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J. Phys. Chem. C 2008, 112, 1468-1472
Orientation-Controlled Synthesis and Ferromagnetism of Single Crystalline Co Nanowire Arrays Xiaohu Huang, Liang Li, Xuan Luo, Xiaoguang Zhu, and Guanghai Li* Key Laboratory of Material Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, People’s Republic of China ReceiVed: October 17, 2007; In Final Form: NoVember 5, 2007
Single crystalline Co nanowire arrays with different orientations were grown within porous anodic alumina membranes by a pulsed electrodeposition technique. It was found that the orientation of nanowires can be controlled by controlling the deposition parameters and the magnetic properties depend strongly on the orientation of the nanowires arrays. The growth mechanism was discussed in detail. High-density, large aspectratio and single crystalline Co nanowire arrays with the proper values of coercivity may find promising application as a perpendicular magnetic recording medium.
1. Introduction
TABLE 1: Deposition Parameters and Hc of Co Nanowire Arrays of Different Orientations
Nanowires have stimulated considerable research interests in recent years, due to their fascinating applications for welldefined interconnects and building blocks for nanodevices.1-3 It has been demonstrated that the properties of nanowires depend strongly not only on the diameter but also on the crystal orientation as well,4,5 and the controlling growth of the nanowire orientation is thus very essential for their future application. Different methods have been used to fabricate preferential oriented nanowire arrays, such as chemical vapor deposition,6 hydrothermal,7 and membrane-based route,5,8 in which the template-assisted electrodeposition shows a promising technique in the controlling of the preferential orientation without changing their nanowire morphology.4 As is well-known, cobalt is an important ferromagnetic material with large coercivity (Hc) and high Curie temperature, and it has two kinds of phases, that is, hexagonal close-packed (HCP) and face-centered cubic (FCC), which show the different properties.4 So the fabrication of Co nanowire arrays has been the focus of much attention because of their attractive application in ultrahigh density magnetic storage.3,4,9 Generally, it is easy to adjust the magnetic property by changing the diameter of the nanowires,3 but the expense is either a change of the storage density or a rise of the interaction between neighboring nanowires, which will severely affect the performance of the potential devices.10 Alternatively, modulating the orientation to control the magnetic property is another feasible route, because it is well-known that the magnetic property of bulk Co differs greatly along the different crystalline orientations.11 It is declared that, as long as the crystalline orientation cannot be controlled, Co is not a suitable candidate for perpendicular storage media.12 Although some groups tried to control the orientation of Co nanowires,13-15 it is still a considerable challenge to do it rationally. In this paper, the growth of single crystalline Co nanowire arrays with controllable orientation and phase structure were reported by the pulsed electrodeposition into anodic alumina membranes (AAMs). The orientation dependence of the magnetic properties of Co nanowires was investigated, and * Corresponding author. E-mail:
[email protected].
Hc (Oe) voltage (V)
current2
1.5 2.4 3.0
0.09 0.64 2.25
(mA2)
pulsed duty time (ms) (%) pH 5 5 7.5
50 50 75
4 4 2
orientation
|
⊥
HCP [101h0] 477 139 FCC [111] 1075 149 FCC [220] 1031 86
the results indicate that Co nanowire arrays are ideal candidates for perpendicular storage media. 2. Experimental Section The AAMs were prepared by a two-step anodization process as described in our previous reports.8a Briefly, high purity aluminum (99.999%) sheets were anodized at 40V DC in 0.3 M oxalic acid electrolyte at 4 °C for 4 h. After the alumina layer produced was removed, the second anodization was conducted under the same conditions as the first one for 12 h. Then, the central aluminum substrate and the alumina barrier layer were removed. Finally, a thin film of Au (about 200 nm in thickness) was sputtered onto the one side of the template to serve as electrode. The electrolyte was comprised of 0.1 M CoSO4‚7H2O and 0.5 M H3BO3. Pulsed electrodeposition was carried out under modulated voltages control. The detailed preparation condition of Co nanowires with different orientations can be found in Table 1. The orientations and microstructure of the nanowire arrays were characterized by X-ray diffraction (XRD) on a Philips X’Pert power X-ray diffractometer using Cu KR (λ ) 1.542 Å) radiation, transmission electron microscopy (TEM), and highresolution transmission electron microscopy (HRTEM, JEM2010) attached with selected area electron diffraction (SAED). Field emission scanning electron microscopy (FE-SEM, FEI Sirion 200) was used to examine the morphology. For FE-SEM observation, the AAM was partially etched in 5% NaOH solution and then washed with distilled water several times. For TEM and HRTEM observations, the AAM was completely dissolved in a 5% NaOH solution, then washed with distilled water several times, and finally dispersed in absolute ethanol by ultrasonic. The magnetic behavior of the nanowires with the
10.1021/jp710106y CCC: $40.75 © 2008 American Chemical Society Published on Web 01/09/2008
Single Crystalline Co Nanowire Arrays
Figure 1. FE-SEM images of Co nanowire arrays etched for different times: (a,b) 6 min, (c) 10 min, and (d) 12 min.
Figure 2. XRD patterns of Co nanowire arrays with different orientations: (1) sample 1, (b) sample 2, and (c) sample 3.
membrane support was investigated on a Quantum Design superconducting quantum interference device (SQUID) magnetometer at 300 K.
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Figure 3. (a) TEM images of a single 40 nm Co nanowires grown along HCP [101h0] (b,c) and the corresponding SAED pattern and HRTEM image.
Figure 4. (a) TEM images of a single 40 nm Co nanowires grown along FCC [111] (b,c) and the corresponding SAED pattern and HRTEM image.
3. Results and Discussion 3.1. Characterization. Figure 1 shows the FE-SEM images of Co nanowires with different magnification after etching the AAM for different times. Apparently, the large-area, highdensity, and highly ordered Co nanowires replicate the pore arrangement of AAM (Figure 1a,c). The diameter of the nanowires is about 40 nm (Figure 1b), and the length is about 40 µm (Figure 1d). The aspect ratio (length/diameter) of the nanowires is up to 1000, which is important to prevent the onset of superparamagnetism in magnetic data storage.9 If one nanowire can be served as one magnetic storage element, the storage density is about 64 Gbits/inch2. These features can meet the geometrical need of high-density magnetic storage. XRD patterns of Co nanowire arrays with different growth orientations are shown in Figure 2. The sharp and narrow diffraction peaks indicate the nanowires have highly preferential orientation. The peak at 2θ ) 41.60° for curve 1 can be indexed to Co HCP [101h0] (JCPDS No. 89-4308), while the peaks at 2θ ) 44.17° and 2θ ) 75.63° are difficult to index, because the Bragg reflection angle of 44.229° for FCC [111] lies close to that of 44.264° for HCP [0002], with the same case for FCC [220] (2θ ) 75.868°) and HCP [112h0] (2θ ) 75.891°) (JCPDS No. 89-4307 and JCPDS No. 89-4308). Although the two predominant peaks in curve 2 and curve 3 lie closer to those of FCC Co, it is still difficult to judge the crystalline phase only from the XRD pattern, which will be identified by TEM characterization in the following text. Figures 3-5 show the TEM microstructure characterizations of Co nanowires with the three different orientations, corresponding to the samples 1-3 shown in Figure 1, respectively. Figure 3a is the TEM image of an individual Co nanowire, from which we can see the nanowire have a smooth surface and uniform diameter of about 40 nm. The corresponding SAED
Figure 5. (a) TEM images of a single 40 nm Co nanowires grown along FCC [220] (b,c) and the corresponding SAED pattern and HRTEM image.
pattern is shown in Figure 3b, it can be well-indexed to HCP Co with growth orientation along [101h0]. The lattice fringes can be clearly seen from the HRTEM image (Figure 3c), in which the interplanar distance is determined to be about 0.217 nm, corresponding to the {101h0} crystal planes of HCP Co. The SAED pattern from the nanowire in Figure 4a can be wellindexed to single crystalline Co with FCC crystalline phase, as shown in Figure 4b. Meanwhile, it can be deduced the preferred orientation is along [111] direction, which was further proved by the lattice fringes in the HRTEM image (Figure 4c). The SAED pattern of the nanowire shown in Figure 5a can be wellindexed to single crystalline Co with FCC crystalline phase, as shown in Figure 5b. It can be deduced that the nanowire is grown along the [220] direction, which was also further confirmed by the HRTEM image (Figure 5c). The combination of these microstructure characterizations with the XRD results (curves 1-3 in Figure 1) can confirm that the Co nanowires are single crystalline with preferential orientations along HCP [101h0], FCC [111], and FCC [220], respectively. The low peak
1470 J. Phys. Chem. C, Vol. 112, No. 5, 2008
Figure 6. (a) TEM image of a single 40 nm Co nanowire grown along FCC [220] direction, the insets are the SAED patterns at different positions along the nanowire. (b) Schematic illustration of the growth process of the nanowires.
labeled by “*” in curve 3 of Figure 2 suggests there may be a little HCP phased impurity in the Co nanowires oriented along FCC [220] direction. 3.2. Growth Mechanism. To further study the growth mechanism of the Co nanowires, a more detailed microstructure analysis was performed. Figure 6 shows the TEM image of a single Co nanowire grown along [220] and the corresponding SAED patterns at different positions along the nanowire. The different contrast along the nanowire might originate from the deformation induced by the ultrasonic treatment used to prepare the TEM sample.16 The SAED pattern taken from the tip of the nanowire shows some weak diffraction rings, and it was found that the diffraction rings correspond to polycrystalline Au electrode. The appearance of Au electrode can be used as an indication of the initial growth of Co nanowires. As shown in the insets of Figure 6, the nanowire is single crystalline in the areas beyond 200 nm from Au electrode, while that in the position near the Au electrode shows a polycrystalline characteristic. These results indicate that the Co nanowire formed in the initial growth stage is polycrystalline and transforms into single crystalline in the subsequent growth, and the transition length is about 200 nm. Actually, this transition from threedimensional nucleation to two-dimensional growth has been observed previously on Sb nanowires,17 and we believe it is a universal phenomenon during the growth of the nanowires via electrodeposition within the AAM (Figure 6b). To further understand how nanowires grow along different preferential orientations, the thermodynamics and dynamic factors should be taken into account. The pulsed time in each pulse cycle is so short that only a small amount of Co ions are reduced at the deposition interface during one pulse. The delayed time is not only for the Co ion concentration to recover before next deposition,8a but also for the surface lattice (and the metal-solution interface) to relax to a more stable state,18 which ensures the growth of the single crystalline nanowires. HCP phase is generally a stable structure
Huang et al. and can be easily obtained by electrodeposition. It is well-known that Co with FCC structure is stable at temperatures above 422 °C. However, it has been reported that FCC Co film could be electrodeposited at high potential.19 Here, we demonstrated that the phase structure of the Co nanowires can be switched from a HCP phase to a FCC phase via tuning the deposition potential from low value to high value, which is consistent with a previous report.20 This is because it is possible to form a metastable Co hydride with a FCC structure at high potential, because of the greatly increased electrodeposition of atomic hydrogen with increasing potential.20,21 Several different factors might affect the orientation of the Co nanowires. The first is the substrate,22 but in the present study, the substrate is always polycrystalline Au film,4 and thus the different orientations are not resulted from the substrate. From Table 1, we can see that, for the nanowires grown along different orientations, the square of deposition currents is very different, and thus, the current is an important factor that affects the orientation of the nanowires. Actually, the current is a very important dynamic factor in electrodeposition.15,23,24 During the electrodeposition, the current is proportional to charge; that is, I ) C1*Q, and the charge is proportional to the number of ions reached to the deposition interface; that is, Q ) C2*N, while the number of ions reached to the deposition interface is proportional to the velocity of the ions; that is, N ) C3*V, so I ) C4*V. It is well-known that the dynamic energy of ions is E ) C5*V2, and from these relationships, we can get that E ) C6*I2. Where C1∼C6 are all constants. This means that the kinetic energy of ions is proportional to the square of the current. Ions with different energies will deposit onto different crystalline planes in order to meet the energy minimum principle due to their different broken-bond number and surface energy, and thus the orientation of the nanowires closely related to the square of current. In this way, the preferential orientation of Co nanowires can be controlled by deposition potential upon which to control the current, as listed in Table 1. The pH value of electrolyte also affects the orientation of the Co nanowires,14,25 as shown in Table 1. There is a competition between the adsorption and the desorption of hydrogen ions during the growth of metallic nanowires,8c and the adsorption of hydrogen ions on the cathode will stabilize the (110) lattice plane and favor the growth of FCC Co nanowires.26 The smaller the pH value, the more hydrogen ions there will be in the electrolyte and the easier for the adsorption of hydrogen ions, and thus the small pH value of about 2 favors the growth of Co nanowires along [220] direction. 3.3. Modulation of the magnetism. Controlling the orientations provides us a new opportunity to manipulate the magnetic property of Co nanowire arrays without changing the diameter. Figure 7 shows the magnetic hysteresis loops for the Co nanowire arrays with different orientations. The maximum value of the applied field is 1.0 T, and the direction of the applied magnetic field is either parallel (|, solid lines) or perpendicular (⊥, dashed lines) to the axis of the nanowires. When the applied field is parallel to the nanowire axis, the loops are relatively square, while when the applied field is perpendicular to the nanowire axis, the loops are sheared, indicating a strong magnetic anisotropy. It is clear from Figure 7a-c that the ferromagnetic properties of the Co nanowires are strongly dependent on their crystalline orientation. The Hc versus the orientation of the nanowires is plotted in Figure 7d. One can see that the Hc parallel to the nanowires gradually increases with the orientation of the nanowires turning from HCP [101h0] to FCC [220] and to FCC [111]. Generally,
Single Crystalline Co Nanowire Arrays
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Figure 7. Magnetic hysteresis loops at room temperature of Co nanowire arrays with the orientation of (a) HCP [101h0], (b) FCC [220], and (c) FCC [111], with the applied field parallel (solid curves) and perpendicular (dashed curves) to the nanowires axes. (d) Plots of the coercivity versus to the orientations with the applied field parallel (solid curves) and perpendicular (dashed curves) to the nanowires axes.
the anisotropic magnetization originates from the corporate contributions of both magnetocrystalline anisotropy (MA) and shape anisotropy (SA) and is determined by the competition between them. The aspect ratio of the nanowire is up to 1000, so if it is considered as an infinite cylinder, then magnetic SA tends to align the magnetization along the axis of the nanowire. For the case of Co nanowires grown along HCP [101h0], the magnetocrystalline easy axis is along [0001], which is perpendicular to the axis of the nanowire.4 Thus, there is a direct competition between MA and SA. The SA energy density (πMs2 ∼ 6 × 106 ergs cm-3) is slightly larger than the MA energy density (K1 ∼ 5 × 106 ergs cm-3). Their competition results in a reduced coercivity and squareness in the HCP Co nanowire arrays. For the case of Co nanowires with a FCC structure, the SA is the same as the HCP Co, because they have the same saturation magnetization (Ms (FCC) ) Ms (HCP)) at room temperature.27 The magnetocrystalline easy axis is along the [111] direction,28 and the MA energy density (K1 ∼ 6.3 × 105 ergs cm-3) is almost 1 order of magnitude smaller than that of SA energy density. Therefore, for Co nanowire grown along [220], there is an angle of about 35.26° between the magnetocrystalline easy axis and the axis of the nanowire. The MA energy will contribute to the total energy partially. While for Co nanowire grown along [111], both MA and SA act cooperatively along the axes of the nanowires,28 so the MA energy will contribute to the total energy totally. Consequently, the effective magnetic anisotropy of the FCC [111] Co nanowire arrays is a little larger than that of FCC [220] Co nanowire arrays, and both of them are much larger than that of HCP [101h0] Co nanowire arrays. Daimon et al. pointed out that an array with coercivity between 500 and 1000 Oe is the most suitable for perpendicular recording medium.29 Previous study indicated that single crystalline Co nanowire arrays oriented along HCP [0002] are not suitable for perpendicular recording medium, because of their high coercivity (1.7∼2.7 kOe).18,30 The present study shows that the Hc are 477, 1031, and 1075 Oe for single crystalline Co nanowire arrays respectively oriented along HCP [101h0],
FCC [220], and FCC [111] directions, which nearly locates within the optimum Hc range. Considering the large aspect ratio of the Co nanowires (about 1000) and the high storage density of the nanowire arrays (64 Gbits /inch2), we believe that Co nanowire arrays with these three orientations are ideal candidates for high-density perpendicular magnetic recording media. 4. Conclusion Single crystalline Co nanowires with controllable orientation and phase structure have successfully been grown by the pulsed electrodeposition technique. It was found that the proper selection of the deposition current and pH value is very important in controlling the orientation and phase structure of the nanowires. The magnetic properties of the nanowires depend strongly on the orientation, which is attributed to the corporate contributions of both magnetocrystalline anisotropy and shape anisotropy. Our results show that single crystalline Co nanowire arrays are ideal material for high-density perpendicular magnetic recording media because of their large aspect-ratio, high density, and proper coercivity. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 10704074), Special Project of Excellent Young Researchers of Anhui Province and Support Project of Excellent President Scholarship of Chinese Academy of Sciences. References and Notes (1) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Brain, M.; Byron, G.; Yin, Y. D.; Franklin, K.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (2) Xu, X. J.; Fei, G. T.; Yu, W. H.; Zhang, L. D.; Ju, X.; Hao, X. P.; Wang, D. N.; Wang, B. Y. Appl. Phys. Lett. 2006, 89, 181914. (3) Sellmyer, D. J.; Zheng, M.; Skomski, R. J. Phys.: Condens. Matter 2001, 13, R433. (4) Zhang, J.; Jones, G. A.; Shen, T. H.; Donnelly, S. E.; Li, G. H. J. Appl. Phys. 2007, 101, 054310. (5) (a) Zhu, Y. G.; Dou, X. C.; Huang, X. H.; Li, L.; Li, G. H. J. Phys Chem. B 2006, 110, 26189. (b) Dou, X. C.; Zhu, Y. G.; Huang, X. H.; Li, L.; Li, G. H. J. Phys. Chem. B. 2006, 110, 21572.
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