Emergence of ferromagnetism due to Ir substitutions in single-crystalline Ba[Co(1 x)Ir(x)]2As2.

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 = 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.