APPLIED PHYSICS LETTERS 104, 012901 (2014)
Influence of direct bias current on the electromagnetic properties of melt-extracted microwires and their composites F. X. Qin,1,a) J. Tang,1 V. V. Popov,2 J. S. Liu,3 H. X. Peng,4 and C. Brosseau5,a) 1
1D Nanomaterials Group, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan 2 Taurida National University, Simferopol, Ukraine 3 School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, China 4 Advanced Composite Centre for Innovation and Science, Department of Aerospace Engineering, University of Bristol, University Walk, Bristol BS8 1TR, United Kingdom 5 Universit e de Brest, Lab-STICC, CS 93837, 6 Avenue Le Gorgeu, 29238 Brest Cedex 3, France
(Received 4 December 2013; accepted 19 December 2013; published online 7 January 2014) We study the influence of a direct bias current on the magnetoimpedance (MI) in melt-extracted amorphous CoFeSiB microwires and the effective electromagnetic properties of epoxy composites filled with these microwires. Our analysis reveals two remarkable features of the current dependence of MI in the range of gigahertz frequencies: a redshift of the dielectric resonance frequency and a decrease of the peak resonance of the effective permittivity as the bias current increases. Both effects are intrinsically linked to the influence of the polymer matrix on the magnetic structure and properties of the microwires. A discussion of these results is proposed in terms of two competing effects of the bias current, i.e., the induced additional effective field in the C 2014 AIP Publishing LLC. plane normal to the wire axis and the stress relief from Joule heating. V [http://dx.doi.org/10.1063/1.4861185] There has been growing interest in the properties of amorphous ferromagnetic microwires, e.g., in the context of high-performance magnetic sensor applications,1 where a precise understanding of the magnetoimpedance (MI) effect is being explored in the MHz range of frequencies. However, there are many advantages associated with increasing the operating frequency for a majority of already well developed applications. Thus, their microwave and millimeter-wave properties, if understood and exploited, make microwires an ideal tool for controlling the electromagnetic wave-matter interaction for a variety of technologies, for example in microwave absorption, and non-destructive test.1–3 At GHz frequencies, the microwires behave like dipoles when they interact with the incident microwave field. The local wire properties largely determine the macroscopic dielectric properties of composites embedding them. It has been demonstrated that the MI of microwires plays a central role in the electromagnetic behaviour of polymer composites.3,4 Several studies have also been performed on the effect of magnetic field on the effective permittivity of these composites.3,5,6 While the most commonly used fabrication technique of microwires is the modified Taylor-Ulitovskiy approach,7 alternative methods have been reported in the literature. For example, the melt-extraction technique has been recently adapted,8 and yields high-quality and high-performance microwires with better mechanical properties, compared with other approaches such as in-rotating water spinning, Taylor-wire, and glass-coated melt spinning. Melt extraction has several advantages: (i) it has the highest solidification or cooling rate, which enables to obtain wires with desirable amorphous structure; (ii) wires without a glass cover are a)
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more suitable for electronic packaging and sensor applications; and (iii) experimental parameters, i.e., linear velocity of wheel, feed rate of the molten, can be well controlled to ensure that the wires have uniform diameter and roundness.9 Above all, as-cast melt-extracted microwires present smaller MI for studying the effect of some specific external stimuli on the electromagnetic properties of microwire composites. External stimuli like bias current offer incisive opportunities to tune competing interactions. Additionally, the development of smart and lightweight polymer composites embedding metal inclusions has sparked vast research efforts linked to emerging technologies.10 In this Letter, we report on the influence of a direct bias current on the MI of melt-extracted microwires at GHz frequencies and the permittivity spectrum of epoxy-based composites containing a single microwire. We will show that: (i) the MI profile of a single microwire exhibits a single peak in the gigahertz frequency range in contrast to the case of glasscoated microwires; (ii) The increase of the bias current has the effect of increasing the MI and reducing the effective permittivity of an epoxy-based composite containing a single microwire. This suggests that the magnetic anisotropy of microwire changes from axial to circumferential; (iii) The applied bias current redshifts the effective permittivity peak resonance, which is correlated to the ferromagnetic resonance (FMR) of the microwire. Microwires with nominal composition of Co68.2Fe4.3B15 Si12.5 (in at. %) were prepared by arc-melting in pure argon and copper mould casting methods. Using a home-built melt extraction facility, the starting alloy was melted by induction coil in a BN crucible. Microwires with diameter of 40 lm were extracted by the edge of a high speed rotating copper wheel under purified argon atmosphere. Wires were then sandwiched between two layers of 913 epoxy prepregs.
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Additional layers of prepregs were used to obtain parallelepipedic samples with dimensions 70 10 1.8 mm3. Finally, the samples were autoclave-cured.11 MI measurements were carried out using the waveguide technique in the frequency range of 8–12 GHz.2 The MI of the sample was obtained from the reflection coefficient of the microwire placed in the shorted sample holder. The distance from the wire to the metallic short was about 1 cm to ensure that the position of the microwire corresponds to the electric field antinode at the measurement frequency. MI at megahertz frequency was measured by Agilent 4294A precision impedance analyser. The MI ratio is defined as DZ=Z ¼ 100 Z ð H Þ Z ðHmax Þ =ZðHmax Þ, where Z(H) and Z(Hmax) represent, respectively, the impedance of a microwire in a magnetic field H and in the maximum field. Room temperature measurements of the effective (relative) complex permittivity e were carried out using a vector network analyzer (Agilent, model 8364A) in the frequency range of 3–18 GHz. The experimental technique is based on the measurement of the transmission and reflection coefficients of an asymmetric microstrip transmission line containing the sample. Software is employed to convert the scattering parameters into the complex permittivity of the material.12 We also performed measurements of e with an applied magnetic field by placing the line between the poles of an electromagnet. The magnetic field, parallel with the wire axis, was swept from 0 to 61 kOe. The dc was applied by connecting the wire to a current source. The electromagnetic measurement was carried out with a wave vector of the electromagnetic field which was perpendicular to the wires. The only mode propagating in the structure is the quasiTEM transverse electromagnetic mode. It is also worth stressing that all samples have a thickness and internal characteristic length of the heterogeneities much smaller than the wavelength of the electromagnetic wave probing the material samples. This suggests that, within the quasi-static analysis, the dominant loss mechanism arises from absorption rather than scattering. An error analysis indicates systematic uncertainties in e0 (
