Friday, March 06, 2009

talk di Tohoku university

pagi di teruja bila dapat email, Sugimoto dari Tohoku University menjemput Thomas untuk memberi talk di universitynya. Dan thomas akan present sebahagian dari result kerja2 ku.

terimalah persembahannya...

Micromagnetics of permanet magnets

Thomas Schrefla,b, Gino Hrkaca, Norlaily M Saidena
aUniversity of Sheffield, UK
bSt Poelten University of Applied Sciences, Austria

Numerical micromagnetic computation is a tool for calculating hysteresis properties of magnetic materials taking into account microstructure and chemical composition. Grain structures and boundary phase distributions can be generated using Voronoi-growth. Solid state molecular dynamics can be used to compute the local arrangement of atoms near surfaces and grain boundaries. From the distortion of the lattice with respect to the bulk equilibrium atomic structure intrinsic magnetic properties such as the magneto-crystalline anistropy constant can be estimated. The grain structure and the local intrinsic magnetic properties are the input for micromagnetic simulations. The total magnetic Gibbs' free energy of the system can be directly integrated using finite element and accelerated boundary element methods. The use of the particular methods depends on the type of magnet to be simulated. Whereas in nanocrystalline permanent magnets the full discretization of the magnet is possible with a fine scale uniform grid, the simulation of large grained sintered magnet requires boundary integration techniques. Here we accurately evalute the magnetic fields and associated energies near grain boundaries and use a coarse grid near the center of the grains. In the talk I will first discuss the relation between microstructure, intrinsic magnetic properties, and hysteresis properties in nano-composite permanent magnets. It will be shown that carefully adjusting the exchange constant may improve coercivity without a loss in remanence. Then the micromagnetic origin of cascade type reversal processes in sintered magnets will be discussed. Finally, the application of exchange spring magnets for use in magnetic data storage will be reviewed.

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