O. Ertl

Recording simulations on graded media for area densities of up to 1Tbit∕in.2

We report recording simulations on graded media with area densities of 1Tbit∕in.2. The media are composed of a nucleation layer exchange coupled to a hard magnetic storage layer. The nucleation layer has an anisotropy K(z) that gradually varies in order to adjust the domain wall propagation field to the write field. Bits were written with a bit length of 12nm and a track width of 53nm on graded media with total thickness of 21nm and maximum anisotropy of 1MJ∕m3. The computed values for transition jitter are around 0.65nm, depending on the intergrain exchange.

Optimization of exchange spring perpendicular recording media

Exchange spring media are proposed for magnetic-recording systems consisting of a hard/soft bilayer. By varying the fraction of thickness for the hard and soft layer and by varying their saturation polarizations, the media properties can be optimized in order to achieve high thermal stability without increase of coercive field. In grains with identical size and coercivity, an optimized bilayer reaches an energy barrier exceeding those of optimized single-phase media by more than a factor of two. Thus, exchange spring media allow to reduce the grain diameter by more than a factor of 1//spl radic/2. Additionally, the lower angular dependence of coercivity of exchange spring media improves the signal-to-noise ratio (SNR) by a factor of 2.5.

A fast level set framework for large three-dimensional topography simulations

We present fast methods to describe the surface evolution of large three-dimensional structures. Based on the sparse field level set method and the hierarchical run-length encoding level set data structure optimal figures for the computation time and for the memory consumption are achieved. Furthermore, we introduce a new multi-level-set technique, which is able to incorporate multiple material regions, and which can also handle material specific surface speeds accurately. We also describe an optimal algorithm for the visibility check for unidirectional etching. The presented techniques are demonstrated on various examples.

Three-dimensional micromagnetic finite element simulations including eddy currents

We developed a micromagnetic eddy current method that allows arbitrary geometries, requires no mesh outside the ferromagnet, and uses a stable integration scheme. We simultaneously solve the Landau–Lifshitz–Gilbert equation and the quasistatic Maxwell equations using a hybrid finite element/boundary element method (FEM/BEM). The eddy current field is directly calculated from the space time behavior of the magnetization rate of change. The boundary conditions of the eddy current field at infinity are taken into account using a FEM/BEM scheme. The resulting system of differential algebraic equations is solved using a backward differentiation method.

Nanostructure calculation of CoAg core-shell clusters

Detailed studies of the structure of magnetic nanoclusters are crucial for understanding their magnetic properties. We have investigated the structure of CoxAg1−x nanoparticles by means of molecular dynamics simulations utilizing the embedded atom method. Starting from a completely random distribution of Co and Ag atoms, the clusters were heated up to 1300K and subsequently cooled down. The size of the resulting particles was 2.8nm (864 atoms). A clear segregation of the Ag atoms on the surface of the Co core was obtained.

Partitioning of the perpendicular write field into head and SUL contributions

We perform multiscale finite element simulation of the write process in perpendicular media. The Landau-Lifshitz-Gilbert equation is solved simultaneously for the head, the data layer, and the soft under layer during the motion of the head. All magnetostatic interactions between head, data layer and soft underlayer are concurrently taken into account. This fully integrated recording model enables a detailed analysis of the head field as seen by the media grains.

Multiscale micromagnetic simulation of giant magnetoresistance read heads

The Landau-Lifshitz-Gilbert equation and quasistatic Maxwell equations were solved simultaneously to calculate the read back signal of giant magnetoresistance read heads with a hybrid finite-element/boundary element method. The finite-element simulations show the influence of the sense current on the linearity of the reader, the effect of the exchange bias field on the sensor performance, and the influence of the Gilbert damping constant on the decay time of the read back voltage. All parts of the system, the layers of the giant magnetoresistance sensor, the hard bias magnets, the shields, and the recording layer are treated micromagnetically. In addition, the influence of the sense current onto the magnetization is taken into account self-consistently. The current distribution in the giant magnetoresistance stack is calculated from local resistivity which depends on the magnetization of the free and of the pinned layer.

Influence of eddy current on magnetization processes in submicrometer permalloy structures

In this paper, we determine the critical particle size and conductivity range that leads to pronounced eddy current effects on magnetization reversal of magnetic nanostructures. An extended finite-element micromagnetic solver which includes eddy currents is used to calculate the magnetization behavior of a permalloy nanocube of 27 nm side under the influence of an applied field. It is shown that for high-conductivity /spl sigma/>10/sup 6/ (/spl Omega/ m)/sup -1/ in the intermediate state of reversal it takes the magnetization longer to fully align in the direction of the applied field.

Influence of eddy currents on the effective damping parameter

A method for the calculation of the effective damping parameter as a function of particle size and electric conductivity is presented. An extended three-dimensional finite element/boundary element micromagnetic model is used to solve the Landau-Lifshitz-Gilbert equation together with the eddy current diffusion equation. From the change of energy with time an effective damping parameter is derived from which the net eddy current contribution is calculated. It is shown that the eddy current effects are reduced with an increasing applied field and are increased for materials with a low resistivity. Furthermore it is shown that the size of the particles, which designates the coherence of the spin structure, is essential for the eddy current net contribution.

PARALLELIZATION STRATEGY FOR HIERARCHICAL RUN LENGTH ENCODED DATA STRUCTURES

An efficient parallelization strategy is presented for a Hierarchical Run Length Encoded (HRLE) data structure, implemented for the Sparse Field Level Set method. In order to achieve high parallel efficiency, computational work must be distributed evenly over all available CPU threads. Since the Level Set surface must be allowed to deform and evolve, thereby increasing the simulation area, there must exist a way to increase the surface domain while keeping an efficient parallelization strategy in place. This is achieved by processing the same number of calculations across each available CPU. The addition of data to HRLE data structures is only permitted in a sequential or lexicographical order, making parallelization more complex. The presented solution uses as many HRLE data structures as there are CPUs available. Approximately 90% of operations can be performed in parallel when using the presented strategy, leading to an efficiency of up to 96% or 78.5% when using two or sixteen CPU cores of an AMD Opteron 8435 processor, clocked at 2.6GHz, respectively. Topographies with one and two moving interfaces were simulated using multi-threading, showing the speedup and efficiency for the presented strategy.