6,546 research outputs found
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
between islands (dipoles) pointing along the two different lattice directions.
The ground-states and excitations are studied as a function of . We have
found, in qualitative agreement with the results of M\"{o}ller and Moessner,
that the ground-state changes for , where ( 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 increases, in such a way that for
its value is around 20 times smaller than that for . 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 .Comment: 6 pages, 7 figures. Published versio
On topological spin excitations on a rigid torus
We study Heisenberg model of classical spins lying on the toroidal support,
whose internal and external radii are and , respectively. The isotropic
regime is characterized by a fractional soliton solution. Whenever the torus
size is very large, , its charge equals unity and the soliton
effectively lies on an infinite cylinder. However, for R=0 the spherical
geometry is recovered and we obtain that configuration and energy of a soliton
lying on a sphere. Vortex-like configurations are also supported: in a ring
torus () such excitations present no core where energy could blow up. At
the limit we are effectively describing it on an infinite
cylinder, where the spins appear to be practically parallel to each other,
yielding no net energy. On the other hand, in a horn torus () a singular
core takes place, while for (spindle torus) two such singularities
appear. If is further diminished until vanish we recover vortex
configuration on a sphere.Comment: 11 pages, 9 figure
A new data reduction scheme to obtain the mode II fracture properties of Pinus Pinaster wood
In this work a numerical study of the End Notched Flexure (ENF) specimen was performed
in order to obtain the mode II critical strain energy released rate (GIIc) of a Pinus pinaster wood in the RL crack propagation system. The analysis included interface finite elements and a progressive damage
model based on indirect use of Fracture Mechanics.
The difficulties in monitoring the crack length during an experimental ENF test and the inconvenience of performing separate tests in order to obtain the elastic properties are well known. To avoid these
problems, a new data reduction scheme based on the equivalent crack concept was proposed and validated. This new data reduction scheme, the Compliance-Based Beam Method (CBBM), does not require crack measurements during ENF tests and additional tests to obtain elastic properties.FCT - POCTI/EME/45573/200
Finite element analysis of the ECT test on mode III interlaminar fracture of carbon-epoxy composite laminates
In this work a parametric study of the Edge Crack Torsion (ECT) specimen was performed
in order to maximize the mode III component (GIII) of the strain energy release rate for carbon-epoxy laminates.
A three-dimensional finite element analysis of the ECT test was conducted considering a
[90/0/(+45/-45)2/(-45/+45)2/0/90]S lay-up. The main objective was to define an adequate geometry to obtain an almost pure mode III at crack front. The geometrical parameters studied were specimen dimensions, distance between pins and size of the initial crack.
The numerical results demonstrated that the ratio between the specimen length and the initial crack length had a significant effect on the strain energy release rate distributions. In almost all of the tested
configurations, a mode II component occurred near the edges but it did not interfere significantly with the dominant mode III state.FCT - POCTI/EME/45573/200
Magnetic monopole and string excitations in a two-dimensional spin ice
We study the magnetic excitations of a square lattice spin-ice recently
produced in an artificial form, as an array of nanoscale magnets. Our analysis,
based upon the dipolar interaction between the nanomagnetic islands, correctly
reproduces the ground-state observed experimentally. In addition, we find
magnetic monopole-like excitations effectively interacting by means of the
usual Coulombic plus a linear confining potential, the latter being related to
a string-like excitation binding the monopoles pairs, what indicates that the
fractionalization of magnetic dipoles may not be so easy in two dimensions.
These findings contrast this material with the three-dimensional analogue,
where such monopoles experience only the Coulombic interaction. We discuss,
however, two entropic effects that affect the monopole interactions: firstly,
the string configurational entropy may loose the string tension and then, free
magnetic monopoles should also be found in lower dimensional spin ices;
secondly, in contrast to the string configurational entropy, an entropically
driven Coulomb force, which increases with temperature, has the opposite effect
of confining the magnetic defects.Comment: 8 pages. Accepted by Journal of Applied Physics (2009
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