Geometric Frustration and Chemical Tuning of Magnetic Order in the Kagome Lattice System YBaCo$_{4}$O$_{7+x}$

Transition metal oxides containing a Kagome lattice motif of magnetic ions form the basis for exploring geometric frustration and exotic magnetic ground states. Examples of such systems include pyrochlores, spinels, SrCr$_{9p}$Ga$_{12-9}$pO$_{19}$ (SCGO) and jarosite minerals. Joining this class of Kagome lattice antiferromagnets is the recently reported YBaCo$_{4}$O$_{7+x}$ (Y-114), a member of the more general R-114 which can be prepared with small rare-earth ions. The structure is comprised of Kagome layers of CoO$_{4}$ tetrahedra linked in the c direction by a triangular layer of CoO$_{4}$ tetrahedra. We show that appropriate control of oxygen stoichiometry so that x=0.0 yields a long-range antiferromagnet with a unique spin arrangement that seeks to satisfy the 120$^{o}$ ground state of the Kagome net in the a-b plane with a strong collinear interaction along the c-axis. This AFM ground state results because of a structural phase transition that breaks the 6-fold symmetry of the Kagome layers. We also discuss chemical approaches whose objective is to preserve the Kagome symmetry to lowest temperatures with concomitant geometric frustration.