Search

Micromagnetic calculations using the finite element technique describe semi-quantitatively the coercivity of novel rare earth permanent magnets in dependence on grain size, grain shape, grain alignment and composition of grain boundaries and grain boundary junctions and allow the quantitative prediction of magnetic hysteretic properties of rare earth free magnets based on densely packed elongated Fe and Co nanoparticles, which depend on crystal anisotropy, aspect ratio and packing density. The nucleation of reversed domains preferentially takes place at grain boundary junctions in granular sintered and melt-spun magnets independently on the grain size. The microstructure and the nanocompostion of the intergranular regions are inhomogeneous and too complex in order to make an exact model for micromagnetic simulations and to allow a quantitative prediction. The incoherent magnetization reversal processes near the end surfaces reduce and determine the coercive field values of Co- and Fe-based nanoparticles. [ABSTRACT FROM AUTHOR]

In this paper a phenomenological approach, based on a generalization in two dimensions of the classical scalar Preisach model, is exploited and identified to reproduce the magnetization curves obtained by accurate micromagnetic simulations of both isotropic and anisotropic polycrystalline Fe–Si films with different values of the anisotropy constants. The identification problem is realized using a suitable set of analytical equations and performing a best fit procedure to the data obtained from micromagnetic simulations of both scalar and rotational loops. The correct reconstruction of all the magnetization processes, as well as of the associated magnetic losses, is achieved through the choice of a small number of either circular or elliptical hysterons, as well as by the implementation of a simple “moving technique” that is necessary to take into account the non-collinearity between the field and the magnetization that occurs in presence of a global uniaxial anisotropy. [ABSTRACT FROM AUTHOR]

Abstract: Sub-200nm patterned magnetic dots are key elements for the design of magnetic switches, memory cells or elementary units of nanomagnetic logic circuits. In this paper, we analyse by micromagnetic simulations the magnetization reversal, the dissipated energy and the excited spin eigenmodes in bistable magnetic switches, consisting of elliptical nanodots with 100×60nm lateral dimensions. Two different strategies for reversal are considered and the relative results compared: (i) the irreversible switching obtained by the application of an external field along the easy axis, in the direction opposite to the initial magnetization; (ii) the precessional switching accomplished by the application of a short magnetic field pulse, oriented perpendicular to the initial magnetization direction. The obtained results are discussed in terms of deviation from the macrospin behavior, energy dissipation and characteristics of the spectrum of spin eigenmodes excited during the magnetization reversal process. [Copyright &y& Elsevier]

Abstract: Here we perform modeling of hysteresis measurements, described earlier by Crowley et al., of an array of cobalt nanotubes using a single infinite tube simulation. The modeling has been performed under the assumptions that (i) the long axis of the tube is the hard axis of the anisotropy and (ii) in the experiment we deal with a distribution of different tubes, each having a different anisotropy constant. The best fit to the experimental result is achieved using a log-normal anisotropy distribution, with a peak value close to −0.2MJ/m3. The distribution parameters are nearly the same for both experimental temperatures, T exp=1.8 and 300K. These parameters are only weakly dependent on the tube-wall width and the material parameters. [Copyright &y& Elsevier]

We discuss a model to quantify long-time thermally induced magnetization reversal in magnetic systems with distributed properties. Two algorithms, based on kinetic and Metropolis Monte Carlo are introduced. While the former requires the constant recalculation of all energy barriers and is useful when the interactions are weak, the latter uses the Metropolis Monte Carlo to estimate the magnetization trajectory and, consequently, only the most probable transition rates are evaluated. The ridge optimization method is used to evaluate the energy barriers in a multidimensional energy landscape. The algorithms are applied to a granular system modeled by means of Voronoi polyhedra and having random in-plane anisotropy. [Copyright &y& Elsevier]

Physical modeling of magnetic materials should give insights into the basic processes involved and should be able to extrapolate results to new situations that the models were not necessarily intended to solve. Thus, for example, if a model is designed to describe a static magnetization curve, it should also be able to describe aspects of magnetization dynamics. Both micromagnetic modeling and Preisach modeling, the two most popular magnetic models, fulfill this requirement, but in the process of fulfilling this requirement, they both had to be modified in some ways. Hence, we should view physical modeling as an iterative process whereby we start with some simple assumptions and refine them as reality requires. In the process of refining these assumptions, we should try to appeal to physical arguments for the modifications, if we are to come up with good models. If we consider phenomenological models, on the other hand, that is as axiomatic models requiring no physical justification, we can follow them logically to see the end and examine the consequences of their assumptions. In this way, we can learn the properties, limitations and achievements of the particular model. Physical and phenomenological models complement each other in furthering our understanding of the behavior of magnetic materials. [Copyright &y& Elsevier]

Magnetic skyrmions are topologically quasi-particles offering a wide range of applications ranging from memory to oscillator and neuromorphic elements such as neurons and synapses. However, their trajectory driven by currents exhibits the so-called skyrmion Hall angle that is a limiting factor for the above-mentioned applications. Here, we study the stochastic dynamics of synthetic antiferromagnetic skyrmions driven by the spin-Hall effect in a racetrack memory with bifurcation. We show the limit of their application in programmable logic and a possible direction to use this design as a skyrmion based random bit generator. • Skyrmion dynamics driven by the Spin-Hall Effect in a synthetic antiferromagnet was studied by using a micromagnetic framework. • Skyrmions were used as building blocks of random bit generator. [ABSTRACT FROM AUTHOR]