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Bath temperature effect on magnetoelectric performance of Ni–lead zirconate titanate–Ni laminated composites synthesized by electroless deposition
Journal of Magnetism and Magnetic Materials 323 (2011) 422–426
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Bath temperature effect on magnetoelectric performance of Ni–lead zirconate titanate–Ni laminated composites synthesized by electroless deposition W. Wu, Y.G. Wang n, K. Bi College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 July 2010 Received in revised form 16 September 2010 Available online 8 October 2010
Magnetoelectric (ME) Ni–lead zirconate titanate–Ni laminated composites have been prepared by electroless deposition at various bath temperatures. The structure of the Ni layers deposited at various bath temperatures was characterized by X-ray diffraction, and microstructures were investigated by transmission electron microscopy. The magnetostrictive coefficients were measured by means of a resistance strain gauge. The transverse ME voltage coefficient aE,31 was measured with the magnetic field applied parallel to the sample plane. The deposition rate of Ni increases with bath temperature. Ni layer with smaller grain size is obtained at higher bath temperature and shows higher piezomagnetic coefficient, promoting the ME effect of corresponding laminated composites. It is advantageous to increase the bath temperature, while trying to avoid the breaking of bath constituents. & 2010 Elsevier B.V. All rights reserved.
Keywords: Magnetoelectric Laminated structure Electroless deposition Bath temperature
1. Introduction The magnetoelectric (ME) effect is characterized by the appearance of electric polarization in response to an applied magnetic field and/or magnetization in response to an applied electric field [1]. Many potential applications such as sensors, actuators and transducers have been proposed for ME materials based on their unique multifunctionality [2,3]. Up to now there are no single-phase ME materials practically applicable due to their low Ne´el or Curie temperatures and weak ME responses. Alternatively, the ME composites show much stronger extrinsic ME effect at room temperature according to the product effect [4]. Particularly, the laminated ME composites exhibit much higher ME voltage coefficient compared with the particulate ME composites, because many obstacles, such as the leak current of magnetostrictive phase and chemical reactions between phases, have been overcome. In the last decade, the laminated ME composites with Tb1 xDyxFe2 y (Terfenol-D) or ferrites as magnetostrictive phase and with lead zirconate titanate (PZT) or polyvinylidene fluoride as piezoelectric phase have been reported [5–10]. The ME voltage coefficient of the laminated composites is greatly dependent on the coupling between the magnetostrictive and piezoelectric phases besides selecting the suitable material components, so interfacial bonding becomes an important factor. There are various methods for preparing the laminated ME composites with different kinds of interfacial bonding. Ryu et al. [11]
n
Corresponding author. Tel./fax: +86 25 52112626. E-mail address: [email protected] (Y.G. Wang).
0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.09.034
reported Terfenol-D/PZT/Terfenol-D laminated ME composites with unprecedented high ME voltage coefficient of 4.68 V cm 1 Oe 1 by bonding PZT and Terfenol-D disks with epoxy. However, the ME composites with polymer as a binder have the disadvantages of nonrigid contact, fatigue and aging effect. Pan et al. [12–14] realized the giant ME coupling in Ni–PZT and Ni–PZT–Ni structures by electrodeposition. Although one can get rigid contact between the magnetostrictive and piezoelectric phases, electrodes are unavoidably involved in the electrodeposition process and certainly deteriorate the interfacial bonding and weaken the coupling between phases. Thin film methods such as magnetron sputtering, pulsed laser deposition and thermal evaporation can be used to fabricate the laminated ME composites without interlayer, but they are available only for 5 mm-thick layers [15–17]. Electroless deposition is a convenient method for preparing a thick functional film without polymer binder or electrodes. Moreover, it has many other outstanding merits, including low cost, low process temperature and excellent filling capability [18]. Most recently, we have successfully prepared Ni–PZT–Ni laminated composites by electroless deposition to strengthen the coupling between the magnetostrictive and piezoelectric phases [19]. In electroless deposition the operating conditions such as pH value and bath temperature dominate the microstructure of the Ni layers, on which the ME voltage coefficient significantly depends. We have studied the dependence of the ME coupling on the pH value of the bath and found that the ME voltage coefficient increases with the pH value [20]. In this work, we investigated the effect of bath temperature on the ME performance of Ni–PZT–Ni laminated composites derived by electroless deposition.
W. Wu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 422–426
2. Experimental details 2.1. Sample fabrication Prior to electroless deposition, the PZT flakes of size 18 5 0.6 mm3 (L W H) were first cleaned in an alcoholic solution and then roughened in a hydrofluoric acid solution, aiding the adhesion of the subsequent metallic coating to the substrate. Dipped in a solution of stannous chloride–hydrochloric acid and an acidified palladium chloride solution, the PZT flakes were sensitized and activated, respectively. In the activating stage, tin ions reacted with palladium ions as follows: Pd2 + +Sn2 + -Pd + Sn4 + . The seed crystals of palladium acted as a catalyst for the electroless deposition when the PZT flakes were dipped into the nickel bath. Afterwards, the PZT flakes were reduced in a sodium hypophosphite solution, preventing the unnecessary palladium ions’ from entering the deposition baths. Distilled water was used to rinse the PZT flakes at the end of each step. Electroless deposition of Ni was carried out using a solution of NiSO4 6H2O, N2H4 H2O and NaKC4H4O6 4H2O. The concentration of each chemical is described in detail elsewhere [19]. During the process the solution temperature was maintained at 358, 360.5,
423
363 and 365.5 K. The pH values of the baths were adjusted using sodium hydroxide and kept at 10. The thickness of the Ni layers was controlled by the electroless deposition time. The samples were polarized in an electrical field of 30 kV cm 1 applied perpendicular to the sample plane after deposition. The geometry arrangement of the Ni–PZT–Ni laminated composites is shown illustratively in Fig. 1(a). 2.2. Measurements The element composition of the deposited Ni layer was evaluated by EDX analysis. The structure of the deposited Ni layer was obtained by X-ray diffraction (XRD, D8 Advance Bruker, Cu Ka) analysis. The microstructures of the deposited Ni layers were investigated by transmission electron microscopy (TEM, FEI Tecnai G2). Cross-sectional scanning electron microscopy (SEM) was used to verify the structure of the composites. The magnetostrictive property of the deposited Ni layer was measured by a resistance strain gauge with a magnetic field applied parallel to the sample plane. During the ME measurement, the induced voltage dV across the sample was amplified and measured by an oscilloscope. The ME voltage coefficient aE was calculated according to the equation aE ¼ dV/(tPZT dH), where tPZT is the thickness of PZT layer and dH the amplitude of the AC magnetic field generated by Helmholtz coils, which were kept at 1.2 Oe in our experiment. The bias magnetic field Hdc and AC magnetic field dH were applied parallel to the sample plane for the transverse coefficient aE,31. The DC bias magnetic field could be changed from 0 to 2 kOe and the frequency f of AC magnetic field varied in the range 1–150 kHz.
3. Results and discussion As shown in the cross-sectional SEM micrograph in Fig. 1(b) the electroless deposited Ni layer contacts well with the PZT layer and it indicates that Ni–PZT–Ni laminated composites with neither electrodes nor bonding layers have been prepared by electroless deposition. The dependence of the Ni electroless deposition rate on applied bath temperature is shown in Fig. 2. It can be seen that the electroless deposition rate increases approximately linearly with the bath temperature. The linear fit does a good job of predicting the Ni deposition rate dependence on bath temperature (R ¼0.99471). The deposition rates are about 8.40, 11.38, 15.76 and 20.58 mm h 1 when the bath temperatures are 358, 360.5, 363 and 365.5 K, respectively. It is beneficial to increase the bath
Fig. 1. (a) Schematic illustrating the geometry arrangement of the Ni–PZT–Ni laminated composites. Vector P shows the PZT polarization direction. The other vector identifies the direction of applied magnetic field. (b) Cross-sectional SEM micrograph shows the clear structure of the composite electroless deposited at 358 K.
Fig. 2. Dependence of the Ni electroless deposited rate on bath temperature.
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temperature to improve the efficiency of deposition, but too high a temperature will lead to the breaking of bath constituents. Maintenance of appropriate bath temperature is required for ensuring both optimum deposition rate and bath stability. Fig. 3 gives the EDX spectrum of the Ni layer. There are only Ni peaks in the EDX spectrum, which indicate that no impurity exists in the Ni layer. The XRD patterns of Ni layers electroless deposited at various temperatures are shown in Fig. 4. It can be seen that all the deposited Ni layers have similar structure and there is only Ni phase in the samples. Fig. 5 shows the bright field TEM images of Ni layers obtained at various bath temperatures. The grain size distributions are estimated at about 30–50, 25–40, 20–40 and 15–25 nm when the bath temperatures are 358, 360.5, 363 and 365.5 K, respectively. The grain size of Ni layer tends to decrease with increase in the bath temperature. The corresponding selected area electron diffraction (SAED) patterns are shown as insets in Fig. 5. The patterns indicate an fcc-structure of the Ni layers, which agrees well with that observed in XRD patterns. The diffraction rings become more and more continuous with increasing bath temperature, which also indicates that the grain size decreases. As other prepared parameters are the same, the chemical characteristics of the Ni layers obtained at various Fig. 5. TEM micrographs and selected area electron diffraction (SAED) patterns of deposited Ni layers obtained at various bath temperatures: (a) 358 K, (b) 360.5 K, (c) 363 K and (d) 365.5 K.
480
384 Ni
288 Ni
192
96
Ni Ni
Ni
0 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 Energy - keV Fig. 3. EDX spectrum of the electroless deposited Ni layer.
bath temperatures can be regarded as the same and the bath temperature mainly influences the grain size on which the ME properties significantly depend. The in-plane parallel and perpendicular magnetostrictions, l11 and l21, in the range 0–2 kOe for the Ni layers deposited at various temperatures are shown in Fig. 6(a). The typical magnetostrictive coefficient l11 at 2.0 kOe is 37, 39, 41 and 48 ppm when the bath temperature is 358, 360.5, 363 and 365.5 K, respectively. The dependence of l21 on bath temperature is similar to that of l11. Magnetostriction increases with bath temperature as the grain size decreases. Fig. 6(b) shows the magnetostriction at low magnetic field (Hr140 Oe) for the Ni layers deposited at various temperatures. One can observe that l increases nearly linearly with increasing H. The piezomagnetic coefficient q, derived from the magnetostrictive coefficient l by differential calculation, equals the slope of the l–H curve approximately. The sums of in-plane parallel and perpendicular piezomagnetic coefficients (q11 +q21) at low magnetic field for the Ni layers deposited at various temperatures are shown in the inset of Fig. 7. It can be seen that the absolute value of (q11 +q21) at low magnetic field increases with the bath temperature. According to the random anisotropy model [21], the coercive force decreases with the grain size if the grain size is smaller than the exchange length (lex of Ni is about 47 nm [22]). The magnetostriction at low magnetic field increases as the magnetic domain motion becomes easier. Thus, the piezomagnetic property promotes when the grain size of Ni layers decreases. The transverse ME coefficient for low frequency in laminated composites is given by [23]
aE,31 ¼
Fig. 4. XRD patterns of the Ni layers deposited at various temperatures.
p kvðv1Þ d31 ðm q11 þ m q21 Þ p m m p p ð s12 þ s11 Þ 33 kv þ ð s11 þ p s12 Þp 33 ð1vÞ2 d231 kð1vÞ
e
e
where k is the interface coupling parameter, v the volume fraction of the piezoelectric phase, mqij are the piezomagnetic coefficients of the magnetostrictive phase, pd31 is the piezoelectric coefficient of the piezoelectric phase, pe33 is the dielectric constant of the piezoelectric phase, msij are the compliance coefficients of the magnetostrictive phase and psij are the compliance coefficients of the piezoelectric phase. Here k and v can be regarded as constants
W. Wu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 422–426
425
Fig. 6. Dependence of magnetostrictive coefficients l11 and l21 on the magnetic field of (a) 0–2 kOe and (b) 0–140 Oe for the Ni layers deposited at various temperatures.
Fig. 7. aE,31 dependence on Hdc at f¼ 1 kHz for Ni–PZT–Ni laminated composites obtained at various bath temperatures. The inset shows the dependence of (q11 + q21) at low magnetic field on bath temperature.
Fig. 8. Frequency dependence of aE,31 at Hdc ¼110 Oe for the Ni–PZT–Ni laminated composites with bath temperature equal to 365.5 K (tNi ¼ 120 mm). Inset shows the maximum ME coefficients at Hdc ¼ 110 Oe of the Ni layers deposited at various temperatures.
because of the same preparation method and the same magnetostrictive–piezoelectric phase thickness ratio in all the samples. pd31, pe33 and psij are invariant as a result of using the same raw material for the piezoelectric phase. The stiffness coefficient is determined by elastic modulus, which is dependent only on chemical composition when the sample’s cross area is identical. There it appears to be an inverse relationship between compliance coefficient and stiffness coefficient, so msij can be considered as constants. Thus aE,31 is demonstrated to be in proportion to (q11 + q21). Fig. 7 shows the dependence of aE,31 on the bias magnetic field at f¼1 kHz for the Ni–PZT–Ni laminated composites obtained at various bath temperatures. The thickness of all the deposited Ni layers is approximately 120 mm. It shows that aE,31 depends strongly on Hdc for all the samples. aE,31 increases until a maximum critical value is reached around Hdc ¼110 Oe, and then drops rapidly, reducing to zero as a result of the saturation of the Ni layer when Hdc exceeds 1 kOe. aE,31 at the same bias magnetic field increases as the bath temperature increases owing to the improvement of piezomagnetic property of the Ni layers mentioned above. Better piezomagnetic property of Ni layers due to smaller grain size obtained at higher temperature results in stronger ME coupling. The dependence of aE,31 at low frequency on bath temperature shown in Fig. 7 coincides with that of (q11 +q21) at low magnetic field shown in the inset.
Fig. 8 shows the frequency dependence of aE,31 at Hdc ¼110 Oe when the bath temperature equals 365.5 K. One can see that there is no remarkable frequency dispersion except for the resonance region. The resonance peak of aE,31 appears at a frequency of about 105 kHz, which is related to the electromechanical resonance. A maximum of ME voltage coefficient aE,31 ¼ 1.72 V cm 1 Oe 1 at resonance frequency is obtained. The maximum of aE,31 dependence on bath temperature at resonance frequency for Hdc ¼110 Oe is shown in the inset. One observes that the maximum of aE,31 goes up with the bath temperature, which implies that stronger ME effect can be obtained with higher bath temperature. This coincides with the situation at low frequency. It indicates that the ME effect strongly depends on the bath temperature during the process of electroless deposition, which may be attributed to the grain size and magnetic properties of the deposited Ni layers’ dependence on the bath temperature discussed above.
4. Conclusions Ni–PZT–Ni laminated composites with neither electrodes nor bonding layers have been prepared by electroless deposition at various bath temperatures. The bath temperature affects the grain size of the deposited Ni layers and consequently influences the
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performance of ME composites. The structure with smaller grain size displays better ME effect due to its higher piezomagnetic coefficient. The higher bath temperature is beneficial to obtain both stronger ME response and more efficient deposition, but one should also note that too high a bath temperature should be avoided in order to keep the plating bath stable.
Acknowledgements This work is supported by the Natural Science Foundation of Jiangsu Province of China (BK2010505), and the Scientific Research and Innovation Foundation of NUAA. K.B. would like to acknowledge support from the Scientific Research and Innovation Program for Graduate of the Higher Education Institutions of Jiangsu Province (Grant no. CX10B_099Z). References [1] L.D. Landau, E.M. Lifshitz, Electrodynamics of Continuous Media, Pergamon, Oxford, 1960. [2] C.W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
M. Fiebig, J. Phys. D 38 (2005) R123–R152. J.V. Suchtelen, Philips Res. Rep. 27 (1972) 28. T. Li, S.W. Or, H.L.W. Chan, J. Magn. Magn. Mater. 304 (2006) e442. K. Mori, M. Wuttig, Appl. Phys. Lett. 81 (2002) 100–101. C.W. Nan, N. Cai, Z. Shi, J. Zhai, G. Liu, Y. Lin, Phys. Rev. B 71 (2005) 14102. C.W. Nan, L. Liu, N. Cai, J. Zhai, Y. Ye, Y.H. Lin, L.J. Dong, C.X. Xiong, Appl. Phys. Lett. 81 (2002) 3831–3833. J.H. Park, M.G. Kim, S.J. Ahn, S. Ryu, H.M. Jang, J. Magn. Magn. Mater. 321 (2009) 1971. J.G. Wan, J.M. Liu, H.L.W. Chand, C.L. Choy, G.H. Wang, C.W. Nan, J. Appl. Phys. 93 (2003) 9916–9919. J. Ryu, A.V. Carazo, K. Uchino, H.E. Kim, Jpn. J. Appl. Phys. 40 (2001) 4948–4951. D.A. Pan, Y. Bai, W.Y. Chu, L.J. Qiao, Smart Mater. Struct. 16 (2007) 2501–2504. D.A. Pan, Y. Bai, W.Y. Chu, L.J. Qiao, J. Phys.: Condens. Matter 20 (2008) 025203. D.A. Pan, S.G. Zhang, A.A. Volinsky, L.J. Qiao, J. Phys. D: Appl. Phys. 41 (2008) 195004. R. Viswan, S.H. Kim, J.R. Jeong, S.C. Shin, J. Magn. Magn. Mater. 310 (2007) e899–e900. P. Zhao, Z. Zhao, D. Hunter, R. Suchoski, C. Gao, S. Mathews, M. Wuttig, I. Takeuchi, Appl. Phys. Lett. 94 (2009) 243507. J. Zhou, H. He, Z. Shi, C.W. Nan, Appl. Phys. Lett. 88 (2006) 3111. B.D. Barker, Surf. Technol. 12 (1981) 77–88. K. Bi, Y.G. Wang, Solid State Commun. 150 (2010) 248–250. K. Bi, Y.G. Wang, W. Wu, D.A. Pan, J. Phys. D 43 (2010) 132002. G. Herzer, IEEE Trans. Magn. 26 (1990) 1397. R.C. O’Handley, Modern Magnetic Materials: Principles and Applications, Wiley-Interscience, New York, 1999. M.I. Bichurin, V.M. Petrov, G. Srinivasan, J. Appl. Phys. 92 (2002) 7681–7683.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Bath temperature effect on magnetoelectric performance of Ni–lead zirconate titanate–Ni laminated composites synthesized by electroless deposition W. Wu, Y.G. Wang n, K. Bi College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 July 2010 Received in revised form 16 September 2010 Available online 8 October 2010
Magnetoelectric (ME) Ni–lead zirconate titanate–Ni laminated composites have been prepared by electroless deposition at various bath temperatures. The structure of the Ni layers deposited at various bath temperatures was characterized by X-ray diffraction, and microstructures were investigated by transmission electron microscopy. The magnetostrictive coefficients were measured by means of a resistance strain gauge. The transverse ME voltage coefficient aE,31 was measured with the magnetic field applied parallel to the sample plane. The deposition rate of Ni increases with bath temperature. Ni layer with smaller grain size is obtained at higher bath temperature and shows higher piezomagnetic coefficient, promoting the ME effect of corresponding laminated composites. It is advantageous to increase the bath temperature, while trying to avoid the breaking of bath constituents. & 2010 Elsevier B.V. All rights reserved.
Keywords: Magnetoelectric Laminated structure Electroless deposition Bath temperature
1. Introduction The magnetoelectric (ME) effect is characterized by the appearance of electric polarization in response to an applied magnetic field and/or magnetization in response to an applied electric field [1]. Many potential applications such as sensors, actuators and transducers have been proposed for ME materials based on their unique multifunctionality [2,3]. Up to now there are no single-phase ME materials practically applicable due to their low Ne´el or Curie temperatures and weak ME responses. Alternatively, the ME composites show much stronger extrinsic ME effect at room temperature according to the product effect [4]. Particularly, the laminated ME composites exhibit much higher ME voltage coefficient compared with the particulate ME composites, because many obstacles, such as the leak current of magnetostrictive phase and chemical reactions between phases, have been overcome. In the last decade, the laminated ME composites with Tb1 xDyxFe2 y (Terfenol-D) or ferrites as magnetostrictive phase and with lead zirconate titanate (PZT) or polyvinylidene fluoride as piezoelectric phase have been reported [5–10]. The ME voltage coefficient of the laminated composites is greatly dependent on the coupling between the magnetostrictive and piezoelectric phases besides selecting the suitable material components, so interfacial bonding becomes an important factor. There are various methods for preparing the laminated ME composites with different kinds of interfacial bonding. Ryu et al. [11]
n
Corresponding author. Tel./fax: +86 25 52112626. E-mail address: [email protected] (Y.G. Wang).
0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.09.034
reported Terfenol-D/PZT/Terfenol-D laminated ME composites with unprecedented high ME voltage coefficient of 4.68 V cm 1 Oe 1 by bonding PZT and Terfenol-D disks with epoxy. However, the ME composites with polymer as a binder have the disadvantages of nonrigid contact, fatigue and aging effect. Pan et al. [12–14] realized the giant ME coupling in Ni–PZT and Ni–PZT–Ni structures by electrodeposition. Although one can get rigid contact between the magnetostrictive and piezoelectric phases, electrodes are unavoidably involved in the electrodeposition process and certainly deteriorate the interfacial bonding and weaken the coupling between phases. Thin film methods such as magnetron sputtering, pulsed laser deposition and thermal evaporation can be used to fabricate the laminated ME composites without interlayer, but they are available only for 5 mm-thick layers [15–17]. Electroless deposition is a convenient method for preparing a thick functional film without polymer binder or electrodes. Moreover, it has many other outstanding merits, including low cost, low process temperature and excellent filling capability [18]. Most recently, we have successfully prepared Ni–PZT–Ni laminated composites by electroless deposition to strengthen the coupling between the magnetostrictive and piezoelectric phases [19]. In electroless deposition the operating conditions such as pH value and bath temperature dominate the microstructure of the Ni layers, on which the ME voltage coefficient significantly depends. We have studied the dependence of the ME coupling on the pH value of the bath and found that the ME voltage coefficient increases with the pH value [20]. In this work, we investigated the effect of bath temperature on the ME performance of Ni–PZT–Ni laminated composites derived by electroless deposition.
W. Wu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 422–426
2. Experimental details 2.1. Sample fabrication Prior to electroless deposition, the PZT flakes of size 18 5 0.6 mm3 (L W H) were first cleaned in an alcoholic solution and then roughened in a hydrofluoric acid solution, aiding the adhesion of the subsequent metallic coating to the substrate. Dipped in a solution of stannous chloride–hydrochloric acid and an acidified palladium chloride solution, the PZT flakes were sensitized and activated, respectively. In the activating stage, tin ions reacted with palladium ions as follows: Pd2 + +Sn2 + -Pd + Sn4 + . The seed crystals of palladium acted as a catalyst for the electroless deposition when the PZT flakes were dipped into the nickel bath. Afterwards, the PZT flakes were reduced in a sodium hypophosphite solution, preventing the unnecessary palladium ions’ from entering the deposition baths. Distilled water was used to rinse the PZT flakes at the end of each step. Electroless deposition of Ni was carried out using a solution of NiSO4 6H2O, N2H4 H2O and NaKC4H4O6 4H2O. The concentration of each chemical is described in detail elsewhere [19]. During the process the solution temperature was maintained at 358, 360.5,
423
363 and 365.5 K. The pH values of the baths were adjusted using sodium hydroxide and kept at 10. The thickness of the Ni layers was controlled by the electroless deposition time. The samples were polarized in an electrical field of 30 kV cm 1 applied perpendicular to the sample plane after deposition. The geometry arrangement of the Ni–PZT–Ni laminated composites is shown illustratively in Fig. 1(a). 2.2. Measurements The element composition of the deposited Ni layer was evaluated by EDX analysis. The structure of the deposited Ni layer was obtained by X-ray diffraction (XRD, D8 Advance Bruker, Cu Ka) analysis. The microstructures of the deposited Ni layers were investigated by transmission electron microscopy (TEM, FEI Tecnai G2). Cross-sectional scanning electron microscopy (SEM) was used to verify the structure of the composites. The magnetostrictive property of the deposited Ni layer was measured by a resistance strain gauge with a magnetic field applied parallel to the sample plane. During the ME measurement, the induced voltage dV across the sample was amplified and measured by an oscilloscope. The ME voltage coefficient aE was calculated according to the equation aE ¼ dV/(tPZT dH), where tPZT is the thickness of PZT layer and dH the amplitude of the AC magnetic field generated by Helmholtz coils, which were kept at 1.2 Oe in our experiment. The bias magnetic field Hdc and AC magnetic field dH were applied parallel to the sample plane for the transverse coefficient aE,31. The DC bias magnetic field could be changed from 0 to 2 kOe and the frequency f of AC magnetic field varied in the range 1–150 kHz.
3. Results and discussion As shown in the cross-sectional SEM micrograph in Fig. 1(b) the electroless deposited Ni layer contacts well with the PZT layer and it indicates that Ni–PZT–Ni laminated composites with neither electrodes nor bonding layers have been prepared by electroless deposition. The dependence of the Ni electroless deposition rate on applied bath temperature is shown in Fig. 2. It can be seen that the electroless deposition rate increases approximately linearly with the bath temperature. The linear fit does a good job of predicting the Ni deposition rate dependence on bath temperature (R ¼0.99471). The deposition rates are about 8.40, 11.38, 15.76 and 20.58 mm h 1 when the bath temperatures are 358, 360.5, 363 and 365.5 K, respectively. It is beneficial to increase the bath
Fig. 1. (a) Schematic illustrating the geometry arrangement of the Ni–PZT–Ni laminated composites. Vector P shows the PZT polarization direction. The other vector identifies the direction of applied magnetic field. (b) Cross-sectional SEM micrograph shows the clear structure of the composite electroless deposited at 358 K.
Fig. 2. Dependence of the Ni electroless deposited rate on bath temperature.
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temperature to improve the efficiency of deposition, but too high a temperature will lead to the breaking of bath constituents. Maintenance of appropriate bath temperature is required for ensuring both optimum deposition rate and bath stability. Fig. 3 gives the EDX spectrum of the Ni layer. There are only Ni peaks in the EDX spectrum, which indicate that no impurity exists in the Ni layer. The XRD patterns of Ni layers electroless deposited at various temperatures are shown in Fig. 4. It can be seen that all the deposited Ni layers have similar structure and there is only Ni phase in the samples. Fig. 5 shows the bright field TEM images of Ni layers obtained at various bath temperatures. The grain size distributions are estimated at about 30–50, 25–40, 20–40 and 15–25 nm when the bath temperatures are 358, 360.5, 363 and 365.5 K, respectively. The grain size of Ni layer tends to decrease with increase in the bath temperature. The corresponding selected area electron diffraction (SAED) patterns are shown as insets in Fig. 5. The patterns indicate an fcc-structure of the Ni layers, which agrees well with that observed in XRD patterns. The diffraction rings become more and more continuous with increasing bath temperature, which also indicates that the grain size decreases. As other prepared parameters are the same, the chemical characteristics of the Ni layers obtained at various Fig. 5. TEM micrographs and selected area electron diffraction (SAED) patterns of deposited Ni layers obtained at various bath temperatures: (a) 358 K, (b) 360.5 K, (c) 363 K and (d) 365.5 K.
480
384 Ni
288 Ni
192
96
Ni Ni
Ni
0 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 Energy - keV Fig. 3. EDX spectrum of the electroless deposited Ni layer.
bath temperatures can be regarded as the same and the bath temperature mainly influences the grain size on which the ME properties significantly depend. The in-plane parallel and perpendicular magnetostrictions, l11 and l21, in the range 0–2 kOe for the Ni layers deposited at various temperatures are shown in Fig. 6(a). The typical magnetostrictive coefficient l11 at 2.0 kOe is 37, 39, 41 and 48 ppm when the bath temperature is 358, 360.5, 363 and 365.5 K, respectively. The dependence of l21 on bath temperature is similar to that of l11. Magnetostriction increases with bath temperature as the grain size decreases. Fig. 6(b) shows the magnetostriction at low magnetic field (Hr140 Oe) for the Ni layers deposited at various temperatures. One can observe that l increases nearly linearly with increasing H. The piezomagnetic coefficient q, derived from the magnetostrictive coefficient l by differential calculation, equals the slope of the l–H curve approximately. The sums of in-plane parallel and perpendicular piezomagnetic coefficients (q11 +q21) at low magnetic field for the Ni layers deposited at various temperatures are shown in the inset of Fig. 7. It can be seen that the absolute value of (q11 +q21) at low magnetic field increases with the bath temperature. According to the random anisotropy model [21], the coercive force decreases with the grain size if the grain size is smaller than the exchange length (lex of Ni is about 47 nm [22]). The magnetostriction at low magnetic field increases as the magnetic domain motion becomes easier. Thus, the piezomagnetic property promotes when the grain size of Ni layers decreases. The transverse ME coefficient for low frequency in laminated composites is given by [23]
aE,31 ¼
Fig. 4. XRD patterns of the Ni layers deposited at various temperatures.
p kvðv1Þ d31 ðm q11 þ m q21 Þ p m m p p ð s12 þ s11 Þ 33 kv þ ð s11 þ p s12 Þp 33 ð1vÞ2 d231 kð1vÞ
e
e
where k is the interface coupling parameter, v the volume fraction of the piezoelectric phase, mqij are the piezomagnetic coefficients of the magnetostrictive phase, pd31 is the piezoelectric coefficient of the piezoelectric phase, pe33 is the dielectric constant of the piezoelectric phase, msij are the compliance coefficients of the magnetostrictive phase and psij are the compliance coefficients of the piezoelectric phase. Here k and v can be regarded as constants
W. Wu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 422–426
425
Fig. 6. Dependence of magnetostrictive coefficients l11 and l21 on the magnetic field of (a) 0–2 kOe and (b) 0–140 Oe for the Ni layers deposited at various temperatures.
Fig. 7. aE,31 dependence on Hdc at f¼ 1 kHz for Ni–PZT–Ni laminated composites obtained at various bath temperatures. The inset shows the dependence of (q11 + q21) at low magnetic field on bath temperature.
Fig. 8. Frequency dependence of aE,31 at Hdc ¼110 Oe for the Ni–PZT–Ni laminated composites with bath temperature equal to 365.5 K (tNi ¼ 120 mm). Inset shows the maximum ME coefficients at Hdc ¼ 110 Oe of the Ni layers deposited at various temperatures.
because of the same preparation method and the same magnetostrictive–piezoelectric phase thickness ratio in all the samples. pd31, pe33 and psij are invariant as a result of using the same raw material for the piezoelectric phase. The stiffness coefficient is determined by elastic modulus, which is dependent only on chemical composition when the sample’s cross area is identical. There it appears to be an inverse relationship between compliance coefficient and stiffness coefficient, so msij can be considered as constants. Thus aE,31 is demonstrated to be in proportion to (q11 + q21). Fig. 7 shows the dependence of aE,31 on the bias magnetic field at f¼1 kHz for the Ni–PZT–Ni laminated composites obtained at various bath temperatures. The thickness of all the deposited Ni layers is approximately 120 mm. It shows that aE,31 depends strongly on Hdc for all the samples. aE,31 increases until a maximum critical value is reached around Hdc ¼110 Oe, and then drops rapidly, reducing to zero as a result of the saturation of the Ni layer when Hdc exceeds 1 kOe. aE,31 at the same bias magnetic field increases as the bath temperature increases owing to the improvement of piezomagnetic property of the Ni layers mentioned above. Better piezomagnetic property of Ni layers due to smaller grain size obtained at higher temperature results in stronger ME coupling. The dependence of aE,31 at low frequency on bath temperature shown in Fig. 7 coincides with that of (q11 +q21) at low magnetic field shown in the inset.
Fig. 8 shows the frequency dependence of aE,31 at Hdc ¼110 Oe when the bath temperature equals 365.5 K. One can see that there is no remarkable frequency dispersion except for the resonance region. The resonance peak of aE,31 appears at a frequency of about 105 kHz, which is related to the electromechanical resonance. A maximum of ME voltage coefficient aE,31 ¼ 1.72 V cm 1 Oe 1 at resonance frequency is obtained. The maximum of aE,31 dependence on bath temperature at resonance frequency for Hdc ¼110 Oe is shown in the inset. One observes that the maximum of aE,31 goes up with the bath temperature, which implies that stronger ME effect can be obtained with higher bath temperature. This coincides with the situation at low frequency. It indicates that the ME effect strongly depends on the bath temperature during the process of electroless deposition, which may be attributed to the grain size and magnetic properties of the deposited Ni layers’ dependence on the bath temperature discussed above.
4. Conclusions Ni–PZT–Ni laminated composites with neither electrodes nor bonding layers have been prepared by electroless deposition at various bath temperatures. The bath temperature affects the grain size of the deposited Ni layers and consequently influences the
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performance of ME composites. The structure with smaller grain size displays better ME effect due to its higher piezomagnetic coefficient. The higher bath temperature is beneficial to obtain both stronger ME response and more efficient deposition, but one should also note that too high a bath temperature should be avoided in order to keep the plating bath stable.
Acknowledgements This work is supported by the Natural Science Foundation of Jiangsu Province of China (BK2010505), and the Scientific Research and Innovation Foundation of NUAA. K.B. would like to acknowledge support from the Scientific Research and Innovation Program for Graduate of the Higher Education Institutions of Jiangsu Province (Grant no. CX10B_099Z). References [1] L.D. Landau, E.M. Lifshitz, Electrodynamics of Continuous Media, Pergamon, Oxford, 1960. [2] C.W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101.
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