In 2017, scientists found an ultrathin magnetic material just three atoms — or one atomic unit — thick.
The material, called chromium triiodide, had a simple magnetic moment arrangement — or the spin of the electrons within the material all aligned in the same direction — which means it’s not able to store large amounts of information.
Now, Liuyan Zhao, physicist at University of Michigan in Ann Arbor, and her team have developed a way to create a more complex magnetic moment arrangement in chromium triiodide, allowing the faster processing of and more storage for information.
“Over time, people began looking for smaller sizes and more complex forms of magnets in order to make our computers and electronics smaller, thinner and faster,” says Zhao. “To do this, the material that stores data or does information processing needs to also get smaller and smaller, while their magnetic forms should be more and more exotic.”
“In very big, bulky materials, people find all kinds of magnetic forms called spin textures. So, in this ultrathin material, we asked: Can we also create those kinds of complex spin textures so that we can store more information,” she asks.
Zhao and her team created an artificial sample by tearing micron-sized (one millionth of a meter) flake of chromium triiodide into two. The split flake is called a bilayer flake, meaning it is two atomic units thick. They layered the pieces one on top of the other rotated a very slight amount.
The flakes are composed of a crystalline lattice structure, and when one is rotated slightly on top of the other, cause the crystalline structures to form a periodic structure with a longer wavelength. This also creates an angular mismatch between the two flakes, leading to a superlattice with a longer period called a moiré superlattice.
This can be likened to a wave of water. The ripple of one wave equals one period. But within this wave, water doesn’t move forward. Instead, the molecules of water rise and fall in one location. When more energy is added to the wave, the wave crests higher.
When the crystalline structures are layered on top of each other, their wave period is doubled. Then the atoms in the top layer of the material are slightly offset from the atoms in the lower layer of material near the center of rotation. This causes a further cascading of offset atoms throughout the doubled layer of material, which repeats throughout the entire piece of stacked layers at the moiré wavelength.
When offset by a third of the distance between the nearest neighboring chromium atoms, their spins favor in the opposite direction. Then between these two areas, their spins become frustrated, not knowing which of the two ways to follow, and could develop new arrangements. Then, for example, they can become spiraled. The different kinds of spin orientations within the same material creates more opportunities to store information.
“The importance of our work is to demonstrate in these very thin magnets we can design the spin texture by doing this kind of twisting to introduce the moiré superlattices. Different spin arrangements can give quite different physical properties of the magnetic materials we study,” says Zhao.
“As compared to many 3D bulky materials, the atomic arrangements are determined by chemistry during growth: you cannot change or manipulate that much. But here, by changing this twist angle between two layers to change the relative distance between atoms, we have the freedom to design and control magnetic properties in 2D moiré superlattices.”
The collaborative work involves scientists from multiple research institutions. Co-authors include Hongchao Xie, Xiangpeng Luo and Kai Sun in the U-M Department of Physics; Gaihua Ye, Zhipeng Ye, Rui He and Haiwen Ge at Texas Tech University; Suk Hyun Sung and Robert Hovden in the U-M Department of Materials Science and Engineering; Emily Rennich in the U-M Department of Mechanical Engineering; and Shaohua Yan, Yang Fu, Shangjie Tian and Hechang Lei at the Renmin University of China.