M. Carara

Magnetization dynamics as derived from magneto impedance measurements

In this work, the magnetization dynamics of soft magnetic materials is studied with the aid of transverse differential permeability μ(Iac,f,Hdc) spectra. Contributions to the magnetization processes from domain wall motion and rotation of the magnetization can be extracted from the transverse differential permeability data which are in turn obtained from impedance Z(Iac,f,Hdc) spectra. In particular, an iteration method is used to extract μ(Iac,f,Hdc) from Z(Iac,f,Hdc) data. The approach is tested in samples with a very well known domain structure, namely (110)[001]FeSi3%. Permeability spectra μ(Iac,f,Hdc) were obtained in the frequency range (100 Hz⩽f⩽100 kHz), probe current range (0.1⩽Iac⩽50 mA) and dc magnetizing field range (0⩽Hdc⩽500 Oe). It is shown that the method developed in this article can be efficiently used to identify and study different dynamic processes driven by the probe current and controlled by the external dc field. In particular, it is shown that the method provides the tools to sepa...

Co/Ag multilayer film: Role of annealing on the magnetic properties

Co(15 A)/Ag(60 A) multilayers produced by electron beam deposition on a 50 A chromium buffer layer over a Si(111) wafer have been studied by magnetoresistance, saturation magnetization, coercivity, and anisotropy. Annealing at various temperatures produces striking effects on the observed physical properties, such as an improvement on the value of the magnetoresistance associated to a ‘‘back‐diffusion’’ process in the Co/Ag interfaces. This leads one to believe that interface roughness is indeed most important to understanding the connection between giant magnetoresistance and antiferromagnetic coupling, as well as the behavior of coercivity vs interface anisotropy in these materials.

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.

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.

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.

Annealing of Co/Ag multilayers

Abstract We have measured the magnetoresistance and magnetic properties of annealed Co/Ag multilayers. The results show an increase of the magnetoresistance associated to lower interface roughness for these immiscible elements. Higher temperature anneals lead to destruction of the multilayer, but still showing giant magnetoresistance for the composite formed.

Comparison between ac and dc current annealing in CoFeSiB glass-covered amorphous microwires

In this work, we present a study on the magnetic properties of Joule-heated CoFeSiB glass-covered amorphous microwires under applied stress. A comparison between the magnetic properties of four samples is made: (i) as cast, Joule-heated under (ii) dc current or ac current of (iii) 100 Hz or (iv) 500 Hz. The stress modifies the anisotropy and the domain structure in such a way that it can be studied through magnetoimpedance measurements and ferromagnetic resonance (FMR) dispersion relations (DR) extracted from them. From the fitting of the FMR DR, the magnitude and the orientation of the transverse anisotropy field, as well as an insight into the micromagnetic structure of the microwires were obtained. It was concluded that longitudinal anisotropy dominates the microwire behaviour under zero stress. By applying an increasing stress to the microwires, the inner core with longitudinal anisotropy is surrounded by two concentric outer shells with almost circumferential anisotropies. These magnetic configurations can be explained in terms of the residual stress arising from the wires manufacturing procedure and the additional applied stress.

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.

Magnetization process and magnetoimpedance in (110)[001]FeSi3%

The present article investigates the relation between magnetoimpedance (MI) and the evolution of the magnetic domain structure along the magnetization curves of commercial FeSi3% sheets cut at different angles with respect to the easy magnetization axis [001]. The role of the magnetocrystalline anisotropy on MI is studied and the correlation between the basic features of the MI vs H and magnetization (M) vs H curves is established. From data obtained at appropriate frequency ranges, the frequency dependence of the effective transverse differential permeability is obtained. The frequency and field spectra of the impedance allow the identification of the rotation of the magnetization as the main contribution of the magnetization processes to the MI effects in the studied samples.

Stress level in Finemet materials studied by impedanciometry

In this work, a study of the stress relief in Finemet ribbons, Fe73.5Cu1Nb3Si16.5B6, as a function of the annealing temperature is presented. The as melt-spun samples are amorphous and become partially crystallized after annealing at appropriate temperatures. For temperatures TA⩾480 °C the samples are nanocrystalline, with a microstructure composed by α-Fe1−xSix (x∼0.2) crystallites (10 nm average diameter) embedded in an amorphous matrix. Nanocrystallization, associated with stress relief effects, improves the soft magnetic properties of this kind of material. The stress level was quantified using magnetorestriction (measured by SAMR), magnetoelastic anisotropy, and domain wall energy data obtained from impedance spectra measurements. A reduction of the internal stress from 15 to 0.2 MPa was verified when comparing the as-cast to the sample annealed at 580 °C. Improvement of the magnetic softness of the samples was also followed by the increase of the domain wall and magnetization rotation contributions ...