Pulsed electrodeposition and magnetism of two-dimensional assembly of controlled-size Co particles on Si substrates (original) (raw)
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Journal of Magnetism and Magnetic Materials, 2005
A silicon substrate, pre-structured by focused ion beam (FIB), is used for selective electrodeposition of cylindrical cobalt nanodots with a large aspect ratio of depth to diameter of 2. These nanostructures are characterised by magnetic force microscopy (MFM) and ferromagnetic resonance (FMR). Cobalt dots grown in the pulsed mode are single domain in the remnant state with an easy magnetization perpendicular to the substrate. On the contrary, some of the dots grown, in the continuous mode, present a Co cap on top, which leads to the formation of magnetic vortices with an in-plane magnetization component.
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.
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 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.
Nature materials, 2004
The use of magnetic nanoparticles in the development of ultra-high-density recording media is the subject of intense research. Much of the attention of this research is devoted to the stability of magnetic moments, often neglecting the influence of dipolar interactions. Here, we explore the magnetic microstructure of different assemblies of monodisperse cobalt single-domain nanoparticles by magnetic force microscopy and magnetometric measurements. We observe that when the density of particles per unit area is higher than a determined threshold, the two-dimensional self-assemblies behave as a continuous ferromagnetic thin film. Correlated areas (similar to domains) of parallel magnetization roughly ten particles in diameter appear. As this magnetic percolation is mediated by dipolar interactions, the magnetic microstructure, its distribution and stability, is strongly dependent on the topological distribution of the dipoles. Thus, the magnetic structures of three-dimensional assembli...
Journal of Magnetism and Magnetic Materials , 2016
Template assisted growth of Co nanorod arrays through electrochemical route was investigated. During this investigation, the template with nano-pore diameter was kept at a fixed value of 45 nm, whereas, the length of the as grown nanorod array was varied from 25 to 400 nm keeping in mind that the aspect ratio (L/D) covers both below and above the unity. X-Ray diffraction patterns indicate that the nanorod arrays initiates its textured growth with fcc (111) phase, however, the change in growth texture to hcp (100) was observed as it grows above 200 nm in length. The anisotropy fields extracted from the measured magnetization data reveal that a cross-over from in-plane to out-of plane anisotropy takes place for L/D ∼ 2.0. Based on the analytical approach, it seems that the shape anisotropy originated from the demagnetization factor with the change in geometry and magnetostatic interaction among the nanorods cause this crossover. However the micromagnetic simulation yields that both mag-netocrystalline anisotropy and the magnetostatic interaction along with shape anisotropy are very much important to explain the experimental observations.
Effect of magnetic field on self-assembling of colloidal Co magnetic nanoparticles
Applied Surface Science, 2006
In this paper the formation of 3-D structures composed of Co nanoparticles (NPs) is reported. Structures were obtained by drying a droplet of a colloidal solution of NPs in a magnetic field perpendicular to the substrate. The Co nanoparticles were prepared by thermolysis of Co 2 (CO) 8 . The 3-D NP structures were characterized by scanning electron microscopy (SEM) and atomic and magnetic force microscopy (AFM/MFM). It has been found that at the border of the droplet, NPs assemble into hexagonally ordered 3-D columns or they form a labyrinthine structure. The formation of the 3-D structures can be explained by the outflow of NPs to the border of the droplet during the drying process. Within this model the pattern formation is dependent on the concentration of the NPs and the degree of alignment of the magnetic moments of NPs in the 3-D columns. #
Tuning the magnetic anisotropy of Co nanoparticles by metal capping
Europhysics Letters (EPL), 2006
The magnetic anisotropy of Co clusters with diameters ranging from 1.1 nm to 4.5 nm turns out to be significantly larger than in bulk and strongly increasing with decreasing cluster size. The dominating role of the surface can be used to modify the anisotropy by changing the electronic properties of the matrix surrounding the clusters. We find that capping the clusters by a metallic (Cu and Au) layer significantly enhances the anisotropy, thus also stabilizing the magnetization against thermal fluctuations. The observed anisotropy enhancement is attributed to the bonding of the Co 3d electrons to the conduction band of the capping layer, which depends on the electronic band structures of both metals.
Nucleation, growth and properties of Co nanostructures electrodeposited on n-Si(111)
Applied Surface Science, 2012
In the present work, cobalt thin films deposited directly on n-Si(1 1 1) surfaces by electrodeposition in Watts bath have been investigated. The electrochemical deposition and properties of deposits were studied using cyclic voltammetry (CV), chronoamperometry (CA), ex situ atomic force microscopy (AFM), X-ray diffraction (XRD) and alternating gradient field magnetometer (AGFM) techniques. The nucleation and growth kinetics at the initial stages of Co studied by current transients indicate a 3D island growth (Volmer-Weber); it is characterized by an instantaneous nucleation mechanism followed by diffusion limited growth. According to this model, the estimated nucleus density and diffusion coefficient are on the order of magnitude of 10 6 cm −2 and 10 −5 cm 2 s −1 , respectively. AFM characterization of the deposits shows a granular structure of the electrodeposited layers. XRD measurements indicate a small grain size with the presence of a mixture of hcp and fcc Co structures. The hysteresis loops with a magnetic field in the parallel and perpendicular direction and showed that the easy magnetization axis of Co thin film is in the film plane.