Estudo via simulação computacional do comportamento da magnetização de nanoilhas ferromagnéticas elípticas

Abstract

The continuous development of structures fabrication techniques at the nanometer scale with considerable precision and reproducibility has allowed and encouraged scientific research around the basic properties and new technological applications of these systems. Especially from the 90's, there is growing interest of the scientific community in the behavior of nanostructured magnetic systems. In these, the breaking of spatial symmetry due to small dimensionality causes quite different behaviors from those observed in the bulk. The resulting shape anisotropy of the classical interaction between magnetic dipoles allows the formation of exotic magnetic structures in nanomagnets as vortices, skyrmions, single domain walls and even topological excitations similar to magnetic monopoles. The understanding and control of static and dynamic magnetic behavior of these structures is essential for the development of new technological devices based on spintronics. In this work we studied planar elongated nanoparticles in the elliptical shape of soft ferromagnetic material, specifically the Permalloy-79. Such nanoparticles have attracted attention because of their potential to practical application in the development of new sensors, logic devices, high density data storage media and MRAM (Magnetic Random Access Memory) devices. By the bias of basic scientific interest, such ferromagnetic nano-islands are the fundamental unit in two-dimensional magnetic arrangements geometrically frustrated as artificial spin ice systems. In these systems, the geometric arrangement of islands break the degeneracy of the network ground state featuring a state of geometrical frustration that allows excitations with analogous behavior of magnetic monopoles. Under these aspects, it is essential to characterize the magnetic configurations in the ground state and the magnetization reversal processes in individual nanoparticles. The elliptical planar shape generates a strong magnetic anisotropy which defines two basic configurations for the magnetization of the ground state of the nanoparticles: the vortex state or the aligned state along the major axis - type C state. As from an aspect ratio limit value, the magnetization of the ground state is confined in the plane and along the major axis of each nano-island defining mono-domain nanomagnet with two degenerate states of magnetization, useful for the applications previously described. Starting from this purpose we study initially, through simulation by spin dynamics, the competition between the vortex states and aligned type C states as a function of the shape of each elliptical nano-island to build a states diagram. Each magnetic nanoparticle is modeled by magnetic moments that interact by exchange interaction between nearest neighbors and by the classical long-range dipolar interaction. Our theoretical results indicate the possibility to manufacture elongated nano-islands with ground state like aligned C state for aspect ratios less than two. This is an interesting result from the technological point of view because it will be possible to use smaller islands in researches on spin ice and MRAM. Generally, the experimental arrangements are made with nanoparticles of aspect ratio close to three to ensure aligned magnetization in the ground state. Adding to the model a Zeeman interaction term between the magnetic moments and an external magnetic field we study the behavior of the magnetization reversal in nanoparticles. We consider different thickness and two different aspect ratios: one in the usual experimental size and a smaller proposed from our results. Applying sinusoidal magnetic field at different frequencies along the anisotropy axis in directions of ten and forty-five degrees from this, we observed the dependence of the reversal processes on the thickness of the particles and with the direction and frequency of the applied field. The results allow to establish general guidelines about the magnetization reversal behavior of the individual nanoparticles under external magnetic field. Evidently, for the development of possible technological applications, including the control of excitation like magnetic monopoles in spin ice, it is crucial to understand the ultrafast magnetization reversal processes which involves the application of high frequency magnetic fields in carefully defined directions. With this aim, we also studied the magnetization reversal of the nano-islands by short pulses of magnetic field (of the nanosecond order) applied in different directions. We observed a strong dependence on the coherence of the magnetization reversal with the direction of the applied field and a significant difference in the angular dependence of the coercivity compared to those seen in previous studies with applied magnetic fields in quasistatic conditions. Finally, based on our results we propose a method for the control of the coherent magnetization reversal of individual nanoparticles in square artificial spin ice arrays. We believe that our results may be useful in further developments of geometrically frustrated magnetic artificial arrangements and in the control of the topological excitations of these systems.