Single-Molecule Magnetometer Images Spin Interactions

22/05/2019 STM imaging of weak to strong spin interactions between two magnetic molecules. 

A single magnetic molecule can act as a sensing device and produce the most detailed images yet of a material’s magnetic structure at the atomic scale. The magnetometer, which has revealed tunable quantum exchange interactions between two magnetic molecules for the first time, could make for a new microscopy technique that exploits magnetic molecules as local probes.

Ten years ago, researchers succeeded in significantly increasing the lateral resolution of low-temperature atomic force microscopy (AFM) by functionalizing the AFM tip with a single carbon monoxide molecule. This so-called bond-imaging technique was an important landmark for visualizing the atomic structure of single molecules. Researchers at the University of California, Irvine, have now taken inspiration from this strategy and have attached a single magnetic molecule (Ni(cyclopentadienyl)2) to the tip of a scanning tunnelling microscope (STM) to make an atomic-scale spin-sensing device. They did this by carefully approaching the tip to the molecule, which itself was adhered on a Ag(110) substrate surface.

“An STM measures the miniscule electric current (called the tunnelling current) flowing between the tip and a sample when the tip is positioned within a nanometre of the sample,” explains team leader Wilson Ho. “Our magnetic molecule sensor at the tip apex improves this technique by allowing us to detect spin-spin interactions with another magnetic molecule adsorbed on the sample surface by measuring minute changes in the tunnelling current.”

Spins start to couple

The technique works because the spins of both molecules start to couple as a result of, surprisingly large, quantum mechanical exchange interactions as the magnetic single-molecule sensor is brought very close (less than half a nanometre) to the other magnetic molecule, Ho tells Physics World. “This antiferromagnetic exchange coupling, as it is known, causes the two molecules to behave as a combined quantum system.

“We can probe this coupling using a technique, pioneered by our group in 1998, called inelastic electron tunnelling spectroscopy (IETS) within the STM. This technique allows us to quantitatively measure the excitation energies of the combined quantum system made up of the two magnetic molecules.”

By tuning the distance by which the two molecules were separated, in steps of several picometres, the researchers were able to explore how the coupled quantum states evolved as their spin-spin interaction strength changed. They did this by measuring the blueshift in the quantum energy levels of the combined spin states. This shift reveals strong coupling and strongly mixed quantum states.

“In this way, we found that the interaction strength exponentially decays across the (vacuum) gap between the molecules,” says Ho. “What is more, we could visualize the contours of the magnetic interaction strength in 3D. These spectroscopic images prove that the technique can image spin density and map spin quantum state mixing in real space.”

Advancing our understanding of nanoscale magnetism for future applications

“This work demonstrates that a magnetic metalorganic tip can function as a magnetometer and serve as a local spin sensor,” he adds. “We hope that our results will lead to other such experiments to help further advance our understanding of exchange interactions, spin coherence and spin coupling to the local environment – that is, magnetism at the atomic scale. Such knowledge will be essential for developing single-atom and single-molecule magnets for memory storage, as well as quantum bits (qubits) for quantum information processing.”

The researchers, reporting their work in Science DOI: 10.1126/science.aaw7505, say they now plan to investigate other magnetic molecules with different spins. “We are also looking into the time-dependent properties of single- and multiply-interacting spin systems to measure the dynamical interactions of a magnetic molecule with its environment,” reveals Ho.

“Probing the spin interaction with another adsorbed magnetic molecule is not the only thing the new magnetic single-molecule sensor is good for. The atomic-scale magnetometer could also be useful for measuring and imaging localized magnetic fields in novel 2D materials.”






Source: https://bit.ly/2HTp5d5 via PhysicsWorld