R. A. Dias

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...

Magnetic vortex formation and gyrotropic mode in nanodisks

The superparamagnetic limit imposes a restriction on how far the miniaturization of electronic devices can reach. Recently it was shown that magnetic thin films with nanoscale dimensions can exhibit a vortex as its ground state. The vortex can lower its energy by developing an out-of-plane magnetization perpendicular to the plane of the film, the z direction, which can be “up” or “down.” Because the vortex structure is very stable this twofold degeneracy opens up the possibility of using a magnetic nanodisk as a bit of memory in electronic devices. The manipulation of the vortex and a way to control the core magnetization is a subject of paramount importance. Recent results have suggested that the polarity of a vortex core could be switched by applying a pulsed magnetic field in the plane of the disk. Another important effect induced by an external magnetic field due to the component out-of-plane in vortex-core is the gyrotropic mode. The gyrotropic mode is the elliptical movement around the disk center e...

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

In this work, we used numerical simulations to study the effect of a ring of magnetic impurities on the vortex core dynamics in nanodisks of Permalloy. The presence of the ring not only allowed us to modulate the gyrotropic frequency but also provided us a way to confine the vortex core. We observed that the gyrotropic frequency depends on the ring parameters. Moreover, we have noticed that the switching of the vortex core polarity can be obtained from the vortex core-impurity interaction under peculiar conditions, in particular, when the ring works for pinning the vortex core.

Vortex core scattering and pinning by impurities in nanomagnets

The dynamical behavior of a magnetic nanoparticle contaminated by pointlike impurities is studied by using a spin dynamics numerical simulation. It was observed that the impurities can behave both as pinning (attractive) and as scattering (repulsive) sites. A Gaussian profile was observed for the interaction potential energy ranging up to two lattice parameters. Using the known values of the parameters for Permalloy-79 we have calculated the interaction energy of the vortex core with a single defect. We estimated the interaction range as approximately 10nm. Both results agree quite well with experimental measurements.

Phase transition in ultrathin magnetic films with long-range interactions : Monte Carlo simulation of the anisotropic Heisenberg model

Ultrathin magnetic films can be modeled as an anisotropic Heisenberg model with long range dipolar interactions. It is believed that the phase diagram presents three phases: A ordered ferromagnetic phase (I), a phase characterized by a change from out-of-plane to in-plane in the magnetization (II), and a high temperature paramagnetic phase (III). It is claimed that the border lines from phase I to III and II to III are of second order and from I to II is first order. In the present work we have performed a very careful Monte Carlo simulation of the model. Our results strongly support that the line separating phase II and III is of the BKT type.

Transverse domain wall scattering and pinning by magnetic impurities in magnetic nanowires

We used numerical simulations to study the dynamical behavior of the transverse domain wall (TDW) in nanowires made of Permalloy-79 contaminated by pointlike magnetic impurities. It was observed that the magnetic impurities can behave both as pinning (attractive) and as scattering (repulsive) sites for the TDW. We have found that the nearer to the bottom edge of the nanowire is located the magnetic impurity, the larger is the magnitude of the pinning and the scattering energy, which agrees with experimental observation. We also observed that the interaction energy and the range of the interaction potential depend on the width of the nanowire. The presence of magnetic impurity affects the motion of the TDW.

Position of the transverse domain wall controlled by magnetic impurities in rectangular magnetic nanowires

We have performed numerical simulations to demonstrate that the domain wall movement can be controlled introducing a distribution of magnetic impurities in a nanowire. In particular, we have considered two identical impurities equidistant from the nanowire width axis. Pinning and scattering sites for the domain wall can be defined by magnetic impurities, consisting of a local variation of the exchange constant. The domain wall motion was induced by application of a magnetic field pulse and our results indicate that it is possible to control the domain wall position.

Ground state study of the thin ferromagnetic nano-islands for artificial spin ice arrays

In this work, we used numerical simulations to study the magnetic ground state of the thin elongated (elliptical) ferromagnetic nano-islands made of Permalloy. In these systems, the effects of demagnetization of dipolar source generate a strong magnetic anisotropy due to particle shape, defining two fundamental magnetic ground state configurations—vortex or type C. To describe the system, we considered a model Hamiltonian in which the magnetic moments interact through exchange and dipolar potentials. We studied the competition between the vortex states and aligned states—type C—as a function of the shape of each elliptical nano-islands and constructed a phase diagram vortex—type C state. Our results show that it is possible to obtain the elongated nano-islands in the C-state with aspect ratios less than 2, which is interesting from the technological point of view because it will be possible to use smaller islands in spin ice arrays. Generally, the experimental spin ice arrangements are made with quite elo...

Molecular dynamics simulation of Lorentz force microscopy in magnetic nano-disks

In this paper, we present a molecular dynamics simulation to model the Lorentz force microscopy experiment. Experimentally, this technique consists in the scattering of electrons by magnetic structures in surfaces and gases. Here, we will explore the behavior of electrons colliding with nano-magnetic disks. The computational molecular dynamics experiment allows us to follow the trajectory of individual electrons all along the experiment. In order to compare our results with the experimental one reported in literature, we model the experimental electron detectors in a simplified way: a photo-sensitive screen is simulated in such way that it counts the number of electrons that collide at a certain position. The information is organized to give in grey scale the image information about the magnetic properties of the structure in the target. Computationally, the sensor is modeled as a square matrix in which we count how many electrons collide at each specific point after being scattered by the magnetic struct...