Controllable Fabrication and Tunable Magnetism of Nickel

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J. Phys. Chem. C 2009, 113, 3973–3977

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Controllable Fabrication and Tunable Magnetism of Nickel Nanostructured Ordered Porous Arrays Jinling Yang, Guotao Duan, and Weiping Cai* Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, Anhui, People’s Republic of China ReceiVed: October 7, 2008; ReVised Manuscript ReceiVed: December 7, 2008

A simple and flexible route was presented to fabricate nanostructured ordered porous arrays based on the thermal evaporation deposition on the nonclose-packed (NCP) polystyrene colloidal monolayer induced by plasma etching. The pore sizes and pore spacings in the arrays can be easily tuned by etching polystyrene colloidal monolayer. The obtained Ni nanostructured ordered porous arrays exhibit significantly higher coercive force, remnant magnetism than those of the continuous films without pores, and dependence on the pore size. These were attributed to structural anisotropy and damage of continuity of the films. We can thus tune magnetic properties of the film by introduction of pores with controlled pore size and spacing, which should be useful in the next generation of nanostructured devices such as sensing, data storage, etc. 1. Introduction Nanostructured or nanoscaled nickel thin films have received much attention both theoretically and experimentally because of their unique properties and potential applications in catalysis,1-3 sensors,4 electrodes,5 microelectronic devices,6 and surfaceenhanced Raman spectroscopy (SERS).5 Among them, Ni nanostructured ordered porous arrays, i.e., the Ni thin films with periodically arranged through-holes, as the promising candidate for the next generation of ultrahigh-density data storage medium, have attracted considerable interest.7,8 Compared with the arrays consisting of insolated magnetic nanosized objects, such as the arrays of magnetic nanowires,9 magnetic nanodots,10 and selfassembled magnetic nanoparticles,11 the nanostructured ordered porous arrays are of higher Curie temperature and have no superparamagnetic limit, which is usually found in the isolated magnetic nanoblocks.12 Such advantages are important in ultrahigh-density data storage. Stable magnetic domain structures (or the magnetic recording units) can be formed in the skeleton or the areas among adjacent pores in the arrays. The domain structures can be easily tailored by changing the diameters of the pores. Thus, the successful application in ultrahigh-density data storage strongly relies on the size of the pores in the arrays (films). There are many methods available for production of the nanostructured porous arrays, such as electrobeam lithography technologies13-19 and mask lithography via porous alumina.20-23 However, controllable fabrication of the nanostructured ordered porous arrays, by a simple, flexible, and substrate-independent route, remains a challenge. The soft lithography, based on the polystyrene colloidal monolayer, is an optional effective method to fabricate the nanostructured ordered porous arrays. Many semiconductor or metal ordered porous arrays have been produced by solution-dipping24-26 or electrodeposition27 and their mixed methods28,29 based on the colloidal monolayers. However, it is difficult to fabricate the arrays with tunable pore sizes and pore spacings if the arranged period of pores (or the distance between adjacent pore centers) is fixed. * To whom all correspondence should be addressed. E-mail: [email protected].

Figure 1. Schematic illustration of the strategy for fabrication of Ni nanostructured ordered porous arrays. Step I: Plasma etching of the close-packed polystyrene colloidal monolayer on a silicon substrate. Step II: Ni evaporation deposition on the etched colloidal monolayer. Step III: Removal of the etched polystyrene colloidal monolayer (the Ni nanostructured ordered porous array is thus obtained).

Here we present a strategy to fabricate the Ni nanostructured ordered porous arrays with controllable pore sizes and pore spacings based on nonclose-packed (NCP) polystyrene colloidal monolayer, as schematically illustrated in Figure 1. The NCP colloidal monolayer can be obtained by plasma-etching the close-packed polystyrene colloidal monolayer (step I in Figure 1). The ordered porous arrays can thus be fabricated simply by Ni evaporation deposition on the NCP colloidal monolayer and removal of it (steps II and III in Figure 1). The pore sizes and the pore spacings between adjacent pores can be tuned or controlled by the duration of etching of the close-packed colloidal monolayer. Recently we have successfully fabricated the Ni nanostructured ordered porous arrays with tunable pore

10.1021/jp808884e CCC: $40.75  2009 American Chemical Society Published on Web 02/13/2009

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sizes and spacings based on this strategy, and found that the obtained Ni nanostructured arrays exhibit significantly higher coercive force, remnant magnetism than those of the continuous film without pores, and their dependence on the pore sizes. We can thus tune the magnetic properties of the film by introduction of pores with controlled pore size and pore spacing. The details are reported in this article. 2. Experimental Section The polystyrene sphere (PS) suspensions were purchased form Alfa Aesar Corporation. Glass substrates were cleaned according to Dyune’s procedures.30 The hexagonally close-packed PSs colloidal monolayer a square centimeter in size was fabricated on the glass substrates by spin-coating on a custom-built spincoater. Silicon substrates were cleaned in acetone, ethanol, and distilled water for 30 min each. The monolayer on the glass substrate was integrally transferred to the silicon substrate, as illustrated previously,31 followed by heating at 110 °C in an oven for 10 min. Then the hexagonally close-packed monolayer was etched in an argon plasma cleaner (PDC-32G-2) at a pressure of 0.2 mbar and a input power of 100 W for a certain time, then was subsequently heated in an oven at 110 °C for another 10 min. The NCP colloidal monolayers were thus fabricated. The spacings between the etched PSs depend on the argon plasma bombardment (etching) time. The as-prepared NCP colloidal monolayer was then placed in a vacuum chamber at a pressure of 5 × 10-5 Pa for evaporation deposition by heating the Ni in a boat at 1000 °C in the chamber. The deposition thickness was monitored by a quartz crystal monitor. After Ni deposition to 20 nm in thickness, the NCP colloidal monolayer was removed by dissolution in CH2Cl2 solution with ultrasonic vibration. The Ni nanostructured ordered porous arrays were thus obtained. For reference, the continuous Ni thin film of 20 nm thickness was also prepared on the Si substrate without the PSs monolayer by using the same deposition conditions. The morphologies of the samples were observed on a Sirion 200 field-emission scanning electronic microscope (FESEM). X-ray diffraction (XRD) was measured on the Philips X’pert, using a KR line (0.15406 nm). The magnetic hysteresis measurement of the samples was conducted with a superconducting quantum interference device (SQUID) magnetometer at room temperature (300 K). 3. Results and Discussion Argon plasma bombardment will induce the reduction of PSs in size. Figure 2A shows the typical morphology of the PS colloidal monolayer (1000 nm in PS diameter) before etching, which is hexagonally closely packed. After the plasma etching for 20 min, PSs are reduced to 800 nm in diameter, but still connected with necks (Figure 2B) due to isotropic bombardment of the plasma on the PSs. Subsequent heating at 110 °C for 10 min led to disappearance of the necks and the etched PSs were completely isolated from each other, forming the NCP colloidal monolayer, as shown in Figure 2C. The etched PSs are still spherically shaped (see the inset in Figure 2C). The spacing between the etched PSs depends on the etching parameters. The longer etching and subsequent heating give rise to the bigger spacings and the smaller etched PSs. 3.1. Ni Ordered Porous Arrays. After evaporation deposition on the etched monolayer and removal of it, we can obtain the nanostructured ordered porous arrays. Typically, Figure 3A shows the result of the sample (S1) corresponding to the 20min-etched monolayer. The circular through pores are arranged

Figure 2. FESEM images of PS colloidal monolayer: (A) close-packed PS colloidal monolayer; (B) the PS colloidal monolayer after plasmaetching for 20 min; and (C) the PS monolayer etched for 20 min and subsequently heated at 110 °C for 10 min. Inset: The corresponding local cross section.

in hexagonal periodic structure due to the geometry of the etched PSs monolayer. Figure 3B gives the image of the reference sample (S0), showing continuous film with similar surface morphology. The corresponding XRD has confirmed that the as-prepared ordered porous arrays are nickel crystal with facecentered cubic lattice structure, as shown in curve a of Figure 4. The grain size is estimated by Scherrer’s equation32 to be 9.6 nm. High-resolution TEM examination has also indicated that the as-produced Ni ordered porous film is composed of nanocrystallites about 10 nm in size, as illustrated in Figure 5. Further, the samples from the PS monolayers etched for different

Nickel Nanostructured Ordered Porous Arrays

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Figure 5. High-resolution TEM image of the sample shown in Figure 3A.

Figure 3. FESEM images of the as-prepared Ni ordered porous arrays from the monlayer shown in Figure 2C (A) and the corresponding continuous film (B).

Figure 4. XRD patterns of the Ni ordered porous arrays before (a) and after (b) annealing in H2 at 300 °C for 200 min. Bottom: Standard diffraction of Ni powders.

time show similar morphology and microstructure except for the pore’s sizes and spacings. The longer etching leads to the smaller pore size and the bigger pore spacing. 3.2. Magnetic Measurements. The magnetic hysteresis loops of such ordered porous Ni arrays were measured. Figure 6A presents the results of the applied fields parallel and perpendicular to the film surface for the sample (or S1) shown in Figure 3A, which exhibit high coercivity (Hc) and significant magnetic anisotropy. The Hc and remnant magnetism (M0/Ms) values are respectively 147 Oe and 0.7 for the parallel field, and 86 Oe and 0.03 for the perpendicular field. In contrast, the corresponding values for the reference sample (or S0) are respectively 26 Oe and 0.2 for the parallel field, and 43 Oe and 0.25 for the perpendicular field, as seen in Figure 6B, showing a much smaller Hc value and good magnetic isotropy in M0/Ms. The Hc value for S1 is considerably larger than that of bulk Ni,33 which is only 0.7 Oe (120-210 times higher) and also larger than that (Hc ) 104 Oe corresponding to the parallel magnetic fields) of

Figure 6. Magnetic hysteresis loops under the applied magnetic fields parallel (HH) and perpendicular (HV) to the film plane. (A and B) Corresponding to the samples shown in Figure 3, parts A and B, respectively.

Ni micro/nanostructured hollow sphere arrays.27 The M0/Ms contrast of different orientation of the applied fields for S1 is close to that of the electrodeposited 3-dimensional ordered nanostructured porous Ni films (M0/Ms ) 0.67 and 0.02 for the parallel and perpendicular fields, respectively).34 Obviously, such magnetic anisotropy should be mainly dominated by the array’s morphology. Further experiment has shown that the smaller pores (or the larger spacing between the adjacent pores) will lead to decrease of the Hc value under the parallel field for our Ni nanostructured ordered porous arrays, as illustrated in Figure 7A. For the sample (or sample S2) from the monolayer etched for 35 min with a pore size of 650 nm, the Hc value is only 52 Oe, much smaller than that (147 Oe) of sample S1 (800 nm in pore size), but it remains considerably larger than that (26 Oe) of the reference sample (or S0). The remnant magnetism has a slight increase with decreasing pore size (M0/Ms ) 0.7 and 0.8 for S1 and S2, respectively) in this study. Under the perpendicular field,

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Yang et al. of constant pore-arranged period, the larger pores will induce the stronger pinning effect and hence the higher Hc value, showing the dependence of Hc on the pore size, which is in agreement with the results above. On the other side, the pore walls will produce magneton charges during the magnetization, which can be taken as dipoles.36 The interaction between the dipoles on the pore walls will induce the strong demagnetization field and lead to the preferential alignment of the magnetic moments along with the circumference of the pores, giving rise to the enhanced remnant magnetism, compared with the reference sample. Obviously, reduction of the pore size will decrease the dipolar interaction, favoring the alignment of the magnetic moments parallel to each other and hence enhancement of the remnant magnetism. Under the perpendicular field, there should also exist the pore-induced pinning effect for the porous films, which thus leads to more difficult magnetization than that of the continuous film (see Figure 7B). As for annealing treatment, it can improve the crystallinity of the film and increase the Ni grain size, which will lead to a stronger exchange-couple effect between crystal grains and hence an increase of coercive forces, in accordance with Herzer’s random anisotropy theory.37

Figure 7. Magnetic hysteresis loops for different samples (S0, S1, and S2) under the applied magnetic field parallel (A) and perpendicular (B) to the film plane.

Figure 8. Magnetic hysteresis loops for sample S2 before and after annealing under the applied magnetic field parallel to the film plane.

samples S1 and S2 are similar in the values of the coercivity and remnant magnetism, and show the more difficult magnetization and lower remnant magnetism than those of the reference sample, as shown in Figure 7B. If we anneal the as-prepared samples at a low temperature, the Hc value will be significantly increased. Typically, Figure 8 shows the result of sample S2 before and after annealing in H2 at 300 °C for 200 min. The Hc increases from 52 Oe to 158 Oe. The XRD pattern shows that annealing slightly improves the crystallinity of the film, as seen in curve b of Figure 4. The grain size increases to 14 nm (estimated by Scherrer’s equation). 3.3. Structural Dependence of Magnetic Properties. The magnetic properties mentioned above should be associated with the structure of the arrays. The Ni nanostructured ordered porous arrays in this study consist of the periodically arranged pores within the array’s plane and exhibit strong structural anisotropy in the directions parallel and vertical to the film. Such structure will produce inhomogeneous magnetization,35 and lead to the dependence of the magnetism on the orientation of applied fields and much easier parallel magnetization than the vertical one due to the magnetic moment directions in the domains parallel to the film surface (see Figure 6). Also, the periodically arranged pores in the arrays destroy the continuity of the film, and hence can act as the pinning centers that hinder the movement of domain walls in the films, leading to the dramatic increase of Hc value under the parallel field. Obviously, under the condition

4. Conclusions In summary, the Ni nanostructured ordered porous arrays with controllable pore sizes and pore spacings can be fabricated based on the strategy of thermal evaporation deposition on the NCP colloidal monolayer induced by plasma etching. The obtained Ni nanostructured ordered porous arrays exhibit much higher Hc and M0/Ms values than those of the continuous film without pores, and their dependence on the pore sizes, due to the existence of the periodically arranged pores which leads to structural anisotropy of the films and can hinder the movement of domain walls. We can thus control magnetic properties of the film by introduction of pores in the nanostructured films based on the etched PS colloidal monolayer. Also the magnetic ordered porous arrays can be constructed on any desired substrates by the method presented in this paper. Acknowledgment. This work is financially supported by the Natural Science Foundation of China (grant nos. 10874184 and 50831005) and the Major State research program of China “Fundamental Investigation on Micro-Nano Sensors and Systems based on BNI Fusion” (grant no. 2006CB300402). The authors thank Prof. Z. R. Yang, from the Institute of Solid State Physics, Chinese Academy of Sciences, for his helpful discussion. References and Notes (1) Yoo, J.; Hong, Y.-J.; An, S. J.; Yi, G.-C.; Chon, B.; Joo, T.; Kim, J.-W.; Lee, J.-S. Appl. Phys. Lett. 2006, 89, 043124. (2) Park, J.-H.; Kapur, P.; Saraswat, K. C.; Peng, H. Appl. Phys. Lett. 2007, 91, 143107. (3) Simpkins, B. S.; Pehrsson, P. E.; Taheri, M. L.; Stroud, R. M. J. Appl. Phys. 2007, 101, 094305. (4) Yan, W.; Li, H.; Kuang, Y.; Du, L.; Guo, J. J. Alloys Compd. 2008, 449, 210. (5) Huang, Q. J.; Lin, X. F.; Yang, Z. L.; Hu, J. W.; Tian, Z. Q. J. Electroanal. Chem. 2004, 563, 121. (6) Huang, W.; Zhang, L.; Gao, Y.; Jin, H. Microelectron. Eng. 2007, 84, 678. (7) Prieto, P.; Pirota, K. R.; Vazquez, M.; Sanz, J. M. Phys. Status Solidi A 2008, 205, 363. (8) Heyderman, L. J.; Nolting, F.; Quitmann, C. Appl. Phys. Lett. 2003, 83, 1797. (9) Yin, A. J.; Li, J.; Jian, W.; Bennett, A. J.; Xu, J. M. Appl. Phys. Lett. 2001, 79, 1039. (10) Cowburn, R. P.; Koltsov, D. K.; Welland, M. E. J. Appl. Phys. 2000, 88, 5315. (11) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989.

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