Helical Magnetic Anisotropy Research Articles

Off-Diagonal Magnetoimpedance in Annealed Amorphous Microwires with Positive Magnetostriction: Effect of External Stresses

It was observed recently that the giant magnetoimpedance (GMI) effect in Fe-rich glass-coated amorphous microwires with positive magnetostriction can be improved significantly by means of post-annealing. The increase in the GMI is attributed to the induced helical magnetic anisotropy in the surface layer of the microwire, which appears after the annealing. The application of external stresses to the microwire may result in changes in its magnetic structure and affect the GMI response. In this work, we study theoretically the influence of the tensile and torsional stresses on the off-diagonal magnetoimpedance in annealed amorphous microwires with positive magnetostriction. The static magnetization distribution is analyzed in terms of the core–shell magnetic structure. The surface impedance tensor is obtained taking into account the magnetoelastic anisotropy induced by the external stresses. It is shown that the off-diagonal magnetoimpedance response exhibits strong sensitivity to the magnitude of the applied stress. The obtained results may be useful for sensor applications of amorphous microwires.

Extension-Torsion-Inflation Coupling in Compressible Magnetoelastomeric Thin Tubes with Helical Magnetic Anisotropy

An axisymmetric and axially homogenous variational formulation is presented for coupled extension-torsion-inflation deformation in compressible magnetoelastomeric tubes in the presence of azimuthal and axial magnetic fields. The tube’s material is assumed to have a preferred magnetization direction which lie in the radial plane but at an angle from the tube’s axial direction - this imparts helical magnetic anisotropy to the tube. The governing differential equations necessary to solve the above deformation problem are obtained which are shown to reduce to a set of nonlinear algebraic equations in the thin tube limit. This allows us to obtain analytical expressions in terms of the applied magnetic field, preferred magnetization direction and magnetoelastic constants which tell us how these parameters can be tuned to generate unusual coupled deformations such as negative Poisson’s effect. The study can be useful in designing magnetoelastic soft tubular actuators.

Off-diagonal magnetoimpedance in amorphous wires and its application in miniature sensors of weak magnetic fields

Off-diagonal magnetoimpedance in amorphous cobalt-based microwire with helical magnetic anisotropy is investigated using a DC bias current. It is demonstrated that the circular magnetic field generated by the bias current improves the sensitivity to an axial external magnetic field, raises the output voltage peak, and expands the sensed field in which the peak is observed. However, a further increase in the bias current reduces the sensitivity. The experimental results agree qualitatively with the theory for large bias fields when the hysteresis in static magnetization is eliminated.

The peculiarities of magnetization reversal process in magnetic nanotube with helical anisotropy

The magnetization reversal process in a soft magnetic nanotube with a weak helical magnetic anisotropy is studied by means of numerical simulation. The origin of a helical anisotropy is a small off-diagonal correction to the magneto-elastic energy density. The change of the external magnetic field parallel to the nanotube axis is shown to initiate a magnetic hysteresis associated with the jumps of the circular magnetization component of the nanotube at a critical magnetic field Hs. For a uniform nanotube, the critical magnetic field Hs is investigated as a function of geometrical and magnetic parameters of the nanotube. Using 2D micromagnetic simulation, we study the behavior of a nanotube having magnetic defects in its middle part. In this case, the jump of the circular magnetization component starts at the defect. As a result, two bamboo domain walls appear near the defect and propagate to the nanotube ends. Similar effect may explain the appearance of the bamboo domain walls in a slightly non uniform amorphous ferromagnetic microwire with negative magnetostriction during magnetization reversal process.

Development of magnetically soft microwires with GMI effect

Thin amorphous magnetically soft microwires attract recently great attention because of their excellent soft magnetic properties and giant magneto-impedance (GMI) effect, thin dimensions and possibility for applications in magnetic micro-sensors. We overview research in the field of development of microwires with high GMI effect and improved features and report novel results on studies of GMI (diagonal and off-diagonal components) at high frequencies (between 10 MHz and 4 GHz) and its correlation with soft magnetic behaviour of thin amorphous microwires (Co-Fe-rich with nearly-zero magnetostriction constant) with metallic diameter between 3 and 20 μm and on optimization of GMI effect. We studied and analyzed low-field hysteresis of GMI effect and its dependence on circular magnetic field and discuss the nature the low-field hysteresis in terms of helical magnetic anisotropy and the effect of the bias field. Choosing samples composition, annealing conditions and geometry we were able to tailor their magnetoelastic anisotropy and respectively magnetic softness and GMI.

Experimental method for determining the total demagnetizing factor of thin amorphous ribbons

The basis of the method is the influence of the demagnetizing field on the maximum rate of magnetic flux change of the amorphous ribbon. The method allows to determine the demagnetizing factor of thin amorphous ribbons in a wide range of its values. With the proposed method the frequency dependence of the demagnetizing factor of amorphous ribbon with a helical magnetic anisotropy was found out.

Magnetoimpedance sensitive to dc bias current in amorphous microwires

We have investigated the impedance dependence of magnetically soft microwire on the internal circumferential magnetic field HB created by the dc bias current IB and theoretically and experimentally demonstrated that in a conductor with helical magnetic anisotropy, the high frequency impedance depends on the dc bias current IB (or the corresponding bias field HB) and this dependence is hysteretic. We have experimentally observed a change of impedance more than 35% upon changing the bias current. The possible applications of the dc current-driven magnetoimpedance effect are discussed.

Magnetic behavior of rapidly quenched submicron amorphous wires

Results on the magnetic behavior of glass-coated amorphous wires with metallic nucleus diameters in the submicron range are reported for the first time. The magnetic behavior of nearly zero magnetostrictive submicron amorphous wires is different from that observed in amorphous microwires with typical dimensions and similar composition. Submicron wires are bistable, whereas typical microwires show an almost anhysteretic loop. Bistability of submicron wires is maintained even in a very thin surface layer, which was shown to display a helical magnetic anisotropy. The origin of this specific behavior is the shape anisotropy which becomes dominant at lower dimensions of the metallic nucleus. The study of submicron amorphous wires opens up the way for new applications of amorphous wires.

Damping of the domain walls motion in Co-based amorphous ribbons with helical magnetic anisotropy: Part III

The damping of the motion of domain walls of a sandwich domain structure by the eddy currents magnetic fields, the stray fields and the hysteresis friction fields is investigated. The blocking of the motion of domain walls by the eddy currents magnetic fields is discovered.

Dynamics of sandwich domain structure in Co-based amorphous ribbons with helical magnetic anisotropy: Part I

The distribution of axes of easy magnetization close to a homogeneous distribution is revealed in each half-thickness of a ribbon after annealing it in a helical magnetic field. The transition from magnetic reversal of a ribbon by the displacement of two domain walls formed near a middle plane of a ribbon to magnetic reversal of a ribbon by displacement of two domain walls formed near to the main surfaces of a ribbon is found out during each half-period of a magnetic reversal.

Relation between surface magnetization reversal and magnetoimpedance in Co-rich amorphous microwires

We report the investigations on the giant magnetoimpedance (GMI) effect and the surface magnetic structure performed in the series of glass covered Co-rich amorphous microwires with different thicknesses of glass covering. The calculation of the surface hysteresis loops has been performed in the frames of the model which takes into account the helical magnetic anisotropy. The correlation between the value of the GMI ratio, the jump of the circular magnetization, and the value of the angle of helical anisotropy has been established. It was found that the angle of the surface helical anisotropy, for which the highest value of GMI, is close to 60°.

Magneto-optical determination of helical magnetic structure in amorphous microwires

Surface magnetization reversal has been investigated in Co-rich glass-covered microwires. Difference in mechanism of the surface magnetization reversal has been observed in the microwires with different thicknesses of glass covering. Correlation of the surface magnetic properties and the surface helical magnetic anisotropy has been found.

Magnetic field sensor based on asymmetric inverse Wiedemann effect

A new type of magnetic field sensor is introduced. The sensor is based on asymmetric inverse Wiedemann effect in amorphous Co-rich ribbon with helical magnetic anisotropy induced by torque strain. The characterization of the stand-alone sensor and further the sensor with developed electronics for feedback operation was accomplished. Linear response of the sensor without feedback appears in narrow magnetic field range of ±15 μT. For the feedback-loop connection the dynamic range has been enlarged up to ±120 μT. The linearity error FSR in the feedback connection is lower than 0.1% in the range of ±75 μT. The results show that the tested prototype achieves better stability than GMI sensors. The temperature offset drift, measured in temperature range of −20 °C to +80 °C, is lower than 5 nT/K or −28 nT/K for the sensor operated with laboratory instruments only or with the developed simple electronics, respectively. For the magnetometer with sensor operating in the feedback loop the RMS noise of 80 pT was achieved in the frequency range from 30 mHz to 10 Hz.

Influence of torsion and tensile stress on magnetoimpedance effect in Fe-rich amorphous microwires at high frequencies

In this work, we study the magnetoimpedance response under tensile and torsional stress of Fe 76B 13Si 11 glass-coated amorphous microwires (with the metallic nucleus diameter of 17 μm) prepared by Taylor–Ulitovski method. The impedance was evaluated using a Network Analyser in the frequency range of 10–500 MHz. In order to induce a helical magnetic anisotropy that modifies the magnetic domain structure and the magnetic response of the sample, the torsion stresses have been applied. The effect of such torsion on giant magnetoimpedance (GMI) has been studied in a Fe-rich amorphous microwire as a function of the driving current frequency. Unusual behaviour of the GMI with torsion has been observed up to 500 MHz: at this frequency, the change from double-peak to single-peak behaviour has been observed. It could be ascribed to changes on domain structures and surface anisotropy induced by applied torsion. For this frequency range, an abrupt decrease of the impedance with torsion has been observed. We have also studied the tensile-induced changes of GMI effect in a Fe-rich amorphous microwire. The GMI effect increases with applied tensile stress and with the frequency of AC current. Observed dependences could be attributed to the change of the overall magnetic anisotropy under applied torsion.

Torsion and tension stress induced transformation of surface magnetic structure in Co-rich amorphous microwires

The effect of torsion and tensile stress on the surface magnetic properties in Co-rich glass covered amorphous microwires has been studied. The magnetization reversal in microwires has been studied by magneto-optical Kerr effect. Calculations of hysteresis loop was performed taking into account the existence of helical magnetic anisotropy in the outer shell of the microwire. The comparative analysis of the results of the experiment and the calculation demonstrates that the observed transformation of the surface hysteresis is related to the variation of the angle of the helical surface structure.

Influence of the ac magnetic field frequency on the magnetoimpedance of amorphous wire

Experimental and theoretical studies on the influence of ac magnetic field frequency on the axial diagonal (ζzz) and off-diagonal (ζϕz) components of the magnetoimpedance (MI) tensor in (Co0.94Fe0.06)72.5Si12.5B15 amorphous wires have been performed. The frequency (f) of an ac current flowing along the wire was varied from 1 to 20 MHz with the current amplitude less than 15 mA. In order to enhance the ζϕz component, the amorphous wire was submitted to torsion annealing for developing and preserving a helical magnetic anisotropy in the surface of the wire. The experimental measurements show that the value of the impedance is proportional to the square-root of the ac current frequency, , in the vicinity of Hex < HK and this increase is due to the contribution of the resistance (real part of the impedance). The measurements also indicate that the peaks of the MI curve shift slightly towards higher field values with increasing f.In a theoretical study the magnetoimpedance expressions ζzz and ζϕz have been deduced using the Faraday law in combination with the solutions of the Maxwell and Landau–Lifshitz–Gilbert (LLG) equations. By analysing quantitatively the spectra of ζzz and ζϕz, the phenomenon of the shift in the peaks of the MI curve with f has been considered as a characteristic of the helical anisotropy in the domain structure of the wire surface.

Magneto-optic vector magnetometry of CoFeCrSiB amorphous ribbons

A magneto-optic method suitable for investigation of soft magnetic ribbons is proposed that combines the internal magnetic field induced by a current flowing through the ribbon axis with the external magnetic field generated by an air coil. The method eliminates effects of demagnetizing factor and shape anisotropy on measured hysteresis loops. Both in-plane magnetization components are measured by means of magneto-optic vector magnetometry. The method is applied to study magnetic properties of CoFeCrSiB amorphous ribbons showing asymmetric giant magnetoimpedance effects. A coherent rotation of magnetization and in-plane easy axis inclined from the ribbon axis is observed. It corresponds to helical magnetic anisotropy originating from the preparation processes.

Domain walls dynamics in the amorphous ribbon with a helical magnetic anisotropy

The damping mechanism for motion of domain walls, which form the sandwich structure and move from the middle plane of the ribbon to opposite surfaces during the dynamic magnetization reversal, have been investigated. The difference between the real and ideal sandwich domain structure, the actual distribution of the anisotropy easy directions through the ribbon thickness and the M ⇒ s deviation from local easy directions under the action of applied magnetic field have been taken into account. It was revealed that the maximum of the total damping coefficient β tot ( x ) near the half-way of the domain wall run is due to the influence of the magnetic stray fields. These fields have a character of irregular oscillations and are directed approximately perpendicular to the local easy direction of the ribbon layer through which the domain wall propagates. The damping coefficient β e . c . ( x ) determined by eddy-currents has the maximal value close to the ribbon middle and decreases linearly to zero when the domain wall approaches the ribbon surface.

Influence of AC Magnetic Field Amplitude on the Surface Magnetoimpedance Tensor in Amorphous Wire With Helical Magnetic Anisotropy

We have performed experimental and theoretical studies on the influence of ac magnetic field amplitude on the magnetoimpedance tensor in an amorphous wire with helical magnetic anisotropy. For the experimental measurements, we used an amorphous wire of composition (Co/sub 0.94/Fe/sub 0.06/)/sub 72.5/Si/sub 12.5/B/sub 15/ with negative, nearly zero magnetostriction constant, excited either by an ac circular h/sub /spl phi// or by an axial h/sub z/ magnetic field created by an ac electric current. We changed the ac current amplitude from 7.5 to 40 mA and the current frequency f from 1.5 to 20 MHz. The values of the asymmetric giant magnetoimpedance ratio associated with the sweeping direction of the dc field H/sub ex/ and the corresponding sensitivity were 211% and 0.64 V/Oe, respectively, for an ac current of 37.5 mA at 3 MHz. For the theoretical study based on the magnetization rotation, we obtained the second-order harmonic of the ac magnetization m/spl I.oarr//sup (2)/ induced by the relatively high ac magnetic field by solving the Landau-Lifshitz-Gilbert (LLG) equation. We also considered a second-order surface impedance tensor /spl sigmav//spl circ//sup (2)/, which allowed us to analyze quantitatively the influence of the ac magnetic field amplitude on the impedance tensor of the wire. We obtained the domain model of the wire with helical magnetic anisotropy having multidomains and the magnetization vector /spl plusmn/M/sub 0/ directed in the easy direction, and the corresponding static magnetic configurations, by solving the static LLG equation. For the given magnetic configurations, we calculated the second-order impedance tensor /spl sigmav//spl circ//sup (2)/. The results can well explain the irregular field characteristics of the voltage responses at low dc field value, when the wire was excited at high frequency and at large ac magnetic field.

Studies of the magnetoelastic inverse Wiedemann effect in pulsed laser depositedcylindrical cobalt films

Cylindrical soft magnetic Co films, nanostructured with a grain size of 1–2 nm and between and 90 nm in thickness, were produced by the pulsed laser deposition technique. Thesefilms showed a magnetostrictive property. The magnetoelastic inverse Wiedemann effect, orIWE, of longitudinal magnetization as a function of the circular applied magnetic field,Mz–Hϕ, was measured. The IWE made it possible to explain the magnetization processesaccording to an initial reversible rotation of the magnetization followed by magneticwall nucleation and propagation. A magnetization rotation model allowed us tocompare the reversible part and the first irreversible magnetization processescorresponding to the IWE. In this way, the values of the helical magnetic anisotropy,Kh, due to thehelical stresses, σh, generated by an applied torque to the films, were determined.A linear increase of the nucleating magnetic wall field,HN, with the appliedtorque, or with σh, was observed. In addition, the IWE made it possible to establishthe non-dependence of the saturation magnetostriction constant,λs, on the sample thickness and to determine the value of this negative isotropic .