Horia Chiriac

The remagnetization process in thin and ultra-thin Fe-rich amorphous wires

A comparative study of the remagnetization process of highly-magnetostrictive as-cast Fe-rich amorphous wires having different diameters (from 165 to 10 μm) and lengths (between 10 cm and 2 mm) is here reported for the first time. Wires obtained by conventional in-rotating-water quenching (165 down to 70 μm diameter) and by a modified Taylors technique (2.5 μm glass cover on 10 to 15 μm diameter magnetic core) are studied. Macroscopic and local hysteresis loops are measured by induction technique with the help of large and very tiny pick up coils respectively. From the latter, the magnetization profile of these magnetically bistable wires is determined. The critical length to observe a single and large Barkhausen jump when remagnetizing decreases with the diameter of the sample. In particular, for the wires covered by glass the critical length to observe spontaneous bistability is less than 2 mm. Values of those critical lengths are interpreted taking into account the compromise between magnetostatic and magnetoelastic energy terms.

Preparation and characterization of glass covered magnetic wires

Abstract The amorphous glass covered magnetic wires (AGCW) are of great interest due to their specific magnetic properties that can be controlled through the composition as well as through the metallic core diameter and the thickness of the glass cover. The specific magnetic properties are determined by the magnitude of the magnetic anisotropies induced by the mechanical stresses due to the preparation process. This paper presents a series of conditions that are determinant for the preparation process of the AGCW. The basic magnetic characteristics of the highly positive, negative and nearly zero magnetostrictive AGCW are presented as a function of composition and samples state. Recently performed FMR measurements showed that the metal–glass interface influences the magnetic characteristics including the magnetic anisotropy. The magnetic domain structure of the highly positive AGCW presents a single domain axially magnetized with some closure domains at the ends of the wire while for the wires obtained after the glass removal it presents an axially magnetized inner core with a typical maze domain configuration at the surface of the wire.

Ni-Ag thin films as strain-sensitive materials for piezoresistive sensors

Abstract Some results concerning the electrical, electromechanical and structural properties of Ni x –Ag 1− x (for x values between 0.35 and 0.50) thin films in view of their utilization for manufacturing pressure and force sensors are presented. As compared to the well known Ni–Cu (constantan) alloys, which are widely used for these type of sensors, Ni x –Ag 1− x thin films exhibit gauge factor values of the same order of magnitude, but they are much more corrosion resistant and adherent to the substrate. The influence of composition and post-deposition annealing on the electrical resistance, temperature coefficient of resistance and gauge factor of Ni x –Ag 1− x thin films is discussed.

Large Gyromagnetic Effect In FeSiB Amorphous Wires

Results on a new effect observed in FeSiB amorphous wires-the large gyromagnetic effect-are reported for the first time. This effect consists in the appearance of a rotation of the sample around its axis with frequencies from 1 to 60 Hz when the sample is subjected to an axial alternating magnetic field of /spl sim/10/sup 2/ A/m having a frequency of /spl sim/10/sup 4/ Hz. Both the sample rotation frequency and the frequency of the field at which the effect appears depend on the sample length, the characteristics of this dependence indicating that the appearance of the effect is closely related to the formation of a standing wave associated to the oscillation of the magnetic moments. The magnitude of the wires magnetostriction constant is also essential as concerns the appearance of the large gyromagnetic effect.

Fe-based amorphous thin film as a magnetoelastic sensor material

Abstract The aim of this paper is to analyze some Fe-based amorphous thin films as sensing elements for magnetoelastic microsensors. Fe-based amorphous thin films with thickness ranging from 0.1 to 1 μm are prepared by r.f. sputtering method. Their magnetic properties (saturation magnetization M s , magnetic anisotropy constant k u and Curie temperature T C ), determining the performance of the material used as magnetoelastic sensing element, are investigated in correlation with the magnetoelastic properties (saturation magnetostriction λ s , magnetoelastic coupling coefficient b γ ,2 , magnetostrictive strain coefficient D and Δ E -effect). The magnetic and magnetoelastic characterizations of the samples are performed using a torque magnetometer and a capacitive cantilever technique, respectively. Based on the obtained results, we discuss the specific qualities of the Fe-based amorphous thin films in terms of their relevant material characteristics for magnetoelastic sensing application in comparison to the conventional magnetostrictive thin films presently in use.

Magnetic domain structure in amorphous glass-covered wires with positive magnetostriction

Experimental investigation of the domain structure in Fe/sub 77.5/Si/sub 7.5/B/sub 15/ amorphous glass-covered wires by Kerr microscopy is reported for the first time. A single domain configuration with axial easy axis was observed in glass-covered samples. After glass removal, a maze domain configuration appears at the wire surface, while the axially magnetized domain still exists within the wires inner region.

Magneto-surface-acoustic-waves microdevice using thin film technology: design and fabrication process

Abstract In this paper we describe the design and fabrication process of a magneto-surface-acoustic-waves (MSAWs) microdevice using Fe 70 B 15 Si 15 amorphous thin films and Fe 70 B 15 Si 15 /SiO 2 -type multilayers as active media. This simple and small device, able to be used as a sensor or implemented as a part of the magnetomechanical sensing integrated systems, is fabricated using thin film technology combined with photolithographic techniques. In order to use magnetostrictive Fe-metalloid amorphous thin films and (Fe-based magnetostrictive alloy/insulator) N multilayers as active media for (MSAWs) microdevices, the magnetic (saturation specific magnetization σ s and magnetic anisotropy constant k u ) and magnetoelastic (ME) (saturation magnetostriction λ s , magnetoelastic coupling coefficient b γ ,2 , magnetostrictive strain coefficient D and Δ E / E ratio) properties which determine their performance are studied. The possible use of MSAWs microdevice fabricated by us in microsensors applications is illustrated.

Development of Fe–Nb–Cr–B Glassy Alloys With Low Curie Temperature and Enhanced Soft Magnetic Properties

Fe_67.7Nb_0.3Cr₁₂B₂₀ glassy melt-spun ribbons and glass-coated microwires with T_C around 310-320 K (~35-45°C) (the lowest ever reported for metallic glasses) have been prepared by rapid solidification. Excepting the thick melt-spun ribbons of 40 μm, all samples have a glassy structure in the as-quenched state, consisting of very small Fe-Cr and boride (Fe₃B) clusters of sizes ranging from several nanometer up to several tens of nanometer, embedded within the residual amorphous matrix. The ferromagnetic behavior of the glassy materials deteriorates drastically with an increase in the temperature. The melt-spun ribbons become paramagnetic above 34-35°C, whereas the transition temperature for the glass-coated microwires is around 45°C, because of the very specific magnetic domain structure and magnetic anisotropy distribution. The observed stable behavior of low <i>T</i>_C Fe_67.7Nb_0.3Cr₁₂B₂₀ glassy alloys up to very high frequencies (10 GHz) is another important feature for sensing and biomedical applications. The temperature sensor based on low <i>T</i>_C Fe_67.7Nb_0.3Cr₁₂B₂₀ glassy melt-spun ribbons has a very good sensitivity in a narrow temperature range (about 1°C).

Electrochemical deposition of FeGa/NiFe magnetic multilayered films and nanowire arrays

Results concerning the preparation and characterization of electrochemically deposited (Fe1−xGax∕Fe1−yNiy)n (x=0.1–0.3at.%; y=0.4–0.8at.%) multilayered films and nanowires arrays are reported for the first time. The layers have been deposited successively by changing the electrodeposition potential. The combination of Fe1−xGax magnetostrictive material and Fe1−yNiy soft magnetic material shows good magnetic softness (Hc does not exceed 60Gs) and gives novel magnetostrictive behavior caused by the formation of twisted spin structures.

Domain Wall Propagation in Nearly Zero Magnetostrictive Amorphous Microwires

Results on the investigation of the propagating 180deg domain walls in bistable amorphous glass-coated microwires with nearly zero magnetostriction are reported for the first time. As-cast glass-coated microwires are bistable only if their metallic nucleus diameter is larger than 20 mum. Glass removal induces bistability in microwires with metallic nucleus diameters below 20 mum. Nearly zero magnetostrictive glass-coated microwires display larger domain wall velocities and mobilities as compared to positive magnetostrictive microwires. Samples that become bistable after glass removal display smaller values of the wall mobility as compared to as-cast bistable microwires. Mobility can be increased by annealing. The experimental results have been explained based on the damping mechanisms of the domain wall motion, specifically on the spin relaxation damping, whose coefficient is proportional to the anisotropy constant from the microwires inner core. Stress relief determined by glass removal and annealing have been considered. The results are important for future applications of nearly zero magnetostrictive microwires in spintronic devices.