A Mixed-Valence and Mixed-Spin Two-Dimensional Ferromagnetic Metal–Organic Coordination Framework

Abstract

Spin-mixed systems with distinct magnetic sublattices present rich physical behaviors and hold promise for magnetic memory, thermomagnetic recording, and optoelectronics. However, most experimental studies remain confined to molecular magnetic salts rather than monolayer two-dimensional (2D) systems. Here, we report the synthesis and characterization of a 2D metal–organic framework (MOF) of Fe₂(Fe-DPyP)₃, constructed from 5,15-di(4-pyridyl)-10,20-diphenylporphyrin (DPyP) molecules and iron atoms on a Au(111) substrate. Through scanning tunneling microscopy and density functional theory (DFT) calculations, we reveal dual coordination modes: (1) peripheral iron atoms (Fe_c) coordinate with the pyridyl substituents, forming a honeycomb lattice, and (2) central iron atoms (Fe_m) bind within the porphyrin core, creating a kagome lattice. Scanning tunneling spectroscopy highlights distinct spin-flip excitations in the bivalent Fe_m atoms with a magnetic spin state of S = 1, while the monovalent Fe_c atoms display a V-shaped dip around the Fermi level, attributed to the quenched spin excitations or a soft Coulomb gap, with a magnetic spin state of S = 3/2. DFT calculations reveal an in-plane ferromagnetic ground state with spin-polarized Fe d orbitals and molecular p orbitals. This work contributes to the understanding of mixed-valence and mixed-spin 2D coordination networks with implications for the development of next-generation quantum materials and spintronic devices.