Papers by Keyword: Dipolar Interactions

Abstract: Most of simulations often require the calculation of all pairwise interaction in large ensembles of particles, such as N-body problem of gravitation, electrostatic interaction and magnetic dipolar interaction, etc. The main difficulty in the calculation of long-range interaction is how to accelerate the slow convergence of the occurring sums. In this work, we are interested in the dipolar interaction in the two dimensional (2D) magnetic dipolar nanoparticle systems, which have attracted much attention due to both their important technological applications such as high-density patterned recording media and their rich and often unusual experimental behaviours. We develop a high efficiency algorithm based on the Lekner method to evaluate the magnetic dipolar energy for such systems, where the simulation cell is periodically replicated in the plane. Taking advantage of the symmetry of the systems, the dipolar interaction energy is expressed by rapidly converging series of modified Bessel functions in our algorithm. We found that our algorithm is better than the traditional Ewald summation method in efficiency for the regular arrays. Moreover, two simple formulas are obtained to evaluate the self-energy, which is important in the simulation of the dipolar systems.

Abstract: The new phase equilibrium of Fe-C diagram under magnetic field has been theoretically calculated. Results show that the magnetic field mainly shifts the γ⁄α+γ equilibrium line and the eutectoid point to the high carbon and high temperature sides. Based on this result, an experimental setup has been launched to investigate the effect of magnetic field on austenite decomposition in medium carbon and high carbon steels. The thermodynamic and kinetic effects of the high magnetic field on proeutectoid transformation at different cooling rates have been studied. It was found that for medium carbon steels, the magnetic field increases the amount of proeutectoid ferrite and accelerates the diffusional decomposition of austenite at medium and relatively fast cooling rates (10°C/min and 46°C/min). But there is no special grain growth along the field direction. The results led to a proposal of a new rapid annealing under a high magnetic field. However, when cooling is slow (2°C/min), the magnetic field shows a strong tendency to promote the proeutectoid ferrite grains to grow along the field direction through the magnetic dipolar interaction, which leads to the formation of an elongated grain structure. Moreover, the magnetic field also exhibits influence on the austenite decomposition in hypereutectoid steel by changing the amount of secondary cementite and lamellar spacing of pearlite.