Made available in DSpace on 2014-12-17T15:14:55Z (GMT). No. of bitstreams: 1 JoaoMA_TESE.pdf: 3210677 bytes, checksum: b439c16c54cbcf828e9759b7638b198c (MD5) Previous issue date: 2000-01-14 / In this work we have studied the effects of random biquadratic and random fields in spin-glass models using the replica method. The effect of a random biquadratic coupling was studied in two spin-1 spin-glass models: in one case the interactions occur between pairs of spins, whereas in the second one the interactions occur between p spins and the limit p > oo is considered. Both couplings (spin glass and biquadratic) have zero-mean Gaussian probability distributions. In the first model, the replica-symmetric assumption reveals that the system presents two pha?ses, namely, paramagnetic and spin-glass, separated by a continuous transition line. The stability analysis of the replica-symmetric solution yields, besides the usual instability associated with the spin-glass ordering, a new phase due to the random biquadratic cou?plings between the spins. For the case p oo, the replica-symmetric assumption yields again only two phases, namely, paramagnetic and quadrupolar. In both these phases the spin-glass parameter is zero. Besides, it is shown that they are stable under the Almeida-Thouless stability analysis. One of them presents negative entropy at low temperatures. We developed one step of replica simmetry breaking and noticed that a new phase, the biquadratic glass phase, emerge. In this way we have obtained the correct phase diagram, with.three first-order transition lines. These lines merges in a common triple point. The effects of random fields were studied in the Sherrington-Kirkpatrick model consi?dered in the presence of an external random magnetic field following a trimodal distribu?tion {P{hi) = p+S(hi - h0) +Po${hi) +pS(hi + h0))- It is shown that the border of the ferromagnetic phase may present, for conveniently chosen values of p0 and hQ, first-order phase transitions, as well as tricritical points at finite temperatures. It is verified that the first-order phase transitions are directly related to the dilution in the fields: the extensions of these transitions are reduced for increasing values of po- In fact, the threshold value pg, above which all phase transitions are continuous, is calculated analytically. The stability analysis of the replica-symmetric solution is performed and the regions of validity of such a solution are identified / Nesse trabalho estudamos os efeitos de acoplamentos biquadr?ticos e campos aleat?rios em vidros de spins utilizando o m?todo das r?plicas. O efeito do acoplamento biquadr?tico aleat?rio ? estudado em um vidro de spins com spin 1 em duas situa??es diferentes: no caso onde as intera??es s?o entre pares de spins e no caso onde a intera??o ocorre entre p spins no limite p > oo. Ambos os acoplamen?tos (vidro de spins e biquadr?tico) t?m distribui??o de probabilidades gaussianas com m?dia zero. Para intera??es entre pares de spins a solu??o com simetria entre r?plicas fornece as fases paramagn?tica e vidro de spins separadas por uma linha de transi??es cont?nuas. A an?lise de estabilidade da solu??o com simetria entre r?plicas fornece, al?m da instabilidade usual associada com o ordenamento vidro de spins, uma nova fase devido ao acoplamento biquadr?tico entre os spins. Para o caso p = oo, a solu??o com simetria entre r?plicas fornece apenas duas fases, marginalmente est?veis com rela??o ? an?lise de estabilidade de de Almeida e Thouless. Entretanto, uma dessas fases apresenta entropia negativa a baixas temperaturas. Implementamos tamb?m um passo de quebra de sime?tria nos par?metros vidros de spins e vidro quadrupolar e obtivemos o diagrama de fases correto, incluindo a fase vidro de spins. Nesse caso, todas as transi??es s?o de primeira ordem e as tr?s fases coexistem em um ponto triplo. Os efeitos de campos aleat?rios s?o estudados no modelo de Sherrington-Kirkpatrick, onde consideramos um campo aleat?rio com distribui??o de probabilidades trimodal {P(hi) = p+?(hi - h0) +po?(h0) +p-6(hi + ho))- Mostramos que para valores convenientes de po e h0, a fronteira ferromagn?tica pode apresentar transi??es de primeira ordem e pontos tricr?ticos a temperaturas finitas. Verificamos que as transi??es de primeira ordem est?o diretamente relacionadas com a dilui??o do campo: as extens?es dessas transi??es s?o reduzidas quando aumentamos o valor de po- De fato, o valor pl acima do qual todas as transi??es s?o cont?nuas ? calculado analiticamente. Fizemos tamb?m a an?lise de es?tabilidade da solu??o com simetria entre r?plicas e determinamos as regi?es de validade desta solu??o

Made available in DSpace on 2014-12-17T15:14:57Z (GMT). No. of bitstreams: 1 EdimilsonFS_DISSERT.pdf: 2546760 bytes, checksum: 08525df56e96d75f8d221757342883b4 (MD5) Previous issue date: 2012-02-17 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / In this work, we investigated the magnetic properties of a monocrystalline Fe thin film and of Fe(80 ?)/Cr(t)/Fe(80 ?) tri-layers, with the nonmagnetic metallic Cr spacer layer thickness varying between 9 ? < t < 40 ?. The samples were deposited by the DC Sputtering on Magnesium Oxide (MgO) substrates, with (100) crystal orientation. For this investigation, experimental magneto-optical Kerr effect (MOKE) magnetometry and ferromagnetic resonance (FMR) techniques were employeed. In this case, these techniques allowed us to study the static and dynamical magnetization properties of our tri-layers. The experimental results were interpreted based on the phenomenological model that takes into account the relevant energy terms to the magnetic free energy to describe the system behavior. In the case of the monocrystalline Fe film, we performed an analytical discussion on the magnetization curves and developed a numerical simulation based on the Stoner-Wohlfarth model, that enables the numerical adjustment of the experimental magnetization curves and obtainment of the anisotropy field values. On the other hand, for the tri-layers, we analyzed the existence of bilinear and biquadratic couplings between the magnetizations of adjacent ferromagnetic layers from measurements of magnetization curves. With the FMR fields and line width angular dependencies, information on the anisotropy in three layers was obtained and the effects of different magnetic relaxation mechanisms were evidenced. It was also possible to observe the dependence of the epitaxy of the multilayers with growth and sputtering parameters. Additionally it was developed the technique of AC magnetic susceptibility in order to obtain further information during the investigation of magnetic thin films / Nesta disserta??o investigamos as propriedades magn?ticas de um filme monocristalino de ferro e tricamadas com a composi??o Fe(80?)/Cr(t)/Fe(80?), variando as espessuras da camada met?lica n?o magn?tica separadora (Cr) entre 9? < t < 40?. As amostras foram depositadas pela t?cnica Sputtring DC sobre substrato de ?xido de Magn?sio (MgO) com orienta??o cristalina (100). Para esta investiga??o foram utilizadas as t?cnicas experimentais de magnetometria por efeito Kerr Magneto-?ptico (MOKE) e Resson?ncia Ferromagn?tica (FMR). Estas t?cnicas permitiram o estudo das propriedades est?ticas e din?micas da magnetiza??o das multicamadas. Os resultados experimentais foram interpretados com base no modelo fenomenol?gico que leva em considera??o os termos mais relevantes da energia livre magn?tica para descrever o comportamento do sistema. Para o filme monocristalino de ferro, realizamos uma discuss?o anal?tica das informa??es das curvas de magnetiza??o e desenvolvemos uma simula??o num?rica baseada no modelo de Stoner-Wohlfarth, possibilitando o ajuste num?rico das curvas de magnetiza??o experimentais e a obten??o do valor de campo de anisotropia. Para as tricamadas, analisamos a presen?a dos acoplamentos bilinear e biquadr?tico entre as magnetiza??es das camadas ferromagn?ticas adjacentes a partir das medidas das curvas de magnetiza??o. Com as depend?ncias angulares do campo de FMR e da largura de linha obtivemos informa??es sobre as anisotropias presentes nas tricamadas e evidenciamos os efeitos de diversos mecanismos de relaxa??o magn?tica. Tamb?m foi poss?vel observar a depend?ncia da epitaxia das multicamadas com os par?metros de crescimento do Sputtering. Adicionalmente foi desenvolvida, durante o per?odo do mestrado, a t?cnica de susceptibilidade magn?tica AC com o objetivo de obter resultados inovadores na investiga??o de filmes finos magn?ticos