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Author: Morgan Sherburne, University of Michigan

Magnets are used in many items in our daily lives, including mobile phones, credit card holders or hotel keys. They can even power the engine in a vacuum.

Since most computers use magnets to store information, finding thinner magnets is the key to achieving faster and lighter electronic products. Graphene is a material that is only one atom thick. It was discovered in 2004 and won the Nobel Prize in Physics in 2010. Although graphene itself is not magnetic, it has sparked interest in finding atomic-scale thin magnets.

In 2017, scientists discovered an ultra-thin magnetic material that is only three atoms or one atomic unit thick. But this material, called chromium triiodide, has a simple arrangement of magnetic moments—the electron spins in the material are all aligned in the same direction, up or down—which means it cannot store a lot of information.

Now, University of Michigan physicist Liuyan Zhao and her team have developed a method to create more complex magnetic moment arrangements in chromium triiodide, allowing this atomically thin material to store more information and possibly Process information faster. Their results are published in "Nature Physics".

"Over time, people began to look for magnets of smaller size and more complex forms to make our computers and electronic products smaller, thinner, and faster. For this, materials for storing data or processing information are also needed They are getting smaller and smaller, and their magnetic forms should be more and more peculiar," Zhao said. "In very large and bulky materials, people have found various forms of magnetism called spin textures. Therefore, in this ultra-thin material, we asked: Can we also create those complex spin textures in order to Can we store more information?"

To this end, Zhao and her team created artificial samples by tearing micron-sized (one millionth of a meter) chromium triiodide sheet into two halves. The flakes of chromium triiodide are double-layered, which means that the material is two atomic units or six atoms thick. Then, they stacked one on top of the other and rotated it slightly. Each flake is composed of a lattice structure. When one structure is superimposed on another structure with a small amount of rotation, the crystal structures will interfere with each other and form a periodic structure with a longer wavelength. This will also create an angular mismatch between the two flakes and result in a longer superlattice period, which is called a moiré superlattice.

Think of a wave of water. One wave of ripples is equal to one cycle. But in this wave, the water did not actually move forward. Instead, water molecules rise and fall in one location. When more energy is added to the wave, the peak of the wave is higher.

Similarly, when the crystal structures are superimposed on each other, their wave period doubles. Then, due to the small rotation between the two layers, the atoms in the top layer of the material are slightly offset from the atoms in the lower layer of the material near the center of rotation. This further leads to a cascading effect of offset atoms in the entire bilayer material, which recurs in the entire stack at the Moiré wavelength.

Zhao said this would cause two extreme shifts within the structure. When the chromium atoms in one layer are arranged in the center of the chromium atoms in the other layer, their spin directions are the same. When they deviate from one third of the distance between the nearest neighboring chromium atoms, their spin favors the opposite direction. Then between these two areas, their rotation became frustrated, not knowing which of these two methods to follow, and new arrangements could be made. For example, they can then become spirals. Different kinds of spin directions in the same material create more opportunities for storing information.

In order to handle such thin and delicate materials, the team used a set of automatic micromanipulators under an optical microscope. The microscope was placed in a box containing ultra-high purity nitrogen. The box was inert and would not interfere with the research. The materials studied by the staff interact. The researchers used a common staple food for household use-adhesive tape-to peel off the two-dimensional material layer and imprint it on the silica substrate, a technique developed by the 2010 Nobel Prize in Physics winner. Using an optical microscope to observe the process, the researchers controlled a group of robotic arms to lift a layer of material, twist it slightly, and then put it back on top of another layer of material.

"The importance of our work is to prove that in these very thin magnets, we can design spin textures through this distortion to introduce moiré superlattices. Different spin arrangements can provide completeness for the magnetic materials we study. Different physical properties," Zhao said. "Compared to many 3D bulk materials, the arrangement of atoms is determined by the chemistry of the growth process: you can’t change or manipulate so much. But here, the relative relationship between the atoms is changed by changing the torsion angle between the two layers. Distance, we have the freedom to design and control the magnetism of the two-dimensional moiré superlattice." Further exploration of the spin wave detective story redux: Researchers found more surprising behaviors in the two-dimensional magnet More information: Rui He , Twisting engineering of two-dimensional magnetism in a double-layer chromium triiodide homogenous structure, Natural Physics (2021). DOI: 10.1038/s41567-021-01408-8. www.nature.com/articles/s41567-021-01408-8 Journal information: Nature Physics

Citation provided by the University of Michigan: Researchers designed magnetic complexity into atomic-level thin magnets (2021, December 2), retrieved from https://phys.org/news/2021-12-magnetic-complexity- atomically-thin-magnets. html This document is protected by copyright. Except for any fair transaction for private learning or research purposes, no part may be copied without written permission. The content is for reference only.

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