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Previous issue date: 2017-01-25 === O estudo das propriedades magn?ticas de nanoestruturas teve grandes avan?os
ao longo do curso do desenvolvimento da Ci?ncia, e ainda continua sendo
alvo de intensa pesquisa em F?sica nos dias atuais. Do ponto de vista te?rico,
h? interesse no surgimento de novas configura??es magn?ticas em nanoelementos
magn?ticos, oriundas de efeitos dipolares, em sistemas com dimens?es da
ordem de dezenas do comprimento de troca. H? igualmente interesse no controle
das fases magn?ticas destes sistemas para fins de aplica??es tecnol?gicas, como
sensores, nano-osciladores e mem?rias magn?ticas. Motivados por trabalhos que
mostram a possibilidade de confinamento de paredes de dom?nio em nanoan?is
ferromagn?ticos, atrav?s de constri??es geom?tricas como defeitos e assimetrias
e, tamb?m, por estudos que revelam uma depend?ncia da resist?ncia el?trica com
as dimens?es de paredes de dom?nio, propomos uma nova maneira para forma??o
e controle de paredes de dom?nio em nanoan?is, baseada na intera??o magn?tica
dipolar em estruturas do tipo n?cleo-casca cil?ndricas. Neste trabalho, mostramos
que nanoan?is de NiFe t?m seu padr?o magn?tico drasticamente modificado
quando na presen?a de um nanodisco coaxial de Fe ou de NiFe. Com um nanodisco
de Fe como n?cleo, ? observada a forma??o de estados com paredes de
dom?nio em nanoan?is de NiFe, que apresentariam v?rtices magn?ticos quando
isolados. Al?m disso, mostramos que a largura e o padr?o das paredes de dom?nio
podem ser controlados pelos valores das dimens?es geom?tricas do sistema
n?cleo-casca. Quando um nanodisco de NiFe ? usado, nota-se a forma??o de
algumas configura??es magn?ticas em que tanto o padr?o magn?tico da casca ?
influenciado pelo campo dipolar do n?cleo, como o padr?o do n?cleo ? influenciado
pelo campo dipolar da casca, devido ? aus?ncia de anisotropia magn?tica do
NiFe. === Nanostructured magnetic systems and their properties have gone through
great achievement and still keep being focus of intense research effort nowadays.
From the pure theoretical point of view, the interest focus on new magnetic phases
that might emerge from dipolar effects in systems, with geometric constrictions,
with dimensions of the order of tenths of fundamental magnetic lengths. There is
also a large interest from the viewpoint of applications, such as magnetic sensors,
nano-oscillators and new designs of magnetic memories. Motivated by recent
experimental results, showing the possibility of domain wall confinement in ferromagnetic
nanorings, by magnetic constrictions like notches and asymmetries,
in addition to studies which reveal a strong impact of the domain wall width on
the electrical resistance, we propose a new form for creating and tailoring domain
walls in nanorings, by means of the magnetic dipolar interaction in cylindrical
core-shell nanostructures. In this work, we show that NiFe nanorings have their
magnetic pattern drastically changed when put in the presence of a coaxial Fe or
NiFe nanodisk. With a Fe nanodisk as the core, we have found the formation of
domain walls in NiFe nanorings that would hold magnetic vortices when isolated.
Furthermore, we show that large changes in the domain wall width and domain
wall patterns may be achieved by suitable choices of the core-shell geometrical
dimensions. For NiFe-NiFe core-shells, we have found that the core-shell dipolar
interaction affects both the core and shell phases, due to the vanishing anisotropy
of NiFe.
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