A magnetic flux density (magnetization) map, showing DW bimerons, which was obtained using the transport of intensity equation. Image: Masahiro Nagao.
A magnetic flux density (magnetization) map, showing DW bimerons, which was obtained using the transport of intensity equation. Image: Masahiro Nagao.

'Topological defects' are formed when the symmetry of a magnetic material is disrupted. Domain walls (DWs) are a type of topological defect that separates regions of a material with different magnetic orientations. Manipulating these defects could lead to several applications, including in high-performance memory storage devices, energy processing devices and quantum computing.

Recently, the possibility of other topological defects embedded in or combined with DWs has sparked interest for potential applications in different fields of physics. Some examples of these 'defects within defects' include DW skyrmions and DW bimerons. While theoretical models have supported the existence of these defects, they have not been experimentally observed – until now.

In a paper in Nature Communications, Masahiro Nagao, an associate professor from Nagoya University in Japan, and his colleagues report using Lorentz transmission electron microscopy (LTEM) to visualize these defects. This involved firing electrons through a thin magnetic film and observing their deflections. The topological defects were observed as contrasting pairs of bright and dark areas. Using this technique, the team imaged topological defects in a chiral magnetic thin film made of cobalt, zinc and manganese.

When the film was unmagnetized, the researchers observed a single DW defect. But on magnetizing the film by exposing it to a perpendicular magnetic field, they observed the development of two types of DWs. The conventional DWs were seen as black lines, while chains of DW bimerons were seen as bright elliptical dots on the LTEM images. These two types of DWs appeared alternatively and in pairs.

The researchers noted that the DWs increased as the strength of the magnetic field was increased and finally disappeared after a certain threshold was reached. To confirm their discovery, they used the transport of intensity equation to obtain magnetic distributions that revealed opposite magnetizations on both sides of the chain of DWs, confirming them to be DW bimerons.

This allowed the researchers to propose an explanation for these defects and their mechanism of formation. "In our chiral magnet thin films, we show chained and isolated bimerons playing the role of and bound to DWs respectively, which are realized by not only in-plane magnetic anisotropy component but also the combination of Dzyaloshinskii-Moriya interaction, out-of-plane magnetic anisotropy, dipolar interaction and Zeeman effect," Nagao explains.

The team's findings shed light on topological defects in chiral magnets and have implications in fields of physics related to topology, ranging from cosmological length scales to condensed matter.

This story is adapted from material from Nagoya University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.