ac susceptibility of ${\mathrm{Sr}}_3{\mathrm{CuPt}}_x{\mathrm{Ir}}_{1-x}{\mathrm{O}}_6$: A magnetic system with competing interactions and dimensionality

${\mathrm{Sr}}_3{\mathrm{CuPt}}_x{\mathrm{Ir}}_{1-x}{\mathrm{O}}_6$ 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 ${\mathrm{Sr}}_3{\mathrm{CuPt}}_x{\mathrm{Ir}}_{1-x}{\mathrm{O}}_6$ 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 $x=0\mathrm{}$ endpoint, ${\mathrm{Sr}}_3{\mathrm{CuIrO}}_6,$ exhibits long-range ferromagnetic order at $T=20.1\mathrm{}$ 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 ${\mathrm{Sr}}_3{\mathrm{CuPt}}_x{\mathrm{Ir}}_{1-x}{\mathrm{O}}_6$ cannot be simply described by the random quantum spin chain theories that were developed, in part, to address this system.