Abstract
The mobility of magnetic domains forms the link between the basic physical properties of a magnetic material and its global characteristics such as permeability and saturation field. Most commonly, surface domain structure are studied using magneto-optical Kerr microscopy. The limited information depth of approx. 20 nanometers, however, allows only for an indirect interpretation of the internal volume domain structures. Here we show how accumulative high-frame rate dynamic neutron dark-field imaging is able for the first time to visualize the dynamic of the volume magnetic domain structures in grain oriented electrical steel laminations at power frequencies. In particular we studied the volume domain structures with a spatial resolution of ∼100 μm and successfully quantified domain sizes, wall velocities, domain annihilation and its duration and domain wall multiplication in real time recordings at power frequencies of 10, 25 and 50 Hz with ±262.5 A/m and ±525 A/m (peak to peak) applied field.
Highlights
Today’s global electricity market sizes to approximately 5 trillion watts
Grain-oriented electrical steels (GOES) which are typically used in transformer cores are so-called soft magnetic materials where bulk magnetic domain structure and the mobility of the domains walls determine the efficiency of transformers
The disadvantage of Kerr microscopy is that the insulating coating, which is used in most transformer steel cores, has to be removed for the analysis to expose the surface of the steel
Summary
Today’s global electricity market sizes to approximately 5 trillion watts. Large power transformers play an important role in this market, being indispensable for transporting electricity along high voltage transmission lines. The unique capability of neutrons to penetrate materials opaque to other non-destructive techniques, e.g. utilizing on light or electrons, makes it possible to study bulk behavior of magnetic domain walls while keeping the insulating coating intact and without modifying the domain structure. The dark-field image (DFI) of neutron grating interferometry (nGI)[9] recently emerged as a valuable complementary technique to the established domain observation methods because it is the only technique available that enables the spatially resolved analysis of bulk magnetic domain walls[5,12,13,14,15] deep in the volume of materials and provides unique information. The typically long exposure times of several minutes for a single DFI in nGI experiments with standard scintillator based detector systems did not allow to study the dynamic properties of magnetic domain structures in bulk GOES at power frequencies
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