  Ac and dc magnetization and heat-capacity (C) measurements performed on the
pseudo-one-dimensional compound Sr$_3$CuIrO$_6$ reveal a competition between
antiferromagnetic (AF) and ferromagnetic (F) exchange couplings, as evidenced
by frequency dependence of ac susceptibility and by the absence of a C anomaly
at the magnetic transition. The value of the saturation moment (about 0.35
$\mu_B$/formula unit) is much smaller than expected for ferromagnetism from the
two S=1/2 ions (Cu and Ir). Thus, this compound is not a ferromagnet in zero
magnetic field, in contrast to earlier beliefs. Of particular importance is the
finding that the value of the magnetic ordering temperature is sample
dependent, sensitive to synthetic conditions resulting from deviations in
oxygen/Cu content. We propose that this compound serves as a unique model
system to test theories on random AF-F interaction in a chain system,
considering that this competition can be tuned without any chemical
substitution.
