Made available in DSpace on 2014-12-17T15:14:53Z (GMT). No. of bitstreams: 1 FabioHSS_TESE.pdf: 1521622 bytes, checksum: c8f41898af4074d0601d4f7c0e859f67 (MD5) Previous issue date: 2011-02-04 / There is presently a worldwide interest in artificial magnetic systems which guide research activities in universities and companies. Thin films and multilayers have a central role, revealing new magnetic phases which often lead to breakthroughs and new technology standards, never thought otherwise. Surface and confinement effects cause large impact in the magnetic phases of magnetic materials with bulk spatially periodic patterns. New magnetic phases are expected to form in thin film thicknesses comparable to the length of the intrinsic bulk magnetic unit cell. Helimagnetic materials are prototypes in this respect, since the bulk magnetic phases consist in periodic patterns with the length of the helical pitch. In this thesis we study the magnetic phases of thin rare-earth films, with surfaces oriented along the (002) direction. The thesis includes the investigation of the magnetic phases of thin Dy and Ho films, as well as the thermal hysteresis cycles of Dy thin films. The investigation of the thermal hysteresis cycles of thin Dy films has been done in collaboration with the Laboratory of Magnetic Materials of the University of Texas, at Arlington. The theoretical modeling is based on a self-consistent theory developed by the Group of Magnetism of UFRN. Contributions from the first and second neighbors exchange energy, from the anisotropy energy and the Zeeman energy are calculated in a set of nonequivalent magnetic ions, and the equilibrium magnetic phases, from the Curie temperature up to the N?el temperature, are determined in a self-consistent manner, resulting in a vanishing torque in the magnetic ions at all planes across the thin film. Our results reproduce the known isothermal and iso-field curves of bulk Dy and Ho, and the known spin-slip phases of Ho, and indicate that: (i) the confinement in thin films leads to a new magnetic phase, with alternate helicity, which leads to the measured thermal hysteresis of Dy ultrathin films, with thicknesses ranging from 4 nm to 16 nm; (ii) thin Dy films have anisotropy dominated surface lock-in phases, with alignment of surface spins along the anisotropy easy axis directions, similar to the known spin-slip phases of Ho ( which form in the bulk and are commensurate to the crystal lattice); and (iii) the confinement in thin films change considerably the spin-slip patterns of Ho. / Sistemas magn?ticos artificiais s?o foco do interesse em universidades e ind?strias ao redor do mundo. A ci?ncia e tecnologia de filmes finos e multicamadas magn?ticas tem desempenhando um papel central, revelando fases novas, cujas propriedades, n?o raro, d?o origem a quebra de paradigmas e novos ramos industriais antes inimagin?veis. Materiais magn?ticos cujas fases consistem da repeti??o de padr?es envolvendo um conjunto de ?ons magn?ticos s?o mais suscet?veis a efeitos de superf?cie. Novas fases s?o esperadas se a espessura do filme se aproxima do comprimento de periodicidade intr?nseco. Materiais helimagn?ticos s?o prot?tipos, uma vez que o per?odo da h?lice ? o comprimento da c?lula unit?ria magn?tica do volume. O estudo das fases magn?ticas de filmes finos de terras raras, com superf?cies na dire??o (002), ? o foco desta tese de doutorado. O projeto de pesquisa inclui a investiga??o do diagrama de fases de equil?brio de filmes finos de Dy, e de Ho, bem como da histerese t?rmica de filmes de Dy. Ciclos de histerese t?rmica de filmes ultrafinos de Dy est?o sendo investigados em parceria com o grupo experimental do laborat?rio de magnetismo da Universidade do Texas, em Arlington - EUA. O m?todo te?rico, para obter as fases de equil?brio, consiste de um sistema, desenvolvido no Grupo de Magnetismo da UFRN, em que as contribui??es das energias de troca, de primeiros e segundos vizinhos, a energia de anisotropia e a energia Zeeman, s?o calculadas em um conjunto de ?ons magn?ticos inequivalentes, e as fases magn?ticas, desde a temperatura de Curie at? a temperatura de Neel, s?o determinadas de forma auto-consistente, garantindo torque nulo nos ?ons magn?ticos em cada plano do filme. Nossos resultados reproduzem as medidas de magnetiza??o para o volume ( a campo magn?tico fixo e medidas isot?rmicas ) e as fases spin-slip do Ho, e sugerem que: (i)o confinamento em filmes finos d? origem a uma nova fase, de helicidade alternada, respons?vel pela histerese t?rmica medida em filmes de Dy com espessura entre 4nm e 16nm; (ii)filmes finos de Dy exibem fases com preval?ncia da anisotropia ("surface lock-in"), semelhantes ?s fases ("spin-slips") do Ho que s?o comensur?veis com a rede cristalina; e (iii)o confinamento em filmes finos altera o padr?o das fases "spin-slips" do Ho

Umělé spinové systémy jsou vhodným nástrojem pro zkoumání a ovlivňování neobvyklých exotických nízko-energiových stavů přímo v reálném prostoru. Experimentální realizace těchto systémů jsou založeny na výrobě vzájemně interagujících nano-magnetů uspořádaných do požadované geometrie. Prvním a asi i nejvíce studovaným umělým systémem je prostá čtvercová mřížka. V této práci se zabýváme modifikováním této čtvercové geometrie, které umožní zachycení různých magnetických fází založených na modelech ledu. Výsledky ukazují, že vhodným nastaveném této modifikace lze realizovat různé magnetické fáze, včetně neuspořádané spinové kapalné fáze s uvězněnými magnetickými kvazičásticemi (magnetickými monopóly).