NMR investigation of spin correlations in BaCo2As2

We use NMR techniques to investigate the magnetic properties of BaCo2As2 single crystals, the nonsuperconducting end member of the Co-substituted iron-pnictide high-Tc superconductor Ba(Fe1−xCox)2As2 with x=1. We present As75 NMR evidence for enhancement 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 χbulk data uncovers the presence of two separate components of spin susceptibility with distinct temperature dependencies, presumably because multiple electronic bands crossing the Fermi energy play different roles in the electronic properties of BaCo2As2.