Sensing a magnetic rare-earth surface alloy by proximity effect with an open-shell nanographene

Imagine a world where the tiny building blocks of matter, usually thought of as passive or neutral, can be coaxed into acting like tiny magnets. This is the frontier of molecular spintronics, a field that seeks to use the "spin" of electrons—their intrinsic angular momentum, which makes them behave like microscopic bar magnets—as a way to store and process information. While traditional electronics rely on the movement of electric charge, spintronics aims to harness this magnetic property, potentially leading to faster, more efficient, and less energy-hungry devices. For this to work, scientists need to create interfaces where magnetic molecules can sit on a magnetic surface without losing their unique properties or getting chemically destroyed. The challenge has been finding a surface that is magnetic enough to influence the molecule but gentle enough to let the molecule keep its own identity.

In a recent study, researchers have successfully demonstrated a way to make this happen using a very specific combination of materials. They took a flat, triangular molecule made entirely of carbon atoms, known as phenalenyl, which naturally possesses a single unpaired electron, giving it a magnetic spin. They placed this molecule onto a special surface made of gold and a rare-earth element called terbium. This surface, which forms a highly ordered, honeycomb-like pattern, acts as a ferromagnet, meaning its atoms are aligned to create a magnetic field pointing straight up from the surface. The team discovered that when the carbon molecule sits on this magnetic surface, its magnetic state changes in a very specific and measurable way. Instead of behaving as it does on a normal, non-magnetic gold surface, the molecule's magnetic signal splits apart, revealing that the surface has successfully "talked" to the molecule and aligned its spin without destroying it.

The researchers began by creating this unique surface alloy. They heated a gold crystal and deposited terbium atoms onto it, allowing the two elements to mix and form a thin, ordered layer. Using a powerful microscope that can see individual atoms, they confirmed that the surface formed a perfect, repeating pattern of triangles and hexagons, stretching across the entire sample. This pattern is not just a visual curiosity; it arises because the atoms in the new layer do not fit perfectly with the gold atoms underneath, creating a gentle, periodic ripple in the surface height. Next, they introduced the carbon molecules. Because these molecules are fragile and react quickly with air, the team first deposited a slightly different, safer version of the molecule and then used the microscope's tip to carefully remove a hydrogen atom from each one, transforming them into the active, magnetic form right on the surface.

To understand how the surface affected the molecule, the team measured the electrical current flowing between the microscope tip and the molecule. On a standard gold surface, the molecule showed a single, sharp peak in its electrical response right at zero voltage. This peak is a well-known signature of a magnetic molecule interacting with the electrons of a metal, a phenomenon called the Kondo effect. However, when the same molecule was placed on the terbium-gold surface, this single peak split into two distinct peaks, separated by a small but clear gap of about 20 millivolts. This splitting is the key finding. It indicates that the magnetic field from the surface has lifted the energy degeneracy of the molecule's spin, effectively forcing the molecule to choose a specific magnetic orientation. The size of this split is significant, suggesting a strong interaction between the molecule and the surface's magnetic field.

The team did not stop at observation; they wanted to understand why the split happened and if it was the same everywhere on the surface. They combined their experimental data with detailed computer simulations to model the interaction. The simulations revealed that the magnetic influence of the surface is not uniform. Because the surface atoms ripple up and down in a specific pattern, the strength of the magnetic interaction changes depending on exactly where the molecule sits. When the molecule lands on certain parts of the ripple, the split in its energy levels is larger; on other parts, it is smaller. The researchers confirmed this by mapping the surface with their microscope, moving from one spot to another and measuring the split at each location. They found that the size of the split varied in perfect step with the underlying pattern of the surface, growing larger in some areas and smaller in others, just as the computer models predicted.

This work provides a clear demonstration that it is possible to place a delicate, magnetic carbon molecule on a magnetic surface and have them interact in a controlled way. The terbium-gold surface acts as a stable platform that preserves the molecule's magnetic nature while imprinting its own magnetic direction onto it. The study shows that the interaction is driven by the proximity of the two materials, where the magnetic field of the surface penetrates the molecule, rather than by a direct chemical bond that would alter the molecule's structure. By proving that the strength of this interaction can be tuned simply by changing the molecule's position on the surface pattern, the researchers have opened a new path for designing future devices. They have shown that rare-earth metal surfaces can serve as a reliable foundation for building magnetic components out of carbon-based molecules, a crucial step toward integrating these organic materials into the next generation of spin-based technology.