Novel utilization of microscopy and modelling to better understand Barkhausen noise signal

The actual origin of the Barkhausen noise (BN) signal itself is not considered much in production quality control when industrial BN
measurements are done. However, with assistance of electron microscopy, the information of microstructure and magnetic substructure,
called as magnetic domains, from the sample can be gathered. Magnetic domains represent the magnetic substructure similar to the grain
structure of the sample defining the magnetic properties of material. The BN measurement gives indirect information of the movements
of magnetic domain walls (DWs) in the applied magnetic field. Electron microscopy allows us to make direct characterizations of micro-
structural pinning sites (e.g., grain boundaries, dislocations, carbides) hindering the DW motion and to visualize how these pinning sites
interact with DWs thus produce the BN signal. Here, we present a methodology to combine indirect (BN measurement) and direct
(microscopy) studies to better understand how microstructural features affect the BN signal. BN measurements were done in millimeter
scale producing the BN signal of the microstructural state while an external magnetic field was applied to material. Micrometer scale
microstructural and crystallographic information was gained with scanning electron microscopy (SEM) together with electron backscatter
diffraction (EBSD) technique. Down to sub-nanoscale microstructural features were studied by transmission electron microscopy (TEM).
Lorentz electron microscopy in TEM was used to observe DWs and to visualize their motion. We used a simple structure, ferritic steel
with carbides, to demonstrate the methodology. Fig. 1 presents examples how a microstructure and magnetic structure behind the BN
signal can be studied by electron microscopy. The SEM image shows grain boundaries and carbides. Crystallographic information is
commonly collected by TEM, however, TEM studies on the magnetic sample can be challenging and thus orientation and dislocation
information is collected also by EBSD. By Lorentz microscopy, DWs are observed as white and black lines. In the future, the domain
structure will be studied also by magnetic force microscopy (MFM). The influence of the pinning sites on the DW motion can be studied
by Lorentz microscopy using an objective lens of TEM as a source of the applied magnetic field, i.e., the BN measurement can be
visualized, see our earlier study [1]. Coupling experimental data with realistic computational modelling such as micromagnetic simulations
enables, e.g., the study of detailed dependencies of statistical properties of BN and the underlying magnetization dynamics on the material
microstructure, which can be created in the model system using experimental electron microscopy results as input. This novel utilization
of multiscale characterization and modelling gives versatile information how microstructural features manifest in the ensuing BN signal.