Structural and magnetic properties of amorphous Co-W alloyed nanoparticles

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 10 6 erg/cm 3 , 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.

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

“…The fitting procedure of these EXAFS data has been previously described in Ref. 9. In fact, the Co local environment in the Co-W alloy NPs can be seen as an altered Co 3 W structure, in which the Co-Co coordination, N Co−Co , and Co-W coordination, N Co−W , vary according to the Co/W ratio in the alloy.…”

“…4,9 These samples follow the formula Al 2 O 3 /(Al 2 O 3 /Co/W) N /Al 2 O 3 , with N being the number that the sequence is repeated in order to obtain a multilayer system. Two series of samples were studied: a, samples with a fixed nominal thickness of deposited Co, t Co = 0.7 nm, and different amounts of W given by t W = 0.6, 1.5, and 4.5 nm; b, samples with constant t W = 1.5 nm and varying t Co = 0.7 and 1.0 nm.…”

“…The morphological, structural, and magnetic properties of these particles have been extensively described in Ref. 9. Co-W hybridization and electronic transfer from W 5d to Co orbitals are also determined by analysis of x-ray absorption near edge structure (XANES) measurements at the Co K and W L 3 edges.…”

“…The electronic transfer from W 5d toward Co 4p bands, and the delocalization of the Co 3d empty states are qualitatively described as a function of the Co/W ratio in these Co-W NPs samples. 9 The effects of similar 3d−5d hybridization, in particular on 5d induced magnetic moments, have been previously studied both experimentally and theoretically in Fe/W multilayers 10 and bulk Co 100−x Ir x alloys, 11 to cite some examples of metallic systems. A remarkable finding in these two systems is the observation of a breakdown of Hund's third rule of the induced orbital and spin magnetic moments in the 5d metals.…”

“…In particular, among the specific group of magnetic nanoparticles with amorphous structure, the amorphous Co-W alloy NPs constitute a paradigmatic example, where their anisotropy is not due to surface effects but results from the structural short range order (SRO) detected in the particles. 9 They consist of a self-organized array of Co-W alloy particles, uniformly dispersed in an alumina matrix. Their magnetic properties are those of noninteracting superparamagnetic particles with random anisotropy axes and an average moment per particle proportional to the particle volume, governed by the Co/W ratio present in the alloy.…”