CPM Seminar

Quantitative Magneto-mechanical Detection and Control of the Barkhausen Effect

Mark Freeman

University of Alberta

The irregular step-like changes in magnetization discovered by Barkhausen in 1919 are recognized as the first experimental evidence for ferromagnetic domains (postulated to exist by Weiss approximately twelve years earlier). The Barkhausen effect itself is very challenging to fully quantify, however. There are seemingly too few experimental handles for determination of the disorder potentials, which collectively pin domain walls at large numbers of individual locations simultaneously.

The situation changes qualitatively for ferromagnetic disks in the micromagnetic vortex state. The vortex core localizes domain wall energy to a length scale characteristic of individual pinning sites. This opens the door to "scanning vortex probe microscopy", in which minute magnetization changes are recorded while the core position is rastered within the disk through the application of in-plane magnetic fields. The requisite magnetization sensitivity is achieved using nanomechanical torque magnetometry. In this regime, the Barkhausen effect becomes a quantitative, 2D probe of local energetics having spatial and energy resolutions on the scales of a few nanometres and ten millielectron-volts [1].

In addition to the mapping of intrinsic disorder in a polycrystalline permalloy film, the ability to impose quantitatively characterized, artificially patterned energetic landscapes through very low-dose, focussed ion beam writing will be described.

[1] Burgess et al., Science Express 2013 (http://www.sciencemag.org/content/early/2013/01/16/science.1231390)