• Laurson, Lasse
• Vippola, Minnamari
• Santa-Aho, Suvi Tuulikki
• Kaappa, Sami
• Honkanen, Mari Hetti

Abstract

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

Topics

• impedance spectroscopy
• grain
• scanning electron microscopy
• simulation
• carbide
• steel
• transmission electron microscopy
• dislocation
• electron backscatter diffraction
• magnetization
• magnetic domain wall
• magnetic force microscope