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Barkhausen Noise

The Barkhausen effect is the name given to the noise in the magnetic output generated in a ferromagnetic material when the magnetising field applied to it is changed. It is understood that the origin of this noise is due to the fact that the domain wall, which moves in response to the externally applied magnetic field, moves in a jerky manner. This irregular motion is due to the fact that the domain wall can become pinned, at various points, by impurities in the material. By increasing the strength of the external field the domain wall can become locally de-pined and moves forward, only to become trapped once again by more impurities further ahead.   

 The study of the Barkhausen effect is currently one of the most active fields in the science of soft magnetic materials. There is considerable interest from an applied perspective, since the amount of Barkhausen noise for a given material is linked with the amount of impurities and defects it contains. This effect can be used as a non-destructive means of analysing the mechanical properties of a material. However, the success of this approach is contingent on having a flexible and accurate simulation method. The method must be able to incorporate a number of effects which can yield a dramatic change in the morphological and dynamic properties of the domain wall. These include: (i) a cross over in the presence of long range dipolar forces acting on the domain wall, (ii) a cross over upon going from a strictly 1D interface (in a 2D space) to a 2D interface (in 3D space).  

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The above image (from our paper) shows the effect on the morphology of the domain wall depending on the strength of the long range dipolar forces. These long range forces are weak for low values of the saturation magnetisation (left image) and strong for high values of the saturation magnetisation (right image). It is found that  the behaviour of Barkhausen avalanches can be described by two separate universality classes depending on the value of the saturation magnetisation. On the one hand, when the long range diploar interaction is strong the domain wall has saw toothed morphology which has the effect of  reducing the magnetic charge density. On the other hand, by increasing the temperature of the sample it is possible to study the regime in which the saturation magnetisation is negligible. In this case the morphology of the domain wall is dominated by line tension and is observed to form a rough interface free of large zigzags.

Currently I am designing a new approach to simulate the Barkhausen effect using the Surface Evolver (SE) package with which the aforementioned effects can be readily incorporated. The SE models the interface using a finite-element method, representing the surface as a union of simplices. The advantage of this method is that it is free of artefacts which would otherwise be present. These include artefacts introduced by an underlying lattice symmetry (as in the case of methods based on the Ising models). Furthermore, using in-built SE routines it is possible to dynamically refine the domain wall (i.e. change the number of vertices, edges and facets used to describe the domain wall) which ensures there are no artefacts due to the resolution of the interface. To see an example of the simulation in action click here (the animation takes a few moments to get started...): movie.gif

For more information see:

Modelling domain wall dynamics in thin magnetic strips with disorder. L. Laurson, A. Mughal, S. Zapperi and G. Durin. Accepted for publication in the proceedings of Soft Magnetic Materials 19. Also see http://arxiv.org/abs/0909.4659

Effect of dipolar interactions for domain wall dynamics in magnetic thin films.  A. Mughal, L. Laurson, , S. Zapperi and G. Durin. Accepted for publication in the proceedings of Soft Magnetic Materials 19. Also see http://arxiv.org/abs/0910.1040

© Adil Mughal 2012 adil.m.mughal@gmail.com