Robust paramagnetism in Bi_2-xM_xRu₂O₇ (M=Mn,Fe,Co,Ni,Cu) pyrochlore

We report magnetic susceptibility, resistivity, and Seebeck coefficients for Bi_2-xM_xRu₂O₇ pyrochlore. The solid solution exists up to x=0.5 for M=Cu,Ni,Co and up to x=0.1 for M=Fe,Mn. The doped materials do not exhibit ferromagnetism or any localized ruthenium moment behavior. Instead we find the Ru-O and Bi-O sublattices to be essentially independent, with any magnetism resulting from the unpaired first-row transition metal dopant spins. Cobalt substitution for bismuth results in localized Co²⁺ and low-temperature spin-glass transitions in several cases. Nickel moments on the pyrochlore lattice display properties intermediate to localized and itinerant. Finally, copper doping results in an enhancement of the Pauli metallic density of states.