Giant stress-impedance effect in Fe71Cr7Si9B13 amorphous wires

Journal of Alloys and Compounds 381 (2004) 245–249

Giant stress-impedance effect in Fe71Cr7Si9B13 amorphous wires N. Bayri, S. Atalay∗ Physics Department, Science and Arts Faculty, Inonu University, 44069 Malatya, Turkey Received 5 February 2004; received in revised form 8 March 2004; accepted 8 March 2004

Abstract The giant stress-impedance (SI) effect in as-received and furnace annealed Fe71 Cr7 Si9 B13 amorphous wires is studied. Samples were annealed at temperature of 460 ◦ C for times between 0.3 and 300 min to remove internal stresses. The results show that the magnetoimpedance and SI effect change sharply with annealing and a maximum stress-impedance ratio of 255% is observed in the wire annealed for 10 min under an applied tensile stress of 250 MPa. © 2004 Elsevier B.V. All rights reserved. Keywords: Amorphous materials; Magnetoimpedance; Stress-impedance

1. Introduction Amorphous transition metal alloys with wire geometry have become available within the last 20 years. Amorphous wires were originally developed for their exceptional mechanical properties but the investigation of the magnetic properties revealed some unique behaviour in the as-received state as well as providing an alternative geometry for existing amorphous alloys applications [1–6]. Recently, large and sensitive magnetoinductance and magnetoimpedance, MI, effects have been observed in amorphous wires [7–9]. The intensive research into the MI effect is a result of its technological importance in the field of sensor applications. When a magnetic material carrying a low intensity, high frequency alternating current is subjected to an external magnetic field, it exhibits a sharp change in its electrical impedance. This effect is known as the magnetoimpedance (MI) or the giant magneto impedance (GMI) effect [9–13]. The external magnetic field is generally applied along the direction of the current flow. The changes in the impedance are a consequence of changes in the interaction between the magnetisation of the material and the alternating magnetic field generated by the current. These changes occur due to ∗ Corresponding author. Tel.: +90-422-3410010; fax: +90-422-3410037. E-mail address: [email protected] (S. Atalay).

0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.03.077

the externally applied magnetic field. The key to understanding the MI effect is the effective permeability (µeff ) or effective susceptibility (χeff ) [µeff = χeff + 1] of the magnetic material. The magnetic field dependence of the impedance is controlled by the ability of the magnetisation to respond to the magnetic field generated by the current. This is governed by the effective susceptibility of the material in the direction of the field produced by the current. The application of the external field simply alters this effective susceptibility, which leads to the changes in the impedance. It is also well known that applied tensile stresses, σ, can also change the effective susceptibility. Therefore, one can expect a change of the magnetoimpedance response under the effect of the applied tensile stresses. Up to now, the magnetic field and stress dependence of MI of Fe77.5 Si7.5 B15 , Co72.5 Si12.5 B15 and (Fe0.06 Co0.94 )72.5 Si12.5 B15 compositions have attracted more attention because they exhibit positive, negative and nearly zero magnetostriction, respectively thus allowing the effect of magnetostriction on the magnetic properties to be investigated more rigorously [14–19]. Recently it has been reported that FeCrSiB wires have more superior mechanical properties than other amorphous and HT steel wires [20] and show a very large GMI [21]. Also, the magnetostriction of this wire is about 12 ppm, making it ideally suited for stress measurements. In this article, therefore the tensile stress dependence of MI effect in Fe71 Cr7 Si9 B13 wires was investigated.

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2. Experimental details

3. Results and discussion

Fe71 Cr7 Si9 B13 amorphous wires with a mean diameter of ∼123 ␮m were kindly supplied from Unitika Company. The crystallisation temperature of the wire was measured to be 565 ◦ C using a DTA-50 Shimadzu differential thermal analysis system. The annealing temperature was kept below the crystallisation temperature. Samples were cut to 10 cm length and annealed in air in a non-inductive tube furnace at temperature of 460 ◦ C for times between 0.3 and 300 min. The M–H curves were obtained using a dc digital system; the coercivity, Hc , was derived from the M–H curves. In the impedance measurements, the ends of sample were clamped to allow the application of tensile stresses and connection to the impedance measurement probe. The external magnetic field and tensile stress were applied perpendicular to the Earth’s magnetic field. The impedance was measured under the effect of tensile stresses and axial magnetic field, using a Hawlett-Packard 4294 A impedance analyser with 42941 A impedance probe. The system allows the separation of the real and imaginary part of impedance. The MI and SI data were obtained at a frequency of 1 MHz, with a constant amplitude ac current of 5 mA. The stepping of the dc current through the selenoid, and hence of the applied dc external axial field, was controlled by a computer. Data from the impedance analyser was also collected and averaged out by the computer at each step of the magnetic field. The averaging of impedance data leads to a large decrease in the noise to the signal ratio, and therefore more clear MI effect curves. MI ratio as a function of the applied magnetic dc field, H, is defined with respect to maximum applied field, Hmax = 3400 A/m, as Z/Z (%) = 100[Z(H) − Z(Hmax )]/Z(Hmax ). The saturation magnetostriction constant, λs , was measured to be 12 ppm using the small-angle magnetisation rotation method.

The variation of coercivity versus annealing time for Fe71 Cr7 Si9 B13 wire after heat treatment at fixed temperature is plotted in Fig. 1. Annealing treatments were carried out for various annealing times at 460 ◦ C. After each annealing treatment, the M–H loop was measured and from this loop the coercivity of the sample was derived. The M–H curves indicated that the as-received wire shows a large Barkhausen jump at low field (∼4.3 A/m), similar to that observed in FeSiB amorphous wires [2]. This suggests that the domain structure of Fe71 Cr7 Si9 B13 wire is similar, consisting of an inner core magnetised axially, and an outer shell magnetised radially. The relief of stress occurred very rapidly when the wire was annealed at 460 ◦ C; after 5 min the large Barkhausen jump disappeared, showing that most of the internal stressed quenched during the production process had been removed. The coercivity of as-received wire is quite high, as expected. After 60 min of annealing, coercivity was found to be 0.84 A/m, which is the minimum value of Hc found in this study. Further annealing time increases the coercivity. The low values of the Hc data show that stress and strain relaxation in the sample is nearly completed. We can see from the same Fig. 1 that after 90, 210 and 240 min annealing times, the coercivities increase to values of 1.34, 34 and 40.5 A/m, respectively. We interpret these higher values as being due to the onset of surface crystallisation. This is justified below by SEM pictures (Fig. 2). Fig. 3 shows the magnitude of the MI effect as a function of annealing time for the wires annealed at 460 ◦ C. The MI effect increased sharply after a short annealing time, at longer annealing times a large decrease in the magnitude of MI was observed. In the stress-impedance study, three wires from different regions in Fig. 3 were studied. Figs. 4–6 show the MI effect in as-received and annealed wire at zero and various applied tensile stresses. The results at zero tensile stress indicated that in the as-received state the circumferential permeability of the wire is not large because the

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Hc (A/m)

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Annealing time (min) Fig. 1. Variation of the coercivity values of the wire annealed at 460 ◦ C.

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Fig. 3. Variation of the magnitude of MI effect as a function of annealing time. Fig. 2. Scanning electron microscope images of wires annealed at 460 ◦ C for 120 min. Figure shows the existence of sub-micron crystallites on the surface of the wire.

domain structure is predominantly radial and the anisotropy is high due to the quenching stresses. Partly removal of the quenched stresses with annealing increases all components of the permeability, simply because of the reduced anisotropy. The enhancements of the MI in the stress relieved and surface crystallised wires are particularly striking. The MI effect increases sharply after annealing of the wire at 460 ◦ C for 10 min. Figs. 5 and 6 show that after 10 and 195 min annealing time Z/Z (%) increases to values of 130 and 80, respectively. The magnitude of this impedance change is comparable with that observed in the as-received and annealed nearly zero magnetostrictive wires, but the required applied field about a few times greater, as might be expected from the greater anisotropy. The longer annealing times increase the percentage of the crystalline fraction, and consequently the anisotropy. A large decrease in the magnitude of the MI effect was therefore observed. The magnitude of the MI effect in the surface crystalline wire is much higher than that in the as-received wire, because the circumferential permeability of this wire is quite high. This can be explained in terms of the circumferential domain structure

suggested for Fe-based amorphous wire on the basis of the surface crystallisation and surface stresses [22]. The shape of the MI effect plot, with a split peak, of the 90 min annealed wire is consistent with the model in [8] at high frequency range. This MI data confirms the existence of circumferential anisotropy in the surface crystallised wire. In order to test the hypothesis that surface crystallisation is responsible for the appearance of circumferential anisotropy in annealed wires, a sample previously annealed at 460 ◦ C for 90 min to induce surface crystallisation was etched for 20 min in HCl. The MI results showed the disappearance of the split peaks and the collapse of the circumferential anisotropy when the surface layer is removed. Asymmetric MI curves at low field region at a frequency of 1 MHz were observed in the surface crystallised wires with high coercivity. After etching of the wire, the asymmetric in the MI curves disappeared and MI curves showed only a single peak at this frequency. On the other hand, the increase in the magnitude of the MI effect after etching can be related to the decrease in anisotropy with the removal of the crystalline surface. The magnetoimpedance ratio, Z/Z (%), was measured for as-received and annealed wires, with the applied field, under the influence of different tensile stresses varying from 0 up to 250 MPa. The single peak behaviour of the MI effect

Fig. 4. Magnetic field dependence of the impedance values for as-received Fe71 Cr7 Si9 B13 wires under the effect of various tensile stress.

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Fig. 5. Magnetic field dependence of the impedance values for 10 min annealed at 460 ◦ C Fe71 Cr7 Si9 B13 wires under the effect of various tensile stress.

Fig. 6. Magnetic field dependence of the impedance values for 195 min annealed at 460 ◦ C Fe71 Cr7 Si9 B13 wires under the effect of various tensile stress.

Fig. 8 shows the tensile stress dependence of the stress-impedance ratios for as-received and annealed wires. The SI ratio is defined as Z/Zσ (%) = 100[Z(σ) − Z(σmax )]/Z(σmax ), where Z(σ max ) is the impedance at the 140 120

Asreceive 10 min

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∆Z/Z (%)

was observed in as-received and 10 min annealed wire. The maximum value of the MI effect was observed at σ = 0 and the magnitude of the MI effect decreases with the increase of σ. The single peak behaviour of the MI effect in as-received wire did not change as the applied tensile stress was increased. The peak values Z/Z (%) of as-received and 10 min annealed wires reduced from 14.7% at σ = 0 MPa to 1.6% at σ = 250 MPa and 127.7% at σ = 0 MPa to 2.9% at σ = 250 MPa, respectively (Fig. 7). The wires annealed for 195 min showed an asymmetric MI curve. The asymmetric MI curve gradually diminishes with the increase of tensile stress. At 138 MPa stress, the asymmetric MI curves had disappeared and single peak behaviour was observed again. The peak value Z/Z (%) of the 195 min annealed wires reduces from 80% at σ = 0 MPa to 2% at σ = 250 MPa (Fig. 7). The magnitude of the MI effect decreases with increasing applied tensile stress in all measurements, because the applied stress increases the magnetoelastic anisotropy imposing an easy axis close to the wire axis direction.

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Stress (MPa) Fig. 7. Z/Z (%) ratios as a function of applied tensile stress for as-received and annealed Fe71 Cr7 Si9 B13 wires.

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in the frame work of the Young Scientist Award Program (SA-TUBA-GEBIP/2001-1-1).

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References

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Fig. 8. Stress-impedance ratios versus applied tensile stress for as-received and annealed Fe71 Cr7 Si9 B13 wires.

external tensile stress of σ = 250 MPa and Z(σ) is for any given applied tensile stress σ. The SI effect was found to be largest in 10 min annealed wire.

4. Conclusion We found that annealing of Fe71 Cr7 Si9 B13 amorphous wires at a temperature of 460 ◦ C has a great influence on the MI response of wire. The magnitude of the MI effect of the wire annealed at 460 ◦ C for 10 min can nearly exceed 130% at 1 MHz at a maximum external dc magnetic field of 3400 A/m. In this study, it was also shown that the stress-impedance effect can be as large as 255%, which is among the highest change reported in amorphous materials. It was concluded that the direct decrease of the magnetoimpedance ratio, Z/Z (%) and Z/Zσ (%) as a function of tensile stress implies that the applied tensile stresses increases the magnetoelastic anisotropy leading to changes in the domain structure of wire.

Acknowledgements This work was supported by TUBITAK with the project number TBAG-2114 and Turkish Academy of Sciences,

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