Adatoms are single atoms that get adsorbed onto the surface of a solid material and are known to hop randomly from one spot to another. In a recent study published in Nature Communications, a group of scientists from Germany demonstrated that single atoms can be steered in a chosen direction at near absolute zero temperatures (4 Kelvin), provided the surface being used is magnetic in nature—a discovery that can open up new possibilities for precise control of atomic motion, a sought-after ability in the field of nanotechnology, data storage and functional materials.
The researchers placed individual cobalt, rhodium, and iridium atoms on a 1-atom-thick manganese surface to create a magnetically well-defined surface and studied the migration behavior of adatoms using a scanning tunneling microscope (STM) at a temperature of 4 K.
According to established findings from nonmagnetic surfaces, atomic movement is usually governed by surface symmetry. In a hexagonal manganese monolayer like the one used in the study, atoms would be expected to migrate randomly in any of six directions. Yet in a surprising twist, researchers found that when a short, localized voltage pulse from the STM was applied, the atoms consistently moved in just one direction.
Atomic and molecular diffusion on surfaces is a fundamental process in surface chemistry as it is crucial for the growth of nanostructures and films and underpins a wide range of chemical reactions, including catalysis and molecular self-assembly. Understanding the real-time dynamics of atom motion and their behavior under various surface conditions and external stimuli is essential for optimization in surface science.
Studies exploring atomic motion on non-magnetic surfaces have found that the underlying surface symmetry plays a crucial role in guiding their movement, often exhibiting isotropic diffusion (the same diffusion in every direction) on high-symmetry surfaces. While non-magnetic surfaces and surface symmetry have been extensively studied, the influence of magnetism on diffusion remains largely unexplored experimentally. This is despite theoretical predictions suggesting that magnetism has a strong influence on adatom diffusion, particularly on ferromagnetic and antiferromagnetic surfaces.
In the study, the researchers designed a magnetically ordered surface by vapor-depositing a single layer of manganese (Mn) atoms onto a rhenium (Re) substrate. This precise arrangement created a hexagonal crystal structure with a row-wise antiferromagnetic state, where the magnetic moments alternate direction from one atomic row to the next.
Individual atoms of the magnetic element cobalt (Co) and the non-magnetic elements rhodium (Rh) and iridium (Ir) were deposited onto the Mn/Re surface, and their motion was triggered using local voltage pulses from the STM tip. Images from STM showed that the atoms strictly moved one-dimensionally along rows of parallel spins, and cobalt was the one to move the furthest when initiated by voltage pulses.
Simulations reveal that atoms move more easily along the magnetic rows of the surface than across them due to magnetic interactions between the adatom and the surface atoms, where both act like tiny bar magnets. In the case of magnetic cobalt atoms, this interaction occurred due to their own magnetic moment. However, for nonmagnetic atoms like rhodium or iridium, the movement is guided by a small magnetic moment induced by the surface itself.
This study reveals a novel way to control the movement of individual atoms using magnetism. Unlocking directional control at the atomic level is a game-changer for designing powerful technologies with atomic precision, from nano-engineered materials to quantum circuits.
