Antiferromagnetic order and crystalline electric field in noncentrosymmetric hexagonal R E Ru 6 As 4 ( R E = Sm and Yb )

We investigated the electronic properties of $RE{\mathrm{Ru}}_{6}{\mathrm{As}}_{4}(RE=\mathrm{La}, \mathrm{Ce}, \mathrm{Sm}, \text{and} \mathrm{Yb})$ using single-crystalline samples grown by the Bi-flux method. These compounds crystallize in the noncentrosymmetric hexagonal ${\mathrm{LiCo}}_{6}{\mathrm{P}}_{4}$-type structure, featuring chain-like and triangular arrangements of magnetic $RE$ ions. From the electrical resistivity, magnetic susceptibility, and specific heat measurements, we have found that ${\mathrm{SmRu}}_{6}{\mathrm{As}}_{4}$ and ${\mathrm{YbRu}}_{6}{\mathrm{As}}_{4}$ order antiferromagnetically below the N\'eel temperature of 1.5 and 9.5 K, respectively. Despite the relatively high transition temperature of ${\mathrm{YbRu}}_{6}{\mathrm{As}}_{4}$, its magnetic field-temperature phase diagram reveals that the antiferromagnetic state collapses at a low field of ${\ensuremath{\mu}}_{0}{H}_{\mathrm{c}}=1.7\phantom{\rule{0.28em}{0ex}}\mathrm{T}$, accompanied by an unusual nonlinear magnetization. The small critical field and characteristic magnetization curve in ${\mathrm{YbRu}}_{6}{\mathrm{As}}_{4}$ suggest an important role of competing interactions associated with magnetic frustration.