Made available in DSpace on 2014-06-12T18:07:44Z (GMT). No. of bitstreams: 2 arquivo7979_1.pdf: 1162128 bytes, checksum: e48cdf9eab0eaa4de2840f161367280d (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2003 / Nesta dissertação investigamos propriedades dinâmicas de válvulas de spin e mecanismos de relaxação em filmes de Permalloy. Foi utilizada a técnica de ressonância ferromagnética operando na banda X de microondas, à temperatura ambiente. As estruturas de válvulas de spin foram depositadas por sputtering dc e possuem composição do tipo Si(001)/NiO(900Å)/Py(100Å)/Cu(tcu)/Py(100Å)/Cu(45Å). Analisamos as propriedades de ressonância ferromagnética para quatro amostras com diferentes espessuras da camada espaçadora de Cobre. Mostramos que existe uma competição entre o acoplamento indireto entre as camadas de Py, mediado pela camada de Cobre, com o acoplamento direto entre a camada presa de Py e a camada de NiO. Os resultados experimentais obtidos para estas estruturas foram interpretados através de um modelo fenomenológico que leva em conta a energia livre magnética do sistema. Com o objetivo de estudar os processos de relaxação em filmes de ferromagnéticos, foram preparadas amostras de Permalloy com camadas adjacentes de materiais diversos. Verificamos que a dependência da largura de linha de FMR apresenta comportamentos que dependem do material adjacente. Filmes de Permalloy depositados diretamente sobre substratos de Si apresentam uma dependência com a espessura do Py do tipo 1/t2. Filmes de Py sanduichados por camadas de Paládio apresentam uma dependência do tipo 1/t. Concluímos que diferentes mecanismos de relaxação atuam nas amostras. Fizemos uma conexão dos nossos resultados com as teorias fenomenológicas existentes

Magnetoimpedance has been proved to be an excellent tool to study the magnetization dynamics and the ferromagnetic resonance (FMR) linewidth provides a convenient way for measuring damping parameters in magnetic materials. The FMR linewidth depends on intrinsic magnetic damping and additional magnetic inhomogeneities, but complete understanding of the origin of these damping parameters is still unaccomplished. Besides the fundamental physics interest, the study of damping term and magnetization dynamics is very important for the development of any device which has its physical effect associated with the reversal of magnetization. Furthermore, the FMR linewidth is a very sensitive way to study the structural quality of magnetic samples, in both bulk and thin film geometries. In this work the magnetic relaxation of CoFeSiB glass-covered microwire was investigated by ferromagnetic resonance (FMR) linewidth measurements. We have identified the main damping mechanisms and quantified these damping terms, showing the effect of annealing temperature to them. The study have shown that there are three main damping mechanisms responsible for the FMR linewidth, the Gilbert damping parameter, a damping mechanism due to anisotropy dispersions and two-magnon scattering. The Gilbert damping parameter is almost constant and not influenced by the annealing. The FMR linewidth is very sensitive to anisotropy dispersions and this mechanism has a great contribution to the magnetic relaxation. The two-magnon scattering is an assignment of the inhomogeneities present in the samples and its contribution to the FMR linewidth decrease with the annealing temperature until a critical value, as a result of a decrease of inhomogeneities due to a reduction of the internal stress level. A further increase in the annealing temperature produces an increase in the two-magnon scattering contribution which is an indication of the growing of nanocrystals acting as scattering centers to the spin waves. / Magnetoimpedância tem provado ser uma excelente ferramenta para estudar a dinâmica de magnetização e a largura de linha da ressonância ferromagnética (FMR) fornece uma maneira conveniente para medir parâmetros de amortecimento em materiais magnéticos. A largura de linha FMR depende do amortecimento magnético intrínseco e adicionais inomogeneidades magnéticas, mas o completo entendimento da origem destes parâmetros de amortecimento ainda está inacabado. Além do interesse da física fundamental, o estudo da dinâmica de amortecimento e magnetização é muito importante para o desenvolvimento de qualquer dispositivo que tem seu efeito físico associado com a inversão da magnetização. Além disso, a largura de linha FMR é uma forma muito sensível para estudar a qualidade estrutural de amostras magnéticas. Neste trabalho é apresentado um estudo da relaxação magnética de microfios amorfos de CoFeSiB recobertos por vidro, mostrando o efeito da temperatura de recozimento sobre as propriedades magnéticas do microfio. Foram identificados e quantificados os principais mecanismos de amortecimento, mostrando o efeito da temperatura de recozimento para eles. Os estudos mostraram que existem três principais mecanismos de amortecimento responsável pela largura de linha FMR: o parâmetro de amortecimento de Gilbert; um mecanismo de amortecimento devido à dispersões na anisotropia; e o amortecimento devido ao espalhamento de magnons. O parâmetro de amortecimento de Gilbert é quase constante e não influenciado pelo recozimento. A largura de linha FMR é muito sensível a dispersões da anisotropia e esse mecanismo tem uma grande contribuição para o relaxamento magnético. A dispersão de magnons é uma atribuição de inomogeneidades presentes nas amostras e tem contribuição para a diminuição da largura de linha com a temperatura de recozimento até um valor crítico, como resultado de uma diminuição de inomogeneidades devido a uma redução do nível de estresse interno. Um aumento na temperatura de recozimento produz um aumento na contribuição de espalhamento dos magnons, que é uma indicação do crescimento de nanocristais atuando como centros de dispersão para as ondas de spin.

No description available.

A recent International Technology Roadmap for Semiconductors (ITRS) report (2.0, 2015 edition) has shown that Moore’s law is unlikely to hold beyond 2028. There is a need for alternate devices to replace CMOS based devices, if further miniaturization and high energy efficiency is desired. The goal of this dissertation is to experimentally demonstrate the feasibility of nanomagnetic memory and logic devices that can be clocked with acoustic waves in an extremely energy efficient manner. While clocking nanomagnetic logic by stressing the magnetostrictive layer of a multiferroic logic element with with an electric field applied across the piezoelectric layer is known to be an extremely energy-efficient clocking scheme, stressing every nanomagnet separately requires individual contacts to each one of them that would necessitate cumbersome lithography. On the other hand, if all nanomagnets are stressed simultaneously with a global voltage, it will eliminate the need for individual contacts, but such a global clock makes the architecture non-pipelined (the next input bit cannot be written till the previous bit has completely propagated through the chain) and therefore, unacceptably slow and error prone. Use of global acoustic wave, that has in-built granularity, would offer the best of both worlds. As the crest and the trough propagate in space with a velocity, nanomagnets that find themselves at a crest are stressed in tension while those in the trough are compressed. All other magnets are relaxed (no stress). Thus, all magnets are not stressed simultaneously but are clocked in a sequentially manner, even though the clocking agent is global. Finally, the acoustic wave energy is distributed over billions of nanomagnets it clocks, which results in an extremely small energy cost per bit per nanomagnet. In summary, acoustic clocking of nanomagnets can lead to extremely energy efficient nanomagnetic computing devices while also eliminating the need for complex lithography. The dissertation work focuses on the following two topics: Acoustic Waves, generated by IDTs fabricated on a piezoelectric lithium niobate substrate, can be utilized to manipulate the magnetization states in elliptical Co nanomagnets. The magnetization switches from its initial single-domain state to a vortex state after SAW stress cycles propagate through the nanomagnets. The vortex states are stable and the magnetization remains in this state until it is ‘reset’ by an external magnetic field. 2. Acoustic Waves can also be utilized to induce 1800 magnetization switching in dipole coupled elliptical Co nanomagnets. The magnetization switches from its initial single-domain ‘up’ state to a single-domain ‘down’ state after SAW tensile/compressive stress cycles propagate through the nanomagnets. The switched state is stable and non-volatile. These results show the effective implementation of a Boolean NOT gate. Ultimately, the advantage of this technology is that it could also perform higher order information processing (not discussed here) while consuming extremely low power. Finally, while we have demonstrated acoustically clocked nanomagnetic memory and logic schemes with Co nanomagnets, materials with higher magnetostriction (such as FeGa) may ultimately improve the switching reliability of such devices. With this in mind we prepared and studied FeGa films using a ferromagnetic resonance (FMR) technique to extract properties of importance to magnetization dynamics in such materials that could have higher magneto elastic coupling than either Co or Ni.

A greater understanding of precessional dynamics in magnetic systems is central to several emerging technologies. This thesis presents the design, construction and development of a Vector Network Analyser based Ferromagnetic Resonance measurement instrument (VNA-FMR), and its application in characterising dynamic material properties in hybrid anisotropy [CoPd]8-NiFe films, produced by remote plasma sputtering. Potential applications for hybrid films include Spin Torque Oscillators (STOs) or Vortex Oscillators (VO) for use as microwave emitters in, for example in Microwave Assisted Magnetic Recording (MAMR). The VNA-FMR system was first used to measure thin films of NiFe (permalloy) which allowed its capabilities to be quantified and compared to systems reported in the literature. The instrument demonstrated the capability of measuring permalloy films down to a thickness of 3 nm and was used to measure resonance and damping behaviour which agreed well with theory. The results obtained forMs were in agreement with measurement using Vibrating Sample Magnetometry. The effect of interlayer exchange on FMR was explored in hybrid films using a sample series with varying Pd spacer layer thickness, t, [Co/Pd]-Pd(t)-NiFe. As Pd spacer thickness increased, a transition was observed from near complete coupling with a single resonance mode to separate acoustic and optical branches of resonance. As spacing was further increased, the branches converged towards the resonances of the individual component layers of the hybrid films. The results suggest exchange coupling has a range of less than 2 nm, and is completely extinguished at 5 nm, in agreement with previous measurements. However, a change in damping behaviour was observed between 10 and 20 nm spacer thickness, independent of field orientation.

This thesis compiles results of research in two mutuallydependent parts: 1) development of ferromagnetic resonance(FMR) spectrometer to study microwave properties offerromagnetic materials, and 2) characterization of new irongarnets: pulsed laser deposited Y3Fe5O12and Bi3Fe5O12films and Ce:Y3Fe5O12single crystal. First part describes a novelBroadbandFMRSpectrometerdesigned to characterize thin ferromagneticfilms. The spectrometer uses two probeheads: one is the X-bandmicrowave reflection cavity for room temperature measurementsand the in-cryostat microstrip line probe to perform FMRexperiments in the frequency range from 50 MHz to 40 GHz. Veryuniform and stable magnetic field up to 2.4 T, temperatures 4 Kto 420 K, and continuous frequency scan performed byHP8722Dvector network analyzer provide various modes ofoperation. Both probeheads are equipped with two-circlegoniometers to ensure accurate study of magneticanisotropy. The spectrometer was used to make express-analysis ofquality thus to optimize processing parameters of epitaxialiron garnet films grown by pulsed laser deposition (PLD).Comprehensive study of uniaxial and cubic magnetocrystallineanisotropy has been performed for Ce:Y3Fe5O12bulk crystal as well as for Y3Fe5O12and Bi3Fe5O12films grown on different substrates by PLD andreactive ion beam sputtering techniques. BroadbandFMR-spectroscopy revealed difference in spectra of domain wallresonances: instead of“soft”spin modes in filmsgrown by liquid phase epitaxy, PLD-made films show“diffuse”transformation of domains near thesaturation field. This effect indicates non-uniformity ofsaturation magnetization and field of uniaxial anisotropy inPLD-iron garnets. Spin wave resonances in comparison withuniform FMR have been studied to evaluate“localquality”of ferromagnetic films. The resonance field andFMR linewidth behavior were studied at various crystallographicdirections determined by X-ray diffraction. FMR was used to choose PLD-made YIG films with low losses atmicrowave frequencies and to build magnetostatic surface wavesmicrowave bandpass filter. The filter was designed as a planarfilm structure with a microstrip line for transducers. It is afirst demonstration of feasibility to introduce PLD processingtechnique to magnetostatic wave technology. Magneto-optical study of Ce:Y3Fe5O12single crystal complements results ofFMR-spectroscopy of new garnets. <b>Keywords:</b>ferrites, thin films, ferromagnetic resonance,microwaves, FMR spectrometer, magnetic anisotropy,magnetostatic waves.

<p>This thesis compiles results of research in two mutuallydependent parts: 1) development of ferromagnetic resonance(FMR) spectrometer to study microwave properties offerromagnetic materials, and 2) characterization of new irongarnets: pulsed laser deposited Y₃Fe₅O₁₂and Bi₃Fe₅O₁₂films and Ce:Y₃Fe₅O₁₂single crystal.</p><p>First part describes a novel<i>Broadband</i>FMR<i>Spectrometer</i>designed to characterize thin ferromagneticfilms. The spectrometer uses two probeheads: one is the X-bandmicrowave reflection cavity for room temperature measurementsand the in-cryostat microstrip line probe to perform FMRexperiments in the frequency range from 50 MHz to 40 GHz. Veryuniform and stable magnetic field up to 2.4 T, temperatures 4 Kto 420 K, and continuous frequency scan performed by<i>HP8722D</i>vector network analyzer provide various modes ofoperation. Both probeheads are equipped with two-circlegoniometers to ensure accurate study of magneticanisotropy.</p><p>The spectrometer was used to make express-analysis ofquality thus to optimize processing parameters of epitaxialiron garnet films grown by pulsed laser deposition (PLD).Comprehensive study of uniaxial and cubic magnetocrystallineanisotropy has been performed for Ce:Y₃Fe₅O₁₂bulk crystal as well as for Y₃Fe₅O₁₂and Bi₃Fe₅O₁₂films grown on different substrates by PLD andreactive ion beam sputtering techniques. BroadbandFMR-spectroscopy revealed difference in spectra of domain wallresonances: instead of“soft”spin modes in filmsgrown by liquid phase epitaxy, PLD-made films show“diffuse”transformation of domains near thesaturation field. This effect indicates non-uniformity ofsaturation magnetization and field of uniaxial anisotropy inPLD-iron garnets. Spin wave resonances in comparison withuniform FMR have been studied to evaluate“localquality”of ferromagnetic films. The resonance field andFMR linewidth behavior were studied at various crystallographicdirections determined by X-ray diffraction.</p><p>FMR was used to choose PLD-made YIG films with low losses atmicrowave frequencies and to build magnetostatic surface wavesmicrowave bandpass filter. The filter was designed as a planarfilm structure with a microstrip line for transducers. It is afirst demonstration of feasibility to introduce PLD processingtechnique to magnetostatic wave technology.</p><p>Magneto-optical study of Ce:Y₃Fe₅O₁₂single crystal complements results ofFMR-spectroscopy of new garnets.</p><p><b>Keywords:</b>ferrites, thin films, ferromagnetic resonance,microwaves, FMR spectrometer, magnetic anisotropy,magnetostatic waves.</p>

This thesis investigates the recently developed technique of spin-torque-driven ferromagentic resonance (ST-FMR). Contrary to conventional FMR techniques where the magnetodynamics are excited by torques on the magentic moments produced by microwave fields, ST-FMR uses the spin-transfer torque acting on a nanomagnet. Here we present two experiments which exploit ST-FMR in the standard AFM/FM/NM/FM exchange-biased spin valve (EBSV) structures, where the ferromagnetic (FM) layers are separated by a nonmagnetic (NM) spacer, and one ferromagnet is pinned with an antiferromagnetic (AFM) layer. In our experiments microwave currents are applied to a mechanical point contact between a sharpened Cu tip and a SV (IrMn/Py/Cu/Py) multilayer film. While most ST-FMR experiments require noncollinear orientation of FM-layer magnetizations, we studied ST-FMR in SVs above saturation, where the two FM layers have parallel magnetizations. The resulting magnetodynamics are detected electrically by a small rectified dc voltage, which appears across the structure during resonance. Studies of the resonance frequencies, amplitudes, line widths, and line shapes as a function of microwave power, microwave frequency, dc current and magentic field are presented. The results are analyzed in terms of ST-FMR and rectification based on GMR. However the origin of the observed voltage cannot be fully explained by the resistance changes which come from the giant magnetoresistance (GMR) effect of the spin valve. To investigate other sources of rectified dc voltages at resonance we have performed the second set of measurements with lithographically patterned pairs of (Py/Cu/Co/IrMn)-SV microstripes. These measurements also revealed a resonance in the rectified voltage at FMR frequencies, and showed additional structures which might be related to spin wave excitations. The observations can be tentatively attributed to additional rectification effects due to anisotropic magnetoresistance (AMR). The line pair structure allows us to use different measurement geometries to investigate the magnetodynamics in the SV. In this experiment FMR can either be excited by spin transfer or by a rf magnetic field created by the microwave current, depending on the used geometry. Qualitative studies of the FMR dependencies and characteristics are presented for different measurement geometries. / text

Thesis (M.S.)--Miami University, Dept. of Physics, 2009. / Title from first page of PDF document. Includes bibliographical references (p. 38).

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.