Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In the present work, we study the magnetic properties in the site diluted two-state Ising model on a body-centered cubic lattice. As an application of this study, we has been obtained the phase diagram for Fe-Al alloys and comparisons between theoretical model and experimental data are made by using the exchange interaction between Fe atoms in the alloy up to the third power dependence on concentration q of Al atoms. We have used the differential operator technique to expand the analytical expressions that arise from the diluted Ising model and describe such behavior by means of magnetic clusters with one and two spins. A good agreement between the theoretical model and experimental data is also obtained for both the magnetization and to the lattice parameter, r as a function of q. In this approach, it is shown as a good alternative to other analytical and computational techniques used for the study of diluted magnetic systems. The analytical results obtained for bcc lattice were compared with the Monte Carlo technique to determine the degree of applicability of the model and confirm the validity of the results obtained analytically. / Neste trabalho, aplicamos o modelo de Ising com diluição por sítios ao estudo das propriedades magnéticas de uma rede cúbica de corpo centrado (do inglês, body centered cubic, bcc), cujos sítios podem ser ocupados por spins. Como aplicação desse estudo, traçamos alguns diagramas de fase da liga Fe-Al, com comparações entre o modelo teórico utilizado e dados experimentais e propomos uma interação de troca entre os atomos de Fe, na liga, com dependência at´e terceira potência da concentração (q) de atomos de Al, possibilitando um melhor ajuste entre teoria e experimento para o diagrama T-q desta liga. Tomamos como base a t´ecnica do operador diferencial para expandir as express oes anal´ıticas que surgem do modelo de Ising dilu´ıdo e descrevemos tal comportamento magn´etico por meio de aglomerados com 1 e 2 spins. Um bom acordo entre o modelo te´orico proposto e dados experimentais tamb´em ´e obtido tanto para a magnetização como função de q quanto para o par ametro de rede, r, como função de q, sendo que esta abordagem mostra-se como uma boa alternativa a outras t´ecnicas anal´ıticas e computacionais utilizadas para o estudo de sistemas magn´eticos dilu´ıdos. Os resultados anal´ıticos obtidos para a rede bcc foram comparados com a t´ecnica de Monte Carlo para verificar o grau de aplicabilidade do modelo bem como confirmar a validade dos resultados encontrados analiticamente.

The correlation of vacancy concentration with microhardness of Fe-Al alloys was studied on samples quenched from 1000 řC and subsequently annealed at lower temperatures. Using X-ray diffraction the lattice parameter and crystal structure were determined for samples of Fe-Al alloys. By measurements of positron lifetime was revealed the high concentration of vacancies in quenched samples and subsequent annealing caused significant decrease in vacancy concentration while in samples with Al content above 39% also the decrease of microhardness was measured. Measurements of coincidence Doppler broadening of annihilation peak helped to distinguish the annihilations coming from positron trapped or delocalized annihilated by electrons of both atoms, Fe and Al. Comparison of measured results with theoretical quantum-mechanics calculations performed in this diploma thesis determined the most probable defect type as a vacancy on A-sublatice of B2 structure.

Title: Investigation of vacancies in Fe-Al alloys Author: RNDr. František Lukáč Department: Department of Low Temperature Physics Supervisor: doc. Mgr. Jakub Čížek, Ph.D., Department of Low Temperature Physics, Faculty of Mathematics and Physics Abstract: Fe-Al alloys exhibit relatively high vacancy concentrations, which significantly influence the mechanical properties of these alloys. Positron annihi- lation spectroscopy was employed for investigations of vacancies in this work and the vacancy concentrations in Fe-Al alloys were determined by positron lifetime spectroscopy and measurements of positron diffusion length of implanted mono- genergetic positrons. The correlation of hardness with the vacancy concentration was characterized by determination of the vacancy hardening coefficient in Fe- Al alloys. The thermal evolutions of hardness and the vacancy concentration were compared during the annealing of the Fe-Al alloys with various composi- tions. Coincidence Doppler broadening of annihilation peak was employed for determination of the chemical composition of quenched-in vacancies in Fe-Al al- loys. Increased concentration of Al atoms in surrounding vacancies in Fe-Al alloys was related to the attractive interaction between the Al atoms and the vacancy. Quantum mechanical ab initio calculations were...