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Glassy alloy composites for information technology applications
Intermetallics 18 (2010) 1983e1987
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Glassy alloy composites for information technology applications N. Nishiyama a, *, K. Takenaka a, N. Togashi a, H. Miura a, N. Saidoh a, Akihisa Inoue b a b
RIMCOF Tohoku University Laboratory, R&D Institute of Metals and Composites for Future Industries, Sendai 980-8577, Japan Tohoku University, Sendai 980-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 November 2009 Received in revised form 29 January 2010 Accepted 27 February 2010 Available online 7 April 2010
With the aim of developing a novel preparation process for bit-patterned-media, nano-patterned glassy alloy composite composed of zero-magneto-striction CoeFeeB amorphous alloy thin film, PdeCueNieP glassy alloy and Co/Pd multi-layer with perpendicular magnetization was prepared. Using the commercial imprinting mold, periodic nano hole array with a hole diameter of 90 nm and a pitch of 180 nm was successfully prepared on the surface of Pd-based glassy alloy thin film. By overlaying the Co/ Pd multi-layer with perpendicular magnetization and surface polishing, isolated Co/Pd magnetic dots embedded into nano hole array can be obtained. These magnetic dots act as single magnetic domains. In addition, the change in magnetization direction was confirmed under magnetic fields of 20 kOe. These results suggest that the glassy alloy nano-composite is suitable for BPM of high data density HDD. It is therefore concluded that the preparation method is candidate process for the preparation of BPM of high data density HDD. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Composite B. Magnetic properties C. Thin films G. Magnetic applications
1. Introduction In our daily life, we benefit from information technology (IT) equipments such as laptop personal computer (PC), cellphone, mobile music player, car navigation system, digital camera and so on. The functionality of these IT equipments continues to develop by miniaturizing the size and heightening the integration. For example, a hard disk drive (HDD) media, which is conventionally used for data storage in PC is required to have mush higher plane data density. According to the requirement, magnetic recording system has been changed from in-plane magnetization type to perpendicular magnetization type [1]. At present, this perpendicular magnetization type HDD media with a plane data density of several hundreds of Gbit/in2 are already commercialized. To further enhance the data density, bit-patterned-media (BPM) type recording [2] is expected to be adopted and commercialized in the next decade. For instance, high data density of 1 Tbit/in2 will be achieved when precisely arrayed single magnetic domains for recording are realized with a domain diameter of about 12 nm and a pitch of 25 nm. Recently, much effort has been devoted to develop BPM with high data density. In particular, several methods such as lithography [3,4], anodic alumina [5e7], Al-Si film [8] were developed for the fabrication of highly ordered nano hole array.
* Corresponding author. Tel.: þ81 22 215 2840; fax þ81 22 215 2841. E-mail address: [email protected] (N. Nishiyama). 0966-9795/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.02.027
However, it is still difficult to prove the feasibility of such nanopatterned array by highly efficient production process. It is well known that glassy alloys exhibit superior properties such as ultrahigh strength, large elastic elongation of 2%, low Young's modulus, good soft magnetic properties or corrosion resistance [9]. By utilizing such superior properties of monolithic glassy alloys, several innovative products have been successfully developed and commercialized [10]. In addition, glassy alloys exhibit nano-level imprint-ability in its supercooled liquid (SCL) region [11e13] above the glass transition temperature (Tg). By utilizing the nano-imprint ability of glassy alloy, novel composite can be prepared with a combination of nano-patterned glassy alloy and other crystalline materials could be prepared. Such nanocomposite is expected to apply to IT applications. In this paper, we intend to present the selection of constituent materials for making nano-composites. Nano-patterning of glassy alloy prepared by thermal imprinting are also reported. Furthermore, the application possibility of nano-composites produced by stacking of a nano-patterned glassy alloy and a crystalline alloy thin film to BPM of high data density HDD system are discussed. 2. Experimental Fig. 1 shows the schematic illustration of expected composite construction for BPM of high data density HDD system. Initially, a soft magnetic under layer (SUL) is deposited onto Si substrate. For the SUL, zero-magneto-strictive Co79.5Fe5.5B15 amorphous alloy [14] was selected to avoid the stress change in magnetic properties
1984
N. Nishiyama et al. / Intermetallics 18 (2010) 1983e1987
Fig. 3. Typical in-plane and perpendicular magnetization curves of the zero-magnetostrictive Co79.5Fe5.5B15 amorphous alloy thin film with a thickness of 100 nm.
Fig. 1. Schematic illustration of expected composite construction for BPM of high data density HDD system.
during thermal imprinting. Then the SUL is overlaid by a glassy alloy layer, and nano-patterning is carried out by thermal imprinting to make nano hole array. A PdeCueNieP alloy for nanopatterning layer was chosen because of its high resistivity against crystallization and its low viscosity in the superccoled liquid region [15]. The Pd-based glassy alloy thin film (GA-TF) with a thickness of ranging from 20 nm to 1 mm was prepared by a pulsed-laserdeposition (PLD) method [16] to avoid phosphorus evaporation. Detailed deposition condition was described elsewhere [17]. For the magnetic recording, a Co/Pd multi-layer with high magnetic anisotropy field (Hk) and high saturation magnetization (Ms) is embedded into the nano-holes. It was 10 times periodically stacked by Co and Pd with a thickness of 0.3 and 1.0 nm, respectively. For the recording layer, Co-based amorphous alloy with a thickness of 100 nm and Co/Pd multi-layer with a total thickness of about 14 nm were fabricated by a magnetron sputtering method. In deposition of the both layers, sputtering atmosphere and DC power was 0.4 Pa of high purity argon gas and 100 W, respectively. Finally, the surface is chemicalemechanically polished (CMP) to flat and isolate the embedded Co/Pd magnetic dots.
Fig. 2. SEM image from right overhead of the dot array mold with a dot diameter of 90 nm, pitch of 180 nm and a dot height of 100 nm.
Structure and thermal characteristics of all the thin films were investigated using X-ray diffractometry (XRD) and differential scanning calorimeter (DSC), respectively. Magnetic properties of the SUL and Co/Pd multi-layer were individually measured using a vibrating sample magnetometer (VSM) and coercivity meter, respectively. To determine the imprinting condition, the viscosity of Pd-based glassy rod in its SCL region was evaluated using a thermal mechanical analyzer (TMA) under a compressive stress. A electron-beam lithographed Si mold (NTT-AT Nanofabrication) having a dot array pattern with a dot diameter of 90 nm, 180 nm in pitch and 100 nm in height was used as a nanoimprint mold in the present study. Fig. 2 shows the SEM image of the right overhead view of the mold, revealing that the dots have a diameter of 90 nm and the dot pitch is at even intervals. Using the Pd-based GA-TF and the Si mold, nano-imprint test was carried out by imprinting equipment (Nano Craft Technologies: NI-1075). The Pd-based GA-TF was heated at a rate of 1.67 K/s in an argon atomosphere. Then imprinting test was carried out at the temperature of 609 K, and in the pressing time of 10 s. The structures of the imprinted Pd-based GA-TF were identified using XRD. The morphologies and the microstructures of imprinted Pd-based GATF were examined using a field-emission scanning electron microscopy (FE-SEM: Carl Zeiss Inc. ULTRA55).
Fig. 4. In-plane and perpendicular magnetization curves of the Co/Pd multi-layered thin film with a total thickness of 14 nm.
N. Nishiyama et al. / Intermetallics 18 (2010) 1983e1987
Fig. 5. DSC profile of PdeCueNieP GA-TF prepared by a PLD method.
3. Results and discussion Fig. 3 shows the typical in-plane and perpendicular magnetization curves of the zero-magneto-strictive Co79.5Fe5.5B15 amorphous alloy thin film with a thickness of 100 nm. In the case of in-plane magnetization, the curve exhibits typical soft magnetism with a steep magnetization. On the other hand, the perpendicular magnetization curve exhibits gentle magnetization. Slight hysteresis can be detected while the perpendicular anisotropy is negligible for SUL layer [17]. Using the value of the saturation magnetization, the saturation flux density (Bs) is evaluated to be 1.3 T. Besides, relatively high coercivity (Hc) of 56 A/m is estimated using coercivity meter. This relatively high Hc is attributed to the residual stress generated during deposition. In addition, the value
1985
of magneto-striction (ls) is evaluated to be 3 107. These magnetic properties are considered to be suitable for SUL. Fig. 4 shows the in-plane and perpendicular magnetization curves of the Co/Pd multi-layered thin film with a total thickness of 14 nm. The film construction is 10 times sequence of Co with a thickness of 0.3 nm and Pd with a thickness of 1 nm on the foundation layer of Pd with a thickness of 1 nm. Different from the CoeFeeB amorphous alloy thin film, the magnetization of the Co/ Pd multi-layer shows perpendicular magnetization behavior. From the curves, saturation magnetization (Ms) and anisotropy field (Hk) of the Co/Pd multi-layer in the as-deposited plane form is evaluated to be 540 emu/cc and 7.2 kOe, respectively. These values have high resistivity for magnetic instability due to heat fluctuation [18]. The Hk will be increased by high aspect magnetic dot due to nanopatterning. It is therefore can be said that the Co/Pd multi-layer is applicable for magnetic recording layer of BPM. Most important layer for realization of nano-patterned BPM is the imprint layer. The imprint layer will be required to have high thermal resistivity against crystallization and low viscosity in its SCL region. Since stable and easy glass-forming alloys are generally multi-component systems, a PLD method with high-energy plasma induced by laser beam was employed to prepare GA-TF with a complicated composition. Fig. 5 shows the DSC profile of PdeCueNieP GA-TF with a thickness of 1 mm. Glass transition temperature (Tg), onset of crystallization temperature (Tx) is evaluated to be 599 K and 638 K, respectively. These values are slightly higher than those of glassy alloy target with a composition of Pd42.5Cu30Ni7.5P20. Compositional analysis by an inductively coupled plasma atomic emission spectroscopy (ICP) reveals that Pd-based GA-TF has an atomic composition of Pd46Cu33Ni7P14. The slightly higher Tg and Tx are attributed to the composition deviation between the GA-TF and the target. However, distinct endothermic reaction due to glass transition can still be detected, proving the possibility of imprint applicability. Consequently, it can be concluded that the three different thin films to construct composite for BPM have been successfully prepared. Using the imprint mold shown in the Fig. 2, imprinting tests for Pd-based GA-TFs were carried out. Imprinting temperature was determined to be 609 K (Tg þ 10 K), where the viscosity is evaluated to be about 1011 Pa s. The applied imprint stress and pressing time
Fig. 6. Surface SEM image of imprinted nano hole array with a hole diameter of 90 nm (a), enlarged hole image (b) and cross-sectional profile of hole measured by AFM (c).
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N. Nishiyama et al. / Intermetallics 18 (2010) 1983e1987
were optimized to be 30 MPa and 10 s, respectively. Fig. 6 shows the SEM surface image of the imprinted nano hole array with a hole diameter of 90 nm (a), enlarged hole image (b) and cross sectional profile of hole measured by atomic force microscope (c:AFM). As seen in (a), periodic holes are formed on the Pd-based GA-TF. It is also found that the hole bottom is flat as shown in (b). This flat bottom morphology is suitable for overlying the Co/Pd multi-layer with perpendicular magnetization. The AFM profile as shown in (c), depth of the hole is evaluated to be about 40 nm. The Co/Pd multilayer on the Pd-based GA-TF nano hole array was then polished by CMP. Fig. 7 shows the SEM overview of polished surface (a) and elemental line analysis obtained by an energy dispersive X-ray spectroscopy (EDS) (b). The embedded Co/Pd multi-layer can be observed as a different contrast shown in (a). As seen in (b), it is found that Co is enriched at the location of embedded Co/Pd multilayer. On the other hand, the other regions show higher Cu content and less Co content. These results suggest that the embedded Co/Pd multi-layers are isolated and may act as magnetic recording dots. To confirm the possibility of realization, magnetization behavior of prototype BPM using glassy alloy composite was investigated under magnetic fields of 20 kOe. Fig. 8 shows the AFM (a) and magnetic force microscopy (b: MFM) images of polished BPM surface using glassy alloy composite under þ20 kOe, respectively. Fig. 8(c) and (d) also show AFM and MFM images of the same sample under 20 kOe. As seen in (a) and (c), AFM images are almost the same, suggesting that the surface morphologies are very close. However, MFM images shown in (b) and (d) exhibit opposite contrast. This
Fig. 8. AFM (a and c) and MFM (b and d) images of surface polished BPM using glassy alloy composite under 20 kOe.
result suggests that the Co/Pd isolated dots under 20 kOe were magnetized in the opposite magnetization direction. Furthermore, it is important to note that no subdivided contrast could be seen in each of the magnetized dots, revealing that each of the dots acts as a single magnetic domain. Therefore the isolated Co/Pd magnetic dots using patterned glassy alloy and CMP finishing are suitable for perpendicular magnetic recoding domains. These results suggest that the nano-composite using patterned Pd-based GA-TF is suitable for BPM of high data density HDD and this preparation method is a candidate process for preparing BPM of high data density HDD. 4. Conclusions With the aim of confirming the application possibility for BPM of high data density HDD, the composite of GA-TF and other crystalline thin films was prepared and thermally nano-imprinted. The obtained results are summarized as follows. (1) For SUL and magnetic recording layers, zero-magneto-striction CoeFeeB amorphous alloy thin film and Co/Pd multi-layer were successfully prepared by MGS. For imprinting layer, PdeCueNieP GA-TF was prepared by PLD. It was found that each of the layers has sufficient properties to construct composites for BPM of high data density HDD. (2) Nano-patterning for Pd-based GA-TF was carried out using commercial nano-imprint mold with a dot diameter of 90 nm. As a result, periodic nano hole array could be formed on the surface of the Pd-based GA-TF (3) By combining the overlaid Co/Pd multi-layer and CMP, prototype composite for BPM was prepared. It was found that the isolated Co/Pd magnetic dots acts as single magnetic domains and change in magnetization direction was confirmed under magnetic fields of 20 kOe. Acknowledgements
Fig. 7. SEM overview of polished surface (a) and elemental line analysis obtained by an energy dispersive X-ray spectroscopy (EDS) (b).
The authors are grateful to Prof. M. Futamoto, Prof. H. Fujimori and Prof. Y. Hirotsu for stimulating discussion. Funding by “New Energy and Industrial technology Development Organization
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(NEDO)” under “Technological Development of Innovative Components Based on Enhanced Functionality Metallic Glass” project is also gratefully acknowledged. References [1] Iwasaki S, Nakamura Y. An analysis for the magnetization mode for high density magnetic recording. IEEE Trans Mag 1977;13:1272e7. [2] Chou SY, Krauss PR, Renstrom PJ. Imprint of sub-25 nm vias and trenches in polymers. Appl Phys Lett 1995;67:3114e6. [3] Koike K, Matsuyama H, Hirayama Y, Tanahashi K, Kanemura T, Kitakami O, et al. Magnetic block array for patterned magnetic media. Appl Phys Lett 2001;78:784e6. [4] Moritz J, Landis S, Toussaint JC, Batle-Guillemaud PB, Rodmacq B, Casali G, et al. Patterned medias made from pre-witched wafers: a promising route toward ultrahigh-density magnetic recording. IEEE Trans Mag 2002;38:1731e6. [5] Zeng H, Zhang M, Skomski R, Sellmyer DJ, Liu Y, Menon L, et al. Magnetic Properties of self-assembled Co nanowire of varying length and diameter. J Appl Phys 2000;87:4718e20. [6] Masuda H, Nagae M, Morikawa T, Nishio K. Long-rang ordered anodic porous alumina with reduced hole interval formed in highly concentrated sulfuric acid solution. Jpn J Appl Phys 2006;45:L406e8. [7] Yasui N, Ichihara S, Nakamura T, Imada A, Saito T, Ohasshi Y, et al. Characterization of high-density patterned media fabricated by a new anodizing process. J Appl Phys 2008;103:07C515.
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[8] Yasui N, Nakamura T, Fukutachi K, Saito T, Aiba T, Ohashi Y, et al. Perpendicular recording media using phase-separated AlSi films. J Appl Phys 2005;97:10N103. [9] Inoue A. Stabilization of metallic supercooled liquid and bulk amorphous alloys. Acta Mater 2000;48:279e306. [10] Nishiyama N, Amiya K, Inoue A. Novel applications of bulk metallic glass for industrial products. J Non-Cryst Solids 2007;353:3615e21. [11] Saotome Y, Itoh K, Zhang T, Inoue A. Superplastic nanoforming of Pd-based amorphous alloy. Scr Mater 2001;44:1541e5. [12] Saotome Y, Hatori T, Zhang T, Inoue A. Superplastic micro/nano-formability of La60Al20Ni10Co5Cu5 amorphous alloy in supercooled liquid state. Mater Sci Eng 2001;A304eA306:716e20. [13] Saotome Y, Noguchi Y, Zhang T, Inoue A. Characteristic behavior of Pt-based metallic glass under rapid heating and its application to microforming. Mater Sci Eng 2004;A375eA377:389e93. [14] Fujimori H, Arai KI, Shirae H, Saito H, Masumoto T, Tsuya N. Magnetostriction of FeeCo Amorphous alloys. Jpn J Appl Phys 1976;15:705e6. [15] Nishiyama N, Inoue A. Glass transition behavior and viscous flow working of Pd40Cu30Ni10P20 amorphous alloy. Mater Trans JIM 1999;40:64e71. [16] Shen J, Gai Z, Kirscner J. Growth and magnetism of metallic thin films and multilayers by pulsed-laser-deposition. Surf Sci Rep 2004;52:163e217. [17] Takenaka K, Sugimoto T, Nishiyama N, Makino A, Saotome Y, Hirotsu Y, et al. Structure, morphology and magnetic properties of FeeBeSieNb glassy alloy thin film prepared by a pulsed laser deposition method. Mater Lett 2009;63:1895e7. [18] Charap SH, Lu PL, He Y. Thermal stability of recorded information at high densities. IEEE Trans Mag 1997;33:978e83.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Glassy alloy composites for information technology applications N. Nishiyama a, *, K. Takenaka a, N. Togashi a, H. Miura a, N. Saidoh a, Akihisa Inoue b a b
RIMCOF Tohoku University Laboratory, R&D Institute of Metals and Composites for Future Industries, Sendai 980-8577, Japan Tohoku University, Sendai 980-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 November 2009 Received in revised form 29 January 2010 Accepted 27 February 2010 Available online 7 April 2010
With the aim of developing a novel preparation process for bit-patterned-media, nano-patterned glassy alloy composite composed of zero-magneto-striction CoeFeeB amorphous alloy thin film, PdeCueNieP glassy alloy and Co/Pd multi-layer with perpendicular magnetization was prepared. Using the commercial imprinting mold, periodic nano hole array with a hole diameter of 90 nm and a pitch of 180 nm was successfully prepared on the surface of Pd-based glassy alloy thin film. By overlaying the Co/ Pd multi-layer with perpendicular magnetization and surface polishing, isolated Co/Pd magnetic dots embedded into nano hole array can be obtained. These magnetic dots act as single magnetic domains. In addition, the change in magnetization direction was confirmed under magnetic fields of 20 kOe. These results suggest that the glassy alloy nano-composite is suitable for BPM of high data density HDD. It is therefore concluded that the preparation method is candidate process for the preparation of BPM of high data density HDD. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Composite B. Magnetic properties C. Thin films G. Magnetic applications
1. Introduction In our daily life, we benefit from information technology (IT) equipments such as laptop personal computer (PC), cellphone, mobile music player, car navigation system, digital camera and so on. The functionality of these IT equipments continues to develop by miniaturizing the size and heightening the integration. For example, a hard disk drive (HDD) media, which is conventionally used for data storage in PC is required to have mush higher plane data density. According to the requirement, magnetic recording system has been changed from in-plane magnetization type to perpendicular magnetization type [1]. At present, this perpendicular magnetization type HDD media with a plane data density of several hundreds of Gbit/in2 are already commercialized. To further enhance the data density, bit-patterned-media (BPM) type recording [2] is expected to be adopted and commercialized in the next decade. For instance, high data density of 1 Tbit/in2 will be achieved when precisely arrayed single magnetic domains for recording are realized with a domain diameter of about 12 nm and a pitch of 25 nm. Recently, much effort has been devoted to develop BPM with high data density. In particular, several methods such as lithography [3,4], anodic alumina [5e7], Al-Si film [8] were developed for the fabrication of highly ordered nano hole array.
* Corresponding author. Tel.: þ81 22 215 2840; fax þ81 22 215 2841. E-mail address: [email protected] (N. Nishiyama). 0966-9795/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.02.027
However, it is still difficult to prove the feasibility of such nanopatterned array by highly efficient production process. It is well known that glassy alloys exhibit superior properties such as ultrahigh strength, large elastic elongation of 2%, low Young's modulus, good soft magnetic properties or corrosion resistance [9]. By utilizing such superior properties of monolithic glassy alloys, several innovative products have been successfully developed and commercialized [10]. In addition, glassy alloys exhibit nano-level imprint-ability in its supercooled liquid (SCL) region [11e13] above the glass transition temperature (Tg). By utilizing the nano-imprint ability of glassy alloy, novel composite can be prepared with a combination of nano-patterned glassy alloy and other crystalline materials could be prepared. Such nanocomposite is expected to apply to IT applications. In this paper, we intend to present the selection of constituent materials for making nano-composites. Nano-patterning of glassy alloy prepared by thermal imprinting are also reported. Furthermore, the application possibility of nano-composites produced by stacking of a nano-patterned glassy alloy and a crystalline alloy thin film to BPM of high data density HDD system are discussed. 2. Experimental Fig. 1 shows the schematic illustration of expected composite construction for BPM of high data density HDD system. Initially, a soft magnetic under layer (SUL) is deposited onto Si substrate. For the SUL, zero-magneto-strictive Co79.5Fe5.5B15 amorphous alloy [14] was selected to avoid the stress change in magnetic properties
1984
N. Nishiyama et al. / Intermetallics 18 (2010) 1983e1987
Fig. 3. Typical in-plane and perpendicular magnetization curves of the zero-magnetostrictive Co79.5Fe5.5B15 amorphous alloy thin film with a thickness of 100 nm.
Fig. 1. Schematic illustration of expected composite construction for BPM of high data density HDD system.
during thermal imprinting. Then the SUL is overlaid by a glassy alloy layer, and nano-patterning is carried out by thermal imprinting to make nano hole array. A PdeCueNieP alloy for nanopatterning layer was chosen because of its high resistivity against crystallization and its low viscosity in the superccoled liquid region [15]. The Pd-based glassy alloy thin film (GA-TF) with a thickness of ranging from 20 nm to 1 mm was prepared by a pulsed-laserdeposition (PLD) method [16] to avoid phosphorus evaporation. Detailed deposition condition was described elsewhere [17]. For the magnetic recording, a Co/Pd multi-layer with high magnetic anisotropy field (Hk) and high saturation magnetization (Ms) is embedded into the nano-holes. It was 10 times periodically stacked by Co and Pd with a thickness of 0.3 and 1.0 nm, respectively. For the recording layer, Co-based amorphous alloy with a thickness of 100 nm and Co/Pd multi-layer with a total thickness of about 14 nm were fabricated by a magnetron sputtering method. In deposition of the both layers, sputtering atmosphere and DC power was 0.4 Pa of high purity argon gas and 100 W, respectively. Finally, the surface is chemicalemechanically polished (CMP) to flat and isolate the embedded Co/Pd magnetic dots.
Fig. 2. SEM image from right overhead of the dot array mold with a dot diameter of 90 nm, pitch of 180 nm and a dot height of 100 nm.
Structure and thermal characteristics of all the thin films were investigated using X-ray diffractometry (XRD) and differential scanning calorimeter (DSC), respectively. Magnetic properties of the SUL and Co/Pd multi-layer were individually measured using a vibrating sample magnetometer (VSM) and coercivity meter, respectively. To determine the imprinting condition, the viscosity of Pd-based glassy rod in its SCL region was evaluated using a thermal mechanical analyzer (TMA) under a compressive stress. A electron-beam lithographed Si mold (NTT-AT Nanofabrication) having a dot array pattern with a dot diameter of 90 nm, 180 nm in pitch and 100 nm in height was used as a nanoimprint mold in the present study. Fig. 2 shows the SEM image of the right overhead view of the mold, revealing that the dots have a diameter of 90 nm and the dot pitch is at even intervals. Using the Pd-based GA-TF and the Si mold, nano-imprint test was carried out by imprinting equipment (Nano Craft Technologies: NI-1075). The Pd-based GA-TF was heated at a rate of 1.67 K/s in an argon atomosphere. Then imprinting test was carried out at the temperature of 609 K, and in the pressing time of 10 s. The structures of the imprinted Pd-based GA-TF were identified using XRD. The morphologies and the microstructures of imprinted Pd-based GATF were examined using a field-emission scanning electron microscopy (FE-SEM: Carl Zeiss Inc. ULTRA55).
Fig. 4. In-plane and perpendicular magnetization curves of the Co/Pd multi-layered thin film with a total thickness of 14 nm.
N. Nishiyama et al. / Intermetallics 18 (2010) 1983e1987
Fig. 5. DSC profile of PdeCueNieP GA-TF prepared by a PLD method.
3. Results and discussion Fig. 3 shows the typical in-plane and perpendicular magnetization curves of the zero-magneto-strictive Co79.5Fe5.5B15 amorphous alloy thin film with a thickness of 100 nm. In the case of in-plane magnetization, the curve exhibits typical soft magnetism with a steep magnetization. On the other hand, the perpendicular magnetization curve exhibits gentle magnetization. Slight hysteresis can be detected while the perpendicular anisotropy is negligible for SUL layer [17]. Using the value of the saturation magnetization, the saturation flux density (Bs) is evaluated to be 1.3 T. Besides, relatively high coercivity (Hc) of 56 A/m is estimated using coercivity meter. This relatively high Hc is attributed to the residual stress generated during deposition. In addition, the value
1985
of magneto-striction (ls) is evaluated to be 3 107. These magnetic properties are considered to be suitable for SUL. Fig. 4 shows the in-plane and perpendicular magnetization curves of the Co/Pd multi-layered thin film with a total thickness of 14 nm. The film construction is 10 times sequence of Co with a thickness of 0.3 nm and Pd with a thickness of 1 nm on the foundation layer of Pd with a thickness of 1 nm. Different from the CoeFeeB amorphous alloy thin film, the magnetization of the Co/ Pd multi-layer shows perpendicular magnetization behavior. From the curves, saturation magnetization (Ms) and anisotropy field (Hk) of the Co/Pd multi-layer in the as-deposited plane form is evaluated to be 540 emu/cc and 7.2 kOe, respectively. These values have high resistivity for magnetic instability due to heat fluctuation [18]. The Hk will be increased by high aspect magnetic dot due to nanopatterning. It is therefore can be said that the Co/Pd multi-layer is applicable for magnetic recording layer of BPM. Most important layer for realization of nano-patterned BPM is the imprint layer. The imprint layer will be required to have high thermal resistivity against crystallization and low viscosity in its SCL region. Since stable and easy glass-forming alloys are generally multi-component systems, a PLD method with high-energy plasma induced by laser beam was employed to prepare GA-TF with a complicated composition. Fig. 5 shows the DSC profile of PdeCueNieP GA-TF with a thickness of 1 mm. Glass transition temperature (Tg), onset of crystallization temperature (Tx) is evaluated to be 599 K and 638 K, respectively. These values are slightly higher than those of glassy alloy target with a composition of Pd42.5Cu30Ni7.5P20. Compositional analysis by an inductively coupled plasma atomic emission spectroscopy (ICP) reveals that Pd-based GA-TF has an atomic composition of Pd46Cu33Ni7P14. The slightly higher Tg and Tx are attributed to the composition deviation between the GA-TF and the target. However, distinct endothermic reaction due to glass transition can still be detected, proving the possibility of imprint applicability. Consequently, it can be concluded that the three different thin films to construct composite for BPM have been successfully prepared. Using the imprint mold shown in the Fig. 2, imprinting tests for Pd-based GA-TFs were carried out. Imprinting temperature was determined to be 609 K (Tg þ 10 K), where the viscosity is evaluated to be about 1011 Pa s. The applied imprint stress and pressing time
Fig. 6. Surface SEM image of imprinted nano hole array with a hole diameter of 90 nm (a), enlarged hole image (b) and cross-sectional profile of hole measured by AFM (c).
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N. Nishiyama et al. / Intermetallics 18 (2010) 1983e1987
were optimized to be 30 MPa and 10 s, respectively. Fig. 6 shows the SEM surface image of the imprinted nano hole array with a hole diameter of 90 nm (a), enlarged hole image (b) and cross sectional profile of hole measured by atomic force microscope (c:AFM). As seen in (a), periodic holes are formed on the Pd-based GA-TF. It is also found that the hole bottom is flat as shown in (b). This flat bottom morphology is suitable for overlying the Co/Pd multi-layer with perpendicular magnetization. The AFM profile as shown in (c), depth of the hole is evaluated to be about 40 nm. The Co/Pd multilayer on the Pd-based GA-TF nano hole array was then polished by CMP. Fig. 7 shows the SEM overview of polished surface (a) and elemental line analysis obtained by an energy dispersive X-ray spectroscopy (EDS) (b). The embedded Co/Pd multi-layer can be observed as a different contrast shown in (a). As seen in (b), it is found that Co is enriched at the location of embedded Co/Pd multilayer. On the other hand, the other regions show higher Cu content and less Co content. These results suggest that the embedded Co/Pd multi-layers are isolated and may act as magnetic recording dots. To confirm the possibility of realization, magnetization behavior of prototype BPM using glassy alloy composite was investigated under magnetic fields of 20 kOe. Fig. 8 shows the AFM (a) and magnetic force microscopy (b: MFM) images of polished BPM surface using glassy alloy composite under þ20 kOe, respectively. Fig. 8(c) and (d) also show AFM and MFM images of the same sample under 20 kOe. As seen in (a) and (c), AFM images are almost the same, suggesting that the surface morphologies are very close. However, MFM images shown in (b) and (d) exhibit opposite contrast. This
Fig. 8. AFM (a and c) and MFM (b and d) images of surface polished BPM using glassy alloy composite under 20 kOe.
result suggests that the Co/Pd isolated dots under 20 kOe were magnetized in the opposite magnetization direction. Furthermore, it is important to note that no subdivided contrast could be seen in each of the magnetized dots, revealing that each of the dots acts as a single magnetic domain. Therefore the isolated Co/Pd magnetic dots using patterned glassy alloy and CMP finishing are suitable for perpendicular magnetic recoding domains. These results suggest that the nano-composite using patterned Pd-based GA-TF is suitable for BPM of high data density HDD and this preparation method is a candidate process for preparing BPM of high data density HDD. 4. Conclusions With the aim of confirming the application possibility for BPM of high data density HDD, the composite of GA-TF and other crystalline thin films was prepared and thermally nano-imprinted. The obtained results are summarized as follows. (1) For SUL and magnetic recording layers, zero-magneto-striction CoeFeeB amorphous alloy thin film and Co/Pd multi-layer were successfully prepared by MGS. For imprinting layer, PdeCueNieP GA-TF was prepared by PLD. It was found that each of the layers has sufficient properties to construct composites for BPM of high data density HDD. (2) Nano-patterning for Pd-based GA-TF was carried out using commercial nano-imprint mold with a dot diameter of 90 nm. As a result, periodic nano hole array could be formed on the surface of the Pd-based GA-TF (3) By combining the overlaid Co/Pd multi-layer and CMP, prototype composite for BPM was prepared. It was found that the isolated Co/Pd magnetic dots acts as single magnetic domains and change in magnetization direction was confirmed under magnetic fields of 20 kOe. Acknowledgements
Fig. 7. SEM overview of polished surface (a) and elemental line analysis obtained by an energy dispersive X-ray spectroscopy (EDS) (b).
The authors are grateful to Prof. M. Futamoto, Prof. H. Fujimori and Prof. Y. Hirotsu for stimulating discussion. Funding by “New Energy and Industrial technology Development Organization
N. Nishiyama et al. / Intermetallics 18 (2010) 1983e1987
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