Short-range ferromagnetic order due to Ir substitutions in single-crystalline Ba(Co_{1− x} Ir _x )₂As₂ (0 ⩽ x ⩽ 0.25)

Abstract

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 ( C o 1 − x I r x ) 2 A s 2 (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 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 μ 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−x Ir x )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.

Alloying-driven transition between ferromagnetism and antiferromagnetism in UTGe compounds: UCo1−xIrxGe

“…One cannot entirely exclude the possibility of specific antiferromagnetic phases in this concentration range of the UCo 1-x Ir x Ge system similar to those proposed in the case of the UCo 1-x Pd x Ge compounds [28]. Rather, however, we tend to assume that the highest substitution disorder (see also [35]) in the intermediate concentration range causes the distribution of exchange interactions that lead to complex magnetic correlations in the paramagnetic state (correlated paramagnet -(CPM)) and ultimately to the freezing of incoherent spin arrangements (spin-glass-like, cluster glass [29] with possible short-range magnetic order (SRMO)). These phenomena lead to responses of the system like these we have observed in the concentration range 0.7  x  0.4.…”

“…Further increase of the Ir content reinforces the magnetic correlations resulting in a strange state in the interval 0.3 ≤ x ≤ 0.7. In our view, we ascribe magnetic behavior in this concentration range to the paramagnetic state with strong FM/AFM correlations (a correlated paramagnet -(CPM)) and/or to the freezing of incoherent spin arrangements (spin-glass-like, cluster glass [29] with possible short-range magnetic order (SRMO)). Ordinary antiferromagnetism has been detected in the interval 0.8 ≤ x ≤ 1 with an abrupt increase of T N towards the parent UIrGe.…”

“…Each sample was characterized by X-ray powder diffraction (XRPD) at room temperature on a Bruker D8 Advance diffractometer. The obtained data were evaluated by the Rietveld technique [28] using FULLPROF/WINPLOTR software [29,30], given the previously published crystallographic data of the isostructural UCoGe [31] and UIrGe [24] compounds as starting models.…”