- Email: [email protected]
Injection of synthesized FePt nanoparticles in hole-patterns for bit patterned media
Journal of Magnetism and Magnetic Materials 324 (2012) 303–308
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Injection of synthesized FePt nanoparticles in hole-patterns for bit patterned media Takuma Hachisu a,b, Wataru Sato a, Shugo Ishizuka a, Atsushi Sugiyama a, Jun Mizuno a, Tetsuya Osaka a,b,n a b
Faculty of Nanoscience and Nanoengineering, Graduate School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Waseda Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
a r t i c l e in f o
abstract
Available online 28 December 2010
FePt nanoparticles of uniform sizes, compositions, and crystal structures can be obtained by chemical synthesis. Additionally, the nanoparticles can be well dispersed by the adsorption of a surfactant on the nanoparticle surface. Previously, the immobilization of FePt nanoparticles on a thermal oxide Si substrate was carried out by chemical synthesis, utilizing the Pt–S bonding between the -SH functional group in (3-mercaptopropyl)trimethoxysilane, MPTMS and Pt in FePt nanoparticles. However, controlling FePt nanoparticle arrays by this synthesis method was very difficult. In the present study, we attempted to control the distortion of the arrangement of FePt nanoparticles using an MPTMS layer modified with a silane coupling reaction and a geometrical structure prepared by ultraviolet nanoimprint lithography (UV-NIL). In this study, the hole-patterns used for the geometrical structure on Si(1 0 0) were 200 nm wide, 40 nm deep, and had a 500 nm pitch. The 5.6 nm FePt nanoparticles were used to coat the hole-patterns by using a picoliter pipette. An XHR-SEM image clearly revealed that the FePt nanoparticles were successfully arranged as a single layer with an average pitch of 10.0 nm by Pt–S bonding in the hole-patterns on Si(1 0 0). & 2010 Elsevier B.V. All rights reserved.
Keywords: Bit patterned media Synthesized FePt nanoparticle Chemical bonding Geometrical structure
1. Introduction Bit patterned media (BPM) is a perpendicular magnetic recording system, where the magnetic domains are isolated physically. BPM offers a potential path for developing next-generation ultra-high density data storage with capacities of 45 Tbit in 2. Magnetic arrays of BPM are fabricated using additive or subtractive processes. Additive processes involve the deposition of magnetic material into a template using electrochemical deposition or evaporation and liftoff. In a subtractive process, the magnetic recording layer is formed first by sputtering, and then etched using ion milling. For the lithographic fabrication of magnetic nanostructures, pattern-drawing BPM technology such as X-ray lithography, imprint lithography, and electron-beam lithography have been widely studied. L10-FePt alloys have a very high uniaxial magnetic crystalline anisotropy energy (Ku ¼7.0 107 erg/cm3). Because superparamagnetic fluctuation of room temperature magnetization can be suppressed even for grains with a diameter of 3.0 nm, the appropriate arrangement of FePt nanoparticles composed of one recording bit one particle is a promising candidate for next-generation BPM with capacities of 410 Tbit in 2. Since Sun et al. [1–4] reported the
n Correspondence to: Department of Applied Chemistry, School of Advanced Science and Engineering, Waseda University Bldg. 55S/Room 601, 3-4-1. Okubo, Shinjuku-ku, Tokyo 169-8555, Japan. Tel.: + 81 3 5286 3202; fax: + 81 3 3205 2074. E-mail address: [email protected] (T. Osaka).
0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.12.023
preparation of FePt nanoparticles monodispersed by chemical synthesis, many researchers have studied these particles for their potential use in a wide range of applications including magnetic recording media [5–8]. Previously, we succeeded in the fabrication of cubic FePt nanoparticles by controlling the reaction temperature in chemical synthesis; the FePt nanoparticles were fixed using chemical bonding on a thermal oxide Si substrate with modified (3-mercaptopropyl) trimethoxysilane, MPTMS as an interlayer between the FePt nanoparticles and the substrate [9,10]. However, the arrangement of the FePt nanoparticles formed irregularities, laminate structures, and aggregates [11]. In this study, to use the synthesized FePt nanoparticles for BPM, we attempted to develop a simple process to control the distortion of the arrangement of FePt nanoparticles using an MPTMS layer modified by a silane coupling reaction and a geometrical structure prepared by ultraviolet nanoimprint lithography (UV-NIL). Our strategy is shown in Fig. 1. In this study, hole-patterns of 200 nm width, 40 nm depth and 500 nm pitch were used as the geometrical structure on Si(1 0 0), and FePt-distributed solution was coated on the geometrical structure using a picoliter pipette (PicoPipet, Altair), which provides droplets down to 50–500 nl. Consequently, the selectivity injection of the particles into holes was observed in the use of PicoPipet. We also confirmed that the FePt nanoparticles were fixed by MPTMS and arranged uniformly in the geometrical structure. Furthermore, we reported the optimum conditions for injection of the FePt nanoparticles into the hole and evaluated the magnetic properties after high-temperature annealing.
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Fig. 1. Strategy to control FePt nanoparticle domains by chemical bonding and geometrical structure.
Fig. 2. Fabrication scheme for uniformity arrangement of FePt nanoparticle by hole-patterns and chemical bonding. Process 1–4: the preparation of hole-patterns on Si (1 0 0) substrate, and process 5–7: the immobilization and arrangement of FePt nanoparticles on the hole-patterns with modified MPTMS.
2. Experimental The FePt nanoparticles were chemically prepared by mixing oleic acid and iron pentacarbonyl (Fe(CO)5) with a benzyl ether/ 1-octadecene solution of acetylacetonato platinum (Pt(acac)2); the mixture was heated to a mixing temperature of 120 1C, and then oleylamine was added. Oleic acid and oleylamine were used as dispersants. The Pt-rich nuclei was formed from the reduction of Pt(acac)2 that occurred simultaneously with partial decomposition of Fe(CO)5 at temperatures between 120 and 185 1C. We selected 245 1C as the growth temperature to obtain FePt nanoparticles with partial L10 phases. Then the mixture was refluxed for 2 h at 245 1C, and it was cooled naturally to room temperature. A black product was precipitated by adding 20 mL of ethanol, and then separated with a centrifuge. The FePt nanoparticles coated by oleic acid and oleylamine were dispersed in two separate solutions of hexane and toluene. Fig. 2 shows a fabrication scheme for a FePt nanoparticle domain by hole-patterns as geometrical structure. We fabricated the holepatterns on a Si substrate by an ultraviolet nanoimprint lithography (UV-NIL) process. The fabrication process is shown in Fig. 2. First, the UV-curable resin was applied to substrates. Then the quartz mold with patterns of 200 nm diameter at 500 nm intervals
was pressed to the resin (process 1 and 2). UV light at a wavelength of 365 nm was used to solidify the resin from above the mold. In a subsequent step, we etched the substrates with a deep reactive ion etching (Deep-RIE) system, using SF6 and C4F8 gases in etching process (process 3). The residual resin was removed by an O2 ashing process (process 4). Before the MPTMS modification, the Si substrate was rinsed in deionized water and then in a sulfuric peroxide mixture (SPM, H2SO4:H2O2 ¼4:1) for 10 min to remove contaminants from the substrate (process 5). Following the SPM cleaning, the MPTMS layer was formed on the Si substrate by immersion in a toluene solution containing 1.0 wt% MPTMS (95%, Sigma-Aldrich) at 60 1C for 10 min. Subsequently, the substrate was rinsed in toluene with ultrasonication to eliminate the additional silane (process 6). The MPTMS layer plays an important role in chemical bonding selectivity of both the substrate and FePt nanoparticles. Finally, the FePt-distributed hexane or toluene was coated by using PicoPipet, Altair (process 7). Surface morphologies at various processing stages were evaluated using an atomic force microscope (AFM; SPM-9600, Shimadzu). The surface view of the FePt nanoparticle array was observed by using an extreme high resolution scanning electron microscope (XHR-SEM; Magellan, FEI company Japan) at an accelerating voltage of 5 kV. The crystal structure of the FePt array before and after annealing was
T. Hachisu et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 303–308
characterized by X-ray diffraction (XRD; RINT-TTR III, Rigaku) with Cu ˚ at a scanning speed of 21/min for a range of Ka radiation (l ¼1.5405 A) 10–901 (2y). Magnetic properties were measured using a superconducting quantum interference device (SQUID; MPMS-7, Quantum Design, Inc.). The maximum applied magnetic field for the SQUID was 70 kOe.
3. Results and discussion First, in order to confirm the hole-patterns as the geometrical structure fabricated by UV-NIL, we measured the surface morphology by AFM. The AFM image is shown in Fig. 3(a). Fig. 3(b) shows the enlarged image of arrowed part in (a) and sectional pattern. From these AFM images, the average vertical depth from the top to the bottom of the hole was calculated to be 44 nm. Second, in order to select the dispersal solvent for the FePt nanoparticles, we tested spreading both the FePt-distributed hexane and toluene by PicoPipet. Fig. 4(a) and (b) shows the AFM images and the sectional pattern after coating the FePt-distributed toluene or hexane by PicoPipet. Comparing Fig. 4(a) with (b), the ruggedness
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morphologies were remarkably different from each other. In Fig. 4(a), the surface morphology of the hole-patterns after coating with the FePt-distributed toluene has the ruggedness in no longer the bottom than the top of the hole-patterns. On the other hand, there was no change of the surface ruggedness before and after coating the FePt-distributed hexane and the depth of the bottom in hole-patterns was changed from 44 to 38 nm. It was suggested that the mobility of FePt nanoparticles on the geometrical structure was dependent on the disperse solvent because of the difference of the surface densities between the geometrical structures. Fig. 5(a) shows the plan view of XHR-SEM images for the holepatterns after coated the FePt-distributed hexane and Fig. 5(b) is the enlarged image of arrowed part in Fig. 5(a). Previously, we confirmed that chemically synthesized FePt nanoparticles were fixed by the interaction between the Pt atom in FePt nanoparticle and the -SH group in MPTMS through possibility [10,11]. It was observed that a single-layer structure of FePt nanoparticles was formed at the bottom of the hole and the near around on Si(1 0 0), though a laminated structure was formed in the top. Additionally, the domain of the FePt nanoparticles was disordered at the top of the hole-patterns. The average pitch of the particles was 10.0 nm in
Fig. 3. AFM image of surface morphology of hole-patterns as geometrical structure on a Si(1 0 0) substrate by UV-NIL (area: 10 mm 10 mm, hole size: 200 nm, hole pitch: 500 nm, average depth: 40 nm), (b) enlarged image of arrowed part in (a) and sectional pattern.
Fig. 4. AFM images of surface morphology of hole-patterns area coated with FePt-distributed solution (a) toluene and (b) hexane using PicoPipet and the sectional pattern.
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Fig. 5. XHR-SEM images of FePt nanoparticles on hole-patterns area with modified (3-mercaptopropyl)trimethoxysilane, MPTMS as an interlayer between the particles and Si(1 0 0) substrate. (a) The plan view, (b) the enlarged image of arrowed part in (a), (c) the tilt view of 451, and (d) the enlarged image of arrowed part in (c).
Fig. 6. AFM images of surface morphology coated with FePt-distributed hexane using PicoPipet and sectional pattern. (a) The hydrophobic surface with no hole-patterns and nontreatment of MPTMS as an interlayer between the particles and Si(1 0 0) substrate and (b) the hole-patterns by UV-NIL without modification of MPTMS.
T. Hachisu et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 303–308
the bottom of hole, as shown in Fig. 5(b). There were many spots of the laminated structure of the FePt nanoparticles in the top of holepatterns when the sample was tilted by 451, as shown in Fig. 5(c). We observed that the FePt nanoparticles were arranged as a single layer in the wall part of the hole as well as the bottom (Fig. 5(b)), as shown in Fig. 5(d). Based on our results, the self-assembly array of the FePt nanoparticles are considered to be formed by the following process: (1) As soon as the nanoparticles solution was injected by a pico-litter pipette, the droplets with a diameter of ca. 30 mm were formed on the solution phase near sample plate. We think that the self-assemble arrays of the nanoparticles were formed in the surface of the pico-litter droplets, as mentioned by Bodnarchuk et al. [12]. (2) Subsequently, soft landing of the nanoparticles onto the hole-patterns modified with MPTMS starts during spontaneous solvent evaporation, and the formed self-assembled array of the nanoparticles were immobilized by Pt–S chemical bonding. The physical guide of hole-patterns plays a role of preventing the fluctuation of the nanoparticles array. In order to clarify the reason for arrangement of the FePt nanoparticles as a single layer, we coated the FePt-distributed hexane on two types of Si(1 0 0) substrate. One type has no holepatterns as the geometrical structure and no treatment of MPTMS
Fig. 7. XRD patterns of FePt nanoparticles on hole-patterns with modified MPTMS as an interlayer between the particles and Si(1 0 0) substrate (a) before annealing and (b) after annealing. The inset shows the enlarged XRD patterns of 38–521. And, the annealing conditions were at 800 1C for 3 h in the forming gas (Ar:H2 ¼ 97:3).
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as interlayer. Another has the hole-patterns as the geometrical structure without modification of MPTMS. The surface morphology of each by AFM measurement was shown in Fig. 6(a) and (b), respectively. The surface ruggedness from the FePt nanoparticles was confirmed and the arrangement of FePt nanoparticles was disordered (shown in Fig. 6(a)). From the AFM image (Fig. 6(a)), it was suggested that the fixation of the FePt nanoparticles would be possible on the surface of the substrate with hydrophobic, if we used a PicoPipet. However, the arrangement of FePt nanoparticles was difficult to control uniformly on non-fabricated geometrical structures that had not been treated with MPTMS. On another, all most the ruggedness from the FePt nanoparticle origin appeared only from the hole bottom (shown in Fig. 6(b)). Additionally, the ruggedness signal from the hole bottom was larger than the size of the particle. That is to say, the FePt nanoparticles were formed as secondary particles by coagulation. From the AFM image (Fig. 6(b)), when the FePt nanoparticles with distributed hexane was spread to the hole-patterns with nontreatment of MPTMS, it was confirmed that the FePt nanoparticles were injected easily in the bottom of hole selectivity. It was suggested that the capillary phenomenon between the hole-patterns as geometrical structure and the FePt nanoparticle-distributed hexane tends to predominate in the holepatterns without modification of MPTMS. Furthermore, the geometrical structure, such as the hole-patterns, plays a significant role in controlling the arrangement of FePt nanoparticles. Finally, we demonstrated the L10-ordering for the arrangement of FePt nanoparticles on the hole-patterns as the geometrical structure with modified MPTMS. The annealing conditions were at 800 1C for 3 h in the forming gas (Ar:H2 ¼97:3). Fig. 7(a) and (b) shows the XRD patterns of the FePt nanoparticles on holepatterns with MPTMS an interlayer between the FePt nanoparticles and Si(1 0 0) substrate before and after annealing, respectively. The extremely sharp peak emerging at the angle of 2y ¼331 was originated from L10-FePt (1 1 0), as shown in Fig. 7(b). In other words, the FePt nanoparticles array took a structure with (1 1 0) preferred orientation. Thus, compared with the peak intensity of L10-FePt (1 1 0), the peak intensities of FePt (1 1 1) and (2 0 0) from the inset in Fig. 7 may become low. On the contrary, we confirmed that the peak from L10-FePt (1 1 0) did not appear on the geometrical structure without modification of MPTMS. We suggest therefore that the Pt–S bonding plays an effective role in L10-ordering by high temperature annealing. Additionally, we observed after annealed FePt nanoparticles were not sintered from XHR-SEM images (Fig. 8(a) and (b)). Unfortunately, it was difficult
Fig. 8. XHR-SEM images of after annealed FePt nanoparticles on hole-patterns area with modified MPTMS as an interlayer between the particles and Si(1 0 0) substrate. (a) The plan view and (b) the enlarged image of arrowed part in (a). The annealing conditions were at 800 1C for 3 h in the forming gas (Ar:H2 ¼97:3).
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to keep the homogeneous array after (thermal) annealing. We recognize that inhomogeneous array is inapplicable to bit patterned media. In order to prevent the particles from disordering the arrangement by (thermal) annealing, we start an attempt to utilize a nonmagnetic layer over coating on FePt nanoparticles.
4. Conclusion In this study, with the aim to control the distortion of the arrangement of the FePt nanoparticles using an MPTMS layer modified by a silane coupling reaction and a geometrical structure prepared by ultraviolet nanoimprint lithography (UV-NIL), we demonstrated to the fixing and arrangement of the FePt nanoparticle uniformity using a PicoPipet, which is a coating tool. We used XHR-SEM imaging to show that the FePt nanoparticles were successfully arranged as a single layer with the average pitch of 10.0 nm by chemical bonding between Pt in the FePt nanoparticle and SH in MPTMS on the geometrical structure of Si(1 0 0). The density of the FePt nanoparticle domain controlled by the geometrical structure was equal up to an areal recording density of 6.45 Tbit in 2.
measurements were performed with the support of Shimadzu Co. Ltd. and FEI Company Japan Ltd., and we wish to express our gratitude. This work was partly supported by the Grant-in-Aid for Specially Promoted Research ‘‘Establishment of Electrochemical Device Engineering’’ from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and by the Global COE program ‘‘Center for Practical Chemical Wisdom’’ from MEXT.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Acknowledgements We deeply thank Mr. Hirafuji, who is a member of Altair, for providing valuable support and suggestions. The XHR-SEM
[10] [11] [12]
S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989. M. Chen, J.P. Liu, S. Sun, J. Am. Chem. Soc. 126 (2004) 8394. M. Chen, J. Kim, J.P. Liu, H. Fan, S. Sun, J. Am. Chem. Soc. 128 (2006) 7132. S. Sun, Adv. Mater. 18 (2006) 393. S. Kang, Z. Jia, S. Shi, D.E. Nikles, J.W. Harrell, Appl. Phys. Lett. 86 (2005) 062503. M.S. Wellons, W.H. Morris, Z. Gai, J. Shen, J. Bentley, J.E. Wittig, C.M. Lukehart, Chem. Mater. 19 (2007) 2483. B. Jeyadevan, K. Shinoda, R.J. Justin, T. Matsumoto, K. Sato, H. Takahashi, Y. Sato, K. Tohji, IEEE Trans. Magn. 42 (2006) 3030. S. Momose, H. Kodama, T. Uzumaki, A. Tanaka, Jpn. J. Appl. Phys. 44 (2005) 1147. T. Hachisu, T. Yotsumoto, A. Sugiyama, H. Iida, T. Nakanishi, T. Asahi, T. Osaka, Chem. Lett. 37 (2008) 840. T. Hachisu, T. Yotsumoto, A. Sugiyama, T. Osaka, ECS Trans 16 (2009) 199. T. Hachisu, W. Sato, A. Sugiyama, T. Osaka, J. Electrochem. Soc. 157 (2010) D514. M.I. Bodnarchuk, M.V. Kovalenko, S. Pichler, G. Fritz-Popovski, G. Hesser, W. Heiss, ACS Nano 4 (2010) 423.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Injection of synthesized FePt nanoparticles in hole-patterns for bit patterned media Takuma Hachisu a,b, Wataru Sato a, Shugo Ishizuka a, Atsushi Sugiyama a, Jun Mizuno a, Tetsuya Osaka a,b,n a b
Faculty of Nanoscience and Nanoengineering, Graduate School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Waseda Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
a r t i c l e in f o
abstract
Available online 28 December 2010
FePt nanoparticles of uniform sizes, compositions, and crystal structures can be obtained by chemical synthesis. Additionally, the nanoparticles can be well dispersed by the adsorption of a surfactant on the nanoparticle surface. Previously, the immobilization of FePt nanoparticles on a thermal oxide Si substrate was carried out by chemical synthesis, utilizing the Pt–S bonding between the -SH functional group in (3-mercaptopropyl)trimethoxysilane, MPTMS and Pt in FePt nanoparticles. However, controlling FePt nanoparticle arrays by this synthesis method was very difficult. In the present study, we attempted to control the distortion of the arrangement of FePt nanoparticles using an MPTMS layer modified with a silane coupling reaction and a geometrical structure prepared by ultraviolet nanoimprint lithography (UV-NIL). In this study, the hole-patterns used for the geometrical structure on Si(1 0 0) were 200 nm wide, 40 nm deep, and had a 500 nm pitch. The 5.6 nm FePt nanoparticles were used to coat the hole-patterns by using a picoliter pipette. An XHR-SEM image clearly revealed that the FePt nanoparticles were successfully arranged as a single layer with an average pitch of 10.0 nm by Pt–S bonding in the hole-patterns on Si(1 0 0). & 2010 Elsevier B.V. All rights reserved.
Keywords: Bit patterned media Synthesized FePt nanoparticle Chemical bonding Geometrical structure
1. Introduction Bit patterned media (BPM) is a perpendicular magnetic recording system, where the magnetic domains are isolated physically. BPM offers a potential path for developing next-generation ultra-high density data storage with capacities of 45 Tbit in 2. Magnetic arrays of BPM are fabricated using additive or subtractive processes. Additive processes involve the deposition of magnetic material into a template using electrochemical deposition or evaporation and liftoff. In a subtractive process, the magnetic recording layer is formed first by sputtering, and then etched using ion milling. For the lithographic fabrication of magnetic nanostructures, pattern-drawing BPM technology such as X-ray lithography, imprint lithography, and electron-beam lithography have been widely studied. L10-FePt alloys have a very high uniaxial magnetic crystalline anisotropy energy (Ku ¼7.0 107 erg/cm3). Because superparamagnetic fluctuation of room temperature magnetization can be suppressed even for grains with a diameter of 3.0 nm, the appropriate arrangement of FePt nanoparticles composed of one recording bit one particle is a promising candidate for next-generation BPM with capacities of 410 Tbit in 2. Since Sun et al. [1–4] reported the
n Correspondence to: Department of Applied Chemistry, School of Advanced Science and Engineering, Waseda University Bldg. 55S/Room 601, 3-4-1. Okubo, Shinjuku-ku, Tokyo 169-8555, Japan. Tel.: + 81 3 5286 3202; fax: + 81 3 3205 2074. E-mail address: [email protected] (T. Osaka).
0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.12.023
preparation of FePt nanoparticles monodispersed by chemical synthesis, many researchers have studied these particles for their potential use in a wide range of applications including magnetic recording media [5–8]. Previously, we succeeded in the fabrication of cubic FePt nanoparticles by controlling the reaction temperature in chemical synthesis; the FePt nanoparticles were fixed using chemical bonding on a thermal oxide Si substrate with modified (3-mercaptopropyl) trimethoxysilane, MPTMS as an interlayer between the FePt nanoparticles and the substrate [9,10]. However, the arrangement of the FePt nanoparticles formed irregularities, laminate structures, and aggregates [11]. In this study, to use the synthesized FePt nanoparticles for BPM, we attempted to develop a simple process to control the distortion of the arrangement of FePt nanoparticles using an MPTMS layer modified by a silane coupling reaction and a geometrical structure prepared by ultraviolet nanoimprint lithography (UV-NIL). Our strategy is shown in Fig. 1. In this study, hole-patterns of 200 nm width, 40 nm depth and 500 nm pitch were used as the geometrical structure on Si(1 0 0), and FePt-distributed solution was coated on the geometrical structure using a picoliter pipette (PicoPipet, Altair), which provides droplets down to 50–500 nl. Consequently, the selectivity injection of the particles into holes was observed in the use of PicoPipet. We also confirmed that the FePt nanoparticles were fixed by MPTMS and arranged uniformly in the geometrical structure. Furthermore, we reported the optimum conditions for injection of the FePt nanoparticles into the hole and evaluated the magnetic properties after high-temperature annealing.
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Fig. 1. Strategy to control FePt nanoparticle domains by chemical bonding and geometrical structure.
Fig. 2. Fabrication scheme for uniformity arrangement of FePt nanoparticle by hole-patterns and chemical bonding. Process 1–4: the preparation of hole-patterns on Si (1 0 0) substrate, and process 5–7: the immobilization and arrangement of FePt nanoparticles on the hole-patterns with modified MPTMS.
2. Experimental The FePt nanoparticles were chemically prepared by mixing oleic acid and iron pentacarbonyl (Fe(CO)5) with a benzyl ether/ 1-octadecene solution of acetylacetonato platinum (Pt(acac)2); the mixture was heated to a mixing temperature of 120 1C, and then oleylamine was added. Oleic acid and oleylamine were used as dispersants. The Pt-rich nuclei was formed from the reduction of Pt(acac)2 that occurred simultaneously with partial decomposition of Fe(CO)5 at temperatures between 120 and 185 1C. We selected 245 1C as the growth temperature to obtain FePt nanoparticles with partial L10 phases. Then the mixture was refluxed for 2 h at 245 1C, and it was cooled naturally to room temperature. A black product was precipitated by adding 20 mL of ethanol, and then separated with a centrifuge. The FePt nanoparticles coated by oleic acid and oleylamine were dispersed in two separate solutions of hexane and toluene. Fig. 2 shows a fabrication scheme for a FePt nanoparticle domain by hole-patterns as geometrical structure. We fabricated the holepatterns on a Si substrate by an ultraviolet nanoimprint lithography (UV-NIL) process. The fabrication process is shown in Fig. 2. First, the UV-curable resin was applied to substrates. Then the quartz mold with patterns of 200 nm diameter at 500 nm intervals
was pressed to the resin (process 1 and 2). UV light at a wavelength of 365 nm was used to solidify the resin from above the mold. In a subsequent step, we etched the substrates with a deep reactive ion etching (Deep-RIE) system, using SF6 and C4F8 gases in etching process (process 3). The residual resin was removed by an O2 ashing process (process 4). Before the MPTMS modification, the Si substrate was rinsed in deionized water and then in a sulfuric peroxide mixture (SPM, H2SO4:H2O2 ¼4:1) for 10 min to remove contaminants from the substrate (process 5). Following the SPM cleaning, the MPTMS layer was formed on the Si substrate by immersion in a toluene solution containing 1.0 wt% MPTMS (95%, Sigma-Aldrich) at 60 1C for 10 min. Subsequently, the substrate was rinsed in toluene with ultrasonication to eliminate the additional silane (process 6). The MPTMS layer plays an important role in chemical bonding selectivity of both the substrate and FePt nanoparticles. Finally, the FePt-distributed hexane or toluene was coated by using PicoPipet, Altair (process 7). Surface morphologies at various processing stages were evaluated using an atomic force microscope (AFM; SPM-9600, Shimadzu). The surface view of the FePt nanoparticle array was observed by using an extreme high resolution scanning electron microscope (XHR-SEM; Magellan, FEI company Japan) at an accelerating voltage of 5 kV. The crystal structure of the FePt array before and after annealing was
T. Hachisu et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 303–308
characterized by X-ray diffraction (XRD; RINT-TTR III, Rigaku) with Cu ˚ at a scanning speed of 21/min for a range of Ka radiation (l ¼1.5405 A) 10–901 (2y). Magnetic properties were measured using a superconducting quantum interference device (SQUID; MPMS-7, Quantum Design, Inc.). The maximum applied magnetic field for the SQUID was 70 kOe.
3. Results and discussion First, in order to confirm the hole-patterns as the geometrical structure fabricated by UV-NIL, we measured the surface morphology by AFM. The AFM image is shown in Fig. 3(a). Fig. 3(b) shows the enlarged image of arrowed part in (a) and sectional pattern. From these AFM images, the average vertical depth from the top to the bottom of the hole was calculated to be 44 nm. Second, in order to select the dispersal solvent for the FePt nanoparticles, we tested spreading both the FePt-distributed hexane and toluene by PicoPipet. Fig. 4(a) and (b) shows the AFM images and the sectional pattern after coating the FePt-distributed toluene or hexane by PicoPipet. Comparing Fig. 4(a) with (b), the ruggedness
305
morphologies were remarkably different from each other. In Fig. 4(a), the surface morphology of the hole-patterns after coating with the FePt-distributed toluene has the ruggedness in no longer the bottom than the top of the hole-patterns. On the other hand, there was no change of the surface ruggedness before and after coating the FePt-distributed hexane and the depth of the bottom in hole-patterns was changed from 44 to 38 nm. It was suggested that the mobility of FePt nanoparticles on the geometrical structure was dependent on the disperse solvent because of the difference of the surface densities between the geometrical structures. Fig. 5(a) shows the plan view of XHR-SEM images for the holepatterns after coated the FePt-distributed hexane and Fig. 5(b) is the enlarged image of arrowed part in Fig. 5(a). Previously, we confirmed that chemically synthesized FePt nanoparticles were fixed by the interaction between the Pt atom in FePt nanoparticle and the -SH group in MPTMS through possibility [10,11]. It was observed that a single-layer structure of FePt nanoparticles was formed at the bottom of the hole and the near around on Si(1 0 0), though a laminated structure was formed in the top. Additionally, the domain of the FePt nanoparticles was disordered at the top of the hole-patterns. The average pitch of the particles was 10.0 nm in
Fig. 3. AFM image of surface morphology of hole-patterns as geometrical structure on a Si(1 0 0) substrate by UV-NIL (area: 10 mm 10 mm, hole size: 200 nm, hole pitch: 500 nm, average depth: 40 nm), (b) enlarged image of arrowed part in (a) and sectional pattern.
Fig. 4. AFM images of surface morphology of hole-patterns area coated with FePt-distributed solution (a) toluene and (b) hexane using PicoPipet and the sectional pattern.
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Fig. 5. XHR-SEM images of FePt nanoparticles on hole-patterns area with modified (3-mercaptopropyl)trimethoxysilane, MPTMS as an interlayer between the particles and Si(1 0 0) substrate. (a) The plan view, (b) the enlarged image of arrowed part in (a), (c) the tilt view of 451, and (d) the enlarged image of arrowed part in (c).
Fig. 6. AFM images of surface morphology coated with FePt-distributed hexane using PicoPipet and sectional pattern. (a) The hydrophobic surface with no hole-patterns and nontreatment of MPTMS as an interlayer between the particles and Si(1 0 0) substrate and (b) the hole-patterns by UV-NIL without modification of MPTMS.
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the bottom of hole, as shown in Fig. 5(b). There were many spots of the laminated structure of the FePt nanoparticles in the top of holepatterns when the sample was tilted by 451, as shown in Fig. 5(c). We observed that the FePt nanoparticles were arranged as a single layer in the wall part of the hole as well as the bottom (Fig. 5(b)), as shown in Fig. 5(d). Based on our results, the self-assembly array of the FePt nanoparticles are considered to be formed by the following process: (1) As soon as the nanoparticles solution was injected by a pico-litter pipette, the droplets with a diameter of ca. 30 mm were formed on the solution phase near sample plate. We think that the self-assemble arrays of the nanoparticles were formed in the surface of the pico-litter droplets, as mentioned by Bodnarchuk et al. [12]. (2) Subsequently, soft landing of the nanoparticles onto the hole-patterns modified with MPTMS starts during spontaneous solvent evaporation, and the formed self-assembled array of the nanoparticles were immobilized by Pt–S chemical bonding. The physical guide of hole-patterns plays a role of preventing the fluctuation of the nanoparticles array. In order to clarify the reason for arrangement of the FePt nanoparticles as a single layer, we coated the FePt-distributed hexane on two types of Si(1 0 0) substrate. One type has no holepatterns as the geometrical structure and no treatment of MPTMS
Fig. 7. XRD patterns of FePt nanoparticles on hole-patterns with modified MPTMS as an interlayer between the particles and Si(1 0 0) substrate (a) before annealing and (b) after annealing. The inset shows the enlarged XRD patterns of 38–521. And, the annealing conditions were at 800 1C for 3 h in the forming gas (Ar:H2 ¼ 97:3).
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as interlayer. Another has the hole-patterns as the geometrical structure without modification of MPTMS. The surface morphology of each by AFM measurement was shown in Fig. 6(a) and (b), respectively. The surface ruggedness from the FePt nanoparticles was confirmed and the arrangement of FePt nanoparticles was disordered (shown in Fig. 6(a)). From the AFM image (Fig. 6(a)), it was suggested that the fixation of the FePt nanoparticles would be possible on the surface of the substrate with hydrophobic, if we used a PicoPipet. However, the arrangement of FePt nanoparticles was difficult to control uniformly on non-fabricated geometrical structures that had not been treated with MPTMS. On another, all most the ruggedness from the FePt nanoparticle origin appeared only from the hole bottom (shown in Fig. 6(b)). Additionally, the ruggedness signal from the hole bottom was larger than the size of the particle. That is to say, the FePt nanoparticles were formed as secondary particles by coagulation. From the AFM image (Fig. 6(b)), when the FePt nanoparticles with distributed hexane was spread to the hole-patterns with nontreatment of MPTMS, it was confirmed that the FePt nanoparticles were injected easily in the bottom of hole selectivity. It was suggested that the capillary phenomenon between the hole-patterns as geometrical structure and the FePt nanoparticle-distributed hexane tends to predominate in the holepatterns without modification of MPTMS. Furthermore, the geometrical structure, such as the hole-patterns, plays a significant role in controlling the arrangement of FePt nanoparticles. Finally, we demonstrated the L10-ordering for the arrangement of FePt nanoparticles on the hole-patterns as the geometrical structure with modified MPTMS. The annealing conditions were at 800 1C for 3 h in the forming gas (Ar:H2 ¼97:3). Fig. 7(a) and (b) shows the XRD patterns of the FePt nanoparticles on holepatterns with MPTMS an interlayer between the FePt nanoparticles and Si(1 0 0) substrate before and after annealing, respectively. The extremely sharp peak emerging at the angle of 2y ¼331 was originated from L10-FePt (1 1 0), as shown in Fig. 7(b). In other words, the FePt nanoparticles array took a structure with (1 1 0) preferred orientation. Thus, compared with the peak intensity of L10-FePt (1 1 0), the peak intensities of FePt (1 1 1) and (2 0 0) from the inset in Fig. 7 may become low. On the contrary, we confirmed that the peak from L10-FePt (1 1 0) did not appear on the geometrical structure without modification of MPTMS. We suggest therefore that the Pt–S bonding plays an effective role in L10-ordering by high temperature annealing. Additionally, we observed after annealed FePt nanoparticles were not sintered from XHR-SEM images (Fig. 8(a) and (b)). Unfortunately, it was difficult
Fig. 8. XHR-SEM images of after annealed FePt nanoparticles on hole-patterns area with modified MPTMS as an interlayer between the particles and Si(1 0 0) substrate. (a) The plan view and (b) the enlarged image of arrowed part in (a). The annealing conditions were at 800 1C for 3 h in the forming gas (Ar:H2 ¼97:3).
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to keep the homogeneous array after (thermal) annealing. We recognize that inhomogeneous array is inapplicable to bit patterned media. In order to prevent the particles from disordering the arrangement by (thermal) annealing, we start an attempt to utilize a nonmagnetic layer over coating on FePt nanoparticles.
4. Conclusion In this study, with the aim to control the distortion of the arrangement of the FePt nanoparticles using an MPTMS layer modified by a silane coupling reaction and a geometrical structure prepared by ultraviolet nanoimprint lithography (UV-NIL), we demonstrated to the fixing and arrangement of the FePt nanoparticle uniformity using a PicoPipet, which is a coating tool. We used XHR-SEM imaging to show that the FePt nanoparticles were successfully arranged as a single layer with the average pitch of 10.0 nm by chemical bonding between Pt in the FePt nanoparticle and SH in MPTMS on the geometrical structure of Si(1 0 0). The density of the FePt nanoparticle domain controlled by the geometrical structure was equal up to an areal recording density of 6.45 Tbit in 2.
measurements were performed with the support of Shimadzu Co. Ltd. and FEI Company Japan Ltd., and we wish to express our gratitude. This work was partly supported by the Grant-in-Aid for Specially Promoted Research ‘‘Establishment of Electrochemical Device Engineering’’ from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and by the Global COE program ‘‘Center for Practical Chemical Wisdom’’ from MEXT.
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Acknowledgements We deeply thank Mr. Hirafuji, who is a member of Altair, for providing valuable support and suggestions. The XHR-SEM
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