Twisted multiferroics discovery makes new kinds of topological spintronic devices possible

Two-dimensional van der Waals materials are one-atom thick compounds that can be used to engineer exotic quantum phases of matter. Stacking and twisting different two-dimensional materials causes a real-space modulation known as a moiré pattern to emerge. This pattern exists everywhere from math and art to the simple example of two chain-link fences overlapping, creating the look of an even more dense metal fence.

For physicists, moiré patterns are a promising way to create the building blocks for new kinds of quantum matter, paving the way for improved quantum technologies like a universal topological quantum computer.

Now a team at Aalto University’s Department of Applied Physics, consisting of Doctoral Researcher Tiago Antão, Postdoctoral Researcher Adolfo Fumega and Assistant Professor Jose Lado, has demonstrated how a new type of twisted van der Waals material, featuring an exotic electric and magnetic state, can be created. 

The team’s findings establish that the moiré modulation—arising from the angle at which the material is twisted—enhances competing magnetic interactions and the inherent multiferroicity in the material, resulting in unconventional properties.

The paper was published in the journal Nano Letters: https://doi.org/10.1021/acs.nanolett.4c04582

'The interplay between the moiré length scale and the magnetic order of the material leads to a complex phase diagram with intriguing states,' says Fumega, last author of the work.

These intriguing states include what the team calls topological kπ-skyrmion lattices. They are novel phases built of a series of ordered dartboard-shaped magnetic textures, which self-organize following the geometry of the twisted material and present potential applications in multi-level memory storage. 

Unprecedented control

The team’s results focused on a twisted bilayer of NiI₂ which has a strong, tunable magnetoelectric coupling. This allows external electric fields to precisely control the material’s magnetic phases, offering a robust way to control different topological magnetic configurations. 

The researchers theoretically showed how external electric fields allows to externally reshape these textures, enabling creating new quantum devices that exploit these exotic and controllable topological states.

'Twisted bilayer NiI₂ represents a highly tunable platform for exploring unconventional magnetism. Its unique magnetoelectric properties could pave the way for next-generation spintronic devices with electric-field-controlled topological spin textures,' Antão says.

The study opens new research possibilities for ultrathin, energy-efficient memory technologies and highlights the vast potential of engineered van der Waals systems in spintronics. It also underscores the importance of combining moiré engineering with multiferroic materials to achieve unprecedented control over magnetic and electric phenomena at the nanoscale.

'Controllable multiferroic materials provide exceptional building blocks to creating exotic quantum states of matter that need to have high tunability. These materials can be one of the key enablers of a whole new family of quantum states not found in natural compounds so far. This may ultimately allow us to build a universal topological quantum computer,' Lado says.

This research was partly funded by the Finnish Quantum Flagship project. Lado also received this December a five-year European Research Council Consolidator grant to engineer a new form of quantum matter known as Fibonacci anyons using super-moire materials. This strategy leverages multiferroic quantum matter as a fundamental enabler for controlling these exotic states.