Short-range ferromagnetic order due to Ir substitutions in single-crystallineBa(Co 1-x Ir x ) 2 As 2 (0 ≤ x ≤ 0.25).

The ternary-arsenide compound BaCo2 As2 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 substitutions for Co on the magnetic and thermal properties of Ba(Co1-x Irx )2 As2 (0 ≤ x ≤ 0.25) single crystals. These compositions all crystallize in an uncollapsed body-centered-tetragonal ThCr2 Si2 structure with space group I 4/ mmm . Magnetic susceptibility measurements reveal clear signatures of short-range FM ordering for x ≥ 0.11 below a nearly composition-independent characteristic temperature T cl ≈ 13 K. The small variation of T cl with x, thermomagnetic irreversibility between zero-field-cooled and field-cooled magnetic susceptibility versus T , the occurrence of hysteresis in magnetization versus field isotherms at low field and temperature, and very small spontaneous and remanent magnetizations < 0.01 μB /f.u. together indicate that the FM response arises from short-range FM ordering of FM spin clusters as previously inferred to occur in Ca(Co1-x Irx )2-y As2 . Heat-capacity C p ( T ) data do not exhibit any clear feature around T cl , consistent with the very small moments of the FM clusters. The C p ( 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 lattice contribution suggests the presence of low-frequency optic modes associated with the heavy Ba atoms in the crystals.