GIANT MAGNETO-IMPEDANCE EFFECTS AND MAGNETIC PERMEABILITY IN CuBe/INSULATOR/CoNiP ELECTROLESS DEPOSITED COMPOSITE WIRES

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1 International Journal of Modern Physics B Vol. 25, No. 1 (2011) c World Scientific Publishing Company DOI: /S GIANT MAGNETO-IMPEDANCE EFFECTS AND MAGNETIC PERMEABILITY IN CuBe/INSULATOR/CoNiP ELECTROLESS DEPOSITED COMPOSITE WIRES J. K. CHENG College of Chemical Engineering, Guizhou University, Caijiaguan, Guiyang, , P. R. China jkcheng99@yahoo.com.cn Z. J. ZHAO, H. L. XIN, Z. M. WU, X. L. YANG and W. Z. YUAN Department of Physics, East China Normal University, 3663 Zhongshan North Rd, Shanghai, , P. R. China wzyuan@phy.ecnu.edu.cn Received 7 October 2008 Giant magneto-impedance (GMI) effects were investigated in CuBe/CoNiP and CuBe/Insulator/CoNiP composite wires prepared by electroless deposition on pure or insulated CuBe wires. Experimental results show that the maximal GMI ratio and the field sensitivity of the composite wires are further improved by adding an insulator layer between the CuBe core and ferromagnetic layer. A distinct MI effect was observed in the composite wires with insulator even at low frequencies. MI ratio is about 18% at 20 khz and 239% at 600 khz. The maximal field sensitivity is 34%/Oe at 600 khz. The properties of GMI in these composite wires are analyzed via complex permeability. Keywords: Giant magneto-impedance; CoNiP; electroless deposition; insulator. 1. Introduction Giant magneto-impedance (GMI) effect has attracted much interest over the past years because of its potential applications in highly sensitive magnetic sensors and magnetic recording heads. 1,2 GMI was first observed in homogenous materials 3,4 and has been discovered in many soft magnetic materials. 5 7 Recently, the study of GMI is focused on heterogeneous materials such as sandwiched films and composite wires It is reported that GMI ratios can be further improved by adding another insulating layer between the conductive layer and ferromagnetic layer in sandwiched films. 12 More attention has been paid to the composite wire for it can be easily connected to external circuit by spot welding and the contact resistance and capacitance are very low. Enhancements of the GMI effect in CuBe/Insulator/NiFeB Corresponding author. 111

2 112 J. K. Cheng et al. composite wires were reported by Wang 13,14 and a theoretical analysis on the effect of insulator layers on GMI in composite wires was developed by Buznikov. 15 However, the physical mechanisms of GMI in composite wires with insulator layers have not been sufficiently understood up to now and further study is needed. The most attractive advantage of electroless deposition is that metallic layers can be deposited on non-conductive substrates. The deposited layers usually have finemicrostructureandgoodcorrosionresistance. 16 Inthispaper,CuBe/CoNiPand CuBe/Insulator/CoNiP composite wires were prepared by electroless deposition, in which the GMI effect, especially at low frequency, is studied. The GMI characteristics are analyzed by considering the magnetization processes and the frequency dependence of magnetization of the composite wires via complex permeability. 2. Experiment The composite wire specimens were produced by electroless depositing CoNiP layer (12 µm in thickness) on CuBe wires of 90 µm in diameter without or with polyester insulator coating, respectively. The pure CuBe wires were cleaned thoroughly in acidic and alkaline solution and activated by Pd/Sn particles before being mounted into an electroless deposition bath. The other pre-treatment procedures of the CuBe wires with insulator coating (5 µm in thickness) were similar to those of the pure CuBe wires, besides, they were dipped into an ethanol solution of silane coupling agent before activation. 17 The main chemicals in the electroless deposition bath were nickel sulfate, cobalt sulfate, ammonium sulfate, sodium hypophosphite, trisodium citrate and sodium tartrate dehydrate. The temperature of the bath, ph value and plating time was kept at 90 C, 9.0 and 3 h, respectively. In order to improve the soft magnetic properties of the ferromagnetic layer, the deposited samples were further annealed at 180 C for 30 min in nitrogen atmosphere to release internal stress that may have been built up during deposition. The magneto-impedance measurement were carried out by using a precise impedance analyzer (HP 4294A). The two terminals of internal CuBe core are connected to the probe adapter alligator clip leads of the impedance analyzer. The length of the composite wire is 4 cm. The RMS value of AC current was kept at 10 ma and its frequency ranged from 10 khz to MHz. The GMI ratio was defined as Z Z % = Z(H ex) Z(H ex = 0) %. (1) Z(H ex = 0) where H ex was the external magnetic field provided by a pair of Helmholtz coils. The maximum external magnetic field used for the measurement was. The direction of H ex was vertical to geomagnetic field and parallel to the longitudinal axis of the composite wire.

3 GMI Effects and Magnetic Permeability in CuBe/Insulator/CoNiP Wires 113 Z/Z(%) khz 20 khz 30 khz 40 khz khz 600 khz 1 MHz 2 MHz 4 MHz Hex(Oe) Fig. 1. Field dependence of the GMI ratio of CuBe/Insulator/CoNiP composite wire. 3. Results and Discussion Figure 1 shows the field dependence of the GMI ratio of CuBe/Insulator/CoNiP composite wire. It can be seen that GMI effect is observed at considerably low frequencies in this composite wire. GMI ratio is 17.7% at 20 khz. The external field corresponding to the peak value increases with increasing frequency which is 8 Oe at 20 khz and 1 at 4 MHz. Figure 2 shows the frequency dependence of the maximal GMI ratio of CuBe/Insulator/CoNiP and CuBe/CoNiP composite wires. It can be seen that CuBe/Insulator/CoNiP CuBe/CoNiP Z/Z (%) f (khz) Fig. 2. Frequency dependence of the maximal GMI ratio.

4 114 J. K. Cheng et al. the GMI ratio increases initially with increasing frequency to a peak value, and then decreases with further increasing frequency. The largest GMI ratios of two kinds of composite wires are observed at the same frequency of about 600 khz. The maximal GMI ratio of CuBe/Insulator/CoNiP composite wire is much higher than that of CuBe/CoNiP composite wire. After adding an insulator layer, the maximal GMI ratio of composite wire increases from 14% to 31% at 30 khz and from 51% to 239% at 600 khz. The GMI ratio is above 200% in the frequency range from 200 khz to 2 MHz and above% in the frequency rangefrom 760kHz to 14 MHz, which indicates that the wire can be used in a wide frequency band. The anisotropy field of CuBe/Insulator/CoNiP composite wire is lower than that of CuBe/CoNiP composite wire at the same frequency. It is and for the composite wires with and without insulator at 600 khz, respectively. Meanwhile, the composite wire with an insulator shows much higher field sensitivity. The maximal field sensitivity is 34%/Oe for CuBe/Insulator/CoNiP composite wire while it is only 3.5%/Oe for CuBe/CoNiP composite wire at 600 khz. Although the insulator layer may prevent the AC driving current from penetrating into the CoNiP layer for CuBe/Insulator/CoNiP composite wire, the CoNiP layer can still be magnetized by the magnetic field induced by AC current. The magnetization changes the driving current distribution of the composite wire. The variation of the magnetic flux in CoNiP layer will induce eddy current. After an external magnetic field is added, the permeability of CoNiP layer will change, which leads to the variation of the eddy current loss and resistance. Thus, the impedance will be changed. The main mechanism of GMI effect is that the dynamic permeability of ferromagnetic layer is in variation with external magnetic field. Using the following Eq. (2), the experimental data of GMI can be converted to those of effective permeability, which is directly related to magnetization processes. 18 Thereafter GMI effect can be investigated by analyzing effective permeability µ = µ jµ = (1/jωG)(Z T Z Cu ), (2) where µ is the effective permeability of the composite wire, Z T is the total impedance (Z T = Z M + Z Cu ), Z M is the impedance of ferromagnetic layer (Z M = Z + jz ), Z Cu is the impedance of CuBe core measured on bare CuBe wires at various frequencies and fields, G is the geometry factor related to the shape of material. Figure 3 shows the frequency dependence of real component µ of magnetic permeability for CuBe/CoNiP and CuBe/Insulator/CoNiP composite wires. As shown by Fig. 3, µ keeps a constant value with the increase of the frequency for CuBe/Insulator/CoNiP composite wires at H ex =. Such behavior indicates that the sample has a preferential circumferential anisotropy H k. 2 Because the magnetic field induced by the driving current is a circumferential field, the magnetization process is mainly dominated by the movement of domain wall. After a DC external magnetic field H ex was added, the magnetic moment begins to deviate from the circumferential direction, which leads to the growing contribution

5 GMI Effects and Magnetic Permeability in CuBe/Insulator/CoNiP Wires CuBe/CoNiP wire µ' CuBe/Insulator/CoNiP wire f (khz) Fig. 3. Frequency dependence of real component µ of permeability. µ" CuBe/CoNiP wire CuBe/Insulator/CoNiP wire f (khz) Fig. 4. Frequency dependence of imaginary component µ of permeability. of moment rotation to magnetic permeability. Hence, µ increases with H ex when H ex is below H k. Then, µ decreases with further increase of H ex since the rotation of magnetic moment is difficult. At H ex, the real component of effective permeability changes obviously with H ex at low and intermediate frequencies. For CuBe/CoNiP composite wires the maximal variation of µ is 320, calculated by equation: µ (H ex = ) µ (H ex = ). For CuBe/Insulator/CoNiP composite wires it is 1590, calculated by equation: µ (H ex = ) µ (H ex = ). In other words, after adding insulator layer the maximal variation of µ increases about 397%.

6 116 J. K. Cheng et al. Figure 4 shows the frequency dependence of imaginary component µ of effective permeability for CuBe/CoNiP and CuBe/Insulator/CoNiP composite wires. As shown by Fig. 4, µ decreases initially and then increases with increasing frequency at H ex =. After adding a DC external magnetic field, µ increases initially and then decreases with H ex. While H ex is equal to the anisotropy field H k, µ reaches the largest value. At intermediate frequencies, µ changes obviously with H ex. For CuBe/CoNiP composite wire the maximal variation of µ is about at 6 MHz, calculated by equation: µ (H ex = ) µ (H ex = ). For CuBe/Insulator/CoNiP composite wire it is about at 6 MHz, calculated by equation: µ (H ex = ) µ (H ex = ). That is to say after adding insulator layer the maximal variation of µ increases about 288%. The great enhancement of GMI effect after adding insulator layer is closely related to the magnetic properties of µ and µ. 4. Conclusions In this paper, CuBe/CoNiP and CuBe/Insulator/CoNiP composite wires were prepared by electroless deposition. Experimental results show that GMI effect is greatly enhanced by adding an insulator layer between the CuBe core and the ferromagnetic layer. The maximal GMI ratio increases from 51% to 239% at 600 khz. The GMI ratio of CuBe/Insulator/CoNiP composite wire is above % in the frequency range from 760 khz to 14 MHz, which indicates that this wire can be used in a wide frequency band. The maximal field sensitivity is 34%/Oe at 600 khz. After adding insulator layer the maximal variation of µ and µ increases about 397% and 288%, respectively. The distinct GMI effect of CuBe/Insulator/CoNiP composite wire is closely related to the magnetic properties of effective permeability. 5. Acknowledgments This work is supported by National Natural Science Foundation of China (Grant No ), the Emphasis Project of Shanghai Nanotechnology Promotion Center (0652nm036) and Shanghai Rising Star program (04QMX1422). References 1. K. Mohri, T. Uchiyama and L. V. Panina, Sens. Actuators A 59, 1 (1997). 2. L. V. Panina, K. Mohri, T. Uchiyama and M. Noda, IEEE Trans. Magn. 31, 1249 (1995). 3. R. S. Beach and A. E. Berkowitz, Appl. Phys. Lett. 64, 3652 (1994). 4. L. V. Panina and K. Mohri, J. Magn. Soc. Jpn. 18, 245 (1994). 5. M. Knobel, J. Schoenmaker, J. P. Sinnecker, R. Sato Turtelli, R. Grössinger, W. Hofstetter and H. Sassik, Mater. Sci. Eng. A , 546 (1997). 6. H. Chiriac, F. Vinai, T. A. Óvári, C. S. Marinescu, F. Barariu and P. Tiberto, Mater. Sci. Eng. A , 646 (1997). 7. S. J. Ahn, C. G. Kim, C. G. Park and S. C. Yu, Mater. Sci. Eng. A , 1026 (2001).

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