  The ternary-arsenide compound BaCo2As2 was previously proposed to be in
proximity to a quantum-critical point where long-range ferromagnetic (FM) order
is suppressed by quantum fluctuations. Here we report the effect of Ir
substitution for Co on the magnetic and thermal properties of
Ba[Co(1-x)Ir(x)]2As2 (0 <= x <= 0.25) single crystals. These compositions all
crystallize in an uncollapsed body-centered-tetragonal ThCr2Si2 structure with
space group I4/mmm. Magnetic susceptibility measurements reveal clear
signatures of FM ordering for x >= 0.11 with a nearly composition-independent
Curie temperature TC = 13 K. The small variation of TC with x, the occurrence
of hysteresis in magnetization versus field isotherms at low field and
temperature, very small spontaneous and remanent magnetizations < 0.01
muB/f.u., and thermomagnetic irreversibility in the low-temperature region
together indicate that the FM response arises from short-range FM ordering of
spin clusters as previously inferred to occur in Ca[Co{1-x}Ir{x}]{2-y}As2.
Heat-capacity Cp(T) data do not exhibit any clear feature around TC, further
indicating that the FM ordering is short-range and/or associated with itinerant
moments. The Cp(T) in the paramagnetic temperature regime 25-300 K is well
described by the sum of a Sommerfeld electronic contribution and Debye and
Einstein lattice contributions where the latter suggests the occurrence of
low-frequency optic modes associated with the heavy Ba atoms in the crystals.
