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Kagome geometry produces magnetism in a 2D organic material

29 Oct 2021 Isabelle Dumé
Kagome lattice structure images
The star-like "kagome" structure of the molecules in this 2D metal-organic material (shown in an STM image on the left and a non-contact AFM image on the right) produces strong electronic interactions. (Courtesy: FLEET)

Strong interactions between electrons can cause local magnetic moments to emerge in two-dimensional (2D) organic materials. This insight comes from a study by researchers at Monash University in Australia, who created a metal-organic nanomaterial with its molecules arranged in a so-called kagome geometry – a star-like shape consisting of corner-sharing equilateral triangles. The material and its unusual magnetic properties could find use in next-generation solid-state electronics.

2D materials with a kagome crystal structure contain electrons that behave in unusual ways. For example, the wavefunctions of the electrons can interfere destructively, resulting in highly localized electronic states in which the particles interact strongly with each other. These strong correlations can lead to a range of quantum phenomena, including magnetic ordering of unpaired electrons spins that can produce, for example, ferro- or antiferromagnetic phases, quantum spin liquids and abnormal topological phases. These phases are all useful for advanced nanoelectronics and spintronics technologies.

While physicists had previously observed strong electron–electron correlations in inorganic kagome crystals, they had not done so in organic systems. Such systems are attractive for materials scientists because they can be synthesized using versatile, tuneable, scalable and cost-effective approaches – via self-assembly and metal-ligand coordination processes, for example.

Magnetism stems from kagome geometry

In the new work, researchers led by Agustin Schiffrin studied a 2D metal-organic framework (MOF) with a structure comprising dicyanoanthracene (DCA) molecules linked in a kagome structure via copper atoms. The 2D MOF was placed on a silver surface. Using atomically precise scanning probe microscopy (SPM) measurements, the researchers found that the MOF hosts magnetic moments confined to specific locations. They backed up these results with theoretical calculations showing that the magnetism is a natural result of the structure’s kagome geometry.

Schiffrin explains that the presence of these local magnetic moments revealed itself experimentally through observations of the Kondo effect. This many-body phenomenon occurs when the magnetic moments are screened by a “sea” of conduction electrons – for example, from an underlying metal. The effect can be detected by SPM, notes team member Dhaneesh Kumar, and its presence implies that the material must be hosting magnetic moments.

Dhaneesh Kumar

The researchers stress that the magnetism is a direct consequence of strong electron-electron interactions that only appear when the normally non-magnetic components of the 2D MOF are arranged in a kagome geometry. These interactions effectively hinder electron pairing, and the spins of these unpaired electrons then produce the local magnetic moments observed.

Organic electronics

Schiffrin and colleagues say that their findings could aid the development of next-generation electronics based on organic materials. This is because the quantum correlations the team unearthed can be tuned to produce a host of magnetic phases, as well as electronic ones, all with different properties.

The researchers, who report their work in Advanced Functional Materials, say they now plan to turn their attention to technological applications. “We will do this by synthesizing such 2D organic and metal-organic materials on substrates other than metals (for example, insulators), incorporating them into devices and controlling electron–electron interactions and quantum phase transitions using external parameters, such as applied electric fields,” Schiffrin tells Physics World.

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