MRI performed on single atoms

Korean and US researchers have performed what is claimed to be the world’s smallest magnetic resonance imaging (MRI) scan — a technique which they used to visualise the magnetic field of single atoms. Their breakthrough has been described in the journal Nature Physics.

Routinely performed in hospitals as a part of diagnostic imaging, MRI scans detect the density of nuclear spins — the fundamental magnets in electrons and protons — in the human body. Traditionally, billions and billions of spins are required for an MRI scan — but researchers from the Center for Quantum Nanoscience (QNS) at Ewha Womans University, collaborating with their US colleagues, have now shown that this process is also possible for an individual atom on a surface.

QNS’s Dr Philip Willke, lead author on the study, explained that the team utilised a scanning tunnelling microscope, which consists of an atomically sharp metal tip that works like a fingertip reading Braille.

“If you scan across the surface and if you have something on the surface, your tip can feel that,” Dr Willke said. This allowed the researchers to image and probe single atoms by scanning the tip across the surface.

The two elements that were investigated in this work, iron and titanium, are both magnetic — and were both readily visible through the microscope. The researchers then used the microscope’s tip like an MRI machine to map the three-dimensional magnetic field created by the atoms, with what is said to be unprecedented resolution. In order to do so, they attached another spin cluster to the metal tip of their microscope. Similar to everyday magnets, the two spins attract or repel each other depending on their relative position. By sweeping the tip spin cluster over the atom on the surface, the researchers were able to map out the magnetic interaction, thus visualising the magnetic field around the atoms with high precision.

“It turns out that the magnetic interaction we measured depends on the properties of both spins — the one on the tip and the one on the sample,” Dr Willke said. “For example, the signal that we see for iron atoms is vastly different from that for titanium atoms. This allows us to distinguish different kinds of atoms by their magnetic field signature and makes our technique very powerful.”

The researchers plan to use their single-atom MRI to map the spin distribution in more complex structures, such as molecules and magnetic materials, with co-author Dr Yujeong Bae noting that many magnetic phenomena take place on the nanoscale — including the recent generation of magnetic storage devices.

The ability to analyse magnetic structure on the nanoscale could help to develop new materials and drugs. Moreover, the research team wants to use this kind of MRI to characterise and control quantum systems, which will be very useful for quantum computing applications.

“I am very excited about these results — it is certainly a milestone in our field and has very promising implications for future research,” said co-author Professor Andreas Heinrich, Director of QNS. “The ability to map spins and their magnetic field with previously unimaginable precision allows us to gain deeper knowledge about the structure of matter and opens new fields of basic research.”

Image caption: Mapping different magnetic interactions between tip and surface atom.

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