Short-range ferromagnetic order due to Ir substitutions in single-crystallineBa(Co1-xIrx)2As2(0 ≤ x ≤ 0.25).

The ternary-arsenide compound BaCo2As2was 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-xIrx)2As2(0 ≤ x ≤ 0.25) single crystals. These compositions all crystallize in an uncollapsed body-centered-tetragonal ThCr2Si2structure with space groupI4/mmm. Magnetic susceptibility measurements reveal clear signatures of short-range FM ordering for x ≥ 0.11 below a nearly composition-independent characteristic temperatureTcl≈ 13 K. The small variation ofTclwith x, thermomagnetic irreversibility between zero-field-cooled and field-cooled magnetic susceptibility versusT, 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-xIrx)2-yAs2. Heat-capacityCp(T) data do not exhibit any clear feature aroundTcl, consistent with the very small moments of the FM clusters. TheCp(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.