Onset of two-dimensional Stoner magnetism in cobalt arsenides*

CaCo_1.86As₂ is a unique example of a metallic square-lattice system with nearly perfect frustration despite the compound having A-type antiferromagnetic (AF) order below a Néel temperature of T_N = 52 K. Our recent neutron diffraction data on Ca(Co_1-xFe_x)_2-yAs₂ show that the A-type AF order is suppressed past x = 0.12, and here we give results from INS experiments made at T = 6 K on single-crystal samples of x = 0.15. We find diffuse spin fluctuations giving rise to walls of magnetic scattering with similar anisotropy to that found for x = 0, however, the fluctuating moment is extremely weak. A simultaneous analysis of magnetic susceptibility and neutron scattering data shows that the total magnetic moment is gradually reduced with increasing x. This suggests that the development of magnetic order has a Stoner-like mechanism where Fe-doping acts to tune (and decrease) the Stoner parameter UN(E_F). This highlights that cobalt arsenides are uniquely frustrated and weakly itinerant magnets.