Comparative Study of Magnetic Properties and Microstructure for As-cast and Square-wave Pulse Current Joule Annealed Wires

Recent investigations attracted much interest and indicated the novel giant magnetoimpedance (GMI) Co-based amorphous microwires are the excellent candidate materials for the weakly magnetic field detection of potential GMI sensor applications1,2. Compared with rotating water-spinning and glass-covered melt-spinning techniques, melt-extracted microwire exhibits higher solidification rate and unique super-soft magnetic properties3-5. Most of all, there exists an important trend for choosing more sensitive GMI materials: exploring novel magnetic microwires with enhanced GMI and magnetic properties. However, this final purpose of melt-extracted Co-based amorphous microwires can be realized by using some efficient Joule annealing techniques, especially alternating current according to the special geometric shape6,7. As previously reported, Zhou et al.8 investigated that the effect of AC Joule-heating annealing for melt-spun CoFeSiB amorphous ribbon on GMI property, influencing the easy direction magnetization, magnetic anisotropy and domain structure, and their [ΔZ/Z0]max increased to about 180% at 900kHz for special annealing conditions of 30 min and an AC current density of 2.8×107 A/m2. Sinha et al.9 observed [ΔZ/Z]max of CoMnSiB glass-coated microwire increased from 66% of as-cast to 129% after short-duration PC annealing with amplitude of 100 mA, due to the increase of outer shell domain volume. Atalay et al.10 investigated that the variation relationship between magnetization and Young’s modulus of PC-treated Fe77.5Si7.5B15 amorphous wires, and measured the coercivity and anisotropic energy density with the variation of current intensity and tensile stress, further revealed the proper PC annealing compared with other traditional annealing can effectively release inner residual stress and improve the soft magnetic property. Generally, the magnetic properties are closely related to microstructures yielded by Joule annealing from structure-property perspective11. Generally, square-wave pulse current Joule annealing (SPCJA) as a distinctive Joule-heating modulation technique for obtaining application-oriented magnetically sensitive materials, in comparison with the other convention current annealing, it also can effectively avoid the induced crystallization phenomenon by relatively large current density, including improving magnetic anisotropy and microstructural relaxation with an excellent GMI effect under high-density pulse or alternating circumferential magnetic field12. However, there also lacks the comparative study on magnetic properties and microstructure of as-cast and SPCJA-ed microwires, so it is worthwhile to explore the related investigation on the relation of GMI properties and microstructure of as-cast and SPCA-ed wire. In this paper, we aim to comparatively present the magnetic properties and microstructure of as-cast and SPCJA-ed microwires by using X-ray diffraction (XRD), Comparative Study of Magnetic Properties and Microstructure for As-cast and Square-wave Pulse Current Joule Annealed Wires


Introduction
Recent investigations attracted much interest and indicated the novel giant magnetoimpedance (GMI) Co-based amorphous microwires are the excellent candidate materials for the weakly magnetic field detection of potential GMI sensor applications 1,2 .Compared with rotating water-spinning and glass-covered melt-spinning techniques, melt-extracted microwire exhibits higher solidification rate and unique super-soft magnetic properties [3][4][5] .Most of all, there exists an important trend for choosing more sensitive GMI materials: exploring novel magnetic microwires with enhanced GMI and magnetic properties.However, this final purpose of melt-extracted Co-based amorphous microwires can be realized by using some efficient Joule annealing techniques, especially alternating current according to the special geometric shape 6,7 .
As previously reported, Zhou et al. 8 investigated that the effect of AC Joule-heating annealing for melt-spun CoFeSiB amorphous ribbon on GMI property, influencing the easy direction magnetization, magnetic anisotropy and domain structure, and their [ΔZ/Z 0 ] max increased to about 180% at 900kHz for special annealing conditions of 30 min and an AC current density of 2.8×10 7 A/m 2 .Sinha et al. 9 observed [ΔZ/Z] max of CoMnSiB glass-coated microwire increased from 66% of as-cast to 129% after short-duration PC annealing with amplitude of 100 mA, due to the increase of outer shell domain volume.Atalay et al. 10 investigated that the variation relationship between magnetization and Young's modulus of PC-treated Fe 77.5 Si 7.5 B 15 amorphous wires, and measured the coercivity and anisotropic energy density with the variation of current intensity and tensile stress, further revealed the proper PC annealing compared with other traditional annealing can effectively release inner residual stress and improve the soft magnetic property.Generally, the magnetic properties are closely related to microstructures yielded by Joule annealing from structure-property perspective 11 .Generally, square-wave pulse current Joule annealing (SPCJA) as a distinctive Joule-heating modulation technique for obtaining application-oriented magnetically sensitive materials, in comparison with the other convention current annealing, it also can effectively avoid the induced crystallization phenomenon by relatively large current density, including improving magnetic anisotropy and microstructural relaxation with an excellent GMI effect under high-density pulse or alternating circumferential magnetic field 12 .However, there also lacks the comparative study on magnetic properties and microstructure of as-cast and SPCJA-ed microwires, so it is worthwhile to explore the related investigation on the relation of GMI properties and microstructure of as-cast and SPCA-ed wire.
In this paper, we aim to comparatively present the magnetic properties and microstructure of as-cast and SPCJA-ed microwires by using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and magnetic property measurement including precision impedance analyzer, and identify the current annealed wires with superior GMI properties for potential sensor applications.

Experimental Details
In this experiment, mother alloy ingot with the nominal composition of Co 68.2 Fe 4.3 B 15 Si 12.5 (in at.%) was prepared by arc-melting in purified argon atmosphere and copper mould casting methods, then setting the top-end of club-shaped alloy into BN crucible with a diameter of 8mm, and it was re-melted by induction coil in precision melt-extraction facility, so the microwires with a diameter of 30μm were extracted by the edge of a high-speed rotating copper wheel also in purified argon.Square-wave pulse current Joule annealed (SPCJA) was conducted by passing through amorphous wire a square-wave alternating current (with a frequency of 50Hz, the average current intensity I m of 75mA and duration time of 480s, air cooling, A.C.) supplied by a mini-type self-designed PC annealing device 12,13 , and the main technique parameters including 50Hz, 75mA and 480s were explored as the optimized indexes by a large number experiments of numerical simulation for the transient temperature rise during SPCJA process, as described in Liu et al. 11 , while it is important to assure the wire still to be amorphous state after this treatment.On the other hand, once the frequency decreases below 50Hz, the improvement of practical action will deteriorate approaching to the effect of DC annealing.Even the time extends for a longer time, it seem to no obvious effect on GMI property as the time is up to the critical times (i.e.480s or 600s) based on a great deal of previous experiments [11][12][13][14] .
As-prepared and SPCJA-ed microwires were examined by X-ray diffraction with CuK α radiation (XRD, Rigaku D/max-γB), high-resolution transmission electron microscopy (HRTEM, JEM 2010F).The magnetic properties were performed by vibrating sample magnetometer (VSM, LakeShore 7407) with a maximum applied field of 0.2T.The impedance measurements were performed using Aligent 4294A precision impedance analyzer at frequencies 40Hz~110MHz.And the effective sample length for impedance measurement is about 22mm.GMI ratio, ΔZ/Z 0 , is defined as 13 : ( ) ( ) and magnetic field response sensitivity, ξ, is expressed as 14 : (%) where Z(H ex ) is the impedance under different external field, H ex is below 4.5Oe supplied by a pair Helmholtz coils paralleling to the wire-axis to avoid the disturbance of geomagnetic field.Z(H 0 ) is the initial impedance at 0Oe.All measurements were conducted at room temperature.Comparative Study of Magnetic Properties and Microstructure for As-cast and Square-wave Pulse Current Joule Annealed Wires

Results and Discussion
Figure 2 reveals the longitudinal M-H magnetic hysteresis loops of as-prepared and SPCJA-ed microwires.Their shapes of hysteresis loops are almost rectangles, which exhibit an excellent soft magnetic feature, so their magnetization can easily reach the saturation state at low magnetic field 15 .According to the magnified central area of loops, the coercivity H c slightly increases from 0.28Oe of as-cast wire to 0.55Oe of SPCJA-ed wire, and the saturation magnetization M s decreases from 51.66 emu/g of as-cast to 39.41 emu/g of SPCJA-ed wire, and the maximum magnetic permeability μ m from 0.71 of as-cast wire to 0.93 of SPCJA-ed wire.While the electrical resistivity ρ m increases from122.71μΩ•cm of as-cast wire to 149.58μΩ•cm of SPCJA-ed wire.Accordingly, the enhanced magnetic properties of SPCJA-ed wire could be attributed to the change of local ordering degree of atomic arrangement and the formation of stably circular magnetic domains, resulting in a notable increase of magnetic moment exchange coupling drived by external magnetic field 16 .
Figure 3 shows the 3D variation images of impedance and their corresponding GMI ratio ΔZ/Z 0 dependence of external magnetic field H ex (0~5.0Oe) in range of frequency (100 kHz~15 MHz) for as-cast and SPCJA-ed (annealing at 75mA for 480s at 50Hz) microwires.Compared to these GMI profiles (including impedance and GMI ratio variations), all of them take on smooth variation and almost have no obviously  treatment can improve atomic diffusion by releasing internal stress and finally forming the regularly arranged atomic micro-regions under the action of thermal activation and high-density alternating current magnetic field energy during SPCJA.Herein, the SPCJA-ed wire is tending to possessing so excellent magnetic properties.Materials Research macroscopic fluctuant variation with increasing magnetic field at different frequency.Meanwhile, the SPCJA-ed microwire has relatively lower values of initial impedance and maximum impedance, namely excellent GMI property (this mainly indicates the larger GMI ratio [ΔZ/Z 0 ]).
Further to explore the specific comparison of GMI property, we conduct the corresponding statistic in following discussion part by using 3D variation images.Herein, Figure 4 shows that the dependence of GMI ratio ΔZ/Z 0 on external magnetic field H ex (0~4.5Oe) of as-cast and SPCJA-ed microwires and frequency dependence of magnetic field response sensitivity ξ max (%/Oe) and its corresponding GMI peak position H p at selected frequencies of SPCJA-ed microwire.It is nearly consistent with above mentioned GMI profiles, all curves of both wires increase rapidly at first (nearly at 0~1.0Oe) then decreases monotonically or keeps small variation with an increase of external magnetic field at the selected frequencies.In details, [ΔZ/Z 0 ] max values of as-cast wire are -28.96%,52.73%, 73.73%, 85.61%, 105.38%, 104.69% and 104.80% at 1MHz, 3MHz, 5MHz, 7MHz, 10MHz, 12MHz, 15MHz, respectively, as shown in Figure 4a.Notably, the ΔZ/Z 0 curve at 1MHz of as-cast wire exhibits the negative GMI effect with increasing field as a result of inhomogeneous distribution of the locally critical field of magnetization rotation.[ΔZ/Z 0 ] max values of SPCJA-ed wire are 28.76%,118.81%, 166.07%, 185.96%, 193.50%, 190.32% and 180.40% at 1MHz, 3MHz, 5MHz, 7MHz, 10MHz, 12MHz, 15MHz, respectively, as seen in Figure 4b.Especially, at 5MHz, SPCJA-ed wire has larger [ΔZ/Z 0 ] max of 166.07%, maximum field sensitivity ξ max of 413.76%/Oe and relatively larger H p =0.9Oe in comparing with as-cast of GMI effect (H p corresponds to the working magnetic field range of sensor 17 , as shown in Figure 4), which are nearly 2.25 times of 73.69% and 1.73 times of 239.5%/Oe and H p =0.8Oe for as-cast wire respectively.At 10MHz, SPCJA-ed wire also has relatively higher [ΔZ/Z 0 ] max of 193.50%, ξ max of 380.81%/Oe and H p =1.0Oe comparing with as-cast of GMI effect, which are nearly 1.84 times of 105.38%, nearly 1.37 times of 278.56%/Oe and H p =1.3Oe for as-cast wire respectively.Importantly, the larger [ΔZ/Z 0 ] max , ξ max and H p at relatively low frequency is tend to enhance the resolution and accuracy of GMI sensor from sensor application perspective, so the better working frequency is chosen as 5MHz according to the sensor applied frequency ranges (5MHz~10MHz) as shown in Figure 4c.Furthermore, SPCJA (annealing at 75mA for 480s at 50Hz) annealed wire possesses an enhanced GMI property at 5MHz in comparison with the corresponding results reported by previously reference 14 , such as the values of [ΔZ/Z 0 ] max , ξ max and H p of ACJA-treated (at 75mA for 480s, 50Hz) Comparative Study of Magnetic Properties and Microstructure for As-cast and Square-wave Pulse Current Joule Annealed Wires achieve to 142.33%, 181.21%/Oe and 0.8Oe at 5 MHz, respectively.Significantly, the square pulse is more effective than "sin" wave annealing owing to both the high-density pulsed circumferential magnetic field and thermal activation energy in relatively long active time for increasing magnetic anisotropy and improving microstructural relaxation.In a word, SPCJA technique is more suitable for applying as the magnetic sensitive materials working at low-frequency and relatively high-magnetic field of GMI sensor.

Conclusions
As a novel class of GMI sensor that contains high-performance sensitive materials, SPCJA-ed microwires exhibit an excellent and attractive engineering magnetic property.In summary, SPCJA treatment has remarkable effect on GMI properties, and the SPCJA (at 75mA for 480s at 50Hz, A.C.) annealed wire possesses the excellent magnetic properties including slightly larger coercivity and smaller saturation magnetization, higher magnetic permeability.At 5MHz, [ΔZ/Z 0 ] max value of SPCJA-ed wire increases to 166.07%, which is nearly 2.25 times of 73.69% for as-cast wire, and the field response sensitivity ξ max of SPCJA-ed wire increases to 413.76%/Oe by more than 1.73 times of 239.5%/Oe for as-cast wire, even the GMI peak position of annealed wire tends to be 0.9Oe and 1.0Oe at 5MHz and 10MHz respectively.This type SPCJA-ed wires show an enhanced GMI property in contrast to AC-treated wire duo to the high-density pulse circumferential magnetic field.Therefore, it can be as a promising candidate sensitive material working at 5MHz~10MHz for potential high-performance GMI sensor applications.

Figure 1 Figure 1 .
Figure 1 displays X-ray diffraction patterns and HRTEM images of as-cast and SPCJA-ed wires.Both XRD patterns consist of one broad diffused diffraction maximum, and these indicate that the microstructure of wires entirely consists of amorphous structure before and after SPCJA.While their surface morphology and is extreme smooth, uniform and almost with no obvious formation of nanocrystalline, which is consistent with the above obtained XRD results, and their corresponding fast Fourier transformation (FFT) images just consist of halo rings, which also exhibits the mainly amorphous feature.Moreover, there is an enhanced degree of local ordering (i.e.atomic arrangement) for SPCJA-ed microwires versus as-cast state statistically calculated by auto-correlation function (ACF) technique of Digital Micrograph software (not shown here).Namely, SPCJA

Figure 2 .
Figure 2. Longitudinal M-H hysteresis loops including a magnified central area of as-cast and SPCJA-ed microwires.The uni-directional arrow indicates the direction of external magnetic field H ex .

Figure 4 .
Figure 4. Variation curves of GMI ratio ΔZ/Z 0 dependence of external magnetic field H ex at selected frequencies of as-cast (a) and SPCJA-ed (b) microwires.Frequency dependence of magnetic field response sensitivity ξ max (%/Oe) and its corresponding GMI peak position H p at selected frequencies of SPCJA-ed microwire.The bidirectional arrow in (c) indicates the sensor applied frequency range of 5MHz~10MHz.