Origin of Long-Range Ferromagnetic Ordering Among Metal-Organic Frameworks with Antiferromagnetic Dimeric-Cu(II) Building Units
Metal-organic frameworks (MOFs) are porous network structures consisting of metal oxide clusters linked by molecular chains. This new class of materials show a number of promising applications in gas storage,1 luminescent materials,2 molecular sieves,3 drug delivery,4 and catalysis.5 Meanwhile, many MOFs exhibit unique magnetic property due to the constitutive open-shell transition metal (TM) ions, which has been attracting much attention because of their potential applications in low density magnetic materials, molecular magnets, and magnetic molecular sensors. By both theoretical and experimental approaches, we uncover the origin of magnetic moments and long-range ferromagnetic (FM) coupling among metal-organic frameworks (MOFs) constructed with antiferromagnetic (AFM) coupled dimeric-Cu(II) building unit. It is revealed that copper vacancies are the origin of local magnetic moments which are coupled via conjugated aromatic linkers. Figure 1 shows the hysteresis loops at 5 and 20 K, indicating the Curie temperature is lower than 5 K and larger than 20 K. Susceptibility (Fig. 1b) measurement indicates that the Curie temperature of the MOF-505 is around 11 K. In order to understand the ferromagnetism mechanism, we used first principle calculations to investigate the origin of ferromagnetism. Fig. 1c shows the perfect structure of MOF-505. No spin density can be observed. However, when Cu vacancy is introduced into the structure, spin density can be observed around the Cu vacancy (Fig. 1d, 1e). Spin density can be both observed in one single or pair Cu vacancies. This work has shown that cation vacancies in MOF-505 can induce ferromagnetism, which may pave a way to explore and realize some new high Tc dimeric-Cu(II) based ferromagnetic MOFs.
Figure 1. (a) Magnetic hysteresis loop of MOF-505 at 5 K and 20 K; (b) Temperature dependent susceptibility of MOF-505. The inset shows the 1/χ −T curve in the temperature ranged from 2 to 65 K. The solid line in the inset is the curve fitted to the Curie-Weiss law;c) A perfect single crystal of MOF505 without spin density; d) and e) show the spin densities of MOF-505 with a single copper vacancy and a pair of copper vacancies, respectively.
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