A. R. Pereira

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.

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

Heisenberg spins on a cone: an interplay between geometry and magnetism

r

Phase transitions in the two-dimensional Anisotropic Biquadratic Heisenberg Model

and

Planar vortex in two-dimensional XY ferromagnets with a nonmagnetic impurity potential

R

On phase transition and vortex stability in the generalized XY models

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

Efficient demagnetization protocol for the artificial triangular spin ice

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

Geometrical pinning of magnetic vortices induced by a deficit angle on a surface: Anisotropic spins on a conic space background

R=0

Magnetic vortex crystal formation in the antidot complement of square artificial spin ice

, the spherical geometry is recovered and we obtain that configuration and energy of a soliton lying on a sphere. Vortexlike configurations are also supported: in a ring torus