Biblio

The relationship between integrated media noise power and linear density in heat-assisted magnetic recording (HAMR) is discussed. A noise plateau for intermediate recording density has been observed in HAMR, similar to that found in perpendicular magnetic recording (PMR). Here, we show, by changing the temperature profile of the heat spot in HAMR, that we can tune the noise plateau regions to different recording densities. The heat spot with sharp temperature gradient favors a plateau at high recording density, while the heat spot with gradual temperature gradient favors a plateau at low recording density. This effect is argued to be a consequence of the competition between transition noise and remanence noise in HAMR.

In this paper, we present a study about the remanence state and coercivity in 1-D chain of cobalt hollow nanospheres, by using micromagnetic simulation. The high coercivity values (Hc is determined in the range of 600-1800 Oe) and the monotonic decrease of remanence are attributed to the shape anisotropy effect due to an increase in the aspect ratio value. The configuration of magnetization in remanence showed the onion state for hollow spheres (HSs) with Re = 15 nm, whereas for Re = 30 nm, appear the curling-vortex (CV) state. Finally for a cluster of chains, constituted by cobalt HSs, with random orientations the CV state is preserved.

A spin-Hall nano-oscillator (SHNO) is a type of spintronic oscillator that shows promising performance as a nanoscale microwave source and for neuromorphic computing applications. Within such nanodevices, a non-ferromagnetic layer in the presence of an external magnetic field and a DC bias current generates an oscillating microwave voltage. For developing optimal nano-oscillators, accurate simulations of the device's complex behaviour are required before fabrication. This work simulates the key behaviour of a nanoconstriction SHNO as the applied DC bias current is varied. The current density and Oersted field of the device have been presented, the magnetisation oscillations have been clearly visualised in three dimensions and the spatial distribution of the active mode determined. These simulations allow designers a greater understanding and characterisation of the device's behaviour while also providing a means of comparison when experimental resultsO are generated.

Micromagnetic simulations of coercivity as a function of external magnetic field direction were performed for a hexagonal array of hemispherical Permalloy nanocaps. The analysis was based on hysteresis loops for arrangements of nanocaps of variable thickness (5 nm and 10 nm). The angular dependence of coercivity had a maximum at about 80° with respect to the arrangement plane. An increase in coercivity with nanocap thickness is related to the magnetization reversal mechanism, where the dipole energy of individual caps generates an effective intermediate axis, locking the magnetic moments. The coercivity has maximum values of 109 Oe for 5 nm and 156 Oe for 10 nm thickness. The remanence decreases monotonically with angle. This is associated with the influence of shape anisotropy, where the demagnetizing field in the plane of the array is much smaller than the demagnetizing field perpendicular to the plane.

We demonstrate how a unique nanomodulation within a continuous ferromagnetic film can induce magnetic dipoles at predefined, submicrometer scale locations, which can tune the global magnetic properties of the film due to dipole-dipole interactions. Arrays of tunable magnetic dipoles are generated with in-plane and out-of-plane directions, which can be rotated in-plane within the three-dimensional (3-D) modulated structure of a continuous film. In-plane magnetic dipole rotation enables a methodology to control overall magnetic properties of a ferromagnetic thin film. Formation of magnetic dipoles and their tunability were studied in detail by magnetic force microscopy, high-resolution magnetic measurements, and micromagnetic simulation of a nanomodulated Ni45Fe55 alloy film. A pattern larger than a single magnetic domain would normally form a vortex in the remanent state. However, here the unique 3-D nanostructure prevents vortex formation due to the competition between in-plane and out-of-plane dipole-dipole interaction giving rise to a metastable state. Experimentally, at zero remanence, the magnetization goes through a transformation from a metastable to a stable state, where the dipole-dipole interaction depends on their geometrical arrangement. Thus, the magnetic properties of the continuous film can be varied by the proposed pattern geometry. A detail analytical study of the dipolar energy for the system agrees well with the experimental and simulated results.

Arrays of nanosized hollow spheres of Ni were studied using micromagnetic simulation by the Object Oriented Micromagnetic Framework. Before all the results, we will present an analysis of the properties for an individual hollow sphere in order to separate the real effects due to the array. The results in this paper are divided into three parts in order to analyze the magnetic behaviors in the static and dynamic regimes. The first part presents calculations for the magnetic field applied parallel to the plane of the array; specifically, we present the magnetization for equilibrium configurations. The obtained magnetization curves show that decreasing the thickness of the shell decreases the coercive field and it is difficult to obtain magnetic saturation. The values of the coercive field obtained in our work are of the same order as reported in experimental studies in the literature. The magnetic response in our study is dominated by the shape effects and we obtained high values for the reduced remanence, Mr/MS = 0.8. In the second part of this paper, we have changed the orientation of the magnetic field and calculated hysteresis curves to study the angular dependence of the coercive field and remanence. In thin shells, we have observed how the moments are oriented tangentially to the spherical surface. For the inversion of the magnetic moments we have observed the formation of vortex and onion modes. In the third part of this paper, we present an analysis for the process of magnetization reversal in the dynamic regime. The analysis showed that inversion occurs in the nonhomogeneous configuration. We could see that self-demagnetizing effects are predominant in the magnetic properties of the array. We could also observe that there are two contributions: one due to the shell as an independent object and the other due to the effects of the array.