New Scanning Probe Microscopy Lifts Magnetic Fingerprints

A Czech and Spanish-led research team has demonstrated the ability to distinguish subtle differences between magnetic ground states using a new form of scanning probe microscopy. In the last few years, a magnetic characterization technique involving scanning tunneling microscopy (STM) measurements with a magnetic nickelocene molecule has been developed. This technique provides insight into magnetic properties based on the interaction between nickelocene and a magnetic sample. Led by researchers at FZU and at IMDEA Nanoscience, the new study, published in the Journal of the American Chemical Society, uses nickelocene-based STM measurements to unambiguously discriminate between magnetic ground states of nanographene molecules, and to image their spin distribution at the atomic scale. The demonstration of these two properties shows that this new technique is a powerful magnetic characterization tool for unveiling the properties of correlated materials. Mixing things up: Extracting magnetic information with nickelocene Measurements using STM involve the "jumping" of electrons between an atomically sharp metal probe and a sample of interest at sub-nanometer distances. At this proximity, when the apex of the probe is decorated with a nickelocene molecule, the nickelocene spin can interact with the spins of a magnetic sample resulting in a mixing of their magnetic properties (via a process called exchange-coupling). The strength of this effect can be carefully controlled by precisely varying the probe-sample distance. The magnetic properties of nickelocene itself are well understood. So, by comparing the mixed magnetic properties of nickelocene and a magnetic sample to models, the authors can extract information about the magnetic properties of the sample itself. "One of my favorite things about this project is that the key to the problem was finding a simple spin model," explains first author Diego Soler Polo, who recently finished his role as a postdoctoral researcher in the Nanosurf Lab group at FZU and began a position at IMDEA Nanoscience. "And not just a heavy ab initio simulation... although of course we also did that." In this study, the authors compared two nanographene molecules with almost identical structures. Nickelocene spectroscopy measurements revealed subtly different signatures for each molecule. This allowed the researchers to conclude that, despite their structural similarity, the molecules have different magnetic ground states. A slice of the pi The nanographene molecules in this study are examples of a class of magnetic materials known as π (pi)-magnets. Some carbon-based materials feature delocalized electrons within so-called π-states—such as the two possible arrangements of alternating double and single bonds in a benzene ring. Unlike conventional magnetic materials, whose magnetism arises from unpaired electrons in metal centers, π-magnets have spins that live within these delocalized π-states. "π-magnets are a recent class of materials which are too reactive to stabilize using traditional chemical methods," explains corresponding author José I. Urgel, a group leader at IMDEA Nanoscience. "Developments in synthetic protocols on surfaces allowed for their synthesis for the first time—opening the door to this new field of magnetism." The nickelocene technique used by the authors is especially useful for studying π-magnets. Along with determining the magnetic ground state, it can also be used to image the spatial distribution of delocalized magnetic properties at the atomic scale. What's next? The demonstrated sensitivity to unique magnetic ground states and atomic-scale resolution makes the nickelocene a promising tool for characterizing correlated materials. As well as further characterization of π-magnets, this technique might be able to shed new light on more exotic magnetic phases in 2D materials.