- Email: [email protected]
Fabrication and magnetic property of binary Co–Ni nanowire array by alternating current electrodeposition
Applied Surface Science 253 (2007) 7203–7206 www.elsevier.com/locate/apsusc
Fabrication and magnetic property of binary Co–Ni nanowire array by alternating current electrodeposition Jinxia Xu *, Yi Xu Department of Materials Science and Engineering, Hohai University, Nanjing 210098, PR China Received 27 January 2007; received in revised form 22 February 2007; accepted 28 February 2007 Available online 3 March 2007
Abstract Ordered binary Co–Ni nanowire arrays with different components have successfully been fabricated by ac electrodeposition. The as-obtained nanowires exhibit a diameter of about 49.2 nm and aspect ratio of more than 30. A highly preferential orientation of the Co–Ni nanowires has been obtained by XRD. The magnetic properties of Co–Ni nanowire arrays determined by VSM are as the function of the Co–Ni components. The maximum value of coercivities perpendicular to the array is 2073 Oe. However, the magnetic properties of such nanowire arrays exhibited a bad thermal stability at the medium temperature of 200 8C. # 2007 Elsevier B.V. All rights reserved. Keywords: Perpendicular magnetic recording; Co–Ni; Nanowire array; Alloy component; Annealing temperature
1. Introduction In recent years, a considerable effort has been made to investigate the magnetic nanowire arrays because of their promising applications in the perpendicular magnetic recording media [1,2]. One promising technique to fabricate such nanowire arrays is the electrodeposition by which ferromagnetic materials can be deposited into the pores of alumina templates. During the process of electrodeposition, the lateral growths of deposited ferromagnetic materials are bounded by the pore walls so as to form the desired nanowire arrays. So far, all the arrays of ferromagnetic single metals (Fe [3], Co [4] and Ni [5]) have been fabricated by the electrodeposition and their magnetic properties have been studied. However, comparing the single metal arrays, it is more interesting for us to fabricate and investigate magnetic alloy nanowire arrays due to their magnetic properties being regulated more easily by varying the components of alloys, which makes them more suitable for an ideal candidate of perpendicular magnetic recording media [6,7]. Co–Ni is an important type of binary ferromagnetic alloys, having many applications in mechanical [8], electrocatalytic
* Corresponding author. Tel.: +86 25 83786046; fax: +86 25 83786046. E-mail address: [email protected] (J. Xu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.191
[9] and magnetic fields. Co2+ and Ni2+ have an approximate value of standard electrochemical potentials (0.28 and 0.23 V, respectively), so that Co–Ni alloy can easily be obtained by co-deposition, which especially important for the template electrodeposition due to the fine nanopores in the alumina template. In present work, the CoNi nanowire arrays with different components have successfully been fabricated in alumina templates by alternating current (ac) electrodeposition. At the same time, the influences of Co–Ni component and thermal annealing on the magnetic properties of the as-obtained nanowire arrays have been investigated. 2. Experiment Al (purity 99.999%) substrates were anodically oxidized in an oxalic acid solution at 0 8C and 40 V to prepare the alumina template, as described in Ref. [10]. After anodization, a technique of reducing the anodization volatage to 10 V in a series of small steps, was applied to thin the barrier layer in order to increase the deposition rate. Then the Co–Ni alloys were electrodeposited in the pores of the alumina templates. The voltages and the frequencies of sine wave used in the ac electrodeposition were 20.0 V and 200 Hz, respectively. The electrolyte had two fixed ingredients: CoSO47H2O 40 g/L and H3BO3 30 g/L. a series of NiSO47H2O were added to fabricate the CoNi nanowire arrays with various components. After
7204
J. Xu, Y. Xu / Applied Surface Science 253 (2007) 7203–7206
deposition, the as-prepared samples were annealed at different temperatures (200, 300, 400, 500 and 600 8C) in a vacuum of about 105 Pa for 2 h, then slowly cooled down to a room temperature. The resulted products are observed by means of scanning electron microscopy (SEM: JEOL JSM-5600 operating at 15 kV) and transmission electron microscopy (TEM: FEI TECNAI20 working at 200 kV). The components of the Co–Ni nanowires were determined by energy-dispersed X-ray spectrometry (EDS) associated with the TEM. The X-ray diffraction instrument (XRD: D/Max-RB diffractometer with Cu Ka radiation) was employed to investigate the microstructure of the as-obtained arrays. The sample for XRD investigation was prepared by dipping the product into a saturated HgCl2 solution to remove the aluminium substrate, and then the remaining film was taken out and put onto a glass layer by layer. The magnetic properties of the as-obtained arrays were measured by the vibrating sample magnetometer (VSM: Lakeshore, Model 7300 series) at room temperature. The applied magnetic field was 10 kOe. 3. Result and discussion Fig. 1 shows the SEM image of an alumina template prepared by a two-step anodic oxidation. From the image, the pores in the alumina template array in a regular style. It should be natural consequence for us to obtain a highly ordered array by means of such a regular template. The typical Co–Ni nanowires prepared by template electrodeposition have been shown in Fig. 2a and Fig. 3. From Fig. 2b, it can be seen that the as-obtained nanowires have a diameter of about 49.2 nm corresponding to the size of pores in the alumina template. To observe the nanowires electrodeposited in the pores by SEM, the arrays have been dipped in 5% NaOH solution for 5 min to dissolve the upper alumina template so as to reveal the nanowires. Due to the loss of sustainment of the alumina template, the nanowires shown in Fig. 2a tend to be gathered into bunched structure. At the same time, the nanowires do not display an identical length, therefore, it can be concluded that
Fig. 1. SEM image of a porous alumina template.
Fig. 2. Typical SEM micrograph of Co–Ni nanowire array by dissolving partly the alumina template in a 5% NaOH solution for 5 min (a); distribution of nanowire diameters (mean = 49.2 1.8 nm, S.D. = 4.0%, the total number of counted nanowires = 134) (b).
the deposition of Co–Ni nanowires is non-homogeneous throughout the alumina template. The aspect ratio (length to diameter) of the as-prepared nanowires, decided by the TEM image of a single nanowire as shown in Fig. 3, is huge and more than 30. The EDS spectra (see Fig. 4) show that the atomic ratios (Co to Ni) of the as-obtained Co–Ni nanowires are 32:68, 43:57 and 71:29, respectively. That is, Co–Ni nanowire arrays with different components have been fabricated in our work. The component of the nanowires intensively depends on the composition of the electrolyte during the ac electrodeposition. By controlling the composition, a desirable component can be attained. The crystallographic structures of the as-obtained Co– Ni nanowire arrays with different components have been investigated by XRD. A meaningful result is that a similar diffraction pattern for the different CoNi nanowire arrays has been obtained. The typical result for the as-obtained Co59Ni41 array has been indicated in Fig. 5. Between 208and 408 angle of diffraction, a sharp amorphous diffraction for alumina template can been seen. Besides, fcc Co (1 1 1) and Co (3 1 1) (JCPDS: 15-0806) have also been indicated. There are no nickel peaks in
J. Xu, Y. Xu / Applied Surface Science 253 (2007) 7203–7206
7205
Fig. 5. XRD pattern of the as-obtained Co59Ni41 array.
the pattern, therefore, Co and Ni exhibit complete solid solubility. Furthermore, because of the loss of other peaks for CoNi, the as-obtained nanowire arrays exhibit a highly preferential orientation. In Fig. 5, an orientation of fcc Co (1 1 1) in the Co59Ni41 array has been indicated. The dependence of the magnetic properties, such as coercivity and squareness, on the components of Co–Ni nanowires before annealing is indicated in Fig. 6. From Fig. 6, due to the marked differences of magnetic properties in and out of plane (i.e. the applied field perpendicular and parallel to the nanowires, respectively), we conclude that the Co–Ni nanowire
arrays have an obvious magnetic anisotropy with easy axis being parallel to the nanowires. Besides, with the increase of nickel content in the Co–Ni nanowires, the coercitivies in and out of plane are dramatically changed. The maximal value of coercivity out of plane is 2073 Oe in case of the content of 29 at.% nickel, and the minimal one out of plane is only 649 Oe in case of pure nickel. In the same way, the maximal and minimal value of coercivity in plane are 849 and 288.2 Oe in case of the content of 68 at.% and pure nickel, respectively. In contrast to this, the squarenesses in and out of plane are slightly affected by the components of Co–Ni nanowires, shown by gentle curves in Fig. 6. Some literatures have reported that there are many factors acting on the magnetic properties of an array, such as the diameter, aspect ratio and components of the nanowires. The magnetic properties are dramatically affected by the aspect ratio, especially in a range of less value. However, when the value is more than 20, the aspect ratio exerts little effect on the magnetic properties of the nanowire arrays [11]. The asobtained Co–Ni nanowire in our experiment has a definite size
Fig. 4. EDS spectra of Co–Ni nanowires arrays.
Fig. 6. The coercivity and squareness as a function of atomic percentage of nickel (out of plane means the applied field is parallel to the nanowires; in plane means the field is perpendicular to the nanowires).
Fig. 3. TEM image of a single Co–Ni nanowire liberating from the alumina template, indicating the as-obtained nanowire with aspect ratio of more than 30.
7206
J. Xu, Y. Xu / Applied Surface Science 253 (2007) 7203–7206
of about 49.2 nm and aspect ratio of more than 30 (above 20). Therefore, the changes of the coercivity and squareness can only be attributed to the components of the as-obtained Co–Ni nanowires. In thin magnetic nanowire, localized reversal process was proposed due to the polycrystalline and imperfection of the wire. Based on Zeng’s research [12], the corresponding coercivity could be expressed as HC ¼
2K 0 a2 DK 2 m M S a2 DK 2 ¼ 0 m0 M S 2Am0 M S 2 2m0 M S
in which A denotes the exchange stiffness, K0 is effective uniaxial anisotropy, a determines the defect’s volume and DK is inhomogenity accompanied by an easy-axis misalignment. The dependence of magnetic properties on the component should be interpreted from the formula above. According to the phase diagram of binary Co–Ni system, a transformation from hcp to fcc of the alloy structure has taken place at a critical point, about 29 at.%, of nickel content in the Co–Ni system. The mixed structure of fcc and hcp for the system with the nickel content of the critical value will be disorder, and induce a notable rise of coercivities in and out of plane due to magneto-crystallize anisotropy. As the content of nickel is further increased up to pure nickel, the MS of Co–Ni nanowire will be reduced so as to cause the decline of coercitivity. Because of the huge aspect ratio of Co–Ni nanowires, the squareness (Mr/MS) is mainly dominated by the shape anisotropy. Therefore the component of Co– Ni nanowire does not obviously affect the squareness, however, the gentle change of squareness can be attributed to the variation of Co–Ni crystalline directions since all the nanowires with different components exhibit an identical aspect ratio. In order to study the annealing effects on the Co–Ni nanowire array, an array with a definite component, Co43Ni57, has been chosen to be annealed. Before annealing, the magnetic properties of the Co43Ni57 nanowire array have been measured at room temperature. The results show that the array has an obvious magnetic anisotropy, of which the squarenesses, coercivities in and out of plane are 0.8175, 1994 Oe and 0.1339, 751.5 Oe, respectively. However, only 200 8C annealing temperature reduces remarkably the magnetic anisotropy, which has been shown in Fig. 7. The squarenesses, coercivities in and out of plane are 0.371, 824.8 Oe and 0.461, 987.6 Oe, respectively. This weak magnetic anisotropy is unsuitable for an ideal candidate of perpendicular magnetic recording media. When the annealing temperature is increased further, relatively stabilized properties can be obtained. We propose that the changes of magnetic properties should be closely related to the variation of microstructure factors. Although the thermal annealing relieves slightly the interval stress in the CoNi nanowires, which increases the coercitivies and squarenesses, the thermal annealing leads to the phase separation in CoNi nanowires, which generates new stress and decreases the magnetic properties. Generally, Co tends to form hcp structure while Ni tends to form fcc structure during the annealing process. Therefore, the drop down of coercivities and squarenesses at the medium temperature of 200 8C is contributed to the phase
Fig. 7. The coercivity and squareness as a function of annealing temperature (out of plane means the applied field is parallel to the nanowires; in plane means the field is perpendicular to the nanowires).
separation in Co–Ni nanowires. Above the annealing temperature of 200 8C, a stable crystallographic structure has formed so that higher temperatures (more than 200 8C) have a little influence on the magnetic properties. 4. Conclusion In conclusion, the arrays of binary CoNi nanowire with different components have successfully been fabricated by ac electrodeposition. The structures and magnetic properties of the as-obtained arrays have been investigated. The results have showed that the as-obtained nanowires exhibit a diameter of about 49.2 nm and aspect ratio of more than 30. Also, a highly preferential orientation has been indicated in the Co–Ni nanowire arrays. The magnetic properties of Co–Ni nanowire arrays are as the function of the Co–Ni components. The maximum value of coercivities perpendicular to the array is 2073 Oe. However, the magnetic properties of such nanowires arrays exhibited a bad thermal stability at the medium temperature of 200 8C. The phase separation during annealing process is proposed to explain the magnetic properties change of samples. References [1] Q.F. Liu, C.X. Gao, J.J. Xiao, D.S. Xue, J. Magn. Magn. Mater. 260 (2003) 151. [2] L. Sun, P.C. Searson, Appl. Phys. Lett. 74 (1999) 2803. [3] S. Yang, H. Zhu, D. Yu, et al. J. Magn. Magn. Mater. 222 (2000) 97. [4] J. Bao, Z. Xu, J. Hong, X. Ma, Z. Lu, Scripta Mater. 50 (2004) 19. [5] S. Kato, H. Kitazawa, G. Kido, J. Magn, Magn. Mater. 272–276 (2004) 1666. [6] Y. Guo, D. Qin, J. Ding, H. Li, Appl. Surf. Sci. 218 (2003) 106. [7] D. Qin, Y. Peng b, L. Cao, H. Li, Chem. Phys. Lett. 374 (2003) 661. [8] D. Golodnitsky, N.V. Gudin, G.A. Volyanuk, Plat. Surf. Finish. 85 (1998) 65. [9] A.N. Correia, S.A.S. Machado, Electrochim. Acta 45 (2000) 1733. [10] J. Xu, X. Huang, G. Xie, et al. Mater. Res. Bull. 39 (2004) 811. [11] H. Zeng, M. Zheng, Rskomski, D.J. Sellmyer, J. Appl. Phys. 87 (2000) 4718. [12] H. Zeng, M. Zheng, R. Skomski, D.J. Sellmyer, J. Appl. Phys. 87 (2000) 4718.
Fabrication and magnetic property of binary Co–Ni nanowire array by alternating current electrodeposition Jinxia Xu *, Yi Xu Department of Materials Science and Engineering, Hohai University, Nanjing 210098, PR China Received 27 January 2007; received in revised form 22 February 2007; accepted 28 February 2007 Available online 3 March 2007
Abstract Ordered binary Co–Ni nanowire arrays with different components have successfully been fabricated by ac electrodeposition. The as-obtained nanowires exhibit a diameter of about 49.2 nm and aspect ratio of more than 30. A highly preferential orientation of the Co–Ni nanowires has been obtained by XRD. The magnetic properties of Co–Ni nanowire arrays determined by VSM are as the function of the Co–Ni components. The maximum value of coercivities perpendicular to the array is 2073 Oe. However, the magnetic properties of such nanowire arrays exhibited a bad thermal stability at the medium temperature of 200 8C. # 2007 Elsevier B.V. All rights reserved. Keywords: Perpendicular magnetic recording; Co–Ni; Nanowire array; Alloy component; Annealing temperature
1. Introduction In recent years, a considerable effort has been made to investigate the magnetic nanowire arrays because of their promising applications in the perpendicular magnetic recording media [1,2]. One promising technique to fabricate such nanowire arrays is the electrodeposition by which ferromagnetic materials can be deposited into the pores of alumina templates. During the process of electrodeposition, the lateral growths of deposited ferromagnetic materials are bounded by the pore walls so as to form the desired nanowire arrays. So far, all the arrays of ferromagnetic single metals (Fe [3], Co [4] and Ni [5]) have been fabricated by the electrodeposition and their magnetic properties have been studied. However, comparing the single metal arrays, it is more interesting for us to fabricate and investigate magnetic alloy nanowire arrays due to their magnetic properties being regulated more easily by varying the components of alloys, which makes them more suitable for an ideal candidate of perpendicular magnetic recording media [6,7]. Co–Ni is an important type of binary ferromagnetic alloys, having many applications in mechanical [8], electrocatalytic
* Corresponding author. Tel.: +86 25 83786046; fax: +86 25 83786046. E-mail address: [email protected] (J. Xu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.191
[9] and magnetic fields. Co2+ and Ni2+ have an approximate value of standard electrochemical potentials (0.28 and 0.23 V, respectively), so that Co–Ni alloy can easily be obtained by co-deposition, which especially important for the template electrodeposition due to the fine nanopores in the alumina template. In present work, the CoNi nanowire arrays with different components have successfully been fabricated in alumina templates by alternating current (ac) electrodeposition. At the same time, the influences of Co–Ni component and thermal annealing on the magnetic properties of the as-obtained nanowire arrays have been investigated. 2. Experiment Al (purity 99.999%) substrates were anodically oxidized in an oxalic acid solution at 0 8C and 40 V to prepare the alumina template, as described in Ref. [10]. After anodization, a technique of reducing the anodization volatage to 10 V in a series of small steps, was applied to thin the barrier layer in order to increase the deposition rate. Then the Co–Ni alloys were electrodeposited in the pores of the alumina templates. The voltages and the frequencies of sine wave used in the ac electrodeposition were 20.0 V and 200 Hz, respectively. The electrolyte had two fixed ingredients: CoSO47H2O 40 g/L and H3BO3 30 g/L. a series of NiSO47H2O were added to fabricate the CoNi nanowire arrays with various components. After
7204
J. Xu, Y. Xu / Applied Surface Science 253 (2007) 7203–7206
deposition, the as-prepared samples were annealed at different temperatures (200, 300, 400, 500 and 600 8C) in a vacuum of about 105 Pa for 2 h, then slowly cooled down to a room temperature. The resulted products are observed by means of scanning electron microscopy (SEM: JEOL JSM-5600 operating at 15 kV) and transmission electron microscopy (TEM: FEI TECNAI20 working at 200 kV). The components of the Co–Ni nanowires were determined by energy-dispersed X-ray spectrometry (EDS) associated with the TEM. The X-ray diffraction instrument (XRD: D/Max-RB diffractometer with Cu Ka radiation) was employed to investigate the microstructure of the as-obtained arrays. The sample for XRD investigation was prepared by dipping the product into a saturated HgCl2 solution to remove the aluminium substrate, and then the remaining film was taken out and put onto a glass layer by layer. The magnetic properties of the as-obtained arrays were measured by the vibrating sample magnetometer (VSM: Lakeshore, Model 7300 series) at room temperature. The applied magnetic field was 10 kOe. 3. Result and discussion Fig. 1 shows the SEM image of an alumina template prepared by a two-step anodic oxidation. From the image, the pores in the alumina template array in a regular style. It should be natural consequence for us to obtain a highly ordered array by means of such a regular template. The typical Co–Ni nanowires prepared by template electrodeposition have been shown in Fig. 2a and Fig. 3. From Fig. 2b, it can be seen that the as-obtained nanowires have a diameter of about 49.2 nm corresponding to the size of pores in the alumina template. To observe the nanowires electrodeposited in the pores by SEM, the arrays have been dipped in 5% NaOH solution for 5 min to dissolve the upper alumina template so as to reveal the nanowires. Due to the loss of sustainment of the alumina template, the nanowires shown in Fig. 2a tend to be gathered into bunched structure. At the same time, the nanowires do not display an identical length, therefore, it can be concluded that
Fig. 1. SEM image of a porous alumina template.
Fig. 2. Typical SEM micrograph of Co–Ni nanowire array by dissolving partly the alumina template in a 5% NaOH solution for 5 min (a); distribution of nanowire diameters (mean = 49.2 1.8 nm, S.D. = 4.0%, the total number of counted nanowires = 134) (b).
the deposition of Co–Ni nanowires is non-homogeneous throughout the alumina template. The aspect ratio (length to diameter) of the as-prepared nanowires, decided by the TEM image of a single nanowire as shown in Fig. 3, is huge and more than 30. The EDS spectra (see Fig. 4) show that the atomic ratios (Co to Ni) of the as-obtained Co–Ni nanowires are 32:68, 43:57 and 71:29, respectively. That is, Co–Ni nanowire arrays with different components have been fabricated in our work. The component of the nanowires intensively depends on the composition of the electrolyte during the ac electrodeposition. By controlling the composition, a desirable component can be attained. The crystallographic structures of the as-obtained Co– Ni nanowire arrays with different components have been investigated by XRD. A meaningful result is that a similar diffraction pattern for the different CoNi nanowire arrays has been obtained. The typical result for the as-obtained Co59Ni41 array has been indicated in Fig. 5. Between 208and 408 angle of diffraction, a sharp amorphous diffraction for alumina template can been seen. Besides, fcc Co (1 1 1) and Co (3 1 1) (JCPDS: 15-0806) have also been indicated. There are no nickel peaks in
J. Xu, Y. Xu / Applied Surface Science 253 (2007) 7203–7206
7205
Fig. 5. XRD pattern of the as-obtained Co59Ni41 array.
the pattern, therefore, Co and Ni exhibit complete solid solubility. Furthermore, because of the loss of other peaks for CoNi, the as-obtained nanowire arrays exhibit a highly preferential orientation. In Fig. 5, an orientation of fcc Co (1 1 1) in the Co59Ni41 array has been indicated. The dependence of the magnetic properties, such as coercivity and squareness, on the components of Co–Ni nanowires before annealing is indicated in Fig. 6. From Fig. 6, due to the marked differences of magnetic properties in and out of plane (i.e. the applied field perpendicular and parallel to the nanowires, respectively), we conclude that the Co–Ni nanowire
arrays have an obvious magnetic anisotropy with easy axis being parallel to the nanowires. Besides, with the increase of nickel content in the Co–Ni nanowires, the coercitivies in and out of plane are dramatically changed. The maximal value of coercivity out of plane is 2073 Oe in case of the content of 29 at.% nickel, and the minimal one out of plane is only 649 Oe in case of pure nickel. In the same way, the maximal and minimal value of coercivity in plane are 849 and 288.2 Oe in case of the content of 68 at.% and pure nickel, respectively. In contrast to this, the squarenesses in and out of plane are slightly affected by the components of Co–Ni nanowires, shown by gentle curves in Fig. 6. Some literatures have reported that there are many factors acting on the magnetic properties of an array, such as the diameter, aspect ratio and components of the nanowires. The magnetic properties are dramatically affected by the aspect ratio, especially in a range of less value. However, when the value is more than 20, the aspect ratio exerts little effect on the magnetic properties of the nanowire arrays [11]. The asobtained Co–Ni nanowire in our experiment has a definite size
Fig. 4. EDS spectra of Co–Ni nanowires arrays.
Fig. 6. The coercivity and squareness as a function of atomic percentage of nickel (out of plane means the applied field is parallel to the nanowires; in plane means the field is perpendicular to the nanowires).
Fig. 3. TEM image of a single Co–Ni nanowire liberating from the alumina template, indicating the as-obtained nanowire with aspect ratio of more than 30.
7206
J. Xu, Y. Xu / Applied Surface Science 253 (2007) 7203–7206
of about 49.2 nm and aspect ratio of more than 30 (above 20). Therefore, the changes of the coercivity and squareness can only be attributed to the components of the as-obtained Co–Ni nanowires. In thin magnetic nanowire, localized reversal process was proposed due to the polycrystalline and imperfection of the wire. Based on Zeng’s research [12], the corresponding coercivity could be expressed as HC ¼
2K 0 a2 DK 2 m M S a2 DK 2 ¼ 0 m0 M S 2Am0 M S 2 2m0 M S
in which A denotes the exchange stiffness, K0 is effective uniaxial anisotropy, a determines the defect’s volume and DK is inhomogenity accompanied by an easy-axis misalignment. The dependence of magnetic properties on the component should be interpreted from the formula above. According to the phase diagram of binary Co–Ni system, a transformation from hcp to fcc of the alloy structure has taken place at a critical point, about 29 at.%, of nickel content in the Co–Ni system. The mixed structure of fcc and hcp for the system with the nickel content of the critical value will be disorder, and induce a notable rise of coercivities in and out of plane due to magneto-crystallize anisotropy. As the content of nickel is further increased up to pure nickel, the MS of Co–Ni nanowire will be reduced so as to cause the decline of coercitivity. Because of the huge aspect ratio of Co–Ni nanowires, the squareness (Mr/MS) is mainly dominated by the shape anisotropy. Therefore the component of Co– Ni nanowire does not obviously affect the squareness, however, the gentle change of squareness can be attributed to the variation of Co–Ni crystalline directions since all the nanowires with different components exhibit an identical aspect ratio. In order to study the annealing effects on the Co–Ni nanowire array, an array with a definite component, Co43Ni57, has been chosen to be annealed. Before annealing, the magnetic properties of the Co43Ni57 nanowire array have been measured at room temperature. The results show that the array has an obvious magnetic anisotropy, of which the squarenesses, coercivities in and out of plane are 0.8175, 1994 Oe and 0.1339, 751.5 Oe, respectively. However, only 200 8C annealing temperature reduces remarkably the magnetic anisotropy, which has been shown in Fig. 7. The squarenesses, coercivities in and out of plane are 0.371, 824.8 Oe and 0.461, 987.6 Oe, respectively. This weak magnetic anisotropy is unsuitable for an ideal candidate of perpendicular magnetic recording media. When the annealing temperature is increased further, relatively stabilized properties can be obtained. We propose that the changes of magnetic properties should be closely related to the variation of microstructure factors. Although the thermal annealing relieves slightly the interval stress in the CoNi nanowires, which increases the coercitivies and squarenesses, the thermal annealing leads to the phase separation in CoNi nanowires, which generates new stress and decreases the magnetic properties. Generally, Co tends to form hcp structure while Ni tends to form fcc structure during the annealing process. Therefore, the drop down of coercivities and squarenesses at the medium temperature of 200 8C is contributed to the phase
Fig. 7. The coercivity and squareness as a function of annealing temperature (out of plane means the applied field is parallel to the nanowires; in plane means the field is perpendicular to the nanowires).
separation in Co–Ni nanowires. Above the annealing temperature of 200 8C, a stable crystallographic structure has formed so that higher temperatures (more than 200 8C) have a little influence on the magnetic properties. 4. Conclusion In conclusion, the arrays of binary CoNi nanowire with different components have successfully been fabricated by ac electrodeposition. The structures and magnetic properties of the as-obtained arrays have been investigated. The results have showed that the as-obtained nanowires exhibit a diameter of about 49.2 nm and aspect ratio of more than 30. Also, a highly preferential orientation has been indicated in the Co–Ni nanowire arrays. The magnetic properties of Co–Ni nanowire arrays are as the function of the Co–Ni components. The maximum value of coercivities perpendicular to the array is 2073 Oe. However, the magnetic properties of such nanowires arrays exhibited a bad thermal stability at the medium temperature of 200 8C. The phase separation during annealing process is proposed to explain the magnetic properties change of samples. References [1] Q.F. Liu, C.X. Gao, J.J. Xiao, D.S. Xue, J. Magn. Magn. Mater. 260 (2003) 151. [2] L. Sun, P.C. Searson, Appl. Phys. Lett. 74 (1999) 2803. [3] S. Yang, H. Zhu, D. Yu, et al. J. Magn. Magn. Mater. 222 (2000) 97. [4] J. Bao, Z. Xu, J. Hong, X. Ma, Z. Lu, Scripta Mater. 50 (2004) 19. [5] S. Kato, H. Kitazawa, G. Kido, J. Magn, Magn. Mater. 272–276 (2004) 1666. [6] Y. Guo, D. Qin, J. Ding, H. Li, Appl. Surf. Sci. 218 (2003) 106. [7] D. Qin, Y. Peng b, L. Cao, H. Li, Chem. Phys. Lett. 374 (2003) 661. [8] D. Golodnitsky, N.V. Gudin, G.A. Volyanuk, Plat. Surf. Finish. 85 (1998) 65. [9] A.N. Correia, S.A.S. Machado, Electrochim. Acta 45 (2000) 1733. [10] J. Xu, X. Huang, G. Xie, et al. Mater. Res. Bull. 39 (2004) 811. [11] H. Zeng, M. Zheng, Rskomski, D.J. Sellmyer, J. Appl. Phys. 87 (2000) 4718. [12] H. Zeng, M. Zheng, R. Skomski, D.J. Sellmyer, J. Appl. Phys. 87 (2000) 4718.