L. F. Schelp

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.

The use of Simmons’ equation to quantify the insulating barrier parameters in Al/AlOx/Al tunnel junctions

We have analyzed the electron transport processes in Al/AlOx/Al junctions. The samples were produced by glow-discharge-assisted oxidation of the bottom electrode. The nonlinear I–V curves of 17 samples were measured at room temperature, being very well fitted using the Simmons’ equation with the insulating barrier thickness, barrier height, and the junction area as free parameters. An exponential growth of the area normalized electrical resistance with thickness is obtained, using just values from I–V curve simulations. The effective tunneling area corresponding to the “hot spots” can be quantified and is five orders of magnitude smaller than the physical area in the studied samples.

Wide frequency range magnetoimpedance in tri-layered thin NiFe/Ag/NiFe films: Experiment and numerical calculation

Given that the magnetoinductive effect (MI), skin effect and ferromagnetic resonance influence magnetic permeability behavior at different frequency ranges, the description of the magnetoimpedance effect over a wide range of frequency becomes a difficult task. To that end, we perform an experimental investigation of the magnetoimpedance effect in a tri-layered thin film over a wide frequency range. We compare the experimental results for a tri-layered thin film with numerical calculus performed using an approach that considers a magnetic transverse susceptibility model for planar systems and an appropriate magnetoimpedance model for a tri-layered system together. The results show remarkably good agreement between numerical calculus and experimental measurements. Thus, we discuss the experimental results in terms of different mechanisms that govern the MI changes observed in distinct frequency ranges and provide experimental support to confirm the validity of the theoretical approach.

Magnetoimpedance effect in structured multilayered amorphous thin films

In this work, we report magnetoimpedance (MI) results for frequencies of up to 1.8 GHz in a series of as deposited nano-structured [F/Cu/F] and [F/i/Cu/i/F] films where F is a Fe-rich alloy/Cu multilayer and i is a SiO2 insulating layer. A high MI ratio (205%) was obtained at 180 MHz for the [F/i/Cu/i/F] structure. The results are analysed in terms of the SiO2 insulating layer geometry and of the widths of the ferromagnetic and conductive layers.

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.

High frequency magnetic behavior through the magnetoimpedance effect in CoFeB/ (Ta, Ag, Cu) multilayered ferromagnetic thin films

Abstract We investigate the structural and magnetic properties and the magnetoimpedance effect of CoFeB/(Ta, Ag, Cu) multilayered thin films grown by magnetron sputtering. It is noticed that the peak of maximum magnetoimpedance ratio values for a given frequency, as well as its position in frequency, is varied according to the used non-magnetic metal. For the CoFeB/Ta and CoFeB/Ag multilayered films, peaks of 30% and 27%, respectively, are observed at localized frequency ranges. For the CoFeB/Cu multilayer, slightly smaller values are verified, but for a wide frequency range. The magnetoimpedance results are analyzed in terms of the mechanisms responsible for the impedance variations at different frequency ranges and the magnetic and structural properties of the multilayer.

Giant magnetoimpedance in glass-covered amorphous microwires at microwave frequencies

In this work, results on the giant magnetoimpedance of glass-covered amorphous microwires with nominal composition Co70.4Fe4.6Si15B10 are presented. The impedance Z=R+iX has been investigated as function of frequency (1 MHz–1.8 GHz) and magnetic field (up to ±400 Oe), using a HP4396B impedance analyzer and an appropriate coaxial microwave cavity. The effects of the thermal treatments (Joule heating, from 1 up to 50 mA for 10 min) for anisotropy induction and/or to induce recrystallization have been investigated.

Barkhausen noise measurements in materials with vanishing magnetoelastic anisotropies

In this work, Barkhausen noise measurements are shown for a series of amorphous ribbons: Metglas 2605TCA, Metglas 2705M, and Finemet materials with composition Fe73.5Cu1Nb3Si18.5B4 under different stress (σ) and annealing temperature (TA) conditions. The dynamical state of the system is characterized by the power spectra and amplitude distribution functions as well as by the Minkowski–Bouligand dimension for each set of samples. Special attention has been paid to the ranges of vanishing anisotropies: low applied stress for the Metglas samples and the nanocrystallized phase of Finemet materials.

Ferromagnetic resonance linewidth mechanisms in annealed CoFeSiB glass-covered microwires

Magnetic relaxation of CoFeSiB glass-covered microwire under applied stress and Joule heating annealing was investigated by ferromagnetic resonance (FMR) linewidth experiments. We were able to identify the main damping mechanisms and to quantify these damping terms, showing the effect of annealing and applied stress on the damping terms. The study has shown that there are three main damping mechanisms responsible for the FMR linewidth: the Gilbert damping parameter, a damping mechanism due to anisotropy dispersions and two-magnon scattering. The Gilbert damping parameter is constant and not influenced by annealing and stress. The FMR linewidth is very sensitive to anisotropy dispersions and this mechanism has a considerable contribution to magnetic relaxation. The two-magnon scattering is an assignment of the inhomogeneities present in the samples.

Spin-dependent tunneling in Co/insulating granular systems: an AC approach

Abstract The spin-dependent tunneling of cobalt clusters embedded in Al2O3 or SiOx has been analyzed as a function of the frequency at room temperature. Two sets of samples, with one or several layers of clusters, have been produced by alternate physical deposition of the metal and the insulator. The impedance versus frequency curves were measured with and without an external magnetic field. The results suggest that when the distance between successive cluster layers is small, some correlations between the cluster positions are present.