The presence of antiferromagnetic (AF) order seems to be a common feature for the parent compounds of cuprate as well as some pnictide high-temperature superconductors. In order to search for antiferromagnetic order in the closely related rare-earth cobalt arsenic oxides ($R$CoAsO), we have measured muon-spin rotation and relaxation (${\mu }^{+}$SR) spectra on a series of powder samples with $R=\text{La}$, Ce, Pr, Nd, Sm, and Gd. It was found that, besides GdCoAsO, all the other five compounds enter into a static ferromagnetic ordered phase below around 70 K ($=T_{\mathrm{C}}$), while additional transitions into a static AF ordered phase were found for NdCoAsO and SmCoAsO. For GdCoAsO, a static ferrimagnetic phase appeared below 60.5 K and then an incommensurate spin-density-wave ordered phase followed below 3.2 K. The AF spin structure was also found to strongly depend on $R$.

• Received 24 June 2011

DOI:https://doi.org/10.1103/PhysRevB.84.184421