Submitted by Automa??o e Estat?stica (sst@bczm.ufrn.br) on 2016-05-03T23:21:33Z No. of bitstreams: 1 PriscilaValdeniaDosSantos_TESE.pdf: 1843795 bytes, checksum: 90c4e994c9a54d049c923beefec400bf (MD5) / Approved for entry into archive by Arlan Eloi Leite Silva (eloihistoriador@yahoo.com.br) on 2016-05-05T22:38:19Z (GMT) No. of bitstreams: 1 PriscilaValdeniaDosSantos_TESE.pdf: 1843795 bytes, checksum: 90c4e994c9a54d049c923beefec400bf (MD5) / Made available in DSpace on 2016-05-05T22:38:19Z (GMT). No. of bitstreams: 1 PriscilaValdeniaDosSantos_TESE.pdf: 1843795 bytes, checksum: 90c4e994c9a54d049c923beefec400bf (MD5) Previous issue date: 2015-07-15 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior (CAPES) / Neste trabalho investigamos o efeito dos campos aleat?rios no modelo Blume-Capel com intera??es de longo alcance. O modelo ? completamente sol?vel no ensemble can?nico, e sua densidade de energia livre nos leva a resultados correspondentes a uma teoria de campo m?dio. S?o estudados os casos para spin S = 1 sob influ?ncia de desordem temperada na presen?a de: (i) um campo cristalino aleat?rio; (ii) de um campo magn?tico aleat?rio; (iii) e de ambos. Para uma escolha adequada do campo aleat?rio, mostramos que o modelo apresenta uma variedade de comportamentos multicr?ticos, linhas de transi??o cont?nuas e de primeira ordem, al?m de fen?menos de re-entr?ncia. Os diagramas de fases, obtidos a partir do c?lculo da energia livre por spin, exibem diversas topologias em fun??o do par?metro que mede o grau de desordem. / In the presente work we investigate the ferromagnetic Blume-Capel (BC) model, for spin 1 and infinite-ranged interactions, under the influence of local quenched disorder. The model is exactly solved within the canonical ensemble. The obtained free energy density lead us to mean-field results. In the first part we study the BC model under the influence of a random crystal-field anisotropy, but otherwise without a magnetic field. In the second part we consider the BC model under a bimodal random magnetic field and a uniform crystal-field anisotropy term. This model was previously studied by Kaufman and Kanner. We give special attention to anisotropy versus temperature phase diagrams which may present reentrant phenomena. Finally, in the third part we consider a generalized version where both local fields - magnetic and crystal-field anisotropy - are diluted and, in the present case, modeled by discrete probability distribution. The phase diagram obtained and presented in this work exhibit a rich variety of multicritical behavior, presenting both continuous and first-order transition lines. Also, for some specific cases there is room for the existence of reentrant effects. This seems to be a characteristic of the Blume-Capel model under the presence of randomness.