Magnetic Domain Structure of Ferromagnetic Steels Studied by Lorentz Microscopy and Magnetic Force Microscopy

Properties of ferromagnetic materials are determined both microstructural and magnetic features. The magnetic structure of ferromagnetic material consists of regions with internal magnetization pointing to a certain direction and these areas are called as magnetic domains. They are separated by boundaries called as domain walls (DWs) where the magnetization direction changes. The magnetic regions are formed by complicated arrangement that is determined by the energy minimization principle. [1] The domains have for example different
sizes; smaller size in martensitic steels which is full of individual nucleation sites (e.g. dislocations) to them compared to simple ferritic steel structure with larger domain size. One industrially relevant technical method, where the physical principle is strongly involving the domain structure and its changes, is the non-destructive testing (NDT) method called magnetic Barkhausen noise (BN) inspection. The DW structures and their differences influence on the BN signal measured when a time-varying magnetic field is applied. The magnetic field forces the internal domain structure to change and orientate towards the applied field. The microstructural details, such as dislocations and carbides, hinder the DW motion. The aim of this study was to compare the magnetic structure in the bulk steel sample studied by magnetic force microscopy (MFM) to the magnetic structure in the thin sample studied by Lorentz microscopy. In MFM, the contrast is produced by the magnetic interaction force between the magnetic tip and sample surface stray fields showing DWs as bright and dark lines [2]. When using Fresnel mode in Lorentz microscopy, deflected beam electrons are superposed
or diverged at the domain boundary showing DWs as white and black lines. In this study, MFM (Nanoscope iCon, Bruker) was utilized for imaging of bulk samples with ferritic and ferritic-pearlitic microstructures. The thin films of both microstructures were studied with TEM (JEM-F200, JEOL) by using Lorentz microscopy. Fig. 1a shows topography of the ferritic bulk sample containing a ferrite matrix with globular cementite (Fe3C) carbides. Based on the MFM studies
(Fig. 1b), the globular Fe3C carbides have their own domain structure appearing with alternating white and black lines as presented also in [2]. Similar type of internal magnetic structure of Fe3C carbides was also observed by Lorentz microscopy in the thin sample (Fig 1c). There are also DWs in the ferrite matrix (Fig 1b and c). More complicated domain structure in the industrially relevant ferrite-pearlite sample was studied. A topography image presented in Fig.
2a shows ferrite grains with thinner and thicker lamellas and globular carbides of cementite (Fe3C). The MFM image (Fig. 2b) shows similar internal contrast for the thicker lamellar and globular Fe3C than in Fig. 1b. Whereas, the thinner Fe3C lamellas appear only as bright/dark lines (Fig. 2b). The Lorentz microscopy image (Fig. 2c) reveals similar internal domain structure in thicker lamellar and globular carbides of cementite than observed by MFM (Fig. 2b). Based on Lorentz microscopy, thinner Fe3C lamellas have no internal domain structure. DWs in the ferrite matrix are mainly parallel and perpendicular to the Fe3C lamellas. In
addition, cross-tie DWs (Fig. 2c) were observed by Lorentz microscopy as they are related to the thin film nature of the TEM samples. To conclude, similar domain structure details were noticed and visualized in both bulk samples by MFM and thin samples by Lorentz microscopy. Both methods, however, have their unique properties for contrast occurrence [3] and therefore, we can only see those DWs oriented favorably towards the electron beam (Lorentz microscopy) and the tip (MFM).