GMI Effect in Co-rich Glass Coated Microwires for Sensor Applications

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1 Sensors & Transducers Magazine, Vol., Issue 3, March 2, pp.7-8 Sensors & Transducers ISSN by IFSA GMI Effect in Co-rich Glass Coated Microwires for Sensor Applications Arcady ZHUKOV,2*, Karin GARCÍA 3, Marek KUZMINSKI, Valentina ZHUKOVA 2, Henryk LACHOWICZ, Julian GONZALEZ,2 and Manuel VAZQUEZ 3 Dpto. Física de Materiales, Fac. Química, UPV/EHU, Apdo. 72, 28 San Sebastián, Spain, Phone: , Fax: , wupzhuka@sp.ehu.es 2 TAMag Iberica S.L., Parque Tecnológico de Miramón, Paseo Mikeletegi 52, ª Planta, 29 San Sebastián, Spain Pnone: , Fax: , tamagiberica@wanadoo.es 3 Instituto de Ciencia de Materiales, CSIC, 289 Cantoblanco, Madrid, Spain Pnone: , Fax: , <klgarcia@icmm.csic.es> Institute of Physics, Polish Academy of Sciences, Al.Lotników 32/6, Warszawa, Poland Pnone: , Fax: Received: 3 March 2 /Accepted: 2 March 2 /Published: 23 March 2 Abstract: GMI effect of soft magnetic Co 67 Fe 3.85 Ni.5 B.5 Si.5 Mo.7 glass-coated amorphous microwires has been investigated. In the as-prepared sample, maximum GMI ratio, Z/Z max depends on the sample geometry, being highest (about 6%) in samples with thinner glass coating. Joule heating has been performed without magnetic filed (CA) under axial magnetic field (MFA). It has been shown, that even in samples with lower Z/Z max the Joule heating enables to enlarge the Z/Z max ratio from 38% up to 5%. A strong dependence of Z/Z max on annealing time, t ann, frequency, f, and amplitude of the driving current, i, has been observed. Z/Z max (t ann ) dependences with f and i as parameters, showed an increase of the anisotropy field, H m, increasing t ann for CA annealing. In contrast, MFA results in an increasing of H m increasing with t ann. The observed dependencies have been interpreted in terms of stress relaxation and changes of the magneto-elastic anisotropy induced by the Joule heating. Keywords: glass-coated microwires, giant magneto-impedance, thermal treatments 7

2 Sensors & Transducers Magazine, Vol., Issue 3, March 2, pp.7-8 The giant magnetoimpedance (GMI) effect has became a topic of great interest for sensor applications owing to the high sensitivity of the impedance to the DC applied magnetic field (usually about 3% in conventional Co-rich amorphous wires) []. Such changes of the impedance of a magnetic conductor have been successfully interpreted in terms of the classical skin effect of magnetic conductor, because of the change of the penetration depth of an AC-current flowing through a sample caused by the applied DC-field []. The frequency (f) of the AC current flowing along the sample is high enough (typically above khz). The electrical impedance, Z, of a magnetic conductor is given by [, 2]: Z = Rdc kr Jo(kr) / 2 J(kr) () with k = (+j)/δ where Jo and J are the Bessel functions, r wire s radius and δ the penetration depth given by: δ = (π σ µφ f) -/2, (2) where σ is the electrical conductivity, f is the frequency of the current along the sample, and µφ is the circular permeability assumed to be constant. The DC applied magnetic field changes the penetration depth through the modification of µ φ, which finally results in a change of the impedance [-3]. Enhanced GMI ratio in magnetically soft amorphous wires has been related with a specific domain structure of soft amorphous wires consisting of an outer domain shell exhibiting high circumferential permeability [, 2]. It should be noted that more detailed description taking into account tensor character of the magnetoimpedence (i.e. taking into account ferromagnetic origin of the samples) is needed to explain some particular features such as AC driving current dependence, torsion impedance etc [, 5]. Such high sensibility to the low magnetic fields makes GMI effect very attractive for numerous sensor applications [6-8]. The presently strong trend to miniaturization of magnetic elements has resulted in the development of the Taylor-Ulitovsky method, which permits to produce tiny ferromagnetic metallic wires ( 3 µm in diameter) covered by an insulating glass coating [6-8]. Recent significant progress in tailoring glass coated microwires fabricated by this method enabled to enhance significantly the GMI ratio (up to about 6%) [6-8]. In this paper we report and analyze the results of tailoring the GMI ratio of Co-rich glass-coated amorphous thin microwires (with the metallic nucleus diameter about 2-22 µm) by choosing the sample geometry (thickness of glass coating) and conditions of heat treatment by Joule heating. 2. Experimental Details Magnetically soft, glass-coated Co 67 Fe Ni.5 B..5 Si.5 Mo.7 microwires of different geometric ratio, ρ, of metallic core diameter to total microwire diameter.789 ρ.98 has been fabricated by the Taylor-Ulitovsky method [3-8]. The sample geometry, i.e. average values of the diameter of the metallic core, d, total diameter, D and the ratio, ρ =d/d, measured at several positions along the sample are presented in Table. The hysteresis loops of the samples have been measured by conventional fluxmetric method. The electrical impedance of the microwire was evaluated by means of the four-point technique. The magneto-impedance ratio, Z/Z, has been defined as: 75

3 Sensors & Transducers Magazine, Vol., Issue 3, March 2, pp.7-8 Z/Z = [ Z (H) - Z (Hmax)] / Z (Hmax), (3) Table. The sample geometrical characteristics Sample Metallic core diameter, d (µm) Total diameter, D(µm) Ratio ρ =d/d A B C An axial DC-field with intensity up to 8 ka/m was supplied by a Helmholtz coils. The dependence of the magnetoimpedance ratio, Z/Z, on the axial field, H, at the driving AC-current, I, ranging from.75 up to of the frequency, f, in the range - 3 MHz, both treated as the parameters, have been investigated. Thermal treatment has been realized by Joule heating passing along the microwire-sample a DC-current of 3 and ma, at various time of this treatment. This Joule heating has been performed without additional external field (CA) and under axial applied magnetic field of about 8 ka/m (MFA). Electrical contacts were made removing mechanically the glass insulating layer at the sample edges and soldering them with the Cu cables. 3. Results and discussion Measured bulk hysteresis loops of three magnetically soft, glass-coated Co 67 Fe 3.85 Ni.5 B.5 Si.5 Mo.7 micro-wires of various geometric ratio.78 ρ.98 are shown in Fig.. From Fig. it should be noted that all the samples exhibit low magnetic anisotropy field (5-2 A/m, see Fig.). As it can be observed in this figure, the anisotropy field, H k, increases with decreasing ρ ratio, i.e. with the increasing of the glass coating thickness. On the other hand, the field H m,, corresponding to the maximum of the GMI ratio, increases and ( Z/Z)max decreases with the ρ ratio, as it is seen in Fig.2. Such a systematic tendency allows to assume that the parameter ρ and not only the diameter of the metallic core and/or the total diameter of the microwire is the most important geometric parameter, which affects both, the hysteretic and GMI features. Therefore, soft magnetic properties and GMI effect can by tailored by the sample geometry. It means that.6 the internal stresses, arising from the difference. between the thermal expansion coefficients of the metallic core and glass coating, strongly affect the.2 aforementioned properties. µ o M(T) ρ=.982 ρ=.86 ρ= H (A/m) Fig.. Bulk hysteresis loop of three samples with ρ as a parameter 76

4 Sensors & Transducers Magazine, Vol., Issue 3, March 2, pp.7-8 Therefore, the relaxation of such internal stresses by means thermal treatment should drastically change both, soft magnetic behaviour and Z/Z(H) dependence. This challenge is of a special importance since the fabrication of the microwires with the glass coating thickness of about.2 µm (sample A) is a difficult task. This is why an identification of the processing procedure, which permits to improve GMI effect of glass-coated microwires, is very important from the viewpoint of applications. The effect of Joule heat treatment was performed using the sample B (see Table ) Z/Z(%) (c) (d) frequency and driving current amplitude, varying solely the Joule heating conditions. These dependencies are presented in Fig.a (annealing current ) and in Fig.b ( ma). Another important parameter affecting the Z/Z(H) dependencies is the driving AC-current amplitude. Fig.5 shows the effect of the driving current amplitude on the GMI behaviour of the as-prepared and annealed micro-wires Fig.3. Z/Z(H) dependences measured in as-prepared microwire at I = ma subjected to CA annealing at for 2 minutes at ma for 2 minutes (c) and at ma for minutes for f= MHz (), MHz (2), 2 MHz (3)and 3 MHz () Z/Z, (%) 6 2 f=mhz ρ=.98 ρ=.86 ρ= H (A/m) Fig.2. Axial field dependence of Z/Z at f= MHz and I =.7 in microwire with ρ as a parameter As expected, the performed Joule heating strongly affects the Z/Z (H) dependence. Fig.3 shows this dependence measured for as -prepared and annealed (CA treatment) samples with the frequency, f, as a parameter. It is well recognized that the maximum value of the GMI ratio, Z/Z max, increases with f as well as after CA treatment. Besides, the value of the axial DC-field, H m, corresponding to the maximum of the GMI ratio, increases also as a result of the Joule heating. In order to illustrate better the effect of the Joule heating only, the Z/Z(H) dependencies have been measured at the same conditions, i.e. at the fixed Z/Z(%) as prepared 2 min min min -2-2 min min 2 min as-prepared Fig.. Z/Z(H) dependences measured at f = 3 MHz and I= ma in microwire to CA annealing at and at ma. 77

5 Sensors & Transducers Magazine, Vol., Issue 3, March 2, pp.7-8 In this way there are few parameter which permit to tailor the GMI effect of glass coated microwires, such as time, t ann, and DC current, I, of Joule heating, frequency, f, and driving AC-current amplitude, I, during the measurements of the GMI effect. The effect of all these parameters on the maximum GMI ratio, Z/Z max, is summarized in the Fig.6. It is worth to notice that not only the maximum of the GMI ratio, Z/Z max, but also the shape of Z/Z(H) dependences change under the effect of the current annealing. In particular, the field corresponding to the maximum of the GMI ratio, H m, depends on the current annealing conditions. Fig.7 summarizes the effect of all the mentioned parameters on H m. Similarly, to the CA, the MFA treatment also induces changes in both hysteresis loops and magnetic field dependence of GMI (see Figs. 8,9). In this case, the external axial magnetic field applied during the MFA treatment induces axial magnetic anisotropy, such as can be appreciated from Fig.8. Consequently, both Z/Z max, and H m after MFA treatment change by the different way owing to such induced magnetic anisotropy (see Fig.9). Experimental dependencies observed at various samples conditions of current annealing can be attributed to the effect of internal stresses, σ, on the domain structure as well as to the magnetic anisotropy. Indeed, the value of the axial DC-field, H m at which the GMI-ratio achieves maximum corresponds to the static circular anisotropy field, H k [, ]. The estimated values of the internal stresses in these amorphous microwires are of the order of - MPa, depending strongly on the thickness of glass coating and metallic core diameter []. It was established that the strength of such internal stresses increase with incrreasing the glass coating thickness. Such large internal stresses give rise to a drastic change of the magnetoelastic energy, K me 3/2 λ s σ i, even for small changes of the glass-coating thickness at fixed metallic core diameter. Consequently, such change of the ρ ratio should be related to the change of the magnetostriction constant []: Z/Z(%) mA 3mA ma mA 3mA 2 ma 3 (c) ma Fig. 5. Effect of driving current amplitude on Z/Z(H) dependences measured at f= MHz in as-prepared, subjected to CA annealing at ma for 2 min and at ma for min (c) microwires. Z/Z(%) max f=3 MHz t ann (min) ma f= MHz f= MHz λ s = (µ ο M s /3)(dH k /dσ), () where µ ο is the free space permeability equals π -7 H/m and M s - the saturation magnetization. In fact, general increase of the H m value with annealing time has been observed for the CA treatment (see Fig.8), Fig. 6. Effect of driving current amplitude on Z/Z max measured in microwires annealed (CA treatment ) at (dot lines) and at ma (solid lines) measured and effect of frequency on Z/Z max measured at I = ma in microwires annealed (CA treatment ) at (dot lines) and at ma (solid lines) 78

6 Sensors & Transducers Magazine, Vol., Issue 3, March 2, pp.7-8 which according to () should be attributed to the increasing of the magnetic anisotropy field with t ann. In contrast general decrease of the H m value with annealing time observed for the MFA treatment (see Fig.9), should be related to the induction of the axial magnetic anisotropy. 5 3 MHz MHz MHz 3 MHz Taking into account the phenomenologically found [2] stress dependence of the magnetostriction λ s (σ)= λ s ()-Aσ, (5) where λ s () is the saturation magnetostriction constant without applied stresses and A is the positive coefficient of the order of - MPa; one can assume that stress relaxation should give rise to a decrease of the H m value. Such contradiction can be attributed to the induction of the circular magnetic anisotropy after the CA due to the effect of the circular magnetic field created by the DC current during the Joule heating. H m (A/m) MHz MHz ma 5 ma t ann (min) Fig. 7. Effect of frequency on H m measured at MHz in microwires annealed (CA treatment) at (dot lines) and at ma (solid lines) and effect of driving AC-current amplitude measured in the same microwires at I= ma M/M s M/Ms,,5, -,5 -, As-cast CA at CA at ma As-cast MFA at MFA at ma Z max /Z H m (Oe) MFA () CA () Annealing time (min) CA () MFA () Fig. 8. Effect of CA and MFA treatments on bulk hysteresis loops of studied microwires. Fig. 9. Effect of CA and MFA treatments on Z/Z max and H m of studied microwires measured at 3 MHz and ma. 5. Conclusions It has been shown that the Joule heating without external magnetic field and under axial magnetic field of amorphous Co 67 Fe 3.85 Ni.5 B.5 Si.5 Mo.7 microwire can significantly change the Z/Z(H) dependencies and significantly improve the Z/Z max ratio. (from 38 up to 5 %). Substantial 79

7 Sensors & Transducers Magazine, Vol., Issue 3, March 2, pp.7-8 dependence of ( Z/Z)(H) on frequency and amplitude of the driving AC-current, DC bias current and geometric ratio, ρ, has also been observed. The shape of the experimental dependencies is noticeably affected by the conditions of current annealing (CA or MFA annealing) which should be attributed to the effect of internal stresses, σ, on the domain structure as well as to the magnetic anisotropy. References [] L.V. Panina and K. Mohri, Appl. Phys. Lett. 65, (99) pp [2] R.L. Sommer and C.L. Chien, J. Appl. Phys. 79, (996) pp [3] R. S. Beach, A. E. Bertowitz, Appl. Phys. Lett. 6 (99) pp [] N.A. Usov, A.S. Antonov and A.N. Lagar`kov, Theory of giant magneto-impedance effect in amorphous wires with different types of magnetic anisotropy J. Magn. Magn. Mat., 85 (998) pp [5] Makhnovskiy D. P., L.V. Panina and D. J. Mapps, Phys.Rev.B, 63 (2) pp.2-7. [6] A. Zhukov, J. González, M. Vázquez, V. Larin and A. Torcunov Nanocrystalline and Amorphous Magnetic Microwires Enciclopedia of Nanoscience and Nanotechnology, Chapter 62, Ed. H.S. Nalwa, American Scientific Publishers (2) p.23. [7] A. Zhukov, J. Magn and Magn, Mater (22) pp [8] Y. Honkura, J. Magn. Magn. Mat. 29 (22) pp [9] L. Kraus, Z. Frait, K. Pirota and H. Chiriac, J. Magn. Magn. Mat , (23) pp [] V. Zhukova, A. Chizhik, A. Zhukov, A. Torcunov, V. Larin and J. Gonzalez, IEEE Trans. Magn. 38, (22) pp [] M. Knobel, C. Gómez-Polo and M. Vázquez, J. Magn. Magn. Mat. 6 (996) p. 23. [2] J. Velázquez, M. Vazquez and A. Zhukov, J. Mater. Res. 299 (996) pp [3] J.M. Barandiaran, A. Hernando, V. Madurga, O.V. Nielsen, M. Vázquez and M. Vázquez - López, Phys. Rev. 35 (987) pp , also J.M. Blanco, L. Domínguez, P. Aragoneses and J. Gonzalez, J. Magn. Magn. Mat. 86 (998) pp Copyright, International Frequency Sensor Association (IFSA). All rights reserved. ( 8