Ferromagnetism is the collective alignment of atomic spins that retain a net magnetic moment below the Curie temperature, even in the absence of externalmagnetic fields. Reducing this fundamental property into strictly two-dimensions was proposed in metal-organic coordination networks, but thus far has eluded experimental realization. In this work, we demonstrate that extended, cooperative ferromagnetism is feasible in an atomically thin two-dimensional metal-organic coordination network, despite only ≈ 5% of the monolayer being composed of Fe atoms (see Fig. 1). The resulting ferromagnetic state exhibits an out-of-plane easy-axis squarelike hysteresis loop with large coercive fields over 2 Tesla, significant magnetic anisotropy, and persists up to Tc ≈ 35 K (see Fig. 2). These properties are driven by exchange interactions mainly mediated by the molecular linkers that we explain with theoretical calculations. Our findings resolve a two-decade search for ferromagnetism in two-dimensional metal-organic coordination networks.

Fig 1. Overview (a) and close-up (b) images of the 2D-metal-organic lattice of Fe and DCA 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).

Fig 2. 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.

Resumen del trabajo presentado a la III International Conference on Novel 2D materials Explored Via Scanning Probe Microscopy &Spectroscopy (2DSPM), celebrada en el País Vasco (España) del 24 al 28 de junio de 2024.

Peer reviewed