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Register a new userDislocation in Motion as the Dynamic Distribution of Elastic Field Singularity
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Added by
Mishreyee Bhattacharya
Publication date
2009
fields
Physics
and research's language is
English
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Plastic deformation of crystals is a physical phenomenon, which has immensely driven the development of human civilisation since the onset of the Chalcolithic period. This process is primarily governed by the motion of line defects, called dislocations. Each dislocation traps a quantum of plastic deformation expressible in terms of its Burgers vector[1,2]. Theorising the mechanisms of dislocation motion at the atomistic scales of length and time remains a challenging task on account of the extreme complexities associated with the dynamics. We present a new concept of modelling a moving dislocation as the dynamic distribution of the elastic field singularity within the span of the Burgers vector. Surprisingly, numerical implementation of this model for the periodic expansion-shrinkage cycle of the singularity is found to exhibit an energetics, which resembles that of a dislocation moving in the presence of the Peierls barrier[1-4]. The singularity distribution is shown to be the natural consequence under the external shear stress. Moreover, in contrast to the conventional assumption, here the calculations reveal a significant contribution of the linear elastic region surrounding the core towards the potential barrier.
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In this letter we propose a model that demonstrates the effect of free surface on the lattice resistance experienced by a moving dislocation in nanodimensional systems. This effect manifests in an enhanced velocity of dislocation due to the proximity of the dislocation line to the surface. To verify this finding, molecular dynamics simulations for an edge dislocation in bcc molybdenum are performed and the results are found to be in agreement with the numerical implementations of this model. The reduction in this effect at higher stresses and temperatures, as revealed by the simulations, confirms the role of lattice resistance behind the observed change in the dislocation velocity.
The propagation of a head-to-head magnetic domain-wall (DW) or a tail-to-tail DW in a magnetic nanowire under a static field along the wire axis is studied. Relationship between the DW velocity and DW structure is obtained from the energy consideration. The role of the energy dissipation in the field-driven DW motion is clarified. Namely, a field can only drive a domain-wall propagating along the field direction through the mediation of a damping. Without the damping, DW cannot propagate along the wire. Contrary to the common wisdom, DW velocity is, in general, proportional to the energy dissipation rate, and one needs to find a way to enhance the energy dissipation in order to increase the propagation speed. The theory provides also a nature explanation of the wire-width dependence of the DW velocity and velocity oscillation beyond Walker breakdown field.
Using _in situ_ transmission electron microscopy (TEM), we have observed nanometre scale dislocation loops formed when an ultra-high-purity tungsten foil is irradiated with a very low fluence of self-ions. Analysis of the TEM images has revealed the largest loops to be predominantly of prismatic 1/2<111> type and of vacancy character. The formation of such dislocation loops is surprising since isolated loops are expected to be highly mobile, and should escape from the foil. In this work we show that the observed size and number density of loops can be explained by the fact that the loops are _not_ isolated - the loops formed in close proximity in the cascades interact with each other and with vacancy clusters, also formed in cascades, through long-range elastic fields, which prevent the escape of loops from the foil. We find that experimental observations are well reproduced by object Kinetic Monte Carlo simulations of evolution of cascades _only_ if elastic interaction between the loops is taken into account. Our analysis highlights the profound effect of elastic interaction between defects on the microstructural evolution of irradiated materials.
Magnetoresistive (xMR) sensors find extensive application in science and industry, replacing Hall sensors in various low field environments. While there have been some efforts in increasing the dynamic field range of xMR sensors, Hall sensors remain to dominate high field applications due to their wide linear range. Using a perpendicular magnetized reference system and an in-plane free layer allows us to overcome this disadvantage of xMR sensors, and, furthermore, investigate spin-canting effects in interlayer exchange coupled perpendicular synthetic antiferromagnets (p-SAF). We created p-SAFs with exchange coupling fields of up to 10 kOe, based on magnetic Co/Pt multilayer systems. The p-SAFs are either designed as single p-SAFs, where two Co/Pt multilayers are interlayer exchange coupled via a 4 {AA} thick Ru spacer, or as double p-SAFs, where an additional Co layer is interlayer exchange coupled to the top multilayer. These p-SAFs are used for giant magnetoresistance (GMR) sensors with wide dynamic field range. By using a p-SAF as the reference system and employing an in-plane magnetic layer as the GMRs free layer, the linear range can be effectively increased limited only by the p-SAFs switching fields. Additionally, the magnetic anisotropy of the in-plane free layer is fully controlled, which allows saturation fields by design. Different configurations were investigated, ranging from free layer magnetic saturation at lower to far higher fields than the p-SAFs switching fields. We can show through micromagnetic simulations that certain GMR transfer curves are dominated by spin-canting effects in the interlayer exchange coupled reference system. Finally, our simulation results lay out the correlation of the p-SAFs design parameters and its magnetization reversal behavior.
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