Atomic-scale spin sensing with STM


Rebecca Pool

Wednesday, November 6, 2019 - 13:15
Image: Functionalising a STM tip with magnetic molecules.
A pan-European team of researchers has used a nickelocene molecule on a scanning tunnelling microscope tip as a sensor to detect magnetic moments with unprecedented spatial resolution
Professor Markus Ternes from the Peter Grünberg Institut at Forschungszentrum Jülich, Germany, and colleagues from  the University of Strasbourg, France, as well as research centres in San Sebastián, Spain, detected moments with atomic resolution and hope to reveal fundamental insights into atomic-scale structures.
Spin-sensing in this way could also help researchers design future atomic-scale devices, such as nanoscale storage devices and quantum simulators.
In the last decade, researchers worldwide have been using single molecules, such as carbon monoxide and hydrogen, to chemically functionalize the probe-tip of a STM and increase spatial resolution.
The success of this approach has now opened the door to using magnetic molecules at the STM tip to detect magnetic spin.
Schematic view of the experiment showing the tip with the molecule attached to the tip apex and the scanned surface. [Forschungszentrum Jülich / Markus Ternes]
With this in mind, Ternes and colleagues attached a nickelocene molecule to a the tip of a STM.
In its ground state, nickelocene possesses no magnetic moment but when in a spin-excited state, this quantum molecular magnet senses moments with high spatial resolution and sensitivity.
As Ternes and colleagues report in Science: “The nickelocene-terminated tip offered the possibility of producing spin excitations on the tip apex of a scanning tunnelling microscope.”
According to the researchers, when the tip was just 100 picometres away from point contact with a surface, the ensuing magnetic effects could be probed through changes in the spin excitation spectrum of nickelocene.
“We used this detection scheme to simultaneously determine the exchange field and the spin polarization of iron atoms and cobalt films on a copper surface with atomic-scale resolution,” says Ternes. 
Topographic image of a small island of cobalt on a copper surface (25 nm x 25 nm). Nc (nickelocene ) was used for functionalising the tip. Top left: Magnetic interaction field from different cobalt atoms. [Forschungszentrum Jülich / Markus Ternes]
Ternes and colleagues claim that their method makes it possible to image surface structures in combination with magnetic properties, at atomic resolution, for the first time.
As they points out, “dark” magnetic moments of complex magnetic structures, which are usually difficult to measure, become accessible.
What's more, because the ground-state of the molecular sensor is non-magnetic, the measurement has a minimal impact on the system under study; important when imaging volatile states at the nanoscale.
Research is published in Science.
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