In a recent study published in the journal Science Advances, a research team from Rice University in the United States, the University of Minnesota, and the Paul Scherrer Institute (PSI) in Switzerland announced a significant breakthrough. They revealed that ruthenium dioxide (RuO₂), previously thought to be non-magnetic, exhibits a new “alternate magnetism” when made into ultra-thin films consisting of only a few atomic layers and subjected to lattice strain.
This discovery marks a breakthrough on three fronts: it not only provides a crucial answer to the long-standing magnetic debate in the physics community but also validates the precise manipulation of quantum states through “strain engineering.” Moreover, it signifies the potential for developing next-generation quantum chips and random access memory (RAM) architectures that are ultra-fast, energy-efficient, and high-density.
Ruthenium dioxide (RuO₂) is a common metal oxide and electrocatalyst that has been predicted by theoretical physicists as one of the candidate materials for emerging “alternate magnetism.” However, past experimental measurements on bulk or thicker crystals mostly failed to demonstrate clear magnetic behavior, sparking heated debates in the scientific community.
Dr. Ming Yi, an associate professor of physics and astronomy at Rice University and one of the study’s authors, explained, “Ruthenium dioxide was one of the earliest materials proposed to exhibit alternate magnetism, but evidence of magnetism was never found in bulk studies. Our research indicates that reducing it to an ultra-thin structure may be the key to awakening its magnetic properties.”
The research team grew ruthenium dioxide films with a thickness of only about 2 nanometers (equivalent to a few atomic layers) on a titanium dioxide (TiO₂) substrate. The controlled lattice strain induced by the differences in crystal structures between the substrate and the thin film completely altered its internal electronic structure.
“Alternate magnetism” is a newly established type of magnetic state that combines the advantages of ferromagnets (easy to read) and antiferromagnets (no external magnetic field leakage, high stability). To confirm the material’s magnetic state, the research team used advanced Spin- and Angle-Resolved Photoemission Spectroscopy (SARPES) to map the spin texture of the ultra-thin ruthenium dioxide in detail.
SARPES is an advanced method that allows the detection of electronic structures at the quantum level. It can not only measure the energy and momentum of electrons (traditional ARPES functionality) but also pinpoint the direction of electron spins (Up or Down), enabling scientists to directly visualize the quantum states within materials.
Spin texture refers to the specific geometric arrangement of electron spins in momentum space (K-space). By observing the symmetry and splitting features of spin texture, scientists can directly determine the material’s magnetic state – whether it belongs to antiferromagnetism, alternate magnetism, or is influenced by the Rashba effect.
The first author of the paper, Yichen Zhang, stated, “Through experimental data analysis combined with theoretical calculations, we found that ultra-thin ruthenium dioxide exhibits spin texture consistent with unconventional magnetism. This confirms that ruthenium dioxide in the bulk state and in the ultra-thin state under appropriate conditions possess distinctly different magnetic properties.”
The experiment also indicated that without lattice strain, the electronic spin behavior of the ultra-thin film would revert to a non-magnetic state similar to that of the bulk material. Yichen Zhang added, “This high dependency on strain suggests that we may use lattice strain as a ‘control knob’ in the future to induce or precisely control alternate magnetism. This has significant practical value in designing next-generation spintronics and memory chips.”
This research highlights the complexity and sensitivity of quantum materials. Dr. Yi emphasized that high-quality sample preparation and rigorous measurement techniques were key to the success of this study. The team not only identified the symmetry of the magnetic state but also found potential pathways to manipulate it in future quantum devices.
With the unveiling of the hidden magnetism in this ultra-thin material, engineers can potentially unlock the potential of low-dimensional quantum materials further through strain engineering, propelling computing devices towards high density, high speed, and low energy consumption.