| 1. |
- Estrader, M., et al.
(författare)
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Robust antiferromagnetic coupling in hard-soft bi-magnetic core/shell nanoparticles
- 2013
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- The growing miniaturization demand of magnetic devices is fuelling the recent interest in bi-magnetic nanoparticles as ultimate small components. One of the main goals has been to reproduce practical magnetic properties observed so far in layered systems. In this context, although useful effects such as exchange bias or spring magnets have been demonstrated in core/shell nanoparticles, other interesting key properties for devices remain elusive. Here we show a robust antiferromagnetic (AFM) coupling in core/shell nanoparticles which, in turn, leads to the foremost elucidation of positive exchange bias in bi-magnetic hard-soft systems and the remarkable regulation of the resonance field and amplitude. The AFM coupling in iron oxide-manganese oxide based, soft/hard and hard/soft, core/shell nanoparticles is demonstrated by magnetometry, ferromagnetic resonance and X-ray magnetic circular dichroism. Monte Carlo simulations prove the consistency of the AFM coupling. This unique coupling could give rise to more advanced applications of bi-magnetic core/shell nanoparticles.
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| 2. |
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| 3. |
- Krycka, K. L., et al.
(författare)
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Correlating material-specific layers and magnetic distributions within onion-like Fe3O4/MnO/gamma-Mn2O3 core/shell nanoparticles
- 2013
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 113:17, s. 17B531-
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic responses of two nanoparticle systems comprised of Fe3O4/gamma-Mn2O3 (soft ferrimagnetic, FM/hard FM) and Fe3O4/MnO/gamma-Mn2O3 (soft FM/antiferromagnetic, AFM/hard FM) are compared, where the MnO serves to physically decouple the FM layers. Variation in the temperature and applied field allows for Small Angle Neutron Scattering (SANS) measurements of the magnetic moments both parallel and perpendicular to an applied field. Data for the bilayer particle indicate that the graded ferrimagnetic layers are coupled and respond to the field as a single unit. For the trilayer nanoparticles, magnetometry suggests a Curie temperature (T-C) approximate to 40 K for the outer gamma-Mn2O3 component, yet SANS reveals an increase in the magnetization associated with outer layer that is perpendicular to the applied field above T-C during magnetic reversal. This result suggests that the gamma-Mn2O3 magnetically reorients relative to the applied field as the temperature is increased above 40 K.
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| 4. |
- Lopez-Ortega, A., et al.
(författare)
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Strongly exchange coupled inverse ferrimagnetic soft/hard, MnxFe3-xO4/FexMn3-xO4, core/shell heterostructured nanoparticles
- 2012
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 4:16, s. 5138-5147
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Tidskriftsartikel (refereegranskat)abstract
- Inverted soft/hard, in contrast to conventional hard/soft, bi-magnetic core/shell nanoparticles of MnxFe3-xO4/FexMn3-xO4 with two different core sizes (7.5 and 11.5 nm) and fixed shell thickness (similar to 0.6 nm) have been synthesized. The structural characterization suggests that the particles have an interface with a graded composition. The magnetic characterization confirms the inverted soft/hard structure and evidences a strong exchange coupling between the core and the shell. Moreover, larger soft core sizes exhibit smaller coercivities and loop shifts, but larger blocking temperatures, as expected from spring-magnet or graded anisotropy structures. The results indicate that, similar to thin film systems, the magnetic properties of soft/hard core/shell nanoparticles can be fine tuned to match specific applications.
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| 5. |
- Estradé, S., et al.
(författare)
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Distinguishing the core from the shell in MnOx/MnOy and FeOx/MnOx core/shell nanoparticles through quantitative electron energy loss spectroscopy (EELS) analysis
- 2012
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Ingår i: Micron. - : Elsevier BV. - 0968-4328 .- 1878-4291. ; 43:1, s. 30-36
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Forskningsöversikt (refereegranskat)abstract
- The structural and chemical characterization of inverted bi-magnetic MnOx(antiferromagnetic)/MnOy(ferrimagnetic) and FeOx(soft-ferrimagnetic)/MnOx(hard-ferrimagnetic) core/shell nanoparticles has been carried out by means of scanning transmission electron microscopy with electron energy loss spectroscopy analysis, (S)TEM-EELS. Quantitative EELS was applied to assess the local composition of the nanoparticles by evaluating the local Mn oxidation state based on the Mn L-3/L-2 peak intensity ratio and the Mn L-3 peak onset. The analysis allows to unambiguously distinguish the core from the shell and to determine the nature of the involved manganese oxides in both cases. The results evidence that the structure of the nanoparticles is, in fact, more complex than the one designed by the synthesis parameters.
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| 6. |
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| 7. |
- Roldan, Manuel A., et al.
(författare)
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Probing the meta-stability of oxide core/shell nanoparticle systems at atomic resolution
- 2021
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Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947 .- 1873-3212. ; 405
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Tidskriftsartikel (refereegranskat)abstract
- Hybrid nanoparticles allow exploiting the interplay of confinement, proximity between different materials and interfacial effects. However, to harness their properties an in-depth understanding of their (meta)stability and interfacial characteristics is crucial. This is especially the case of nanosystems based on functional oxides working under reducing conditions, which may severely impact their properties. In this work, the in-situ electron-induced selective reduction of Mn3O4 to MnO is studied in magnetic Fe3O4/Mn3O4 and Mn3O4/Fe3O4 core/shell nanoparticles by means of high-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy. Such in-situ transformation allows mimicking the actual processes in operando environments. A multi-stage image analysis using geometric phase analysis combined with particle image velocity enables direct monitoring of the relationship between structure, chemical composition and strain relaxation during the Mn3O4 reduction. In the case of Fe3O4/Mn3O4 core/shell the transformation occurs smoothly without the formation of defects. However, for the inverse Mn3O4/Fe3O4 core/shell configuration the electron beam-induced transformation occurs in different stages that include redox reactions and void formation followed by strain field relaxation via formation of defects. This study highlights the relevance of understanding the local dynamics responsible for changes in the particle composition in order to control stability and, ultimately, macroscopic functionality.
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