Jari Jalkanen

First-principles calculations of spin spirals in Ni2MnGa and Ni2MnAl

We report here noncollinear magnetic configurations in the Heusler alloys Ni 2 MnGa and Ni 2 MnAl which are interesting in the context of the magnetic shape memory effect. The total energies for different spin spirals are calculated and the ground-state magnetic structures are identified. The calculated dispersion curves are used to estimate the Curie temperature which is found to be in good agreement with experiments. In addition, the variation of the magnetic moment as a function of the spiral structure is studied. Most of the variation is associated with Ni, and symmetry constraints relevant for the magnetization are identified. Based on the calculated results, the effect of the constituent atoms in determining the Curie temperature is discussed.

Equilibrium shape and dislocation nucleation in strained epitaxial nanoislands.

We study numerically the equilibrium shapes, shape transitions, and dislocation nucleation of small strained epitaxial islands with a two-dimensional atomistic model, using simple interatomic pair potentials. We first map out the phase diagram for the equilibrium island shapes as a function of island size (up to

Atomistic studies of strain relaxation in heteroepitaxial systems

N=105

Equilibrium shape and size of supported heteroepitaxial nanoislands

atoms) and lattice misfit with the substrate, and show that nanoscopic islands have four generic equilibrium shapes, in contrast with predictions from the continuum theory of elasticity. For increasing substrate-adsorbate attraction, we find islands that form on top of a finite wetting layer as observed in Stranski-Krastanow growth. We also investigate energy barriers and transition paths for transitions between different shapes of the islands and for dislocation nucleation in initially coherent islands. In particular, we find that dislocations nucleate spontaneously at the edges of the adsorbate-substrate interface above a critical size or lattice misfit.

Two approaches to dislocation nucleation in the supported heteroepitaxial equilibrium islanding phenomenon

We present a review of recent theoretical studies of different atomistic mechanisms of strain relaxation in heteroepitaxial systems. We explore these systems in two and three dimensions using different semi-empirical interatomic potentials of Lennard-Jones and many-body embedded atom model type. In all cases we use a universal molecular static method for generating minimum energy paths for transitions from the coherent epitaxial (defect free) state to the state containing an isolated defect (localized or extended). This is followed by a systematic search for the minimum energy configuration as well as self-organization in the case of a periodic array of islands. In this way we are able to understand many general features of the atomic mechanisms and energetics of strain relaxation in these systems. Finally, for the special case of Pd/Cu(100) and Cu/Pd(100) heteroepitaxy we also use conventional molecular dynamics simulation techniques to compare the compressively and tensilely strained cases. The results for this case are in good agreement with the existing experimental data.

Shape transition and dislocation nucleation in strained epitaxial islands

AbstractWe study the equilibrium shape, shape transitions and optimal size of strained heteroepitaxial nanoislands with a two-dimensional atomistic model using simply adjustable interatomic pair potentials. We map out the global phase diagram as a function of substrate-adsorbate misfit and interaction. This phase diagram reveals all the phases corresponding to different well-known growth modes. In particular, for large enough misfits and attractive substrate there is a Stranski-Krastanow regime, where nano-sized islands grow on top of wetting films. We analyze the various terms contributing to the total island energy in detail, and show how the competition between them leads to the optimal shape and size of the islands. Finally, we also develop an analytic interpolation formula for the various contributions to the total energy of strained nanoislands.

Magnetically driven shape memory alloys

We study the dislocation formation in 2D nanoscopic islands with two methods the Molecular Static method and the Phase Field Crystal method. It is found that both methods indicate the same qualitative stages of the nucleation process. The dislocations nucleate at the film-substrate contact point and the energy decreases monotonously when the dislocations are farther away from the island-wetting film contact points than the distance of the highest energy barrier.

Stress relaxation in supported metallic nanostructures

We study numerically the equilibrium shape, shape transition and dislocation nucleation in strained epitaxial islands with a two-dimensional atomistic model, using interatomic potentials of Lennard-Jones type. The phase diagram for the equilibrium island shapes as a function of island size and lattice misfit with the substrate is obtained by an energy minimization procedure which does not require predefined faceted shapes. We determine the energy barrier and transition path for transition between different shapes of the islands and for dislocation nucleation in initially coherent islands using a method introduced recently, based on a systematic search of the transition paths for activated events.