47 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
Phase transition in the two-dimensional dipolar Planar Rotator model
In this work we have used extensive Monte Carlo simulations and finite size
scaling theory to study the phase transition in the dipolar Planar Rotator
model (dPRM), also known as dipolar XY model. The true long-range character of
the dipolar interactions were taken into account by using the Ewald summation
technique. Our results for the critical exponents does not fit those from known
universality classes. We observed that the specific heat is apparently
non-divergent and the critical exponents are ,
and . The critical temperature was found to be .
Our results are clearly distinct from those of a recent Renormalization Group
study from Maier and Schwabl [PRB 70, 134430 (2004)] and agrees with the
results from a previous study of the anisotropic Heisenberg model with dipolar
interactions in a bilayer system using a cut-off in the dipolar interactions
[PRB 79, 054404 (2009)].Comment: 6 pages, 8 figures, Submitted to Journal of Physics: Condensed Matte