Uncovering the Kagome Ferromagnet within a Family of Metal-Organic Frameworks.

Kagome networks of ferromagnetically or antiferromagnetically coupled magnetic moments represent important models in the pursuit of a diverse array of novel quantum and topological states of matter. Here, we explore a family of Cu2+-containing metal-organic frameworks (MOFs) bearing kagome layers pillared by ditopic organic linkers with the general formula Cu3(CO3)2(x)3·2ClO4 (MOF-x), where x is 1,2-bis(4-pyridyl)ethane (bpe), 1,2-bis(4-pyridyl)ethylene (bpy), or 4,4'-azopyridine (azpy). Despite more than a decade of investigation, the nature of the magnetic exchange interactions in these materials remained unclear, meaning that whether the underlying magnetic model is that of an kagome ferromagnet or antiferromagnet is unknown. Using single-crystal X-ray diffraction, we have developed a chemically intuitive crystal structure for this family of materials. Then, through a combination of magnetic susceptibility, powder neutron diffraction, and muon-spin spectroscopy measurements, we show that the magnetic ground state of this family consists of ferromagnetic kagome layers that are coupled antiferromagnetically via their extended organic pillaring linkers.

Materialization of a Geometrically Frustrated Magnet in a Hybrid Coordination Framework: A Study of the Iron(II) Oxalate Fluoride Framework, KFe(C2O4)F.

Here we discuss magnetic hybrid coordination frameworks in relation to the realization of new geometrically frustrated magnets. In particular, we present the nuclear and magnetic structures of one such system-the Fe2+-based oxalate fluoride framework KFe(C2O4)F-through analysis of the powder neutron diffraction and muon spectroscopy data. KFe(C2O4)F retains an orthorhombic Cmc21 structure upon cooling to 2 K composed of quasi-one-dimensional iron fluoride chains connected to a distorted triangular network via oxalate anions. Previous magnetometry measurements of KFe(C2O4)F indicate that it is a strongly interacting system with a Weiss constant θ ≈ -300 K that undergoes a magnetic ordering transition at TN ≈ 20 K, yielding a frustration index, f = |θ|/TN ≈ 15, reflective of high-spin frustration. We determine the nature of this frustrated antiferromagnetic ordering below TN and show that the resulting magnetic structure is best described by a model in the Cmc'21' magnetic space group.