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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 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.
Phys. Rev. B. 05/2013; 86(6):064428.
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J Bartolome,
A I Figueroa,
L M Garcia,
F Bartolome,
L Ruiz,
J M Gonzalez-Calbet,
F Petroff, C Deranlot,
F Wilhelm,
A Rogalev,
N Brookes
Low Temperature Physics. 01/2012; 38(9):835-838.
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A. I. Figueroa,
J. Bartolome,
L. M. Garcia,
F. Bartolome,
C. Magen,
A. Ibarra,
L. Ruiz,
J. M. Gonzalez- Calbet,
F. Petroff, C. Deranlot,
S. Pascarelli,
P. Bencok,
N. B. Brookes,
F. Wilhelm,
A. Rogalev
Physical Review B 01/2011; 84:184423. · 3.69 Impact Factor
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ABSTRACT: Co–W nanoparticles formed by sequential sputtering of Co on amorphous alumina substrate and subsequent W capping are studied by high resolution and by scanning transmission electron microscopies, and by superconducting quantum interference device magnetometry. The analysis is focused on W nominal thickness dependence. Results suggest the formation of amorphous Co–W alloy nanoparticles, whose magnetic moment per Co atom is systematically reduced as the nominal thickness of W capping layer increases. The Co–W nanoparticles show superparamagnetic behavior. The activation energy for moment reversal and the effective anisotropy are obtained.
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
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A. I. Figueroa,
J. Bartolomé,
L. M. García,
F. Bartolomé,
C. Magén,
A. Ibarra,
L. Ruiz,
J. M. González-Calbet,
F. Petroff, C. Deranlot,
S. Pascarelli,
P. Bencok,
N. B. Brookes,
F. Wilhelm,
A. Rogalev
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ABSTRACT: W-capped Co nanoparticles dispersed in an alumina matrix are studied by means of high-resolution transmission electron microscopy, extended x-ray absorption fine structure, SQUID-based magnetic measurements, dc magnetization, ac magnetic susceptibility, and x-ray magnetic circular dichroism. Results show the formation of amorphous Co-W alloy nanoparticles, the magnetic properties of which are modified by the amount of W or Co present in the samples. The average Co magnetic moment depends on the number of W atoms surrounding it. Co-W particles show superparamagnetic behavior and are described as an array of noninteracting particles with random anisotropy axes and an average moment per particle proportional to the particle volume and to the average Co moment for each alloy composition. Values of the magnetic anisotropy constant of the particles are on the order of 106 erg/cm3, higher than that of bulk Co. Evidence of short-range ordering within each amorphous particle is found that provides insight of the origin of their magnetic anisotropy.
Phys. Rev. B. 84(18).
