits the alternating current to induce an oscillating current-induced torque (CIT) on the magnetisation. In the frequency range close to the natural resonance frequency of the magnetic moments (gigahertz), CIT can excite precessional motion of the magnetisation, a process known as ferromagnetic resonance (FMR). CIT can be parameterised by an effective magnetic field. By analysing the lineshape of the measured FMR signals, the magnitude and orientation of this effective field have been accurately determined. Moreover, the current-induced fields in these ferromagnetic materials have been observed with symmetries of the Dresselhaus, and for the first time, Rashba spin-orbit coupling. A new class of device-scale FMR technique, named as CIT-FMR, has been established in this Dissertation, with the advantage of simple device structure (only a resistor is required) and scalability (measurements have been performed on devices sized from 4 μm down to 80 nm). This technique is not only limited to magnetic semiconductors, but can also be transferred to study other ferromagnetic systems such as ultrathin metal films. Finally, the CIT-FMR technique is employed to study the magnetic anisotropyin individual (Ga,Mn)As and (Ga,Mn)(As,P) micro-devices. Devices down to 80 nm in width have been measured in (Ga,Mn)(As,P), which show strong strain-relaxation-induced anisotropy, larger than any previously reported cases on (Ga,Mn)As. Furthermore, due to the tensile-strain on the (Ga,Mn)(As,P) epilayers, the anisotropy field due to patterning-induced strain-relaxation in these devices is observed to take the opposite direction compared to that in the compressively-strained (Ga,Mn)As samples.

Orientadores: Carlos Rettori, Pascoal José Giglio Pagliuso / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-22T06:56:36Z (GMT). No. of bitstreams: 1 Lesseux_GuilhermeGorgen_M.pdf: 14003739 bytes, checksum: b737fc7f0d847cdc75e895351974e1e2 (MD5) Previous issue date: 2013 / Resumo: Compostos nanoestruturados têm atraído cada vez mais atenção do ponto de vista tecnológico devido às inúmeras possibilidades em termos de aplicação nas mais diversas áreas. Além da motivação em termos de aplicação, o aumento da proporção de átomos na superfície em relação ao volume e a redução da dimensionalidade nestes compostos trazem consigo novas questões em física. Com base nisso, o estudo científico sistemático destas questões é fundamental para o desenvolvimento da nanociência e da nanotecnologia de forma geral. No presente trabalho são estudadas propriedades magnéticas de três tipos de nanopartículas (NPs): i) NPs de Au assistidas por óxidos do tipo R2O3 (R = Er e Y) que exibem propriedades ferromagnéticas; ii) NPs metálicas, Au e Ag, com a impureza magnética Er3+ diluída permitindo assim a sondagem microscópica de propriedades físicas por Ressonância de Spin Eletrônico; iii) E, por fim, NPs de NaYF4 mono e codopadas com os íons de terras raras RE = Yb3+, Er3+ e Tm3+ nas quais foi possível verificar o fenômeno de upconversion. Com base em adaptação de métodos estabelecidos na literatura, [1_3] foi desenvolvida uma rota química para a obtenção de NPs de Au com propriedades ferromagnéticas acentuadas pela incorporação de óxidos. A partir da magnetização de saturação em 2 K e baseado em uma análise termogravimétrica (TGA) estimou-se um momento magnético efetivo de aproximadamente 0.2 µB por átomos de Au na superfície das NPs. Além da caracterização magnetometrica típica, observou-se uma linha intensa de ESR em banda-X desde 370 K até 4.2 K. Esta ressonância possui intensidade praticamente constante caracterizando a ressonância observada como ferromagnética (FMR). Estes resultados são interpretados com base na ligação entre a capa orgânica (capping), o óxido R2O3 e os átomos de Au gerando uma hibridização efetiva dos orbitais 5d-6s dos elétrons do Au. Esta hibridização seria, então, responsável por tornar a camada 5d do Au magnética devido a spins não compensados nos orbitais 5d. As NPs metálicas com impurezas de Er3+ foram obtidas por uma variação do método utilizado para as NPs de Au ferromagnéticas. Os valores de g e as estruturas hiperfinas observadas indicam que o íon Er3+ está em um sítio cúbico tanto nas partículas de Ag como nas de Au. Os espectros de ESR mostram que não há deslocamento de g e relaxação tipo Korringa devido à interação de troca entre os spins do Er3+ e os dos elétrons de condução, sugerindo assim que esta interação de troca não ocorre em NPs metálicas. Por fim, as NPs de NaYF4 dopadas com RE = Yb3+, Er3+ e Tm3+ foram obtidas por um método estabelecido na literatura. [4, 5] A incorporação, o estado de oxidação e a concentração dos íons magnéticos Er3+ e Yb3+ foram confirmados por medidas de magnetização dc e de ESR. Observou-se emissão visível no verde e no azul para amostras codopadas com 20%Yb3+ / 2%Er3+ e 30%Yb3+ / 0.5%Tm3+, respectivamente, devido ao fenômeno conhecido como upconversion / Abstract: Nanostructured compounds have attracted growing attention from the technological point of view due to numerous possibilities in terms of application in several areas. Besides, the large surface/volume atoms ratio and the reduced dimensionality of these nanocompounds raised new fundamental physical issues. Therefore, a detailed and systematic scientific study regarding these phenomena is crucial for the sake of nanoscience and nanotechnology development. In this dissertation, we thoroughly investigated the magnetic properties of three different types of nanoparticles (NPs): i) Au NPs assisted by oxides R2O3 (R = Er and Y) which present unexpected ferromagnetic properties; ii) diluted magnetic Er3+ impurities in Ag and Au NPs, which allow Electron Spin Resonance to study several microscopic local physical properties and, finally, iii) single and co-doped Yb3+, Er3+ and Tm3+ NaYF4 NPs that allowed to verify the up-conversion phenomena in these NPs. After adapting and improving already established methods reported in the literature, [1_3] we developed a novel chemical route to obtain Au-NPs with enhanced ferromagnetic properties by oxide incorporation. Based on the saturation magnetization at 2 K and thermogravimetric analysis (TGA), we estimated an effective magnetic moment of µeff ¿ 0.2 µB per Au atom on the surface of the NPs. Besides the typical magnetometric characterization, we also carried out X-band ESR experiments. An intense ESR line was observed in the range of 4.2K = T = 370K with an integrated signal intensity which is almost constant in the entire T-range. Based on our results, the observed ESR signal is attributed to a ferromagnetic resonance (FMR). These results are discussed in terms of bonds between the NP-capping ligands and the Au atoms, which give rise to an effective hybridization between the 5d-6s electrons at the surface of the AuNPs. This hybridization might be the responsible mechanism for the Au 5d shell to become magnetic due to uncompensated spins in the 5d orbitals. The metallic Er3+ doped Ag and Au NPs were obtained by a slightly modified method used to get the ferromagnetic Au -NPs. The ESR g-values and observed hyperfine splitting indicate a cubic symmetry for the Er3+ ions in the Ag and Au NPs. Furthermore, we observed no g-shift and Korringa relaxation due to the exchange interaction between the magnetic rare-earth impurities and the conduction electron spins. This fact suggests that such an exchange interaction is negligible in metallic NPs. Finally, the Er3+, Yb3+ and Tm3+ doped NaYF4 NPs were obtained by a method already established in the literature. [4, 5] The incorporation of the Er3+ and Yb3+ ions as well as their oxidation state and concentration were confirmed by T-dependent magnetization and ESR measurements. For the co-doped NaYF4 NPs, we observed by naked eye the expected green and blue emitted lights of Yb/Er and Yb/Tm, respectively, due to a phenomenon known as upconversion / Mestrado / Física / Mestre em Física

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Ferromagnetic (FM) thin-film heterostructures provide opportunities to investigate GHz magnetic dynamics and emerging magnetoelectric devices based on dynamic phenomena. An intriguing direction for these studies is the control of the flow of pure (chargeless) spin current flow in different magnetic systems. In this thesis, we focus on the excitation of pure spin currents using ferromagnetic resonance (FMR), also known as spin pumping, and their transport characteristics in magnetic heterostructures based on Ni₈₁Fe₁₉ (Permalloy, or Py) under a variety of circumstances. In Chapter 2, we present measurements of the anisotropy of the spin pumping effect in the Pt/Py/Pt system via variable-frequency, swept-field FMR. We find a very small anisotropy of enhanced Gilbert damping with sign opposite to a recent theory's prediction from the Rashba effect at the FM/Pt interface. In Chapter 3, we present an experimental and theoretical study of spin dynamics in the antiferromagnetically coupled Py/Ru/Py system. We show that, contrary to the behavior of the uniform mode in a saturated single-layer FM, the symmetric mode in unsaturated synthetic antiferromagnet (SAF) has approximately constant FMR linewidth as a function of frequency. This behavior can be explained mostly semiclassically by our model. In Chapter 4, we present an investigation of interfacial Gilbert damping due to the spin pumping effect in Py/W heterostructures with enriched α phase or β phase W. The spin mixing conductances (SMC) for W at interfaces with Py are found to be significantly lower than those for similar heavy metals such as Pd and Pt, but comparable to those for Ta, and independent of enrichment in the β phase. The experimental results also indicate that W, no matter of which phase, is a good spin sink in Py/W heterostructures. In Chapter 5, we describe explorations of the spin pumping effect in antiferromagnetic insulator (AFI)-based heterostructures using variable-temperature, variable-frequency FMR. We find a spin-pumping-induced damping enhancement for Py/Cu/CoO, Py/Cu/CoO/Cu/Pt and Py/Cu/NiO/Cu/Pt. Broad peaks have been observed in FMR linewidth difference as a function of temperature for Py/Cu/NiO, normalized to the linewidth of the reference Py sample. Our results indicate that some effects previously attributed to spin current flow are better described by a defect-related mechanism. Chapter 6 summarizes the various findings of spin-pumping effects in ferromagnetic thin film heterostructures and possible future work.

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