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

Abstract

<p>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 substitutions for Co on the magnetic and thermal properties of Ba (0 ≤ <em>x</em> ≤ 0.25) single crystals. These compositions all crystallize in an uncollapsed body-centered-tetragonal ThCr2Si2 structure with space group <em>I</em>4/<em>mmm</em>. Magnetic susceptibility measurements reveal clear signatures of short-range FM ordering for <em>x</em> ≥ 0.11 below a nearly composition-independent characteristic temperature <em>T</em>cl ≈ 13 K. The small variation of <em>T</em>cl with <em>x</em>, thermomagnetic irreversibility between zero-field-cooled and field-cooled magnetic susceptibility versus <em>T</em>, the occurrence of hysteresis in magnetization versus field isotherms at low field and temperature, and very small spontaneous and remanent magnetizations <0.01 <em>μ</em>B/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−<em>x</em>Ir<em>x</em>)2−<em>y</em>As2. Heat-capacity <em>C</em>p(<em>T</em>) data do not exhibit any clear feature around <em>T</em>cl, consistent with the very small moments of the FM clusters. The <em>C</em>p(<em>T</em>) 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.</p>