Ferromagnetic resonance studies in granular Co-SiO2 thin films

Properties of thin granular Co-SiO2 films have been studied by means of ferromagnetic resonance (FMR). The obtained FMR results are discussed using sample magnetization, electrical conductivity, and transmission electron microscopy analysis. Co and SiO2 were sequentially deposited for the sample preparation. The general behavior of the applied field for resonance could be described using effective out-of-plane anisotropies. A dipolar interaction model developed for magnetic heterostructures was applied to the interpretation of these anisotropies. The anisotropy terms caused by the magnetic particle shapes and by the film shape can explain the results for two metallic films close to percolation, in which the film shape is the preponderant contribution. In the case of an insulating sample, the consideration of an additional anisotropy term seems to be necessary to explain the results.99

Nanostructure and giant Hall effect in TMx(SiO2)(1-x) (TM=Co,Fe,Ni) granular system

Granular TMx(SiO2)(1-x) (TM=Co,Fe,Ni) thin films were thermally treated at different temperatures and their magnetotransport and structural properties were studied. Hall resistivity decreases with thermal annealing. Structure was analyzed based on small angle x-ray scattering results. A model of polydisperse system of hard spheres was used for obtaining structural parameters. Analysis reveals that a volume fraction of transition-metal atoms (less than 29%) are forming nanospheres. Changes in giant Hall effect upon annealing can depend on a particular combination of nanoparticle diameter, interparticle distance, and size distribution. (C) 2006 American Institute of Physics.99

Blocking phenomena in granular magnetic alloys through magnetization, Hall effect, and magnetoresistance experiments

Magnetization and magnetotransport were measured in CoxAg1-x granular composites as a function of temperature and applied magnetic field. A transition from blocked to superparamagnetic behavior with increasing temperatures can be observed in magnetization, giant magnetoresistance and the extraordinary Hall effect measurements. However, the blocking temperature determined from magnetotransport measurements is systematically lower than the one estimated from magnetic measurements. This is due to the selective magnetic scattering, which is enhanced for smaller particles, while the magnetization probes the whole particle size distribution. (C) 2003 American Institute of Physics.82576376

Giant Hall effect in superparamagnetic granular films

A comprehensive review of the giant Hall effect (GHE) is presented, with emphasis on novel experimental data obtained in Ni-SiO2 and Co-SiO2 films prepared by co-sputtering. GHE is observed close to and on both sides of the metal-insulator transition. From the point of view of microscopic conduction mechanisms, this means a crossover from metallic conductivity with weak localization to tunneling, or hopping, between separate granules across insulating barriers. Magnetic percolation is also interrupted at this concentration of metal, leading to superparamagnetic behavior of the composite and blocking phenomena. Temperature dependencies of magnetization and extraordinary Hall coefficient in the composites near the critical concentration are compared. In single phase magnetic metals and alloys, the extraordinary Hall is believed to be directly proportional to the total magnetization, due to side jumps or skew scattering. In a metal-insulator composite, only those electrons traveling in conduction critical paths can contribute to the Hall signal, thus only magnetization of the material belonging to these paths is important in the Hall measurements. Comparison with the magnetic results leads to new possibilities in understanding both the electronic and magnetic properties of granular nanocomposites. (C) 2003 Elsevier Science B.V. All rights reserved

Structural and magnetic properties of TM-SiO2 (TM = Fe, Co, Ni) films

TMx-(SiO2)(1-x) (TM =Fe, Co, Ni) thin films were prepared in a wide concentration range (0.35 less than or equal to x less than or equal to 1). Structure was Studied with transmission electron microscopy (TEM), X-ray diffraction (XRD) and small angle X-ray scattering (SAXS). Magnetic and magnetotransport properties were investigated by means of magnetization and Hall effect measurements. TEM images display nanometric spherical structures embedded in a SiO2 amorphous matrix, with typical sizes increasing from 3 to 5 nm when TM volume concentration x is increased. SAXS measurements indicate a complex structure formed by nanosized objects. XRD measurements show that the structure is composed by amorphous SiO2 and TM crystallites. Slightly above the percolation threshold all samples display giant Hall effect. The observed magnetic properties are dependent on x, and display an evolution resulting from the progressive increase of the mean particle size. (C) 2003 Elsevier Science B.V. All rights reserved

Ordinary and extraordinary giant Hall effects in Co-SiO2 granular films

Magnetization. resistance and Hall effect are studied in granular magnetic Co-SiO2 nanocomposites in the temperature range 5-300K and fields up to 6T. Relative contributions from spin-independent and spin-dependent processes to the giant Hall effect near the metal-insulator transition are analysed. (C) 2001 Published by Elsevier Science B.V

Transport and magnetotransport properties of Co thin films on Si

Resistance, magnetoresistance and Hall effect were studied in sputtered cobalt films of varying thickness, deposited on silicon substrates, in the temperature range of 5-350 K and magnetic field up to 7 T. Contributions from both metallic cobalt and silicide layers are revealed. The latter undergo a metal-insulator transition in the temperature range of 260-280 K. (C) 2001 Elsevier Science B.V. All rights reserved

Nanostructure of granular Co-SiO2 thin films modified by thermal treatment and its relationship with the giant Hall effect

Granular cosputtered Co-52(SiO2)(48) thin films were thermally treated at different temperatures and their magnetotransport and structural properties were investigated. Hall resistivity increases with annealing temperature (T-a), up to T-a=250 degrees C, and then decreases to a minimum for T-a=400 degrees C. The structural analysis was based on small-angle x-ray scattering results. A model of a polydisperse system of hard spheres was used to retrieve structural parameters. Results reveal that a volume fraction of Co atoms (approximately 25%) are forming nanospheres. The giant Hall effect depends on a particular combination of nanoparticle diameter, size distribution, and interparticle distance.721

Giant Hall effect in Co-SiO2 nanocomposites

Measurements of both the ordinary and extraordinary components of the Hall effect in co-sputtered granular magnetic CoSiO2 nanocomposites are presented. The experiments were done in the temperature range 5-300 K, and fields up to 7 T. Both components show a large enhancement when the metal volume fraction is reduced to the metal-insulator transition. However the enhancement of the ordinary Hall effect is much weaker than that of the extraordinary one. We discuss the implications of this observation for understanding of the giant Hall effect.12143397339