Influence of heat treatment on structure and giant magnetoimpedance (gmi) effect of electrodeposited permalloy (80Ni20Fe)

Journal of Chemistry, Vol. 44 (3), P. 392 - 395, 2006 INFLUENCE OF HEAT TREATMENT ON STRUCTURE AND GIANT MAGNETOIMPEDANCE (GMI) EFFECT OF ELECTRODEPOSITED PERMALLOY (80Ni20Fe) Received 28 June 2005 MAI THANH TUNG1, NGUYEN HOANG NGHI2 1Dept of Electrochemistry and Corrosion Protection, Hanoi University of Technology 2Lab of Amorphous and Nanocrystalline Materials, Hanoi University of Technology SUMMARY Influence of heat treatment on structure and Giant Magnetoimpedance (GMI) effect of permalloy thin films (80Ni20Fe) electrodeposited on insulating substrates was investigated using electron scanning microscope (SEM), X-ray Diffraction and GMI ratio measurement. Results showed that annealing reduced lattice defects of electrodeposited films and facilitated the development of <111> texture, thereby enhanced magnetic properties of the obtained films. The increasing temperature resulted to the increase of maximum GMI ratio (GMIRmax) due to the higher defect reduction. GMIRmax increased with annealing time at both annealing temperatures 250oC and 450oC and reached the saturated value when the annealing time was 120 min. I - INTRODUCTION Materials with the so-called Giant Magnetoimpedance (GMI) effect, which is characterized by changes of electrical impedance when the materials are exposed in magnetic fields, have been intensively studied due to their numerous interesting applications [1, 2]. The GMI materials are particularly suitable for fabrication of high resolution magnetic sensors. In previous studies, we have shown that the permalloy (Fe20Ni80) film with GMI effect can be deposited on insulating substrates by the direct galvanic deposition technique [2, 4, 5]. In order to improve magnetic properties of the electrodeposited permalloy films, it is important to have a suitable heat treatment [1, 2, 6]. However, the effect of heat treatment on GMI of the electrodeposited film is still not well studied in Vietnam. In this study, we present results of study on 392 influence of annealing on structure and GMI effect of the permalloy film deposited on insulating substrates using direct galvanic deposition method. II - EXPERIMENTAL The electrodeposition was performed on alumina ceramic supplied by Good Fellows. The process of direct metallization consists of 2 steps: i) activation ii) metal electrodeposition. The activation is based on the so-called adsorption concept and it includes two steps: i ) adsorption by Co2+ and ib) sulphidication. The conductivity of the activation layer is ranged from 10-2 to 10-1 -1cm-1. The conditions of the process steps are given in table 1. Electro-chemical metal deposition was performed in a conventional electrochemical cell with Ni anode. The thickness of electrodeposited film is 3.5 - 4 µm. The annealing of the electro-deposited permalloy films was carried out in argon atmosphere. Table 1: Conditions of electrodeposition process steps Steps i) Activation Co2+ adsorption Sulphidication ii) Electrodeposition Solution (CoSO4.7H2O, oxidizer) Na2S, H2SO4 0.1 M Ni2+: 34.5 g/l; Fe2+: 1.5 g/l; NaCl: 6 g/l; Sacharine 0.2 g/l; pH 2.5; I = 2.5 A/cm2 Temperature, oC 25 25 25 Current density, mA/cm2 150 Time, minutes 3 1 7 Scanning Electron Microscopy (SEM) image of the obtained electrodeposits was measured using JMS 5410-Jeol (Japan) equipment. GMI curves were measured using GMI apparatus made by Laboratory of Amorphous and Nanocrystalline Materials (Hanoi University of Technology). The external magnetic field the GMI measurement changes from -400Oe to +400Oe. The GMI ratio (GMIR) was defined by the equation: GMIR = Z (1) where Z is impedance as external magnetic field H = 0, Z is change of impedance as external magnetic field H 0. III - RESULTS AND DISCUSSION Figure 1 shows SEM images of electrodeposited permalloy films without annealing (a); annealed at 250oC (b) and annealed at 450oC (c). Results show that increasing annealing temperature results finer crystal and films with fewer defects. It can also be seen that at 450oC, bumps due to the accumulation of electrodeposits tend to disappear. Structures of non-annealed and annealed films were analysed by XRD and results were shown in figure 2. It can be observed that both non-annealed and annealed films are solid solution of FeNi with <200> and <111> textures. However, the intensity ratio Ir = I<111> /I<200> of the annealed film increases following the order: Irnon-ann< Ir250oC < Ir450oC (Fig. 2), that means increasing annealing temperature results a faster development of the <111> texture. Since the <111> texture has higher magnetic orientation, the films with higher content of <111> texture are expected to have better magnetic properties. Figure 1: SEM images of electrodeposited permalloy thin films (annealing time 120 min) (a) non-annealing; (b) annealing at 250oC; (c) annealing at 450oC 393 100 <111> 80 Ir = 6.2 60 40 Ir = 5.5 20 Ir = 4.1 0 Ir = I<111>/I<200> <200> (c) Annealing 450oC -20 (b) Annealing 250oC -40 -60 (a) Non annealing 40 45 50 55 60 2 Figure 2: XRD pattern of electrodeposited permalloy thin films (annealing time 120 min) Figure 3 displays GMIR curves of the electrodeposited permalloy films without annealing (a); annealed at 250oC (b) and annealed at 450oC (c). It can be observed that annealing increases GMIRmax, but decreases the linearity of the GMI curve. In order to increase the linearity, which is important factor for operation of GMI sensor, the shape of electrodeposited films should be further optimized [2]. Figure 4 summarizes the dependence of GMIRmax on temperature and annealing time. It can be seen that annealing increases GMIRmax remarkably and GMIRmax of the film annealed at 450 C is higher than that of the film annealed at 250oC. These results can be explained by the fact that annealing reduces defects of electrodeposited films [6]. On the other hand, annealing facilitates the development of <111> texture, which results better magnetic films with lower magnetic coercivity Hc [2, 3, 6]. Both factors, reduction of defects and development of <111> textures, result the decrease of magnetic coercivity Hc and thereby the increase the GMIRmax of the permalloy films. At 450 C, the defects are reduced more completely, resulting highest GMIRmax of the film. On the other hand, the GMIRmax increases with annealing time, corresponding to the reducton of layer defects. When annealing time longer than 120 min, the number of defects in the films is very low and GMIRmax does not change with increasing annealing time. 50 GMIRmax (c) Annealing 450oC 40 (b) Annealing 250oC 30 (a) Non-annealing 20 Figure 3: GMIR curves of electrodeposited permalloy thin films (annealing time 120 min) 10 0 -300 -200 -100 0 100 200 300 H, Oe 394 50 t=120 min 45 t=180 min 40 35 t=90 min 30 t=60 min 25 20 t=30 min 15 10 0 100 200 300 400 500 Temperature, oC Figure 4: Influences of temperature and annealing time on GMIRmax of the electrodeposited permaloy film IV - CONCLUSIONS Annealing influences on both structure and Giant Magnetoimpedance (GMI) effect of permalloy thin films (80Ni20Fe) electrodeposited on insulating substrates. Results of SEM and XRD investigations show that annealing reduces defects of the electrodeposited films and facilitates the development of <111> texture, thereby enhance magnetic properties of the obtained films. The increasing temperature results the increase of maximum GMI ratio (GMIRmax) due to the higher defect reduction. GMIRmax increases with annealing time at both annealing temperatures 250oC and 450oC and reached the saturated value when the annealing time is 120 min. Acknowledgements: We thank the Basic Research Fund (Project Nr. 81.18.04) and VLIR-HUT Research Fund (Project Nr. VLIR-HUT/IUC/PJ10) for the financial support of this work. REFERENCES 1. N. H. Nghi, N. M. Hong, T. Q. Vinh, N. V. Dung, P. M. Hong. Phys. B, Vol. 327, P. 253 - 258 (2003). 2. M. T. Tung, N. H. Nghi. Report of the Basic Research Project Nr. KHCB-81.18.04 (2004). 3. V. D. Cu. Magnetism, Science and Technology Publishers, Hanoi (1996). 4. T. T. Mai, J. W. Schultze, G. Staikov. Electrochim. Acta, Vol. 49, P. 3021 - 3027, (2003). 5. T. T. Mai, J. W. Schultze, G. Staikov. J. Sol. Stat. Electrochem., Vol. 34, P. 823-830 (2002). 6. P. C. Andricacos, L. T. Romankiw. 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