Winder A. Moura-Melo

Magnetic monopole and string excitations in 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 on the dipolar interaction between the nanomagnetic islands, correctly reproduces the ground state observed experimentally. In addition, we find magnetic monopolelike excitations effectively interacting by means of the usual Coulombic plus a linear confining potential, the latter being related to a stringlike excitation binding the monopoles pairs, which indicates that the fractionalization of magnetic dipoles may not be so easy in two dimensions. These findings contrast this material with the three-dimensional analog, where such monopoles experience only the Coulombic interaction. We discuss, however, two entropic effects that affect the monopole interactions. First, the string configurational entropy may lose the string tension and then free magnetic monopoles should also be found in lower dimensional spin ices; second, in contrast to the string con...

Thermodynamics of elementary excitations in artificial magnetic square ice

We investigate the thermodynamics of artificial square spin ice systems assuming only dipolar interactions among the islands that compose the array. Emphasis is given to the effects of temperature on elementary excitations (magnetic monopoles and their strings). By using Monte Carlo techniques we calculate the specific heat, the density of poles and their average separation as functions of temperature. The specific heat and average separation between monopoles with opposite charges exhibit a sharp peak and a local maximum, respectively, at the same temperature, Tp ≈ 7.2D/kB (here, D is the strength of the dipolar interaction and kB the Boltzmann constant). When the lattice size is increased, the amplitude of these features also increases but very slowly. Really, the specific heat and the maximum of the average separation dmax between oppositely charged monopoles increase logarithmically with system size, indicating that completely isolated charges could be found only at the thermodynamic limit. In general, the results obtained here suggest that, for temperatures T ≥ Tp, these systems may exhibit a phase with separated monopoles, although the quantity dmax should not be larger than a few lattice spacings for viable artificial materials.

A model for structural defects in nanomagnets

A model for describing structural pointlike defects in nanoscaled ferromagnetic materials is presented. Its details are explicitly developed whenever it interacts with a vortexlike state comprised in a thin nanodisk. Among others, our model yields results for the vortex equilibrium position under the influence of several defects along with an external magnetic field that is shown to be in good qualitative agreement with experiments. We also discuss how such defects may affect the vortex motion, such as its gyrotropic oscillation and dynamical polarization reversal.

Topological spin excitations on a rigid torus

We study the Heisenberg model of classical spins lying on a toroidal support, whose internal and external radii are

How hole defects modify vortex dynamics in ferromagnetic nanodisks

r

Miniaturization of vortex-comprising system using ferromagnetic nanotori

and

Heisenberg model on a space with negative curvature: Topological spin textures on the pseudosphere

R

Magnetic vortex-like excitations on a sphere

, respectively. The isotropic regime is characterized by a fractional soliton solution. Whenever the torus size is very large,

Scattering of charge carriers in graphene induced by topological defects

R\ensuremath{\rightarrow}\ensuremath{\infty}

Testing CPT- and Lorentz-odd electrodynamics with waveguides

, its charge equals unity and the soliton effectively lies on an infinite cylinder. However, for