Micromagnetic modelling

Recently, the microstructure of magnetic materials has gained a lot of research interest since the microstructural properties of the material influence the magnetic macroscopic behaviour of the material vastly.  On the one hand, magnetic materials with optimal properties can be designed by adjusting the microscopic parameters.  On the other hand, a change of the macroscopic magnetic properties can indicate a change in microstructure, which can result in a failure of the system.  In both cases, the relations between the microstructural material properties and the macroscopic magnetic behaviour have to be determined.

Up to now, the determination of these relations relies almost exclusively on experimental research combined with macroscopic models. In these experiments the influence of separate microstructural features as grain size, crystal defects, dislocations, internal stresses, etc is hard to examine because it is impossible to alter one parameter without changing other microstructural parameters. Furthermore, the measurement of microstructural parameters is very difficult and time intensive.

In our research, we want to determine the sought relations on a fundamental and systematic way, starting from the basic physical description of the magnetic material.  In the micromagnetic theory, the magnetic material is composed of interacting magnetic dipoles. The dynamics of these magnetic dipoles is described by the Landau-Lifshitz equation on a time scale in the order of ps and a spatial scale in the order of nm.  Robust numerical techniques are used to bridge the gap between these microscopic time and spatial scales on the one hand and the macroscopic time and spatial scales on the other hand.



Design of magnetic materials with optimal parameters

To improve the performance of electromagnetic devices, like electrical machines and transformers the overall losses in general and the iron losses in particular must be minimized.  These losses are related with the surface enclosed by the magnetic hysteresis loop of the ferromagnetic material.  Various microstructural properties influence the shape of the hysteresis loop.  As an example, Fig. 1 shows the simulated hysteresis loops of a ferromagnetic grain with different number of dislocations.  Fig. 2 shows the influence of the orientation of the iron lattice on the shape of hysteresis loop.


Non destructive evaluation of ferromagnetic materials

Ferromagnetic materials under application conditions (e.g. cyclic loading) undergo a degradation that can finally result into failure.  During lifetime, the material should be monitored non-destructively to see how far the material is degraded.  The degradation in the ferromagnetic material is chiefly caused by changes in the stress distribution in the material.  These changes in stress distribution also influence the magnetic behaviour of the material.  In that way, monitoring of the magnetic material properties can serve as a non-destructive evaluation technique to examine the material degradation.  Fig. 3 shows a numerical simulation of the magnetization configuration in two ferromagnetic grains at zero applied field.  The microstresses at the grain boundaries cause chaotic magnetization patterns at the boundaries


Relevant publications

B. Van de Wiele, F. Olyslager, and L. Dupré, “Fast numerical 3D-scheme for the simulation of hysteresis in ferromagnetic grains”, Journal of Applied Physics, Vol. 101, No. 6, 2007.

B. Van de Wiele, F. Olyslager, and L. Dupré, “Fast semi-analytical integration schemes for the Landau-Lifshitz equation”, IEEE Transactions on Magnetics, Vol. 43, No. 6, 2007.

B. Van de Wiele, L. Dupré, and F. Olyslager, "Memory properties in a 3D micromagnetic model for ferromagnetic samples", Physica B: Condensed Matter, Vol. 403, pp. 342-345, 2008.

B. Van de Wiele, L. Dupré, and F. Olyslager, "Accelerated 3D numerical scheme for the evaluation of hysteresis in ferromagnetic grains", IEEE Transactions on Magnetics, Vol. 44, No. 6, pp. 850-853, 2008. 

B. Van de Wiele, F. Olyslager, and L. Dupré, "Application of the fast multipole method for the evaluation of magnetostatic fields in micromagnetic modelling", Journal of Computational Physics, Vol. 227, pp. 9913-9932, 2008.

B. Van de Wiele, A. Manzin, L. Dupré, F. Olyslager, O. Bottauscio, and M. Chiampi, "Comparison of finite-difference and finite-element schemes for magnetization processes in 3-D particles," IEEE Transactions on Magnetics, Vol. 45, No. 3, 2009.

External links 

http://magnet.atp.tuwien.ac.at/scholz/  click further on to ‘Graphics gallery’ to see some animations.




For more information: ben.vandewiele@ugent.be, luc.dupre@ugent.be