Room-Temperature Ferromagnetism in an Iron-Based Zeolitic Imidazolate Framework Glass

Although many metal-organic frameworks (MOFs) display magnetic properties, it remains unclear whether intrinsic weak ferromagnetism (WFM) can occur at room temperature within these materials. Here the discovery of the WFM is reported at room temperature in an iron-based zeolitic imidazolate framework (ZIF) glass, specifically Fe(Im)₂, where Im is imidazolate. It is found that antiferromagnetic behavior in the crystalline Fe-ZIF transforms into WFM upon melt-quenching, i.e., during the transition to a structurally disordered glassy state. This magnetic transition is attributed to the enhanced exchange interactions between adjacent Fe^II nodes, resulting from a reduction in the Fe^II-Fe^II correlation length from 6.2 Å in the crystalline phase to 6.0 Å in the glass. ⁵⁷Fe Mössbauer spectroscopy reveals that the order-to-disorder transition leads to a transition of the low-spin-state in Fe^II to the uniform high-spin state. The modification of the coordination environment induces room-temperature WFM. The finding opens a pathway for the application of MOF glasses in magnetic and spintronic technologies.