We use NMR techniques to investigate the magnetic properties of BaCo2As2 single crystals, the non-superconducting end member of the Co-substituted iron-pnictide high Tc superconductor Ba(Fe1-xCox)2As2 with x = 1. We present 75As NMR evidence for enha
ncement of low frequency spin fluctuations below ~ 100 K. This enhancement is accompanied by that of static uniform spin susceptibility at the wave vector q = 0, suggesting that the primary channel of the spin correlations is ferromagnetic rather than antiferromagnetic. Comparison between the NMR Knight shift 75K and bulk susceptibility chi(bulk) data uncovers the presence of two separate components of spin susceptibility with distinct temperature dependences, presumably because multiple electronic bands crossing the Fermi energy play different roles in the electronic properties of BaCo2As2.
We report synthesis and single crystal measurements of magnetic, transport and thermal properties of single crystalline BaCo$_2$As$_2$ as well as first principles calculations of the electronic structure and magnetic behavior. These results show that
BaCo$_2$As$_2$ is a highly renormalized paramagnet in proximity to a quantum critical point, presumably of ferromagnetic character and that BaFeNiAs$_2$ behaves similarly. These results are discussed in relation to the properties of Ba(Fe,Co)$_2$As$_2$ and Ba(Fe,Ni)$_2$As$_2$, which are superconducting for low Co and Ni concentrations.
