'World's smallest MRI' on single atoms


Rebecca Pool

Thursday, July 4, 2019 - 12:00
Schematic of single magnetic atoms on magnesium oxide; atoms are imaged by the magnetic tip of a scanning tunnelling microscope so researchers can perform an MRI scan of the atom’s field. [Willke et al]
South Korea- and US-based researchers have devised magnetic resonance imaging in a scanning tunnelling microscopy to view the magnetic field of single atoms on a surface.
Using STM with electron spin resonance, Dr Philip Willke and colleagues from the Center for Quantum Nanoscience (QNS), Institute of Basic Science, Ewha Womans University and IBM Almaden Research Center, imaged single, magnetic iron and titanium atoms on a magnesium oxide surface.
The set-up comprised single adatoms adsorbed onto the magnesium oxide and a magnetic STM tip, fabricated by transferring iron atoms from the surface to the apex of the tip, where they form a spin cluster.
Experiments were carried out in a custom ultra-high vacuum STM, at IBM Almaden, with the researchers mapping the 3D magnetic field created by the atoms with unprecedented resolution.
As Willke writes in Nature Physics: “[We have demonstrated] the MRI of individual atoms on a surface.”
“The set-up, implemented in a cryogenic scanning tunnelling microscope, uses single-atom electron spin resonance to achieve sub-ångström resolution, exceeding the spatial resolution of previous MRI experiments by one to two orders of magnitude,” he adds.
According to the researcher, the MRI scans of different atoms with different probe tips, leads to unique signatures in the resonance images.
These signatures reveal the magnetic interactions between the tip and the atom, in particular magnetic dipolar and exchange interaction. 
“The signal that we see for iron atoms is vastly different from that for titanium atoms,” says Willke. “This allows us to distinguish different kinds of atoms by their magnetic field signature and makes our technique very powerful.”
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. [Willke et al]
The researchers plan to use their single-atom MRI to map the spin distribution in more complex structures such as molecules and magnetic materials.
"Many magnetic phenomena take place on the nanoscale, including the recent generation of magnetic storage devices." says Willke's colleague, Dr Yujeong Bae. "We now plan to study a variety of systems using our microscopic MRI."
The researchers want to to characterise and control quantum systems to drive  quantum computing forward.
"I am very excited about these results. It is certainly a milestone in our field and has very promising implications for future research." says 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."
Research is published in Nature Physics.
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