B. V. Costa

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...

Diagram for vortex formation in quasi-two-dimensional magnetic dots

The existence of nonlinear objects of the vortex type in two-dimensional magnetic systems presents itself as one of the most promising candidates for the construction of nanodevices, useful for storing data, and for the construction of reading and writing magnetic heads. The vortex appears as the ground state of a magnetic nanodisk whose magnetic moments interact via the dipole-dipole potential {D∑[Si⋅Sj−3(Si⋅rij)×(Sj⋅rij)]/rij3} and the exchange interaction (−J∑Si⋅Sj). In this work it is investigated the conditions for the formation of vortices in nanodisks in triangular, square, and hexagonal lattices as a function of the size of the lattice and of the strength of the dipole interaction D. Our results show that there is a “transition” line separating the vortex state from a capacitorlike state. This line has a finite size scaling form depending on the size, L, of the system as Dc=D0+1/A(1+BL2). This behavior is obeyed by the three types of lattices. Inside the vortex phase it is possible to iden...

Vortex behavior near a spin vacancy in 2D XY-magnets

The dynamical behavior of anisotropic two dimensional Heisenberg models is still a matter of controversy. The existence of a central peak at all temperatures and a rich structure of magnon peaks are not yet understood. It seems that the central peaks are related, in some way, to structures like vortices. In order to contribute to the discussion of the dynamical behavior of the model we use Monte Carlo and spin dynamics simulations as well analytical calculations to study the behavior of vortices in the presence of nonmagnetic impurities. Our simulations show that vortices are attracted and trapped by the impurities. Using this result we show that if we suppose that vortices are not very much disturbed by the presence of the impurities, then they work as an attractive potential to the vortices explaining the observed behavior in our simulations.

Monte Carlo study of the critical temperature for the planar rotator model with nonmagnetic impurities

We performed Monte Carlo simulations to calculate the Berezinskii-Kosterlitz-Thouless (BKT) temperature

Phase diagrams of a two-dimensional Heisenberg antiferromagnet with single-ion anisotropy

{T}_{\mathrm{BKT}}

Magnetic vortex formation and gyrotropic mode in nanodisks

for the two-dimensional planar rotator model in the presence of nonmagnetic impurity concentration

Dynamics of the vortex core in magnetic nanodisks with a ring of magnetic impurities

(\ensuremath{\rho}).

A model for vortex formation in magnetic nanodots

As expected, our calculation shows that the BKT temperature decreases as the spin vacancies increase. There is a critical dilution

Monte Carlo and spin dynamics study of the anisotropic Heisenberg model in two dimensions

{\ensuremath{\rho}}_{c}\ensuremath{\approx}0.3

Vortex core scattering and pinning by impurities in nanomagnets

at which