abstract: Soft magnetic alloys play a significant role for magnetic recording applications and highly sensitivity magnetic field sensors. In order to sustain the magnetic areal density growth, development of new synthesis techniques and materials is necessary. In this work, the effect of oxygen incorporation during electrodeposition of CoFe alloys on magnetic properties, magnetoresistance and structural properties has been studied. Understanding the magnetic properties often required knowledge of oxygen distribution and structural properties of the grown films. Transmission electron microscopy (TEM) was a powerful tool in this study to correlate the oxygen-distribution nanostructure to the magnetic properties of deposited films. Off-axis electron holography in TEM was used to measure magnetic domain wall width in the deposited films. Elemental depth profiles of Fe, Co, O were investigated by secondary ion mass spectroscopy (SIMS). Magnetic properties have been determined by superconducting quantum interference device (SQUID) measurements. Oxygen content in the CoFe deposited films was controlled by electrolyte composition. Films were deposited on Si 100 substrates and on other substrates such as Cu and Al. However, a good film quality was achieved on Si substrate. Electron energy loss and x-ray spectroscopies showed that the low oxygen films contained intragranular Fe2+ oxide (FeO) particles and that the high oxygen films contained intergranular Fe3+ (Fe2O3) along grain boundaries. The films with oxide present at the grain boundary had significantly increased coercivity, magnetoresistance and reduced saturation magnetization relative to the lower oxygen content films with intragranular oxide. The differences in magnetic properties between low oxygen and high oxygen concentration films were attributed to stronger mobile domain wall interactions with the grain boundary oxide layers. The very high magnetoresistance values were achieved for magnetic devices with nanocontact dimension < 100 nm and oxide incorporation in this nanoconfined geometry. The content of oxide phase in nanocontact was controlled by concentration of the Fe3+ ions in the electrodeposition solution. Magnetic device integrity was improved by varying amount of additive into plating solution. These results indicated that electrodeposited CoFe nanocontact is a novel class of materials with large application for magnetic field sensors. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2012

Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro / Nesta dissertação, microesferas magnéticas não-porosas à base de estireno (STY) e divinilbenzeno (DVB) foram sintetizadas pela técnica da polimerização em suspensão. O material utilizado para conferir as propriedades magnéticas foi o ferro. Dois tipos diferentes de ferro foram utilizados: I ferro tratado com ácido oleico e II ferro sem tratamento. Foram estudados os efeitos do teor dos dois tipos de ferro, a concentração de iniciador e o tipo de agente de suspensão sobre as características das partículas poliméricas obtidas. As microesferas foram caracterizadas quanto ao seu aspecto morfológico, à estabilidade térmica, ao tamanho e à distribuição do tamanho de partículas e quanto à sua propriedade magnética. Foram obtidos copolímeros com morfologia esférica e com propriedades magnéticas próximas de materiais superparamagnéticos. A magnetização de saturação para os copolímeros onde foram adicionados 20% de ferro na síntese foi acima dos valores encontrados na literatura. A distribuição de tamanho das partículas foi mais estreita para os copolímeros sintetizados com 5% de peróxido de benzoíla (BPO). A adição de ferro aumentou a temperatura de degradação quando comparada ao copolímero de STY-DVB sem ferro. vii / In this dissertation, nonporous magnetic microspheres based on styrene (STY) and divinylbenzene (DVB) were synthesized by suspension polymerization technique. In order to confer magnetic properties iron was employed. Two different types of iron were used: I) iron treated with oleic acid and II) iron without treatment. The effect of type of iron, initiator concentration and type of stabilizer was studied. The magnetic microspheres were characterized according to morphology, thermal stability, particle size distribution and magnetic properties. It was verified that the microspheres presented spheric shape and magnetic properties typical for superparamagnetic materials. The saturation magnetization for copolymers with 20 wt% was higher than that values reported in the literature. The particle size distribution was more monodisperse with 5 wt% benzoyl peroxide (BPO). Iron improved the decomposition temperature of STY-DVB copolymers when compared with the material without iron. viii

The magnetic behaviour of thin films of (111) FCC structures and (0001) corundum structured materials were studied by the mean field analysis and some Monte Carlo simulation. These models were conditioned on a mapping from first principles calculations to the Ising model. The effect of the suggested octopolar reconstruction for the polar (111) surfaces of FCC was also examined.

Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro / Nesta dissertação, microesferas magnéticas não-porosas à base de estireno (STY) e divinilbenzeno (DVB) foram sintetizadas pela técnica da polimerização em suspensão. O material utilizado para conferir as propriedades magnéticas foi o ferro. Dois tipos diferentes de ferro foram utilizados: I ferro tratado com ácido oleico e II ferro sem tratamento. Foram estudados os efeitos do teor dos dois tipos de ferro, a concentração de iniciador e o tipo de agente de suspensão sobre as características das partículas poliméricas obtidas. As microesferas foram caracterizadas quanto ao seu aspecto morfológico, à estabilidade térmica, ao tamanho e à distribuição do tamanho de partículas e quanto à sua propriedade magnética. Foram obtidos copolímeros com morfologia esférica e com propriedades magnéticas próximas de materiais superparamagnéticos. A magnetização de saturação para os copolímeros onde foram adicionados 20% de ferro na síntese foi acima dos valores encontrados na literatura. A distribuição de tamanho das partículas foi mais estreita para os copolímeros sintetizados com 5% de peróxido de benzoíla (BPO). A adição de ferro aumentou a temperatura de degradação quando comparada ao copolímero de STY-DVB sem ferro. vii / In this dissertation, nonporous magnetic microspheres based on styrene (STY) and divinylbenzene (DVB) were synthesized by suspension polymerization technique. In order to confer magnetic properties iron was employed. Two different types of iron were used: I) iron treated with oleic acid and II) iron without treatment. The effect of type of iron, initiator concentration and type of stabilizer was studied. The magnetic microspheres were characterized according to morphology, thermal stability, particle size distribution and magnetic properties. It was verified that the microspheres presented spheric shape and magnetic properties typical for superparamagnetic materials. The saturation magnetization for copolymers with 20 wt% was higher than that values reported in the literature. The particle size distribution was more monodisperse with 5 wt% benzoyl peroxide (BPO). Iron improved the decomposition temperature of STY-DVB copolymers when compared with the material without iron. viii

Nesta dissertação, foram sintetizados copolímeros à base de poli(metacrilato de metila) (PMMA), divinilbenzeno (DVB) com propriedades magnéticas pela técnica de polimerização em suspensão. O material utilizado para conferir propriedade magnética foi o ferro. Foi estudada a influência das variáveis reacionais para a formação de microesferas (ordem de adição das fases orgânica e aquosa e a dispersão do ferro antes e após a pré-polimerização), o efeito da concentração de ferro adicionado na polimerização, a influência da razão molar MMA/DVB na formação do copolímero, o efeito do tipo de agente de suspensão e a velocidade de agitação para a síntese dos copolímeros. Os copolímeros foram caracterizados quanto à morfologia, estabilidade térmica, teor de ferro incorporado, distribuição de tamanho de partículas, propriedades magnéticas, área superficial, volume e tamanho de poros. As análises de propriedades magnéticas mostraram que os materiais obtidos não apresentaram ciclos de histerese, estando assim próximos de um material com propriedades superparamagnéticas, com magnetização de saturação entre 8,0 e 13,0 emu/g. Os copolímeros sintetizados com velocidade de agitação 500 RPM, temperaturade 90 C, tempo de polimerização de 24h, monômeros MMA/DVB 50/50 (% molar); razão volumétrica fase orgânica/fase aquosa 1/3, diluente heptano, grau de diluição 100% apresentaram melhor controle morfológico, melhor magnetização de saturação, tamanho de partículas menores e maior tamanho de poros / In this dissertation, copolymers based on methyl methacrylate (MMA) and divinylbenzene (DVB) with magnetic properties were synthesized by suspension polymerization. It was employed iron as magnetic material. The influence of reaction variables for the microspheress formation as order of addition of the organic and aqueous phases, dispersion of iron before and after pre-polymerization, iron concentration added to the polymerization, MMA/DVB molar ratio, type of suspending agent and stirring speed were studied. The copolymers were characterized with respect to morphology, thermal stability, the amount of iron incorporated, particle size distribution, magnetic properties, surface area and volume and size of pores. The materials obtained showed no hysteresis loops, standing near a material which superparamagnetic with saturation magnetization between 8.0 and 13.0 emu / g. The copolymers with agitation speed 500 RPM, temperature of 90 C, curing time of 24 hours, molar ratio MMA / DVB = 50/50 (mol%); volume ratio organic phase/aqueous phase = 1/3, heptane and 100% degree of dilution showed better morphologic control, better saturation magnetization, smaller particle size and larger pores

D.Phil. / Spin-density-wave (SDW) effects were studied in diluted Cr-Si and Cr-Ga alloy single cristals. Ga and Si impurities in Cr are respectively from groups 3 and 4 non-magnetic non-transitional elements from the periodic table. Both influence the magnetic behaviour of Cr in a special way. The alloying of Cr with Si and Ga impurities influences the magnetic phase transition temparatures, namely the Néel temparature (Tn) and the transition temparature (T1c) for the incommensurate to commensurate (C) SDW transition, in a very complex way. The magnetic phase diagrams of both Cr-Si and Cr-Ga show a triple phase where the paramegnetic (P), CSDW and ISDW phases co-exist.

The use of spin polarised neutron scattering as an experimental probe for magnetic phenomena within solid state physics has been demonstrated in two separate studies of superconductors. With the use of three-dimensional polarisation analysis, the magnetic response within a thermal energy window -30meV < Iiw < +30meV has been measured for both insulating and superconducting compositions of the high T c material YBa2Cu306+x. The observed magnetic scattering from the superconducting composition (x=0.9S, T c-90K) at T =300K shows conclusively that there is negligible magnetic scattering in the normal state below 30meV. What little scattering is observed corresponds to a paramagnetic cross-section of (0.048±0.008)barns or to -3.2% of the Cu atoms carrying a spin 1/2. Similar polarised neutron measurements performed on insulating YBa2Cu306.11, above and below TN, reveal a small but finite level of scattering corresponding to -18% of the Cu atoms carrying a spin 1/2 in the paramagnetic state. The observed increase in paramagnetic scattering above T N corresponds to just 20% of that expected due to the thermal disordering of the observed ordered magnetic moment at 300K. These measurements are consistent with a magnetic excitation spectrum driven by quantum rather than thermal fluctuations for which the notion of separate magnetic and charge degrees of freedom (and therefore also the concept of local moments) may not be valid. In addition, measurements conducted on single crystal YBa2Cu307 in its normal state revealed no indication of magnetic enhancement in the vicinity of the (It, It) point up to 30meV. This is in disagreement with theoretical predictions of an enhanced magnetic scattering in this region for spin fluctuation energies of order 10 to 20meV. The second study demonstrates the feasibility of spin polarised neutron scattering as a novel tool for the investigation of the flux line lattice in type 11 superconductors. Measurements conducted in the mixed state of niobium (T =4.SK, B=O.22T) constitute the first experimental observation of lattice distortions due to the presence of flux line vortices. The experimentally determined magnitude of the lattice distortion yields a value approximately three orders of magnitude greater than that expected due to the volume anomaly between the coexisting normal and superconducting regions. An alternative mechanism is suggested on the basis of electron redistribution between the normal and superconducting regions in which electrons are trapped by the flux lines. As a result of this process, a response in the form of a lattice distortion is induced in order to maintain charge neutrality.

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