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Left: hysteresis loop from coordinating Fe atoms measured in-plane (red) and out-of plane (blue). Center: temperature dependence of the XMCD signal showing a Curie temperature of 35 K. Right: representation of the spin orientation in remanence of the 2D metal-organic network.
Cerdanyola del Vallès, 1st March 2024 Ferromagnetism is the collective alignment of atomic spins that retain a net magnetic moment below the Curie temperature, even in absence of external magnetic fields. Downscaling this fundamental property to strictly two dimensions was proposed in metal-organic coordination networks (2D-MOCNs) as they feature all essential ingredients to exhibit 2D ferromagnetism. Among others, these 2D-MOCNs allow selectable magnetic metal atoms with incomplete quenching of the magnetic orbital moment and periodical spacing of these magnetic moments over (non-magnetic) surfaces in a large scale. They also are technically simple to fabricate following self-assembly protocols close to room temperature. Despite these relevant properties, 2D-MOCNs have historically failed to explicitly exhibit 2D ferromagnetism.
Now, a combined experimental and theoretical team led by researchers at the Instituto de Nanociencia y Materiales de Aragón (INMA) in Zaragoza and the Material Physics Centre (CFM) in Donostia-San Sebastian in collaboration with the BOREAS beamline at the ALBA Synchrotron and the Laboratorio de Microscopías Avanzadas of the Universidad de Zaragoza, has demonstrated that extended, cooperative ferromagnetism is feasible in an atom thick two-dimensional metal-organic coordination network of iron (Fe) and anthracene derivatives on Au(111).
The results are remarkable since a scarce amount of iron atoms (~5% of a monolayer) produces an out-of-plane easy-axis square-like hysteresis loop with large coercive fields over 2 Tesla, as well as an extraordinary large magnetic anisotropy. This ferromagnetism is driven by exchange interactions, mainly through the molecular linkers, presenting a phase transition at a Curie temperature of Tc ≈ 35 K. The observed spin and orbital momenta found in the network are consistent with Fe(II) high-spin d6 configuration (L=S=2), which are large enough to give rise to a high uniaxial magnetic anisotropy. Importantly, the magnetic exchange coupling constant found for this system is comparable to the highest reported for the ultrathin 2D-van der Waals ferromagnets and is certainly much higher than all other previous 2D-MOCNs studied at the single-layer limit.
These findings settle over two-decades of search for 2D ferromagnetism in atomically thin MOCNs, thereby representing a clear advance in the transversal fields of magnetism and surface science. It is expected to boost the community’s interest in magnetic 2D-MOCNs and trigger follow-up theoretical and experimental work capable of leading to new ferromagnetic systems that exhibit even higher ordering temperatures with such ultra-low magnetic atom densities.
Overview (a) and close-up (b) images of the 2D-metal-organic lattice of Fe and anthracene derivatives on Au(111). A model of the network structure is overlaid in (b) with the molecules forming a kagome substructure and the Fe adatoms (orange-brown spheres) a honeycomb sublattice. Top and side views of the 2D-metal-orgainc network optimized structure (c) and the corresponding spin density isocontours [red (blue) denotes the majority (minority) spin component] (d).
Reference: Jorge Lobo-Checa, Leyre Hernández-López, Mikhail M. Otrokov, Ignacio Piquero-Zulaica, Adriana Candia, Pierluigi Gargiani, David Serrate, Fernando Delgado, Manuel Valvidares, Jorge Cerdà, Andrés Arnau, and Fernando Bartolomé. Ferromagnetism on an atom-thick & extended 2D metal-organic coordination network. Nature Communications (2024) 15, 1858. https://doi.org/10.1038/s41467-024-46115-z
