Giant Magnetoimpedance effect at GHz frequencies in amorphous microwires

Magnetic properties and GMI effect of amorphous Co- rich microwires reveal that they present GMI effect at GHz frequencies. Frequency dependence of maximum GMI ratio has been evaluated. Magnetic field dependences of GMI ratio and frequency dependence of maximum GMI ratio are affected by the post-processing conditions. In particular, we observed that stress-annealing allows improvement of magnetic softness, GMI ratio and affects the frequency dependence of GMI ratio.


I. INTRODUCTION
tudies of Giant Magnetoimpedance (GMI) effect in different kind of magnetic materials have attracted considerable attention from the point of view of various technological applications as well as from the point of view of basic research [1][2][3][4]. One of the main tasks in this field is the achievement of the highest GMI effect that will allow improvement of the sensitivity of the magnetic sensors and devices utilizing GMI effect. In particularly small size and high magnetic field resolution are the features of magnetic field sensors made from thin magnetically soft microwires that have been proposed for magnetic compass applications in Cell phones [5]. Additionally, high frequency GMI effect is suitable for development of smart composites with tunable magnetic permittivity [2,6] Accordingly, development of thin magnetically soft wires is a key for GMI applications. It must be underlined that the diameter reduction must be associated with the increasing of the optimal GMI frequency range: a tradeoff between dimension and frequency is required in order to obtain a maximum effect [4]. Consequently development of thin soft magnetic materials required for miniaturization of the sensors and devices requires an extension of the frequency range for the impedance toward the higher frequencies (GHz range). On the other hand for a given chemical composition and geometry the optimal frequency for GMI effect is reported [2].
Accordingly, the purpose of this paper is to study the GMI effect in thin amorphous magnetically soft microwires extending the frequency range up to GHz band.

II. EXPERIMENTAL TECHNIQUE
We studied the GMI effect in extended frequency range up to GHz frequencies in Fe3.6Co69.2Ni1B12.5Si11Mo1.5C1.2 (metallic nucleus diameter, d=22.8µm; total diameter, D=23.2µm) microwire with low negative magnetostriction coefficients, λs, prepared using the Taylor-Ulitovsky technique [2]. The impedance, Z, and its magnetic field, H, dependence have been measured from the reflection coefficient, S11, evaluated by the vector network analyzer as described elsewhere [7]. From Z values obtained for different H-values we evaluated the magnetic field dependences of the GMI ratio, ΔZ/Z, defined as: being Z (H max ) -Z-values at maximum magnetic field, H max The frequency dependence of the maximum GMI ratio, ΔZ/Zm, defined as a maximum ΔZ/Z obtained at a given frequency, f, is also evaluated.
Hysteresis loops have been measured using the fluxmetric method previously successfully employed for characterization of magnetic microwires [8]. For better comparison we represent the normalized magnetization, M/M0, versus magnetic field, H, where M is the magnetic moment at a given H and M0 is the magnetic moment of the sample at the maximum magnetic field amplitude Hm.
We measured aforementioned properties in as-prepared and annealed microwires. For conventional and stress annealing we used a conventional furnace and annealing temperature, Tann, 350 o C.

III. EXPERIMENTAL RESULTS AND DISCUSSION
Excellent magnetic softness with coercivity, Hc, of about 5 A/m and magnetic anisotropy field, Hk, of about 150 A/m is observed in as-prepared sample (see Fig. 1a). Almost linear shape of hysteresis loops is characterized by almost zero remanence, Mr/Mo. In this sample ΔZ/Z(H) dependencies resent the double-peak character (for 10 ≤ f ≤1 GHz) typical for wires with circumferential magnetic anisotropy (see Fig. 1b). In spite of rather soft magnetic properties, ΔZ/Zm is slightly above 100% (Figs 1b,c).
Similarly to other Co-rich microwires [2,10], a substantial magnetic hardening and transformation of the linear hysteresis loop into a rectangular is observed upon annealing at 350 o C S (see Fig. 2a). However, stress-annealing at the same Tann allows to prevent such magnetic hardening and even achieve better magnetic softness with Hc ≈2 A/m and Hk ≈70 A/m (see Fig.2a). Consequently, a remarkable improvement of ΔZ/Zm up to 230% is observed upon stress-annealing (see Fig.2b).
Frequency dependence of ΔZ/Zm in as-prepared sample presents a maximum at about 80 MHz (see Fig. 2c). A superior ΔZ/Zm -values, observed in stress-annealed sample in the whole frequency range (up to 1 GHz) are evident for stress-annealed samples compared to as-prepared sample (see Fig.2c). It is interesting that the optimum frequency for stress-annealed sample shifts to about 150 MHz (Fig.2c).

Conclusion
We studied magnetic properties and GMI effect at elevated frequencies in as-prepared and stress-annealed Co-based microwires. Magnetic field dependences of GMI ratio are affected by the annealing conditions. Stress annealing allows remarkable improvement of the GMI ratio in extended frequency range.