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Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: from shape to surface.

Oyarzún S, Tamion A, Tournus F, Dupuis V, Hillenkamp M - Sci Rep (2015)

Bottom Line: Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quantitatively by combining magnetic experiments and advanced data treatment.The obtained values are discussed in the frame of two theoretical models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diameter.

View Article: PubMed Central - PubMed

Affiliation: Institut Lumière Matière, UMR5306 Université Lyon 1-CNRS, Université de Lyon, 69622 Villeurbanne cedex, France.

ABSTRACT
Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quantitatively by combining magnetic experiments and advanced data treatment. The obtained values are discussed in the frame of two theoretical models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diameter.

No MeSH data available.


TOF-MS spectra of cobalt clusters at different mean sizes (1.9 nm–5.5 nm) as produced by the magnetron cluster source.
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f1: TOF-MS spectra of cobalt clusters at different mean sizes (1.9 nm–5.5 nm) as produced by the magnetron cluster source.

Mentions: The samples consist of cobalt nanoparticles embedded in copper matrices, where the mean particle diameter was varied from 1.9 nm to 5.5 nm (cf. Fig. 1). The samples were prepared according to the strategy of cluster-assembled materials. The experimental setup with typical operating conditions is described in detail elsewhere25 and is only sketched briefly. Cobalt cluster ions are generated in the gas phase using a home-built magnetron cluster source (based on the principle introduced by Haberland et al.26) and guided towards the deposition chamber housing the silicon substrate surface with its native oxide layer for sample preparation. Time-of-flight mass spectrometry (TOF-MS) is used for in-situ characterization of the cluster ions in the gas phase and shows no trace of cluster oxidation or other complexes apart form CoAr+ for sizes up to Co150. The kinetic energy of the clusters is adjusted below 0.5 eV/atom in order to ensure soft-landing conditions without fragmentation27. The copper matrix is evaporated in a commercial electron beam evaporator and co-deposited simultaneously at room temperature with the clusters to reach thicknesses around typically 200 nm. The cobalt concentration can be adjusted by varying the matrix deposition rate and is reduced to below 0.5 at.% to avoid magnetic interactions28. No further heat treatment was performed on the samples discussed here. Magnetic measurements were performed in a SQUID MPMS-XL5 from Quantum Design. In order to study the presence of inter-cluster interactions we analyze the difference between the isothermal remanent magnetization (IRM) and the dc demagnetization (DCD) curves producing an extremely sensitive curve termed Δm2429. The values for Δm are close to the noise level in our samples, thus the inter-cluster interactions can be assumed as negligible (cf. supplementary information).


Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: from shape to surface.

Oyarzún S, Tamion A, Tournus F, Dupuis V, Hillenkamp M - Sci Rep (2015)

TOF-MS spectra of cobalt clusters at different mean sizes (1.9 nm–5.5 nm) as produced by the magnetron cluster source.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4593963&req=5

f1: TOF-MS spectra of cobalt clusters at different mean sizes (1.9 nm–5.5 nm) as produced by the magnetron cluster source.
Mentions: The samples consist of cobalt nanoparticles embedded in copper matrices, where the mean particle diameter was varied from 1.9 nm to 5.5 nm (cf. Fig. 1). The samples were prepared according to the strategy of cluster-assembled materials. The experimental setup with typical operating conditions is described in detail elsewhere25 and is only sketched briefly. Cobalt cluster ions are generated in the gas phase using a home-built magnetron cluster source (based on the principle introduced by Haberland et al.26) and guided towards the deposition chamber housing the silicon substrate surface with its native oxide layer for sample preparation. Time-of-flight mass spectrometry (TOF-MS) is used for in-situ characterization of the cluster ions in the gas phase and shows no trace of cluster oxidation or other complexes apart form CoAr+ for sizes up to Co150. The kinetic energy of the clusters is adjusted below 0.5 eV/atom in order to ensure soft-landing conditions without fragmentation27. The copper matrix is evaporated in a commercial electron beam evaporator and co-deposited simultaneously at room temperature with the clusters to reach thicknesses around typically 200 nm. The cobalt concentration can be adjusted by varying the matrix deposition rate and is reduced to below 0.5 at.% to avoid magnetic interactions28. No further heat treatment was performed on the samples discussed here. Magnetic measurements were performed in a SQUID MPMS-XL5 from Quantum Design. In order to study the presence of inter-cluster interactions we analyze the difference between the isothermal remanent magnetization (IRM) and the dc demagnetization (DCD) curves producing an extremely sensitive curve termed Δm2429. The values for Δm are close to the noise level in our samples, thus the inter-cluster interactions can be assumed as negligible (cf. supplementary information).

Bottom Line: Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quantitatively by combining magnetic experiments and advanced data treatment.The obtained values are discussed in the frame of two theoretical models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diameter.

View Article: PubMed Central - PubMed

Affiliation: Institut Lumière Matière, UMR5306 Université Lyon 1-CNRS, Université de Lyon, 69622 Villeurbanne cedex, France.

ABSTRACT
Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quantitatively by combining magnetic experiments and advanced data treatment. The obtained values are discussed in the frame of two theoretical models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diameter.

No MeSH data available.