arXiv2023
Ferromagnetism (FM) is the cornerstone of permanent magnets, data storage and other technologies that directly impact our everyday life by their implementation in standard applications and devices. When downscaling bulk materials into their two-dimensional (2D) magnetic isotropic form, the Mermin-Wagner theorem precludes this collective state mediated by short-range exchange interactions at finite temperatures. Interestingly, this prediction fails when significant magnetic anisotropy is present in the material, as recently demonstrated in single layered van der Waals crystals. Before the latter, single layer metal-organic frameworks (MOFs) grown on metallic supports were one of the earliest candidates for achieving 2D-FM. Such high expectations were based on the chemical and spacing control of the 2D-MOF magnetic centers, the tunability of the organic linkers and the rich self-assembled architectures displayed. However, despite many attempts, extended FM in 2D-MOFs has been experimentally elusive. In this work, we demonstrate that extended, cooperative FM takes place in an atom thick 2D-MOF consisting of 9,10-dicyanoanthracene (DCA) molecules and Fe adatoms grown on Au(111). We show this by means of an experimental multitechnique approach that is endorsed by state-of-the art first-principles calculations. Particularly, this 2D ferromagnet follows a first order transition with TC ~ 35 K, which is driven by exchange interactions mainly through the molecular linkers (J=2 meV) and exhibits an out-of-plane square-like hysteresis loop. The strict periodicity of our 2D-MOF allows us to envision the fabrication of ultra-dense single atom magnetic memories and opens the way to explore periodic magnetic 2D-models that could considerably increase the fundamental superparamagnetic limit.