A detailed investigation of the core@shell structure of exchanged coupled magnetic nanoparticles after performing solvent annealing

Abstract

Thanks to important advances in synthesis techniques, a wide collection of bimagnetic core-shell nanoparticles with tunable properties was reported in the literature. Such nanoparticles may combine two phases with different...

Magnetic anisotropy engineering in onion-structured metal oxide nanoparticles combining dual exchange coupling and proximity effects

“…The pronounced reduction in both H E (10 K) and T onset even for CSSa (for which we estimated above a modest decrease in CoO content from 63% to 51%, see Table 1) suggested that the conversion of CoO into CoFe 2 O 4 was accompanied by the loss of UCS in the remaining CoO, a likely effect of the recently reported effective annealing provided by the third decomposition step. 24 The same effects can be expected from a progressive The decreasing number of CoO UCS (both pinned and rotatable) was conrmed by the difference between the H C values [H C (FC) − H C (ZFC)], which was highest for CS (3.4 kOe) and decreased markedly for the CSS particles (#1 kOe). In short, the concurrent structural ordering of the CoO phase and its hybridization with Co-ferrite from CS to CSSc were consistent with the large reduction in H E across the CS-CSS series (roughly an order of magnitude) while essentially preserving the high coercivity (with a mere 20% decrease in the 10 K loops).…”

“…Here, the intensity of the S4 peak yielded 23% of the uncompensated Co spins in CS, which mainly consisted of a CoFe 2 O 4 layer localized at the Fe 3− δ O 4 /CoO interface. 24 In CSS nanoparticles, the S4 peak indicated there were much higher amounts of uncompensated Co spins (42%, 54%, and 68% in CSSa, CSSb, and CSSc, respectively). This result unambiguously confirmed the increasing fraction of CoFe 2 O 4 in the nanoparticles at the expense of CoO, completing a consistent structural picture of the particles, as graphically summarized in Fig.…”

“…46 It is worth noting that the I 1 /I 2 ratio calculated from the XAS spectra of CS was higher than that for Fe 3 O 4 , which could be attributed to the presence of an additional fraction of Fe 2+ in the Wüstite phase, as observed recently. 24 Therefore, we expect the Wüstite shell to consist of Co 1−x Fe x O resulting from the cocrystallization of Co 2+ and Fe 2+ , with the latter resulting from partial solubilization at the early stages of the Wüstite shell formation. As the CoO shell is also partially solubilized during thermal annealing in a liquid medium, this resulted in a strong decrease in the I 1 /I 2 for CSSa.…”

“…, due to the formation of an intermediate layer of Co-doped ferrite at the Fe 3− δ O 4 /CoO interface. 7,23,24,42…”

“…14,21,22 Synthetic processes can also favor the formation of ferrites, since high temperatures usually favor cation mobility through interfaces 20,23 and partial solubilization. 7,24 Therefore, Fe 3 O 4 @-CoO nanoparticles can include an intermediate layer, such as a CoFe 2 O 4 shell, and can be better described as a core@shell@shell structure. 22,23 Such an onion-type structure offers the possibility to generate additional so-hard interfaces to enhance exchange coupling.…”