Recently, a research team led by Professor Shilei Zhang from the School of Physical Science and Technology of ShanghaiTech University realized the folding and unfolding of information storage carriers in three-dimensional space. Using the properties of topological defects in a special magnetic material system, the research team successfully folded the three-dimensional topological spin configuration into a zero-dimensional singularity and restored its structure in reverse. In the study, published in the journal Nano Letters, the team showed how the various stages of the intricate folding process were uncovered using soft X-ray magnetic scattering.
The formation and evolution of topological defects is a fundamental concept in physics and a natural consequence of spontaneous symmetry breaking. For example, defects inevitably arise in crystals, and it are those "singularities" that interrupt long-range order. These point defects can gradually evolve into one-dimensional line defects or two-dimensional surface defects, eventually leading to fracture. In other words, the failure of a crystal under load can be traced back to the generation of topological singularities; and it is the formation of point defects that determines the location of the fracture.
The same concept is said to hold true in cosmology, namely, that in the early universe, spacetime is filled with magnetic monopole singularities (caused by the Higgs mechanism). These magnetic monopoles evolve into one-dimensional strings and two-dimensional domain walls, and then form abundant elementary particles and ultimately determine the spacetime of the universe in its present form. In other words, the full information of the delicate structure of the universe can be traced back to the earliest topological singularity. In fact, this kind of inference conforms to the basic property of topology: information is not related to the scale of space. As long as they are topologically homologous, complex configurations can either occupy a huge space or be collapsed to an infinitesimal point.
This general concept can bring a new perspective to information storage. For magnetic storage, in order to limitlessly increase the storage density, it is necessary to continuously reduce the space occupied by a single bit in terms of materials and engineering. However, if considered from a topological point of view, this kind of problem seems to become simpler: it only needs to fold the magnetic configuration of the bit into an infinitesimal topological singularity, which is generated and stored in a material.
Magnetic skyrmions are a magnetically ordered phase with a vortex configuration. Because of the topological properties of the vortex structure, they are excellent information carriers for magnetic memory devices. In recent years, research has been focusing on reducing the size of magnetic skyrmions via material optimization. For example, common skyrmions that can be used for magnetic memory are on the order of tens of nanometers in size. However, from a topological point of view, magnetic skyrmions are also "shrinkable". As shown in the figure, in terms of their spatial distribution, the vector field distribution of a skyrmions is equivalent to the one-dimensional topological defect-string in string theory, and the origin of strings can be traced back to topological singularities - magnetic monopoles. If a skyrmion can be folded into a structure similar to a magnetic monopole, its footprint will be greatly reduced.
Experimentally, the research team used the helical magnet-ferromagnetic multilayer heterojunction to achieve precise control of the skyrmion string length. It was shown that the limit of one-dimensional string shrinkage is the zero-length point floating at the material interface (as shown in the figure). The research team used the three-dimensional analytical capability of the synchrotron radiation based technique of soft X-ray magnetic scattering at the RASOR endstation of beamline I10 at Diamond to directly observe the process of the skyrmion string being gradually folded into a topological singularity. Importantly, the folded information can be unfolded again to reveal the complete structure of the string and restore its original configuration and size through the reverse process. This discovery provides a new research idea for understanding the origin of topological magnetic structures and topological magnetic storage.
Prof Shilei Zhang’s team from the School of Physical Science and Technology at ShanghaiTech included colleagues from RIKEN Research Center, Nanjing University, the Institute of Physics of the Chinese Academy of Sciences, Diamond Light Source and Oxford University.
Corresponding author Prof Shilei Zhang explains his excitement:
It is always intriguing to think about that information can be folded like an umbrella. This is, perhaps, the ultimate way to store data in the most compact space imaginable.
For more information on the subject matter, please contact either
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or Professor Thorsten Hesjedal (Oxford Physics): Thorsten.Hesjedal@physics.ox.ac.uk.
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Haonan Jin, Wancong Tan, Yizhou Liu, Kejing Ran, Raymond Fan, Yanyan Shangguan, Yao Guang, Gerrit van der Laan, Thorsten Hesjedal, Jinsheng Wen, Guoqiang Yu, and Shilei Zhang, Evolution of Emergent Monopoles into Magnetic Skyrmion Strings, Nano Letters (2023), DOI: 10.1021/acs.nanolett.3c01117
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