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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.


Related in: MedlinePlus

Magnetic characterization of the 2.7 nm sample.(a) Experimental ZFC/FC curves at 5 mT and m(H, T) at T = 300 K (points) with fits (solid lines); (b) comparison between geometric size distribution as derived from TEM (histogram) and log-normal magnetic size distribution as derived from triple fit; (c) IRM data (points) and fits with uniaxial (dashed) and biaxial anisotropy (solid line); (d) low temperature experimental m(H, T) data (points) and simulation using the parameters obtained from the fits (line).
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f2: Magnetic characterization of the 2.7 nm sample.(a) Experimental ZFC/FC curves at 5 mT and m(H, T) at T = 300 K (points) with fits (solid lines); (b) comparison between geometric size distribution as derived from TEM (histogram) and log-normal magnetic size distribution as derived from triple fit; (c) IRM data (points) and fits with uniaxial (dashed) and biaxial anisotropy (solid line); (d) low temperature experimental m(H, T) data (points) and simulation using the parameters obtained from the fits (line).

Mentions: In order to investigate the evolution of the magnetic anisotropy as a function of cluster size, a complete magnetic characterization was performed for all samples based on a series of SQUID measurements, namely zero-field-cooled/field-cooled (ZFC/FC) susceptibility, magnetization m(H, T) and low temperature IRM curves. The data treatment to determine the magnetic size distribution and the effective anisotropy constant Keff is based on the simultaneous fit of the ZFC/FC susceptibility curves and a high temperature magnetization curve22. Recently this multiple fit procedure was extended to include the low temperature IRM response in order to include an anisotropy distribution described by a Gaussian function centered at 24 as well as a biaxial contribution K2 to the anisotropy23. Note that namely the ZFC and IRM protocols measure different magnetization reversal processes, the first being thermally activated and thus always passing via the lowest lying potential barrier between the two energy wells corresponding to a macrospin orientation along the easy axis, the second forcing the macrospin close to the field direction. Consequently the IRM measurement probes contributions beyond the uniaxial hypothesis. The obtained set of parameters is finally used to simulate the magnetization cycle at 2 K using a modified Stoner-Wohlfarth model23. The agreement with the experimental data is very good and the obtained values for all samples are listed in Table 1. As an example the complete set of curves, fits and simulation for the 2.7 nm sample is shown in Fig. 2, the data for the other samples can be found in the supporting 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)

Magnetic characterization of the 2.7 nm sample.(a) Experimental ZFC/FC curves at 5 mT and m(H, T) at T = 300 K (points) with fits (solid lines); (b) comparison between geometric size distribution as derived from TEM (histogram) and log-normal magnetic size distribution as derived from triple fit; (c) IRM data (points) and fits with uniaxial (dashed) and biaxial anisotropy (solid line); (d) low temperature experimental m(H, T) data (points) and simulation using the parameters obtained from the fits (line).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: Magnetic characterization of the 2.7 nm sample.(a) Experimental ZFC/FC curves at 5 mT and m(H, T) at T = 300 K (points) with fits (solid lines); (b) comparison between geometric size distribution as derived from TEM (histogram) and log-normal magnetic size distribution as derived from triple fit; (c) IRM data (points) and fits with uniaxial (dashed) and biaxial anisotropy (solid line); (d) low temperature experimental m(H, T) data (points) and simulation using the parameters obtained from the fits (line).
Mentions: In order to investigate the evolution of the magnetic anisotropy as a function of cluster size, a complete magnetic characterization was performed for all samples based on a series of SQUID measurements, namely zero-field-cooled/field-cooled (ZFC/FC) susceptibility, magnetization m(H, T) and low temperature IRM curves. The data treatment to determine the magnetic size distribution and the effective anisotropy constant Keff is based on the simultaneous fit of the ZFC/FC susceptibility curves and a high temperature magnetization curve22. Recently this multiple fit procedure was extended to include the low temperature IRM response in order to include an anisotropy distribution described by a Gaussian function centered at 24 as well as a biaxial contribution K2 to the anisotropy23. Note that namely the ZFC and IRM protocols measure different magnetization reversal processes, the first being thermally activated and thus always passing via the lowest lying potential barrier between the two energy wells corresponding to a macrospin orientation along the easy axis, the second forcing the macrospin close to the field direction. Consequently the IRM measurement probes contributions beyond the uniaxial hypothesis. The obtained set of parameters is finally used to simulate the magnetization cycle at 2 K using a modified Stoner-Wohlfarth model23. The agreement with the experimental data is very good and the obtained values for all samples are listed in Table 1. As an example the complete set of curves, fits and simulation for the 2.7 nm sample is shown in Fig. 2, the data for the other samples can be found in the supporting 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.


Related in: MedlinePlus