ac susceptibility of Sr 3 CuPt x Ir 1-x O 6 : A magnetic system with competing interactions and dimensionality

${\mathrm{Sr}}_{3}{\mathrm{CuPt}}_{x}{\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{O}}_{6}$ has been cited as an example of a one-dimensional quantum spin chain with competing ferromagnetic and antiferromagnetic interactions. We have measured the ac susceptibility of ${\mathrm{Sr}}_{3}{\mathrm{CuPt}}_{x}{\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{O}}_{6}$ with $x=0,$ 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7, in magnetic fields of 0--60 kOe, and at temperatures down to 0.275 K. Our data show that the $x=0$ endpoint, ${\mathrm{Sr}}_{3}{\mathrm{CuIrO}}_{6},$ exhibits long-range ferromagnetic order at $T=20.1$ K, contrary to results from dc susceptibility studies which indicated that it remained a one-dimensional ferromagnet to below 4 K. When platinum is substituted for iridium, antiferromagnetic couplings are introduced, and the susceptibility shows a diminishing signature of the three-dimensional ferromagnetic transition. Furthermore, the low-temperature susceptibility exhibits peaks which appear and evolve as x is increased. These results lead to a rich phase diagram in temperature and Pt concentration space. We find that the behavior of ${\mathrm{Sr}}_{3}{\mathrm{CuPt}}_{x}{\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{O}}_{6}$ cannot be simply described by the random quantum spin chain theories that were developed, in part, to address this system.