J.C. Denardin

Giant Hall effect in Co-SiO2 nanocomposites

Measurements of both the ordinary and extraordinary components of the Hall effect in co-sputtered granular magnetic Co-SiO2 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.

Probing two-dimensional magnetic switching in Co∕SiO2 multilayers using reversible susceptibility experiments

The magnetic switching behavior of Co∕SiO2 multilayers has been studied using reversible susceptibility experiments performed along different orientations in the sample’s plane. A sensitive method for critical curve determination of two-dimensional magnetic systems was proposed. It was shown that this method, based on reversible susceptibility’s singularities detection, is general and can be applied independent of the expression of free energy describing the magnetic system under study. It is found that as the Co∕SiO2 ratio increases in the samples, the switching mechanism is governed by a noncoherent rotation mechanism.

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.

Magnetic and Magnetotransport Properties of Co Thin Films on Si

A systematic study of magnetic and magnetotransport properties of thin Co films on Si is reported in this work. The Co films of thicknesses 30, 160 and 440 A were prepared by magnetron sputtering from a Co target onto a Si(100) substrate held at room temperature. Resistance, magnetoresis-tance and Hall effect were measured in the temperature range 5-350 K. Magnetization was measured in the same temperature range using a SQUID magnetometer. Complete surface hysteresis loops were measured from 4 to 300 K by means of the magneto-optical Kerr effect, in order to follow the behavior of the coercivity and magnetic easy axes of the samples. The transport and magnetotransport properties display a peculiar effect as functions of temperature. The films behave as pure metallic Co below 250 K. However, the resistance drops with heating from 250 to 280 K, regaining its typical metallic behavior at temperatures higher than 280 K. The thinner the film, the larger is the resistance drop in the temperature interval 250-280 K, reaching a factor of 4.5 for the 30 A thick film. The Hall effect contains both ordinary and extraordinary contributions. From ordinary Hall effect measurements, one finds that the conduction is electronic at low temperatures and turns to hole-like above the transition point. Magnetoresistance changes from negative at low T to positive at high temperatures. These facts indicate that the observed effect is related to the metallic layer, which undergoes a conducting channel switching when the temperature is increased.

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.

Magnetotransport, magnetic, and structural properties of TM–SiO2 (TM=Fe, Co, Ni) granular alloys

TM{sub x}(SiO{sub 2}){sub 1-x} (transition metals, or TM=Fe, Co, Ni) thin films were prepared in a wide concentration range with the purpose of studying the giant Hall effect (GHE). The structure was studied using transmission electron microscopy (TEM), X-ray diffraction (XRD), and small angle X-ray scattering (SAXS). Magnetic, transport, and magnetotransport properties were investigated by means of magnetization, resistivity, and Hall effect measurements. TEM images show nanometer-sized spherical structures embedded in an SiO{sub 2} amorphous matrix, with typical sizes ranging from 2 to 7 nm when TM volume concentration x is increased. SAXS measurements show a broadened peak. XRD measurements show that the system consists of amorphous SiO{sub 2} and a crystalline peak, corresponding to small TM crystallites. The observed magnetic properties are strongly dependent on x, and clearly display an evolution resulting from the progressive increase in the mean particle size. Above the percolation threshold region all samples display GHE. Relationships between structure and magnetotransport properties are discussed.

Study of interactions in Co–SiO2 granular films by means of MFM and magnetization measurements

A study of Co[SiO2] granular films using magnetic force microscopy (MFM), transmission electron microscopy and DC magnetization is presented. MFM shows large strip magnetic domains in the percolated system and allows a more direct investigation of the magnetic interaction among grains in the film with the lowest metal volume fraction.

Effects of Magnetic Interactions in Superparamagnetic Granular Films

Granular magnetic systems are formed by magnetic grains whose size is of the order of a few nanometers, embedded in a nonmagnetic (insulating or metallic) matrix. These ultrafine particle systems present size, shape, and anisotropy distributions, besides randomly orientated easy directions. Magnetic interactions always exist, being stronger or weaker according to the volume concentration and the matrix type. These systems have shown interesting magnetotransport properties, such as giant magnetoresistance and giant Hall effect. Owing to the inherent complexity of the nanostructure, the magnetization can be analytically calculated only in two limiting cases: when T = 0 (Stoner‐Wohlfarth model) or for high temperatures (Langevin model). The Langevin model presents very good results when applied at temperatures higher than the mean blocking temperature hTBi of the system. However, this adequacy can be just apparent: the obtained structural parameters are very different from the real ones, as we show in this w...