I. Betancourt

Correlations between low-field microwave absorption and magnetoimpedance in Co-based amorphous ribbons

Microwave power absorption measurements at 9.4 GHz were carried out on as-cast amorphous ribbons of nominal composition Co66Fe4B12Si13Nb4Cu. Two absorptions were observed: a small signal at a low dc field (<0.01T) and another one at a high dc field (∼0.1682T). The high-field signal shows all the features corresponding to ferromagnetic resonance. The low-field absorption (LFA) signal exhibits different characteristics such as hysteresis and a minimum in power absorption at zero magnetic field. A correlation between this LFA signal and magnetoimpedance measurements showed that both electromagnetic processes are associated with the same phenomenon.

High-resolution analytical imaging and electron holography of magnetite particles in amyloid cores of Alzheimer’s disease

Abnormal accumulation of brain metals is a key feature of Alzheimer’s disease (AD). Formation of amyloid-β plaque cores (APC) is related to interactions with biometals, especially Fe, Cu and Zn, but their particular structural associations and roles remain unclear. Using an integrative set of advanced transmission electron microscopy (TEM) techniques, including spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM), nano-beam electron diffraction, electron holography and analytical spectroscopy techniques (EDX and EELS), we demonstrate that Fe in APC is present as iron oxide (Fe3O4) magnetite nanoparticles. Here we show that Fe was accumulated primarily as nanostructured particles within APC, whereas Cu and Zn were distributed through the amyloid fibers. Remarkably, these highly organized crystalline magnetite nanostructures directly bound into fibrillar Aβ showed characteristic superparamagnetic responses with saturated magnetization with circular contours, as observed for the first time by off-axis electron holography of nanometer scale particles.

Magnetization dynamics of pinned domain walls in partially crystallized ribbons

The frequency dependence of the permeability of Co- and Fe-rich amorphous ribbons (Vitrovac 6025(R) and Metglas(R) 2605 SC, respectively), submitted to a variety of thermal treatments involving different degrees of crystallization, was measured in the 5 Hz-13 MHz frequency range. The quasi-static initial permeability, /spl mu//sub i/, and the domain wall relaxation frequency, f/sub x/, exhibited an inverse relationship, /spl mu//sub i//spl middot/f/sub x/=constant. These results are interpreted in terms of variations in the average distance between pinning points of domain walls, and the effects of such variations on the vibrational properties of pinned domain walls.

Mapping the magnetic and crystal structure in cobalt nanowires

Using off-axis electron holography under Lorentz microscopy conditions to experimentally determine the magnetization distribution in individual cobalt (Co) nanowires, and scanning precession-electron diffraction to obtain their crystalline orientation phase map, allowed us to directly visualize with high accuracy the effect of crystallographic texture on the magnetization of nanowires. The influence of grain boundaries and disorientations on the magnetic structure is correlated on the basis of micromagnetic analysis in order to establish the detailed relationship between magnetic and crystalline structure. This approach demonstrates the applicability of the method employed and provides further understanding on the effect of crystalline structure on magnetic properties at the nanometric scale.

Enhanced coercivity in REFeAl-based bulk amorphous alloys

A systematic study of die-cast REFeAl (RE=Nd+Pr) bulk amorphous alloys with small B, Cu, and Dy additions is presented. Differential scanning calorimetry and x-ray diffraction data are consistent with an amorphous microstructure and a small amount of hcp Nd-type crystallites embedded in the amorphous matrix for B, Cu+B-, and Dy-containing alloys. Coercivity, Hc, exhibits a progressive enhancement upon B, B+Cu, and Dy addition (from 284 kA/m for Nd30Fe60Al10 up to 388 kA/m for Dy substituted alloy). This Hc improvement is associated not only with domain wall pinning on the crystallites, but also with the increase of nucleation sites and thus, with the number of RE and RE–Fe–Al clusters. In agreement with the latter, a superparamagnetic behavior, which is characteristic of a clustered structure, was observed above 451 K for all the alloy compositions.

Giant magnetoimpedance, skin depth and domain wall dynamics

Summary form only given, as follows. Giant magnetoimpedance (GMI) has raised a strong interest due to its technological applications in magnetic field and dc current sensors. In this work, a detailed analysis of the impedance response of a typical material (CoFeBSi amorphous wires with high circumferential permeability and low negative magnetostriction) on a wide frequency range, is presented. By using the quasistatic circumferential permeability (actually measured on samples at 100 kHz) the skin depth is calculated, which is larger than the wire diameter for frequencies up to tens of kHz. The analysis of the real and imaginary contributions to impedance in this frequency range is consistent with a magnetization process dominated by domain wall movements. For frequencies higher than the domain wall relaxation, where only the magnetization process is spin rotation, the behavior of impedance can be modeled by means of the general expression based on first order Bessel equations. Impedance calculated in this way, showed a good agreement with experimental results. The domain wall relaxation process was also studied; its unusually large frequency extension can be explained in terms of the Foucault microcurrents generated by the wall movements, which give an additional damping factor.

Domain model for the magnetoimpedance of metallic ferromagnetic wires

Complex inductance formalism (L) is used to calculate the complex circular permeability (μcirc) in a domain model for the magnetoimpedance (MI) of soft ferromagnetic wires. An excellent agreement between calculated and experimental values of μcirc as a function of frequency is observed. In addition, a very good agreement is also exhibited between experimental and calculated plots of μcirc as a function of an applied dc magnetic field before and above the relaxation frequency (also known as single- and double-peak MI effect). These results confirm the validity of L as an alternative approach to MI phenomena in soft ferromagnetic wires.

Giant magnetoimpedance in Co-based microwires at low frequencies (100 Hz–13 MHz)

A systematic study of the complex inductance response and the giant magnetoimpedance effect of as-cast CoFeBSiMoNi microwires (30 μm diameter) as a function of frequency (100 Hz–13 MHz) and circular field amplitude (21–212 A/m rms on the wire’s surface) is presented. The microwire magnetization mechanisms are discussed in terms of complex inductance plots (in both real and imaginary parts). The analysis of the experimental results showed evidence of pinning, bulging, and displacement of circumferential domain walls. Higher relaxation frequencies together with a larger unpinning field (3–5×106 Hz and 127 A/m, respectively), compared with conventional amorphous wires, were explained in terms of the reduced dimension of the microwire. Total impedance plots as a function of a bias Hdc field showed an asymmetric character associated with an induced anisotropy during the wire fabrication.

Magnetocaloric effect in as-cast Gd1−xYx alloys with x = 0.0, 0.1, 0.2, 0.3, 0.4

In this report, we present the magnetocaloric effect of Gd1−xYx alloys (0.0 ≤ × ≤ 0.4) prepared by arc-melting from high purity Gd and Y precursors in inert atmosphere. The formation of Gd1−xYx solid solutions was verified by means of X-ray diffraction analysis across the compositional series; also, residual secondary phases Gd and Y were present. Magnetic characterization performed by Vibrating Sample Magnetometry at a maximum applied field of 3.0 T showed a drastic reduction of the magnetization saturation (from 233 emu/g for x = 0.0 to 183 emu/g for x = 0.4), due to a dilution effect of the Y alloying, together with a marked Curie temperature decrease from 296 K to 196 K between x = 0.0 and x = 0.4. The second-order character of the magnetic transition was established by Arrot plots for all the cases. On the other hand, the magnetic entropy variation, determined from numerical integration of Maxwell relation displayed excellent values above 5.30 J/kg K for alloys with x < 0.3 due to the steep transitio...

Magnetic Properties of B-Rich Nano-Composite REyTM 90-y-xNbxB10 Alloys

Abstract We report and discuss the results of a study of the structures and magnetic properties of B-rich (10 at%) nanocomposite alloys, based on the formula RE y TM 90-y-x Nb x B 10 (RE=Nd, Nd+Pr, TM=Fe, Fe+Co; y=8,10,12, x=0,2,4) and processed by devitrification of melt-spun amorphous precursors. Considerable enhancement of the intrinsic coercivity, together with good to excellent remanence and energy density values were observed for Co-containing compositions with y =10 and x =2,4. The intrinsic coercivity and remanence enhancements are ascribed to the grain refining effects of Nb, particularly with respect to the soft magnetic phases, and thus to more complete exchange coupling of the soft grains to the hard phase grains than for the Nb-free alloy.