Breakdown of Hund’s third rule in amorphous Co-W nanoparticles and crystalline Co3W alloys

We present x-ray absorption near edge structure (XANES) and x-ray magnetic circular dichroism (XMCD) measurements performed at the Co K and W L2,3 edges, on amorphous Co-W alloy nanoparticles, and a comparison with those on a bulk Co3W alloy. A strong hybridization between the 4p and 3d orbitals in Co and the 5d band in W are observed, resulting in an induced magnetic moment in the W atoms. The orbital to spin moment ratio in W of all these Co-W systems is positive, suggesting a parallel orientation of the two moments. This is opposite to the expected antiparallel coupling for atoms with a less-than-half-filled 5d band, according to Hund’s third rule. These findings are supported by calculations of the electronic density of states projected at the Co 3d and W 5d orbitals, as well as XANES spectra and XMCD signals at the Co K and W L2,3 edges in a Co3W system.