Conclusions and outlook

The Langevin dynamics approach proved to be a suitable method to model the effects of thermal activation in magnetic materials.

• Simulations of a single rigid magnetic moment showed, that the Heun scheme is a suitable time integration method, which allows a time step size one order of magnitude larger than that for the Milshtein scheme. Moreover the stochastic Landau-Lifshitz equation of motion in Stratonovich interpretation leads to the correct thermal equilibrium properties.

• The magnetization switching behaviour found for a small cubic particle is identical for the finite difference and finite element model, even though their method of calculating the effective field is substantially different. The finite element method is better suited for the simulation of particles with curved or very complex surfaces and allows the modeling of polycrystalline grain structures.

• For a small cubic ferromagnetic particle magnetization reversal by coherent rotation has been found. As a result, is switching dynamics is well described by the Arrhenius-Néel law for reversal over a single energy barrier.

• Complex magnetization reversal mechanisms have been found for small spherical magnetic particles. The magnetocrystalline anisotropy and the strength of the external determine the switching mechanism and three different regimes have been identified. For fields, which are smaller than the anisotropy field, magnetization by coherent rotation has been observed. If an external field comparable to the anisotropy field is applied single droplet nucleation occurs and for higher fields multi-droplet nucleation is the driving reversal process.

• The interaction of two particles changes the dynamic behaviour, depending on the position and distance of the particles. Two spherical particles aligned along their easy axes can stabilize each other, whereas two horizontally aligned particles exhibit a reduced metastable lifetime.

As the bit size shrinks, the bit density grows and the read/write frequencies increase, the effects of thermal perturbations become more and more important. The design of magnetic recording media has to guarantee reliability, stability of the stored information and a fast switching mechanism for high frequency writing processes. Micromagnetic simulations based on the programs developed and tested in this thesis will provide important information about the magnetization distribution, the dynamics of the reversal process and the interaction between the elements.

• A detailed study of the shape and arrangement of nano-elements for quantum disks can optimize their thermal stability.

• For high frequency recording applications the switching mechanism can be investigated and the design optimized to minimize the switching time.

• Interaction effects are determined by the pattern, in which the elements are arranged. The distance and position of neighbouring elements has been found to influence the switching time, which has to be considered in the design of quantum discs.

• In thin film recording media the grain structure plays a vital role for the magnetic properties [64]. The finite element package allows the design of arbitrary shapes of the grains and any material composition.

• Predictions of the activation volume will enhance the understanding how microstructural features influence thermally activated magnetization reversal [62].

Thus, the simulations will provide theoretical insight for the development of future recording media.