Neutron Scattering from Magnetically Frustrated Ruthenium Pyrochlores

Spin-1 ruthenium pyrochlores feature strong exchange interactions and deeply suppressed N\'{e}el ordering. In addition a doping induced metallic phase has been demonstrated. I discuss neutron scattering experiments that explore the strongly frustrated quantum magnetism of Y$_{2}$Ru$_{2}$O$_{7}$ [1] and Pr$_{2-x}$Bi$_{x}$Ru$_{2}$O$_{7}$ [2]. In Y$_{2}$Ru$_{2}$O$_{7}$ ($\Theta _{CW}$= --1100 K, T$_{N}$=77 K) much of the magnetic spectral weight is concentrated in a 20 meV spectral peak above an 11 meV low temperature gap in the excitation spectrum. In Pr$_{2-x}$Bi$_{x}$Ru$_{2}$O$_{7}$ magnetic order which occurs for T$<$T$_{N}$=160 K when $x$=0, is suppressed when the material becomes a metal for $x\approx $1. I discuss the anomalous low energy spin dynamics associated with praseodymium for $x$=0 and $x$=1 and relate the results to other rare-earth pyrochlores. \newline \newline [1] J. van Duijn, N. Hur, J. W. Taylor, Y. Qiu, Q. Z. Huang, S.-W. Cheong, C. Broholm, and T. G. Perring, Submitted to Phys. Rev. B Rapid Communications, cond-mat 0707.2678v1 (2007). \newline [2] J. van Duijn, K. H. Kim, N. Hur, D. Adroja, M. A. Adams, Q. Z. Huang, M. Jaime, S.-W. Cheong, C. Broholm, and T. G. Perring, Phys. Rev. Lett.\textbf{ 94}, 177201 (2005).