Helical Magnetic Anisotropy Research Articles

Magnetic means for determining torsional yield strength

The presence of a unique residual stress distribution at zero applied torque, in shafts that have been previously subjected to torque levels high enough to cause yielding, is made apparent by readily detected helical magnetic anisotropy in the surface regions. Following application of ever higher torques, and upon relaxation to zero torque, consecutive current pulses of respectively opposite polarity are conducted axially through the sample shaft. The appearance and rapidly rising values of integrated EMFs induced in a small encircling search coil signifies the presence of axial components in the remanent magnetization being reversed by the circumferential field of the current.

Torsional impedance effect in Fe-rich amorphous wires

An amorphous ferromagnetic wire with a highly positive saturation magnetostriction constant, made of Fe 73.5Si 13.5B 9Nb 3Cu 1, was simultaneously submitted to both, an AC current flowing through it and a torsional stress, in order to induce a helical magnetic anisotropy that modifies the magnetic domain structure and therefore the magnetic response of the sample. We study the torsion-impedance effect in an Fe-rich amorphous wire when it is submitted to different applied torsional stresses in both senses, up to 0.8 π rad/cm, as a function of the drive current frequency, in the range of 10 Hz–2 MHz. The experimental results are explained on the basis of the core-shell magnetic structure model combined with the skin effect.

Torsional stress dependence of reactance and resistance in Fe-rich amorphous wires at low frequencies

Amorphous ferromagnetic wire with a highly positive saturation magnetostriction coefficient, made of Fe 73.5Si 13.5B 9Nb 3Cu 1, was simultaneously submitted to both, an AC current passing through it and a torsional stress, in order to induce a helical magnetic anisotropy in the wire that modifies its domain structure and therefore the magnetic response of the sample. The aim of this work is to study the reactive, resistive and impedance behaviour in this Fe-rich wire, submitted to different applied torsional stresses, and for several values of the AC current amplitude through the wire (5–20 mA rms), in the low frequency range, f<MHz. The experimental results are explained on the basis of the core-shell model.

Effect of annealing under torsion stress on the field dependence of the impedance tensor in amorphous wires

The total impedance tensor in a 120 μm-diameter (Co 0.95Fe 0.05) 72.5Si 12.5B 15 amorphous wire subjected to annealing under torsion stress has been measured as a function of a DC axial field H ex with the DC bias current I as a parameter. Helical magnetic anisotropy has been induced by heat treatment under torsional stress. The modification to the impedance-field characteristics caused by a DC bias current has been investigated in conjunction with the static magnetisation processes. The combination of the helical anisotropy and the DC bias field induced by a current I results in asymmetry in the static magnetic configuration. The modifications in the impedance plots due to I are of the same kind as those for the DC magnetisation loops: with increasing I the hysteresis loop becomes narrower and asymmetrical. This shows clearly that the asymmetry in the impedance-field (MI) behaviour is related to the static magnetic structure that originates from a combination of the helical anisotropy and DC circumferential field. Introducing asymmetry enhances the MI sensitivity and is also important to realise linear sensing.

Effect of Applied Mechanical Stressses on the Impedance Response in Amorphous Microwires with Vanishing Magnetostriction

The effect of external (torsion and tensile) stresses on the electrical impedance of nearly-zero magnetostrictive amorphous microwires (conventional and glass covered) is reported. Glass coated Co 68.5 Mn 6.5 Si 10 B 15 microwire exhibits a maximum relative change in magnetoimpedance ratio up to around 130% at a frequency of 10 MHz, magnetic dc field of about 180 A/m and under tension of 60 MPa. This giant magnetoimpedance (GMI) effect of the microwire is affected by the magneto-elastic anisotropy induced in the sample by applying tensile stress. In addition, from the linear variations of the magnetic field H m corresponding to the maximum AZ/Z ratio, with the applied tensile stress, that is the H m (σ) curve, the magnetostriction constant of this amorphous microwire (λ s -2 × 10 -7 ) is estimated. The torsion giant impedance (TGI) ratio (ΔZ/Z) ξ has been investigated in as-cast and annealed (Co 0.94 Fe 0.06 ) 72.5 B 15 Si 12.5 conventional amorphous microwire. As-cast conventional microwire shows a slightly asymmetric Torsion Giant Impedance (TGI) and such asymmetry can be eliminated by current annealing or enhanced by current annealing under torsion. The asymmetry of (ΔZ/Z) ξ in the as-cast state and after torsion annealing could be ascribed to the spontaneous or induced helical magnetic anisotropy, which can be compensated by the application of certain torsional stress.

Equation of the domain wall motion in amorphous ribbon with helical magnetic anisotropy

The magnetic field of eddy currents corresponding to the Matteucci emf was included in the equation of the domain-wall motion while magnetizing amorphous ribbons with helical magnetic anisotropy. The obtained equations determine the dependence of the damping coefficient β r on the domain wall coordinate. A sharp maximum of β r( x) in the middle section of the domain wall run is discovered.

The magnetizing mechanism of the amorphous ribbons with the helical magnetic anisotropy

A helical magnetic anisotropy in amorphous ribbon has been induced by annealing in helical magnetic field. The ribbon magnetizing with the help of large Barkhausen jump in the ‘sandwich structure’ model is realised by the appearing two domain walls near the middle of the ribbon and their next movement to the opposite ribbon surfaces. The domain wall velocity near the middle of the ribbon is higher by an order of magnitude than near the ribbon surface.

Influences of the helical anisotropy on the bistable behaviour of amorphous wires

We investigate coercive field Hc and switching field H∗ for three compositions of bistable amorphous wires: FeSiB, CoFeSiB and CoSiB. When a strong helical magnetic anisotropy is induced in the wires, Hc and H∗ depend on applied torsion in a stair-like way. The effect is explained as a consequence of finite circumference of a wire and a helical structure of magnetic domains.

Magnetic anisotropy in FeSiB amorphous glass-covered wires

Results on the helical magnetic anisotropy in positive magnetostrictive amorphous glass-covered magnetic wires are presented. Measurements of the circumferential magnetization component in Fe/sub 77.5/Si/sub 7.5/B/sub 15/ amorphous glass-covered wires show the existence of a non diagonal anisotropy component that determines an inclination of the easy axis of magnetization in the inner region of as-cast wires. Applied torque leads to an increase of the azimuthal magnetization and contributes to the increase of the easy axis inclination, determining an increase of the switching field at which the axial magnetization reversal occurs in these wires. The torsion sensitivity of wires after glass removal is higher than that of glass-covered wires.

Helical magnetic anisotropy induced by current annealing under torsion in Co-rich amorphous wires

The structural relaxation and helical magnetic anisotropy induced by current annealing in Fe-based amorphous alloy wires are investigated. Measurements of these effects have been carried out using the variation of the hysteresis loop and the inverse Wiedemann effect, respectively. Annealing at 550 mA for 1 min results in a fully square hysteresis loop, indicating a complete relaxation. Helical magnetic anisotropy has been induced by annealing under a constant torsional strain. The influence of preannealing and relaxation on the helical anisotropy was also investigated. The behavior of the helical magnetic anisotropy seems to confirm the assumption of separability of the helical anisotropy into at least two terms of opposite directions. >

Helical magnetic anisotropy induced by current annealing under torsion in amorphous wires

The structural relaxation and helical magnetic anisotropy induced by current annealing in Fe-based amorphous alloy wires are investigated. Measurements of these effects have been carried out using the variation of the hysteresis loop and the inverse Wiedemann effect, respectively. Annealing at 550 mA for 1 min results in a fully square hysteresis loop, indicating a complete relaxation. Helical magnetic anisotropy has been induced by annealing under a constant torsional strain. The influence of preannealing and relaxation on the helical anisotropy was also investigated. The behavior of the helical magnetic anisotropy seems to confirm the assumption of separability of the helical anisotropy into at least two terms of opposite directions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magnetic observations of austenite-martensite transformations induced by torsional strain

Wire specimens of a TRIP steel and several commercial austenitic stainless steels were torsionally strained both by continuous monotonic twist to fracture and by following various pause and reverse twist regimens. Progress of the phase transformation to ferromagnetic martensite, induced by the extensive plastic strain, was monitored by in situ dynamic measurements of the cyclic axial magnetizations due to alternating circular and alternating axial magnetic fields. By these means, various anisotropic features, as well as the relative concentration of the transformation products, were indicated. In the TRIP steel the dominant anisotropy is found to be magnetoelastic, whereas the Cr-Ni steels are found to develop a helical magnetic anisotropy, interpreted as resulting from the shape of the emergent martensite. While the TRIP steel transformation followed closely upon the advent of plastic strain, the Cr–Ni steels showed strain thresholds. Measurements following incremental surface metal removal of post fracture specimens indicated a negative radial gradient in martensite concentration from the surface, correlating with the similar gradient in accumulated strain.