A.D.C. Viegas

CORRELATION BETWEEN MAGNETIC INTERACTIONS AND GIANT MAGNETORESISTANCE IN MELT-SPUN CO10CU90 GRANULAR ALLOYS

The effects of annealing on the structural, magnetic, and magnetotransport properties of melt-spun Co10Cu90 granular alloys were investigated. The interaction effects were studied from both remanent magnetization and magnetotransport data, using two different methods to reach the demagnetized state, ac and dc demagnetization. The analysis of the structural evolution and interaction strength between the magnetic clusters clearly shows the role of some structural parameters (particle size and density, interparticle distance) and the degree of magnetic correlation in the magnetic field response of the resistance in these inhomogeneous systems.

Negative remanent magnetization of fine particles with competing cubic and uniaxial anisotropies

The magnetic properties of noninteracting single-domain particles whose anisotropy is made up of a cubic magnetocrystalline and a uniaxial components were investigated. Various directions of the uniaxial anisotropy were considered and the dependencies of the reduced remanence as a function of the ratio between the two anisotropies were obtained. It was found that for sufficiently strong uniaxial anisotropy and random arrangement of the particle orientations, reduced remanence lower than 0.5 is, in general, an intrinsic property of the system due to the negative remanent magnetization of some of the particles.

Angular dependence of asymmetric magnetoimpedance in exchange biased NiFe/IrMn multilayers

We investigate the angular dependence of asymmetric magnetoimpedance of NiFe/IrMn multilayers and explore the possibility of tuning the linear region of the magnetoimpedance curves by modifying the angle between the applied external magnetic field and exchange bias field, and probe current frequency. We quantify the sensitivity by calculating the ratio |ΔZ|/|ΔH| at low magnetic fields as a function of the frequency for samples cut with different orientations and show that considerable values in the linear region around zero field can be reached. The results extend the possibilities for application of exchange biased magnetoimpedance multilayers as probe element for the development and improvement of auto-biased linear magnetic field sensors.

Structural and magnetic behavior of Ar+‐implanted Co/Pd multilayers: Interfacial mixing

The magnetization behavior of Co/Pd multilayers has been analyzed as a function of the degree of interfacial mixing among the Co and Pd layers. Controlled atomic mixing was induced by low‐dose and low‐flux ion implantation and a follow‐up of the structural status of the samples was made by simulation of the high‐angle x‐ray‐diffraction data. Values of the saturation magnetization as a function of the broadness of Co concentration profile are presented and explained by a simple model based on the parameters obtained from the x‐ray simulations.

Magnetoimpedance of single and multilayered FeCuNbSiB films in frequencies up to 1.8GHz

In this work, very high magnetoimpedance measured at frequencies up to 1.8GHz in nanostructured Fe73.5Cu1Nb3Si13.5B9∕Cu films is reported. Magnetoimpedance ratios of 280% and 50% were observed in multilayered and sandwiched samples, respectively. For the last one, the magnetoimpedance ratio exhibited a nearly constant value in the frequency range of 10–200MHz. The results are discussed in terms of the anisotropies present in all samples and by considering the connection of magnetoimpedance effects with ferromagnetic resonance phenomena.

Giant magnetoresistance and remanence in granular CoCu codeposited films

We present experimental magnetoresistance, magnetization, and remanence data for CoCu granular films obtained by electron beam codeposition, subsequently submitted to a thermal treatment. The magnetization measurements show coexistence of superparamagnetic and ferromagnetic (or blocked superparamagnetic) Co grains. By modeling the experimental magnetization curves, the distribution function of the superparamagnetic particle size, the volume fraction of the blocked part of the Co particles, and their saturation magnetization are obtained. δM plots are constructed using the remanence curves. A correlation between the changes of the δM parameters and those of the giant magnetoresistance, caused by the annealing, is discussed.

Study of CoFeSiB glass-covered amorphous microwires under applied stress

Magnetoimpedance-based ferromagnetic resonance (FMR) studies of annealed Co68.15Fe4.35Si12.5B15 glass-covered amorphous microwires under stress are reported. The applied stress modifies the anisotropy and the domain structure present in each sample, in such a way that it can be studied through magnetoimpedance measurements and FMR dispersion relations extracted from them. From the fitting of the FMR dispersion relations, the magnitude and the orientation of the transverse anisotropy field, as well as an insight on the micromagnetic structure of glass-covered microwires, were obtained. From these studies, it can be concluded that a longitudinal anisotropy dominates the microwire behavior under zero stress. By applying an increasing stress to the microwires, an inner core with a longitudinal anisotropy surrounded by an outer shell with a circumferential anisotropy develops and dominates its magnetic behavior.

Correlation between giant magnetoresistance and magnetic interactions in a CoAg multilayered/granular system

Magnetizing, demagnetizing, and remanent magnetization curves for [Co(12 A)/Ag(60 A)]×15 multilayered/granular films thermally treated have been measured. The changes of the giant magnetoresistance and the interaction effects have been explained as a result of the structural, morphological, and magnetic evolution of the samples as a function of the thermal treatment. It has been inferred that for samples annealed at temperatures lower than 360 °C, the changes of the giant magnetoresistance come from the misalignment of the magnetic moments of the Co particles formed during annealing. A strong correlation between the giant magnetoresistance and the magnetic interaction effects has been found for samples annealed at temperatures higher than 360 °C: the giant magnetoresistance is degraded as the demagnetizing interparticle interactions are increased.

Quantifying magnetic anisotropy dispersion : theoretical and experimental study of the magnetic properties of anisotropic FeCuNbSiB ferromagnetic films

The Stoner-Wohlfarth model is a traditional and efficient tool to calculate magnetization curves and it can provides further insights on the fundamental physics associated to the magnetic properties and magnetization dynamics. Here, we perform a theoretical and experimental investigation of the quasi-static magnetic properties of anisotropic systems. We consider a theoretical approach which corresponds to a modified version of the Stoner-Wohlfarth model to describe anisotropic systems and a distribution function to express the magnetic anistropy dispersion. We propose a procedure to calculate the magnetic properties for the anisotropic case of the SW model from experimental results of the quadrature of magnetization curves, thus quantifying the magnetic anisotropy dispersion. To test the robustness of the approach, we apply the theoretical model to describe the quasi-static magnetic properties of amorphous FeCuNbSiB ferromagnetic films. We perform calculations and directly compare theoretical results with longitudinal and transverse magnetization curves measured for the films. Thus, our results provide experimental evidence to confirm the validity of the theoretical approach to describe the magnetic properties of anisotropic amorphous ferromagnetic films, revealed by the excellent agreement between numerical calculation and experimental results.

Magnetic Characterization of Co Doped Cu

In this work we have studied the magnetic properties of room temperature electrodeposited Cu2O layers doped with Co. These layers showed anhysteretic ferromagnetic behavior with Curie temperature of 550 K. Structural characterization indicated the incorporation of Co+2 ions to the Cu2O lattice. No evidences for the existence of superparamagnetic particles and contaminations were observed, respectively, from (zero field cooling) ZFC and (field cooling) FC curves and (electron paramagnetic resonance) EPR measurements.