Magnetic and topographic correlations in Co nanoparticles (original) (raw)
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Morphological and magnetic properties of Co nanoparticle thin films grown on Si3N4
Journal of Applied Physics, 2007
The morphological and magnetic properties of Co nanoparticles deposited by triode sputtering on Si3N4 at 550 °C are reported. The nominal thickness of Co ranges from 2 up to 15 nm, and two different capping layers, Au and Pt, are used. The nanoparticles were characterized by x-ray diffraction and atomic force microscopy. Morphological and structural studies show that the nanoparticles grow in a well-defined nanostructured pattern and adopt a hexagonal closed packed crystalline structure. Moreover, the average particle size and the particle size dispersion increase as the thickness increases, due to percolation. Experimental characterization of effective anisotropy field was carried out with transverse susceptibility. Transverse susceptibility measurements reveal an in-plane isotropic magnetic behavior. Both the effective anisotropy field and the coercive field increase as the particle size increases, following a D6 dependence, which is typical for three-dimensional structures in the framework of the random anisotropy model. The relationship between the particle size distribution and the anisotropy field distribution is shown, explaining the significant dependence of the magnetic behavior on the Co layer thickness. On the other hand, different capping layers give rise to a change in the magnetic response due to the modification of the interparticle interaction.
Physical Review B, 2008
Magnetic and magneto-optical properties of Co films are studied as a function of the morphology and the capping layer. We show that the nanoparticulate structure of the Co films has a clear influence on the magnetic and magneto-optical properties of the system. Kerr measurements combined with x-ray magnetic circular dichroism provide evidence of a strong correlation between the collective magnetic behavior of the system and the individual atomic magnetic response. The influence of the magnetic nature of the capping layer ͑Al, Au, and Pt͒ is also analyzed. Polarized capping layers, such as Pt, magnetically couple the nanostructures and not only increase the effective anisotropy of the system but also enhance the atomic magnetic moment of Co and the global magneto-optical activity.
MAGNETIC DYNAMICS OF CO NANOSPHERES: ORIGIN OF THE ENHANCED ANISOTROPY
NATO Science Series II: Mathematics, Physics and Chemistry, 2006
The present work deals with the observation of enhancement of the magnetic anisotropy of Co nanoparticles and its origin. The samples were granular multilayer samples prepared by sequential deposition, by sputtering, of amorphous Al 2 O 3 and Co layers on a Si substrate. Co nanoparticles are selforganized in a quasi-regular spatial order of approximately hexagonal closepacked symmetry. The particles studied range in average diameter between 0.7 nm and nearly 5 nm, with a narrow size distribution. This special morphology has enabled us to circumvent ambiguities in sample configuration and, by means of a simple model for fluctuating moments, explain the dynamics of the Co particle moments in terms of an activation energy with contributions from anisotropy K eff , dipole-dipole interactions E dip , and a bias magnetic field H. The anisotropy is enhanced by one to two orders of magnitude with respect to the bulk fcc Co due to strong pinning of the surface Co magnetic moments anisotropy, and increase as 1/D as the particle diameter decreases. The origin of this enhancement is related to an enhancement of the orbital magnetic moment at the surface atoms. Capping the Co nanospheres with a Cu film increases further the particle anisotropy and the orbital magnetic moment of the surface atoms.
Magnetic and structural properties of Co nanocluster thin films
Physical Review B, 2005
In this work we report on the magnetic characterization of thin films composed of gas-phase cobalt nanoclusters deposited on surfaces. Measurements of magnetization curves at ambient temperature indicate a strong exchange interaction between the clusters, while at cryogenic temperatures an exchange bias field appears. The latter confirms the existence of a ferromagnetic/antiferromagnetic core-shell system. Temperature-dependent magnetization measurements under zero-field-cooled conditions showed a rather broad maximum situated around 200 K. Magnetic force microscopy indicates the formation of a correlated super-spin-glass ͑CSSG͒ resulting from the frustration between the interparticle exchange interaction and the randomly oriented intraparticle anisotropy. The approach to saturation of the magnetization curves at 295 K is consistent with a CSSG.
Magnetic anisotropy of embedded Co nanoparticles: Influence of the surrounding matrix
Physical Review B, 2010
We report on the magnetic properties of Co clusters embedded in four different matrices ͑Ag, Au, Si, and amorphous carbon͒. The recently developed "triple fit" method for treating conventional magnetometry data allows, together with micro-superconducting quantum interference device ͑-SQUID͒ investigations, the detailed study of the influence of the surrounding matrix on the magnetic volume and the magnetic anisotropy of Co nanoparticles. While interdiffusion between matrix and Co atoms cannot be excluded in Si and amorphous C matrices, the structure of clusters embedded in the metallic matrices remains intact. Ag and Au matrices increase significantly the magnetic anisotropy energy of the Co clusters.-SQUID experiments indicate that the magnetic anisotropy of embedded clusters is not affected by a magnetically dead layer and that an anisotropy dispersion must be taken into account for size-selected nanoparticles.
2011
Co-based nanostructures ranging from core/shell to hollow nanoparticles were prepared by varying the reaction time and the chemical environment during the thermal decomposition of Co 2 (CO) 8 . Both structural characterization and kinetic model simulation illustrate that the diffusivities of cobalt and oxygen determine the growth ratio and the final morphology of the nanoparticles. Exchange coupling between Co and Co-oxide in core/shell nanoparticles induced a shift of field-cooled hysteresis loops that is proportional to the shell thickness, as verified by numerical studies. The increasing nanocomplexity, when passing from core/shell to hollow particles, also leads to the appearance of hysteresis above 300 K due to an enhancement of the surface anisotropy resulting from the additional spin-disordered surfaces.
Magnetic properties of arrays of interacting Co nanocrystals
Journal of Magnetism and Magnetic Materials, 2002
Monodisperse Co FCC nanocrystals with 12 nm diameter were self-assembled into regular quasi-two-dimensional triangular periodic arrays on carbon substrates from a toluene-based colloidal suspension. At 300 K the regular arrays show a collective magnetic behaviour due to dipolar coupling. A remanent magnetization with an easy axis in the filmplane and an uniaxial in-plane anisotropy field of 0.037 T were determined by SQUID magnetometry and angular dependent ferromagnetic resonance. r
Magnetic anisotropy study of triangular-shaped Co nanostructures
Journal of Magnetism and Magnetic Materials, 2008
Atomic force microscopy (AFM), X-ray magnetic circular dichroism (XMCD), magnetic force microscopy (MFM) and vibrating sample magnetometry (VSM) have been used to measure the magnetic and geometrical characteristics of triangular-shaped Co structures of lateral size 730 nm and thickness 32 nm, prepared by nanosphere lithography (NSL). Evidence of in-plane six-fold magnetic anisotropy induced by the symmetry of the structure has been found. By means of XMCD measurements, performed at remanence after applying a pulsed field, a structure rotation angle-dependent oscillation of about 15% with a periodicity of 601 has been observed for both the orbital and spin moments. Furthermore, the system exhibits the angular hysteresis effect. The magnetic measurements performed by MFM show a reduction of the magnetic configurations to only two states, one quasi-single domain Y state and second, a combination of vortex and Y state.