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revised on 26.05.2020 and posted on 28.05.2020by Jesse Park, Brianna Collins, Lucy Darago, Tomce Runcevski, Michael Aubrey, Henry Z. H. Jiang, Ever Velasquez, Mark Green, Jason Goodpaster, Jeffrey R. Long
that combine magnetic order with other desirable physical attributes offer to revolutionize
our energy landscape. Indeed, such materials could find transformative applications
in spintronics, quantum sensing, low-density magnets, and gas separations. As a result, efforts to design multifunctional magnetic materials have recently
moved beyond traditional solid-state materials to metal–organic solids. Among
these, metal–organic frameworks in particular bear structures that offer
intrinsic porosity, vast chemical and structural programmability, and tunability
of electronic properties. Nevertheless, magnetic order within metal–organic
frameworks has generally been limited to low temperatures, owing largely to challenges
in creating strong magnetic exchange in extended metal–organic solids. Here, we employ the phenomenon of itinerant ferromagnetism to realize magnetic
ordering at TC = 225 K in
a mixed-valence chromium(II/III) triazolate compound, representing the highest ferromagnetic
ordering temperature yet observed in a metal–organic framework. The itinerant
ferromagnetism is shown to proceed via a double-exchange mechanism, the first such
observation in any metal–organic material. Critically, this mechanism results
in variable-temperature conductivity with barrierless charge transport below TC and a large negative
magnetoresistance of 23% at 5 K. These observations suggest applications for double-exchange-based
coordination solids in the emergent fields of magnetoelectrics and spintronics. Taken together, the insights gleaned from these results are expected to provide
a blueprint for the design and synthesis of porous materials with synergistic
high-temperature magnetic and charge transport properties.