Developing a new generation of quantum hard drives using 2D magnets

Using theoretical and experimental analysis, researchers aim to better understand the novel and intriguing magnetic properties of 2D materials for the next generation of information technologies.

Since the discovery of graphene in 2004 - which won its inventors the Nobel Prize and launched a new field of materials research - two dimensional (2D) materials that are one atom thick have promised to revolutionized technology as a result of their unique and sometimes bizarre properties.

However, one crucial property to data storage and electronics has, for a long time, remained elusive in these materials: magnetism.

For a while, many experts thought that 2D magnetism existed only in theory, but in 2017, a breakthrough came with the first measurements of ferromagnetism were made in chromium triiodide (CrI₃) and Cr₂Ge₂Te₆ compounds. This discovery sparked a renewed interest in the magnetism of these materials, for which many unknowns still remain. "For one," said Dr Elton Santos from the School of Physics and Astronomy, "it is still unclear how magnetism develops in 2D."

Dr Santos is part of a collaborative team of researchers who suggest that as a result of magnetic domains found in 2D CrI₃, this material shows properties that may go beyond those in conventional magnets, with quantum effects playing an important role in large area and denser amounts of information that may be stored. The researcher’s findings have recently featured the cover of Advanced Materials.

The interplay between different interactions usually align the spins of the atoms in small spatial regions on the material, with different orientations forming what are called ‘magnetic domains’. In the case of monolayer CrI₃, however, we observed that the domains cluster themselves in patches that can be large enough to cover the material’s entire surface, like in a single-domain particle but with no magnetic fields. Materials that show such a large area of magnetism can be used in different magnetic processes, such as information storage, but in a much finer fashion—in an area about 10,000 times thinner than the human hair.

When first discovered in 2017, 2D materials like CrI₃ were initially classified as Ising magnets, which is the simplest mathematical model that may describe magnets in a honeycomb lattice of spins, where each site can be either spin up or spin down. Each spin acts like a mini magnet with its own magnetic moment; if all the spins are aligned, then the whole lattice behaves like a big magnet with a net magnetic moment.

But Santos and his colleagues felt this classification did not fully capture the properties of these materials, as they had observed some contradictory phenomenon.

CrI3 was assigned as an Ising magnet, but at the same time it shows properties that would not be compatible with this characteristic, i.e., spin-waves—continuous variations of the orientation of the spins in a wave form. Spin waves have been measured by different techniques, including neutron scattering and Raman spectroscopy, and by different groups worldwide. So, we thought something was not quite right.

What Santos and his group uncovered is the presence of domain walls within the 2D materials, which evolve following the low-dimensionality of the layer, which is not commonly found. To give an example, in conventional magnets like Fe (iron), the domain walls just separate the magnetic domains but they don’t change. They are mostly static in time.

The team now is working on the implementation of these thin magnets in different device-platforms. For instance, where the control of magnetic domain walls is mediated by electrical currents. Initially designed by Stuart Parkin and his team at IBM in 2008, this phenomenon was used to build what are called racetrack memories, which organize data in a 3D microchip and may someday replace conventional data storage.

Dr Santos commented:

We believe that the implementation of our findings in racetrack structures is a matter of time. Since the domain wall is very narrow in these 2D magnets, it is ideal for such processes, and they can also be transferred on different surfaces which facilitates integration with current semiconductor components. This could result in devices that are practically unbreakable because the 2D material is so flexible, as well as light and thin enough to be carried around in your pocket but with the capacity to store thousands of huge files on the go. However, we would need to be cautious since there is a long way before we will reach that level in terms of application.