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

Abstract

We present x-ray absorption near edge structure (XANES) and x-ray magnetic circular dichroism (XMCD) measurements performed at the Co K and W L 2,3 edges, on amorphous Co-W alloy nanoparticles, and a comparison with those on a bulk Co 3 W 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 L 2,3 edges in a Co 3 W system.

Visualizing chemical states and defects induced magnetism of graphene oxide by spatially-resolved-X-ray microscopy and spectroscopy

“…Furthermore, the negative sign of the C K -edge XMCD feature, implying positive orbital magnetic moment on C atoms, also demonstrates the breakdown of the Hund’s 3 rd rule since the spin magnetic moment on C atoms is parallel to the external magnetic field and is thus positive, although the less-than-half electron occupation case as C 2 p may exhibit antiparallel spin and orbital magnetic moments according to the Hund’s 3 rd rule. However, such breakdowns were observed in many amorphous/nano systems with induced magnetic moments 63 64 65 and few bulk systems with intrinsic moments 66 . The nano-crystalline nature and/or presence/modification of chemical environment of GO possibly the reason for such violation of the Hund’s 3 rd rule.…”

Structural and magnetic properties of granular Co-Pt multilayers with perpendicular magnetic anisotropy

“…For Pt, we have derived n h from the white line difference with a Pt foil at the L 2,3 -edge XANES, similarly to the procedure followed for XANES at the W L 2,3 edges for Co-W NPs in Ref. [59]. We have obtained n h = 1.80(2) holes in the 5d band, m S = 0.14(1) μ B , m L = 0.027(2) μ B , and m L /m S = 0.19(1) for the Pt atoms in our Co-Pt granular films.…”

XMCD and ab initio study of interface-engineered ultrathin Ru/Co/W/Ru films with perpendicular magnetic anisotropy and strong Dzyaloshinskii–Moriya interaction

“…Integrating the 〈 l Z 〉 and 〈2 s Z 〉 spectra and taking X-ray absorption cross-section per hole from ref. 35, one obtains m L < ±0.002 and m S ∼ −0.023 ± 0.004 for the three studied samples. Similar values were also obtained in ref.…”

“…4, we reproduce the normalized XANES and XMCD spectra recorded at the L 2,3 -edges of W in Co/W/Ru trilayers and compared to those recorded for the bulk Co 3 W alloy. 35 For the XANES spectra, the the tungsten L 3 / L 2 ratio was taken to be equal to 2.19/1 according to ref. 36.…”

“…We have applied the same procedure to the spectra recorded in ref. 35 on the bulk Co 3 W alloy. The results of the application of differential magneto-optical sum-rules, that is 〈 l Z 〉 and 〈2 s Z 〉 5d density of states (DOS) of tungsten for the three measured samples compared to the Co 3 W alloy, are presented in Fig.…”