Efectos de matriz y tamaño en las propiedades magnéticas y magnetoópticas de nanopartículas magnéticas

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 23-02-200

Capping layer effects in the structure and composition of Co nanoparticle ultrathin films

In this work, we present the correlation of the magnetic and structural properties of Co nanoparticles deposited by sputtering on Si3N4 substrates at different temperatures, and covered with different capping layers, two insulators, AlN and MgO, and a metal, Pt. High-resolution transmission electron microscopy shows the formation of CoPt3 and Co2N, for the Pt and AlN capping layers, respectively, giving to a significant change of the magnetic behavior. When using a cap of MgO, energy-filtered transmission electron microscopy shows an oxidized shell covering the Co nanoparticles with thickness decreasing as the deposition temperature increases, explaining the changes in the magnetic response induced by the MgO capping layer.This work has been funded by the Spanish Project No.MAT2002-04484-C03-02. Y.H. acknowledges the Consejo Superior de Investigaciones Científicas CSIC and Ramón y Cajal Program for financial support.Peer reviewe

Control of the perpendicular magnetic anisotropy of FePd films via Pd capping deposition

We have investigated the influence of two capping layers (MgO and Pd) on the magnetic anisotropy of highly anisotropic L10 FePd films. While we mainly found perpendicular magnetic anisotropy in MgO capped films, we observed that the Pd capping layer induces formation of an additional new phase near the FePd/Pd interface that exhibits in-plane magnetic anisotropy. The combined effect of these two anisotropies results in global canted magnetic anisotropy. Thus, our findings illustrate a mechanism to influence the magnetic anisotropy in FePd highly ordered alloys via adequate choice of capping layer materials.Peer reviewe

Growth and magnetic characterization of Co nanoparticles obtained by femtosecond pulsed laser deposition

We present a detailed study on the morphology and magnetic properties of Co nanostructures deposited onto oxidized Si substrates by femtosecond pulsed laser deposition. Generally, Co disks of nanometric dimensions are obtained just above the ablation threshold, with a size distribution characterized by an increasingly larger number of disks as their size diminishes, and with a maximum disk size that depends on the laser power density. In Au/Co/Au structures, in-plane magnetic anisotropy is observed in all cases, with no indication of superparamagnetism regardless of the amount of material or the laser power density. Magnetic force microscopy observations show coexistence of single-domain and vortex states for the magnetic domain structure of the disks. Superconducting quantum interference device magnetometry and x-ray magnetic circular dichroism measurements point to saturation magnetization values lower than the bulk, probably due to partial oxidation of the Co resulting from incomplete coverage by the Au capping layer.Work was supported in part by the U.S. Department of Energy, Basic Energy Sciences (Grant No. DE-FG02-06ER46273), NSF FOCUS Center, the Spanish Ministerio de Educación y Ciencia (References No. PR2005-0017 and No.MAT2005-05524-C02), Comunidad de Madrid (Reference No. S-0505/MAT/0194 NANOMAGNET), and CSIC (Reference No. 200650I130). Support from the SRS staff during the XMCD experiments is greatly acknowledged. Y.H. and L.M. also acknowledge financial support from the “Ramón y Cajal” and “Juan de la Cierva” programs, respectively, from the Spanish Ministerio de Investigación y Ciencia and Consejo Superior de Investigaciones Científicas (CSIC).Peer reviewe

Capping-layer-induced magnetic coupling in a two-dimensional nanostructured system

The magnetic polarization of a Pt capping layer leads to an increase of the interisland magnetic coupling in a two-dimensional array of Fe islands. For small superparamagnetic islands, Pt deposition leads to a superparamagnetic–ferromagnetic transition. For larger ferromagnetic but weakly coupled islands, Pt deposition produces a stronger interisland coupling. Polar Kerr spectroscopy measurements and simulations evidence the magnetic polarization of Pt in contact with Fe. The described effects and their interpretation are supported by the use of a nonpolarizable Al capping, where both the superparamagnetic-to-ferromagnetic transition and the increase of the interisland coupling are absent.Work carried out under financial support of the Spanish CICyT, CAM Scientific Agencies and Ramón y Cajal program.Peer reviewe

Magnetic coupling between Fe nanoislands induced by capping-layer magnetic pola

A study on the interislands interaction in granular Fe(110) thin films grown on c-sapphire as a function of the islands size and the capping layer induced magnetization is presented. Islands size (ranging from 10 to 50 nm in diameter) and physical contact between them can be monitored with the deposition time. While Al and MgO cappings do not modify the magnetic hysteresis loop of free islands surface, Pd and Pt give rise to a superparamagnetic-ferromagnetic transition in structures formed by small islands and a stronger interisland coupling in those formed by larger ferromagnetic islands. This improvement in the exchange interactions between islands is due to the induced magnetization of Pt and Pd localized at the interfaces between Pt-Fe(Pd-Fe) as evidenced by polar Kerr spectroscopy measurements and simulations.This work has been partially financed by the Spanish Commission of Science and Technology, Comunidad Autónoma de Madrid, and the Spanish Ministerio de Ciencia y Tecnología. Y.H. acknowledges the Consejo Superior de Investigaciones Científicas (CSIC) and Ramón y Cajal program for financial support.Peer reviewe

Cobalt nanoparticles deposited and embedded in AlN: Magnetic, magneto-optical, and morphological properties

8 pages, 10 figures, 2 tables.-- PACS: 78.20.Ls; 75.60.Ej; 81.15.Cd; 61.46.+w; 75.50.Tt; 75.50.Cc; 68.37.Ps; 68.37.Rt; 81.05.Bx; 81.07.-b.-- et al.We present a structural, morphological, magnetic, and magneto-optical study of cobalt nanoparticles deposited on 50 Å AlN/c-sapphire substrates and embedded in an AlN matrix. The dependence of the properties of Co nanoclusters deposited on AlN with growth temperature and amount of deposited Co are studied and discussed. Also we directly compare the properties of as grown and AlN embedded Co nanoclusters and show that the AlN matrix has a strong impact on their magnetic and magneto-optical properties.This work was partially financed by the Spanish Commission of Science and Technology [Comisión Interministerial para la Ciencia Y la Tecnología (CICYT)], the Spanish Ministerio de Ciencia y Tecnología, and the Comunidad de Madrid (Project No. 07N/0108/2002). Y.H. acknowledges the Consejo Superior de Investigaciones Científicas (CSIC) and Ramón y Cajal program for financial support.Peer reviewe

Morphological and magnetic properties of Co nanoparticle thin films grown on Si3N4

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.This work was supported in part by Universidad de Oviedo. One of the authors (B.P.) gratefully thanks Dr. Luis Eugenio Fernandez-Outon and Dr. Gonzalo Vallejo- Fernandez for the support received in the particle size analysis. Also, B.P. acknowledges financial support received from Gobierno del Principado de Asturias under Grant No. BP05-015. J.M.G.M. wishes to thank the financial support from CSIC under Project No. PIE 200650 130. C.C. acknowledges the Ministerio de Educación y Ciencia and FPI program for financial support.Peer reviewe

Order and phase nucleation in nonequilibrium nanocomposite Fe-Pt thin films with perpendicular magnetic anisotropy

We report on the time evolution of mass transport upon annealing nonequilibrium Fe-Pt nanocomposite films, leading to nucleation of L10 chemically ordered phase. The nonequilibrium nanocomposite films were fabricated by applying Fe+ ion implantation to epitaxial Pt films grown on (001) MgO substrates, yielding Fe nanoclusters embedded in a Pt matrix at a tailored penetration depth. Time-resolved x-ray diffraction studies were carried out using synchrotron radiation, allowing determination of the activation energy for nucleation of the FePt L10 phase within the segregated nanoclusters during annealing. The growth of the segregated L10 ordered phase was modeled using ideal grain-size law and found to be dominated by strain-driven surface nucleation. The activation energies were found to correlate with the nanocluster size. Magnetic characterization of selected annealed samples indicates perpendicular magnetic anisotropy with high coercive field coincident with high value of the chemical order parameter of the ordered phase within the magnetic nanoclusters.Funding from NSF (Grant No. DMR-0355171), Research Corporation Cottrell Scholar Award, and the American Chemical Society under Grant No. PRF-41319-AC10 is acknowledged. Funding from different Spanish Institutions, CM (Grant No. S-0505/MAT/0194) (NANOMAGNET) and MEC (Grant No. MAT2005-05524-C02-01), is also acknowledged. Use of the Advanced Photon Source was supported by the Office of Science, Office of Basic Energy Sciences, U.S. Department of Energy under Contract No. DEAC02-06CH11357. The authors also acknowledge R. Irving, M. Brown, and M. Mitra for assistance during ion implantation at the Toledo Heavy Ion Accelerator (THIA).Peer reviewe

Control of the perpendicular magnetic anisotropy of FePd films via Pd capping deposition

We have investigated the influence of two capping layers (MgO and Pd) on the magnetic anisotropy of highly anisotropic L10 FePd films. While we mainly found perpendicular magnetic anisotropy in MgO capped films, we observed that the Pd capping layer induces formation of an additional new phase near the FePd/Pd interface that exhibits in-plane magnetic anisotropy. The combined effect of these two anisotropies results in global canted magnetic anisotropy. Thus, our findings illustrate a mechanism to influence the magnetic anisotropy in FePd highly ordered alloys via adequate choice of capping layer materials.Peer reviewe