Physicists Use MRI to Scan Individual Atoms

A team of experimental physicists from the Institute for Basic Science, the Ewha Womans University and the IBM Almaden Research Center has performed the world’s smallest magnetic resonance imaging (MRI) to visualize the magnetic field of single atoms. The results are published in the journal Nature Physics.

Single magnetic atoms are deposited on a surface of magnesium oxide; they are imaged by the magnetic tip of a scanning tunneling microscope which allows researchers to perform an MRI scan of the atom’s magnetic field. Image credit: Willke et al.

Single magnetic atoms are deposited on a surface of magnesium oxide; they are imaged by the magnetic tip of a scanning tunneling microscope which allows researchers to perform an MRI scan of the atom’s magnetic field. Image credit: Willke et al.

“An MRI is routinely done in hospitals nowadays as a part of imaging for diagnostics,” said Dr. Philip Willke, first author of the study.

“MRI detects the density of spins — the fundamental magnets in electrons and protons — in the human body. Traditionally, billions and billions of spins are required for an MRI scan.”

“We show that this process is also possible for an individual atom on a surface.”

Dr. Willke and his colleagues combined scanning tunneling microscopy with electron spin resonance to perform MRI scans on single-atom spin centers.

“We used a scanning tunneling microscope, which consists of an atomically sharp metal tip that allows researchers to image and probe single atoms by scanning the tip across the surface,” they explained.

“The two elements that were investigated in this work, iron and titanium, are both magnetic. Through precise preparation of the sample, the atoms were readily visible in the microscope.”

MRI scans on top of a titanium atom taken at different energies; the bright areas mark positions where the atom’s magnetic field is the same. Image credit: Willke et al.

MRI scans on top of a titanium atom taken at different energies; the bright areas mark positions where the atom’s magnetic field is the same. Image credit: Willke et al.

The physicists used the microscope’s tip like an MRI machine to map the 3D magnetic field created by the atoms with unprecedented resolution.

“In order to do so, we attached another spin cluster to the sharp metal tip of their microscope,” they said.

“Similar to everyday magnets, the two spins would 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.

“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,”

“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 team now plans to use the single-atom MRI to map the spin distribution in more complex structures such as molecules and magnetic materials.

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Philip Willke et al. Magnetic resonance imaging of single atoms on a surface. Nature Physics, published online July 1, 2019; doi: 10.1038/s41567-019-0573-x

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