Magnetic domains on unpolished and polished surfaces of titanium‐rich titanomagnetite

Abstract

Magnetic domains on unpolished and polished surfaces of synthetic aluminium‐magnesium titanium‐rich titanomagnetite (AMTM60) have been investigated with the dried Bitter method and scanning electron microscopy (SEM). The high magnification and superior resolution of the SEM afforded easy measurements of characteristic domain features, including domain wall widths. The large depth of field of the SEM also allowed domain patterns on unpolished sample surfaces to be studied. It was found that analogous domain structures were observed on unpolished, unpolished but acid‐etched, and ionically polished surfaces. However, even on unpolished surfaces, the domain style in AMTM60 was different from that expected for this material based on its cubic magnetocrystalline anisotropy. Features commonly observed on unpolished surfaces included (1) closely spaced stripe domains, (2) sinusoidally wavy walls, (3) rows of circular spike domains, and (4) complicated wavy wall patterns consisting of star like features enclosed by undulating and branching walls. All these features corresponded to domain structures found in uniaxial materials whose anisotropy Ku and saturation magnetization Ms satisfy the condition 2πMs2/Ku<1. The source of the uniaxial anisotropy in AMTM60 was most likely due to residual stress. Quantitative estimates of the exchange and anisotropy constants in AMTM60 were obtained from analysis of domain patterns using a theoretical model for sinusiodally wavy walls. Furthermore, as predicted by theory, wall widths increased and anisotropy decreased as the geometric complexity of the domain pattern increased from simple stripe domains to complicated wavy wall domains in response to a decrease in local residual stresses. Finally, it was observed that there was an important difference between domain style on polished and unpolished surfaces. On ionically polished surfaces only, many regions exhibited a “patchy” appearance, in which the domain style changed abruptly or in which domain walls disappeared completely at the edge of the patch and suggested that stresses produced by mechanical polishing may not have been removed completely or uniformly by ionic polishing. Consequently, it is uncertain how much of the intrinsic domain structure in titanium‐rich titanomagnetites survives mechanical polishing.