R.B. da Silva

Tailoring the magnetoimpedance effect of NiFe/Ag multilayer

The magnetoimpedance (MI) effect was investigated in NiFe/Ag multilayered (ML) and ML/SiO2/Ag/SiO2/ML structured multilayered (SD) ferromagnetic films grown by magnetron sputtering. The MI measurements were performed with an impedance analyzer over a wide frequency range, from 10 MHz to 1.8 GHz. Sample geometries are mainly responsible for the different MI behaviours and by considering the entire frequency range, distinct mechanisms responsible for MI changes were associated. For the ML sample, a maximum value of 80%, associated with the appearance of ferromagnetic resonance (FMR), was reached at around 1 GHz. For the SD sample, the striking feature is the existence of two distinct frequency ranges with high MI% values of 80% at around 100 MHz, related to the skin and magnetoinductive effects, and of 120% at around 1 GHz, associated with the strong skin and FMR effect.

High frequency magnetoimpedance in Ni81Fe19/Fe50Mn50 exchange biased multilayer

Gigahertz magnetoimpedance (MI) curves obtained in an exchange biased Ni81Fe19/Fe50Mn50 multilayer are reported. Experimental MI curves are shifted by the exchange bias field (HEB), following the features presented by the hysteresis curves. Theoretical MI curves, calculated using the classical expression for the impedance of a planar magnetic conductor, describe well the experimental data. The results open possibilities for application of exchanged biased MI multilayered materials for the development of autobiased linear magnetic field sensors.

Thickness dependence of the high-frequency magnetic permeability in amorphous Fe73.5Cu1Nb3Si13.5B9 thin films

Field-dependent transverse permeability characterization of amorphous thin films with nominal composition Fe73.5Cu1Nb3Si13.5B9 was performed for frequencies in the range of 100kHz–1.8GHz. Dynamic and static magnetic properties were investigated in films with thickness in the range from 21to5000nm. Samples with thicknesses below 85nm exhibit a well-defined in-plane uniaxial anisotropy and uniform ferromagnetic resonance modes. Samples thicker than 85nm were found to be magnetically isotropic in the plane, with complex magnetic dynamics depicted by several ferromagnetic resonance modes detected at relatively low fields. The results are discussed in terms of the stress contribution to the magnetic anisotropy of the samples.

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.

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.

Magnetoimpedance effect at the high frequency range for the thin film geometry: Numerical calculation and experiment

The magnetoimpedance effect is a versatile tool to investigate ferromagnetic materials, revealing aspects on the fundamental physics associated to magnetization dynamics, broadband magnetic properties, important issues for current and emerging technological applications for magnetic sensors, as well as insights on ferromagnetic resonance effect at non-saturated magnetic states. Here, we perform a theoretical and experimental investigation of the magnetoimpedance effect for the thin film geometry in a wide frequency range. We calculate the longitudinal magnetoimpedance for single layered, multilayered or exchange biased systems from an approach that considers a magnetic permeability model for planar geometry and the appropriate magnetic free energy density for each structure. From numerical calculations and experimental results found in literature, we analyze the magnetoimpedance behavior, and discuss the main features and advantages of each structure. To test the robustness of the approach, we directly compare theoretical results with experimental magnetoimpedance measurements obtained in a wide range of frequencies for an exchange biased multilayered film. Thus, we provide experimental evidence to confirm the validity of the theoretical approach employed to describe the magnetoimpedance in ferromagnetic films, revealed by the good agreement between numerical calculations and experimental results.

Origin of spin-glass and exchange bias in La1∕3Sr2∕3FeO3−γ nanoparticles

The structure and the magnetic properties of nanopowdered samples of La1∕3Sr2∕3FeO3−γ with average particles size d in the range of 67-367 nm prepared by a sol-gel method were investigated in detail. The samples were characterized by X-ray diffraction, scanning electron microscopy, specific heat, Mossbauer spectroscopy, ac susceptibility, and magnetization measurements. Exchange bias with vertical magnetization shift was found in all samples. Charge ordering and antiferromagnetism were observed close to 200 K for large particles (d ≥ 304 nm) samples, while for particles with intermediated and smaller values (d ≤ 156 nm) a cluster-glass like behaviour and a short range charge ordering were seen near 115 K and 200 K, respectively. The spin-glass like and exchange bias behaviour in nanopowdered samples of La1∕3Sr2∕3FeO3−γ are associated to compact Fe3+ antiferromagnetic (AF) clusters caused by an oxygen deficiency, which was found to be higher in the samples with the smallest average particles sizes. The eff...

Discrimination between coupling and anisotropy fields in exchange-biased bilayers

In the framework of models that assume planar domain wall formed at the antiferromagnetic part of the interface of exchange-biased bilayers, one cannot distinguish between the cases of high or low ratios between the coupling and the antiferromagnet’s anisotropy fields by using hysteresis loop measurement, ferromagnetic resonance, anisotropic magnetoresistance, or ac susceptibility techniques applied on one and the same sample. The analysis of the experimental data obtained on a series of FeMn/Co films indicated that once the biasing is established the variation in the coercivity with the FeMn layer thickness could be essential for solving this problem. If the coercivity decreases with the thickness then the interlayer exchange coupling is the parameter that varies while the domain-wall energy of the antiferromagnet remains practically constant.

Domain structure in Joule-heated CoFeSiB glass-covered amorphous microwires probed by magnetoimpedance and ferromagnetic resonance

Impedance spectra (100 kHz⩽f⩽1.8 GHz) were measured at different magnetic fields, and magnetoimpedance and magnetization measurements were performed in as-produced and Joule-heated Co70.4Fe4.6Si10B15 glass-covered amorphous microwires. From the magnetization curves and ferromagnetic resonance features obtained from the impedance spectra, it was possible to propose a domain structure in the as-produced microwires, as well as to follow its evolution with the annealing current. The inner core and outer shell domain structure were observed. The outer shell domains evolve from a circumferential, in the as-cast sample, to a helical structure in the case of the annealed samples. On the other hand, the inner core evolves from a domain structure typical of a radial anisotropy to a longitudinal one. This domain structure evolution is explained in terms of the combined effects of the stress, promoted by the annealing temperature, and the magnetic field caused by the annealing current.