Kinetic Monte Carlo Simulations of Heat-assisted Magnetization Reversal in Ultrathin FilmsPer Arne Rikvold, Florida State University, DMR 0802288 Heat-assisted Magnetization Reversal (HAMR) is a promising method to increase the thermal stability of magnetically stored information without sacrificing writing speed. The idea is to use a high-coercivity, ultrathin ferromagnetic film, but temporarily decrease its coercivity during writing by localized heating by a laser beam. In the heated region, the magnetization of the medium easily aligns with the writing field. Kinetic Monte Carlo simulations of HAMR were performed with FSU graduate student W.R. Deskins, former FSU Ph.D. student S.H. Thompson (Northrop Grumman Corp.), and Dr. G. Brown (Oak Ridge National Laboratory). W.R. Deskins, G. Brown, S.H. Thompson, P.A. Rikvold, e-print arXiv:1105.5155. Snapshot of the simulated spin system during magnetization reversal. The reversal starts in the central area, which was briefly heated at the beginning of the simulation.
Crossover between a short-range and a long-range Ising modelPer Arne Rikvold, Florida State University, DMR 0802288 An effective long-range interaction is realized among local bistable variables (spins) in systems where elastic interactions cause spin ordering. An example is spin-crossover materials, which are important candidate materials for ultrahigh density memory applications. As long as a system has nonzero long-range interactions, it shows criticality in the mean-field universality class. Here we study by Monte Carlo simulations the crossover from the pure short-range interacting model to an infinite-range; in particular how the critical temperature changes as a function of the strength of the long-range interaction. Collaboration with the group of Prof. S. Miyashita, University of Tokyo. T. Nakada, P.A. Rikvold, T. Mori, M. Nishino, S. Miyasyhita, Phys. Rev. B 84, 054433 (2011). The dependence of the relative distance of the critical temperature from that of the pure Ising model, shown as a function of the relative strength of the long-range interaction. The red dots are based on the peak position of the susceptibility, and the blue dots on the fourth-order cumulant method. The dashed, blue line is our numerically exact theoretical result, not a fit to the Monte Carlo data.
Floridian high-voltage power-grid partitioning with cluster optimization by simulated annealingPer Arne Rikvold, Florida State University, DMR 0802288 Intentional Intelligent Islanding (I3) is a method to limit cascading power-grid failures. The grid is divided into separate “islands” or clusters that are densely connected internally, but only sparsely connected to each other. In addition, each island should be as self-sufficient with power as possible. Here we present an I3 procedure for the Floridian high-voltage grid. Loads are connected to “nearby” generators, and the resulting islanding is optimized by simulated annealing to simultaneously maximize the internal connectivity and power self-sufficiency of individual islands. Collaboration with FSU postdoc I. Abou Hamad (now with BP R&D) and Prof. S.V. Poroseva, University of New Mexico Mechanical Engineering. I. Abou Hamad, P.A. Rikvold, S.V. Poroseva, Physics Procedia, in press. • Florida high-voltage grid partitioned by • connecting loads (ovals) to their “nearest” • generators (squares). (b) Grid clustering • after the simulated annealing algorithm • is applied to the partitioning shown in (a). • Nodes of the same color and label belong • to the same cluster.
Kinetic Monte Carlo Simulations of Heat-assisted Magnetization Reversal in Ultrathin FilmsPer Arne Rikvold, Florida State University, DMR 0802288 Education: Three undergraduate students, three graduate students, and one postdoc were supported in part by this grant. Collaborations:1. The PI spent one month in the Summer of 2011 as a visiting Professor in the Department of Applied Physics, University of Tokyo, Japan. 2. Prof. Gloria M. Buendía from Universidad Simón Bolívar in Venezuela visited the PI’s group for collaborations, 2008, ’09, ’10, and ‘11. 3. Collaborated on research projects with various groups at Mississippi State University, University of New Mexico, The University of Tokyo, Japan, and McGill University and Université de Montréal, Canada. Outreach: The PI demonstrates some of his LEGO physics creations at the FSUPhysics Department’s 2009open house. For more on the LEGO project, see:http://www.physics.fsu.edu/users/rikvold/info/legostuff.html