Magnetization reversals in a disk-shaped small magnet with an interface

We consider a nanodisk possessing two coupled materials with different ferromagnetic exchange constant. The common border line of the two media passes at the disk center dividing the system exactly in two similar half-disks. The vortex core motion crossing the interface is investigated with a simple description based on a two-dimensional model which mimics a very thin real material with such a line defect. The main result of this study is that, depending on the magnetic coupling which connects the media, the vortex core can be dramatically and repeatedly flipped from up to down and vice versa by the interface. This phenomenon produces burst-like emission of spin waves each time the switching process takes place.Comment: 11 pages, 10 figure

Correção de deficiência de boro em videira.

bitstream/item/25707/1/Cit41.pdf; bitstream/item/75015/1/CNPUV-CIR.-TEC.-41-03.pd

The multipliers of periodic points in one-dimensional dynamics

It will be shown that the smooth conjugacy class of an $S-$unimodal map which does not have a periodic attractor neither a Cantor attractor is determined by the multipliers of the periodic orbits. This generalizes a result by M.Shub and D.Sullivan for smooth expanding maps of the circle

Conditions for free magnetic monopoles in nanoscale square arrays of dipolar spin ice

We study a modified frustrated dipolar array recently proposed by M\"{o}ller and Moessner [Phys. Rev. Lett. \textbf{96}, 237202 (2006)], which is based on an array manufactured lithographically by Wang \emph{et al.} [Nature (London) \textbf{439}, 303 (2006)] and consists of introducing a height offset $h$ between islands (dipoles) pointing along the two different lattice directions. The ground-states and excitations are studied as a function of $h$. We have found, in qualitative agreement with the results of M\"{o}ller and Moessner, that the ground-state changes for $h>h_{1}$, where $h_{1}= 0.444a$ ($a$ is the lattice parameter or distance between islands). In addition, the excitations above the ground-state behave like magnetic poles but confined by a string, whose tension decreases as $h$ increases, in such a way that for $h\approx h_1$ its value is around 20 times smaller than that for $h=0$. The system exhibits an anisotropy in the sense that the string tension and magnetic charge depends significantly on the directions in which the monopoles are separated. In turn, the intensity of the magnetic charge abruptly changes when the monopoles are separated along the direction of the longest axis of the islands. Such a gap is attributed to the transition from the anti to the ferromagnetic ground-state when $h=h_1$.Comment: 6 pages, 7 figures. Published versio