Atomic-scale spin sensing with a single molecule

Scanning tunneling microscopy (STM) is a technique in which a fine conducting probe is held close to the surface of a sample. Electrons tunnel between the sample and the probe, producing an electrical signal. The probe is slowly moved across the surface and raised and lowered so as to keep the signal constant. The precision reached in approaching the STM tip apex toward the surface permits for a controlled electronic contact with a single surface atom or molecule. A profile of the surface is thus produced, and a computer-generated contour map is created.

The invention of the scanning tunneling microscope by Binnig et al. in 1982 opened a new era in surface science. It is now a standard microscopy technique for real-space imaging of the electronic structure of conducting surfaces with picometer resolution.

In conventional scanning tunneling microscopy, the magnetic ground state of an isolated atom or molecule is inferred by collecting spin-related fingerprints in the conductance measured with a metallic tip. Isolated atoms or molecules can also serve as spin detectors when controllably moved on the surface with the help of the tip within their local magnetic environment. The magnetic ground state can change in the presence of a magnetic coupling. Exchange- and surface-mediated interactions have been spatially mapped in this way by monitoring the zero-bias peak in the differential conductance associated with several effects.

A well-calibrated sensor attached to the tip apex would allow the tip to be freely positioned above a surface target. This detection scheme eliminates surface-mediated interactions and benefits from the vertical-displacement sensitivity of the STM as the sensor-target distance is no longer imposed by the surface corrugation.

Probing a magnetic exchange interaction across a vacuum gap is experimentally demanding because scanning probe techniques suffer from poor structural and magnetic characterization of the tip apex. Now, a team of researchers has overcome ¹ these limitations by introducing spin sensitivity through the functionalization of the tip apex with a single magnetic molecule.

The researchers used a tip decorated by a spin S = 1 nickelocene molecule, which comprises a Ni atom sandwiched between two C₅H₅ cyclopentadienyl rings. First-principles calculations showed that they captured the junction geometry accurately.

When the nickelocene-tip was 100 picometers away from point contact with a surface-supported object, magnetic effects could be probed through changes in the spin excitation spectrum of nickelocene. This detection was used to simultaneously determine the exchange field and the spin polarization of iron atoms and cobalt films on a copper surface with atomic-scale resolution.

A large variety of magnetic systems can be investigated using this scheme, ranging from systems having resolvable magnetic quantum states to systems having a magnetic moment but nonresolvable quantum states. The latter could include single atoms and organometallic molecules on magnetic surfaces or surfaces with complex magnetic structures.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper.