“Topological defects” occur when the symmetry of a magnetic material is disturbed. Domain walls (DWs) are a type of topological defect that separates regions with different magnetic orientations. As a widely studied phenomenon, the manipulation of these defects has potential applications in high performance storage devices, energy processing devices, and quantum computers.
Recently, the possibility of other topological defects embedded in or combined with DWs has attracted attention due to their possible applications in various areas of physics. Some examples of these “defects within defects” are referred to as DW skyrmions and DW bimerons. Although theoretical models have supported the existence of these defects, they have not yet been observed experimentally.
In a new study published in Nature communication, Associate Professor Masahiro Nagao of Nagoya University, Japan, and colleagues used Lorentz transmission electron microscopy (LTEM) to visualize these defects. They could do this by letting electrons pass through and watching their deflections through a thin magnetic film. The topological defects were observed as contrasting pairs of light and dark areas. Using this technique, the team mapped topological defects in a chiral magnetic thin film made of cobalt, zinc and manganese.
Initially, the researchers observed a single DW defect when the film was not magnetized. When the film was magnetized by passing a magnetic field perpendicular to the film, they were able to observe 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 alternately and in pairs. The researchers found that these DWs increased as the strength of the magnetic field increased and eventually disappeared after a certain threshold was reached. To confirm their discovery, the researchers used the transport-of-intensity equation to get the magnetic distributions that showed opposite magnetizations on either side of the DW chain, confirming that they were DW bimerons.
The researchers were finally able to propose an explanation for these defects and their mechanism of origin. As Prof. Nagao explains: “In our chiral magnetic thin films we show chained and isolated bimerons that play the role of DWs or are bound to them, which is not only due to the magnetic anisotropy component in the plane, but also due to the combination of the Dzyaloshinskii Moriya interaction is realized, magnetic anisotropy out of plane, dipolar interaction and Zeeman effect. ”
The team’s results shed light on topological defects in chiral magnets and have implications for areas of physics related to topology, ranging from cosmological length scales to condensed matter.
The article “Observation of domain wall bimerons in chiral magnets” was published in the journal. released Nature communication on June 9, 2021, at DOI: 10.1038 / s41467-021-23845-y.
About Nagoya University, Japan
Nagoya University has roughly 150 years of history, with roots in a makeshift medical school and hospital established in 1871, which was officially established in 1939 as the last imperial university in Japan. Though modest compared to the largest universities in Japan, Nagoya University has strived for excellence since its inception. Six of the 18 Japanese Nobel Prize winners since 2000 have done all or part of their Nobel Prize-winning work at Nagoya University: four in physics – Toshihide Maskawa and Makoto Kobayashi in 2008 and Isamu Akasaki and Hiroshi Amano in 2014; and two in chemistry – Ryoji Noyori in 2001 and Osamu Shimomura in 2008. In mathematics, Shigefumi Mori did his Fields Medal-winning work at the university. A number of other important discoveries were also made at the university, including the Okazaki DNA fragments by Reiji and Tsuneko Okazaki in the 1960s; and exhaustion from Sho Asakura and Fumio Oosawa in 1954.
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