C. L. Shen a,*, P. C. Kuo a, G. P. Lin a, Y. S. Li a, J. A. Ke a, S. T. Chen a, and S. C. Chen b

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1 Available online at Physics Procedia 32 (2012 ) th International Vacuum Congress Microstructure and Magnetic Properties of Nano-Island CoPt Thin Films C. L. Shen a,*, P. C. Kuo a, G. P. Lin a, Y. S. Li a, J. A. Ke a, S. T. Chen a, and S. C. Chen b a Institute of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan b Department of Materials Engineering, Ming-Chi University of Technology, Taipei 24301, Taiwan Abstract The separated CoPt nano-size islands were fabricated on amorphous glass substrates in this work. The microstructures and magnetic properties of CoPt thin films with thicknesses between 1 and 20 nm after annealing at 700 o C for 30 minutes were investigated. The morphology of CoPt thin film would change from a well-distributed and discontinuous nano-size CoPt islands into a continuous one as increasing the film thickness from 1 to 20 nm. The formation mechanism of the CoPt islands may be due to the surface energy difference between the glass substrate and CoPt alloy. This nano-size CoPt thin film may be a good candidate for ultra-high density magnetic recording media Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). Open access under CC BY-NC-ND license. PACS: Type pacs here, separated by semicolons ; Keywords: CoPt; magnetic recording media; nano-island; perpendicular; surface energy 1. Introduction CoPt film with L1 0 phase (face centered tetragonal phase) has been investigated for ultra-high magnetic recording media application due to its high thermal stability and high magnetocrystalline anisotropy constant which is about erg/cm 3 [1-3]. The as-deposited CoPt film possesses fcc (face centered cubic) phase which would be transferred to fct phase by introducing a proper under layer beneath the CoPt film and post-annealing at 600~700 [4-6]. For high density magnetic recording media application, a lot of relevant properties are associated with reducing magnetic particle size. The grain size of the magnetic film has to be smaller than 10 nm to increase the recording density [7]. Moreover, the exchange coupling effect should be minimized in order to lower the transition noise. Therefore, a granular microstructure of CoPt film is preferred to reducing the inter-grain interaction. Usually, the granular film should consist of magnetic grains embedded in non-magnetic matrix. However, as magnetic recording density increasing to ultra-high density (1 Tbit/in 2 ), the grain size of thin film recording media must be very small. Thermal fluctuation will overcome the magnetic anisotropy energy and disarrange the magnetic moments of the recording bits. This phenomenon is known as the superparamagnetic effect. Patterned media have * Corresponding author. Tel.: ; fax: address: d @ntu.edu.tw Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). Open access under CC BY-NC-ND license. doi: /j.phpro

C. L. Shen et al. / Physics Procedia 32 ( 2012 ) 412 416 413 been suggested as a potential solution for this physical limit [8,9].

2 C. L. Shen et al. / Physics Procedia 32 ( 2012 ) been suggested as a potential solution for this physical limit [8,9]. In this work, we fabricated the separated CoPt nano-size islands on amorphous glass substrates. Patterned media with single-domain magnetic dots could also be obtained by fabricating discontinuous magnetic films as shown in this work. The continuous CoPt magnetic recording medium is replaced by isolated nano-size magnetic islands for ultra-high density recording media. 2. Experimental CoPt films with different thicknesses (1~20 nm) were prepared by dc magnetron sputtering (base pressure = torr) of Co and Pt targets alternately to fabricate a (Co/Pt) n multilayer on amorphous glass substrates and annealed at 700 o C for 30 min. The chemical composition of the CoPt films was Co 50 Pt 50. The dc sputter power densities of the Co and Pt targets were and W/cm 2, respectively, the Ar flow rate was 20 sccm at a process pressure of 10 mtorr for all sputtered films. The film thickness was controlled by sputtering time and measured with an atomic force microscope. The composition of the film was estimated by energy dispersive spectrometry. The magnetic properties of the films were characterized by a vibrating sample magnetometer (VSM, Model 7407, LakeShore, USA) with applied magnetic fields up to 20 koe at room temperature. The crystalline structure was analyzed by X-ray diffraction (XRD) using Cu K radiation ( = nm) at 40 KV, 30 ma. The microstructures were observed by a 200 KV field emission gun high resolution transmission electron microscope (FEG-TEM). 3. Results and discussion The X-ray diffraction patterns of various thickness CoPt films after annealing at 700 o C for 30 min are shown in Figure 1. We found that the diffraction peaks of the ultra-thin CoPt films (thinner than 7.5 nm) were too weak to be detected by XRD [10]. The intensities of the CoPt (111) peak increased with increasing CoPt thickness from 7.5 nm to 20 nm. The origin of fct-copt (111) texture evolution in the CoPt films was due to that the (111) surface is the lowest energy surface of fct-copt film [11]. The higher intensity of fct-copt (111) peak presented in 20 nm-copt film was due to that the thicker CoPt film could provide more nucleation sites of fct-copt (111), which reveals that the better crystallization of the film could be obtained. Therefore, the ordering fct-copt (111) could be found above 7.5-nm thick CoPt film significantly after annealing at 700 o C for 30 min. Fig. 1. The X-ray diffraction patterns of the CoPt films with various thicknesses after annealing at 700 o C for 30 min. Figure 2(a)~2(f) shows the FEG-TEM bright-field images of the CoPt films with different thickness after annealing at 700 o C for 30 min. It can be seen that island-like CoPt grains formed on the glass substrates as the film was thinner than 10 nm. When the film thickness was 1 nm, the particle size distribution was uniform, as shown in Fig. 2(a), this might be attributed to that the cluster coalescence was suppressed by the thinner film thickness. However, the increase of the film thickness would result in the grain growth, cluster growth and coalescence of the CoPt grains, and finally formed a continuous CoPt film, as shown in Figs. 2(e) and 2(f). This is due to the more

414 C. L. Shen et al. / Physics Procedia 32 ( 2012 ) 412 416 amount of CoPt alloy could be provided for the grain growth, cluster growth and coalescence as the film thickness was increased.

3 414 C. L. Shen et al. / Physics Procedia 32 ( 2012 ) amount of CoPt alloy could be provided for the grain growth, cluster growth and coalescence as the film thickness was increased. The morphology of the film changed during the annealing process might be due to the surface energy difference between the glass substrate and CoPt alloy. The composition of the glass substrate was SiO 2. The surface energy of amorphous SiO 2 (about 0.3 J/m 2 ) [12] is much smaller than that of ordered L1 0 CoPt crystal (about 12.4 J/m 2 ) [13]. The surface energy difference between the glass substrate and CoPt alloy would cause the CoPt films to form islandlike shapes to reduce the surface energy of the samples during annealing [14]. When the film thickness was increased, the grain growth, cluster growth, and coalescence were occurred obviously. The interconnected and elongated CoPt islands could be seen in Figs. 2(c)~2(e). Fig. 2. The FEG-TEM bright-field images of the CoPt films annealed at 700 o C for 30 min with thickness of (a) 1 nm, (b) 2.5 nm, (c) 5 nm, (d) 7.5 nm, (e) 10 nm and (f) 20 nm. Figure 3 shows the variations among out-of-plane coercivity (H c ), in-plane coercivity (H c// ) and CoPt film thickness. It can be seen that the H c and H c// were increased as the film thickness was increased from 1 to 7.5 nm. The H c and H c// values of CoPt film with thickness of 1 nm were as low as 1.2 and 1.0 koe, respectively. By calculating the D p (minimal stable particle diameter) with anisotropy energy density (K u = erg/cm 3 ) and anisotropy field (H k =30 koe) of CoPt [15], we obtained that the D p is about 8.8 nm. As the particle size is smaller than 8.8 nm, the thermal fluctuation effect (superparamagnetic effect) will cause the H c and H c// of CoPt films decrease drastically. It is suggested that some CoPt particles below the D p would cause a decrease of H c and H c// values, as shown in Figs. 2(a) and 2(b). The H c and H c// values were increased to about 12.0 koe as the film thickness was increased to 7.5 nm. The particle size distribution of 2.5 nm CoPt was broader than that of 1 nm. According to Fig. 2(c), we found that partial CoPt grains were still isolated and their size was larger than D p. Therefore, the magnetization reversal mechanism of CoPt film may be the domain rotation [16] and it will lead to higher H c and H c// values. When the film thickness was further increased to 10 nm, the H c value was decreased gradually to 11.7 koe and H c// value was increased gradually to 12.3 koe, respectively. When the film thickness was increased to 20 nm, the H c value of the CoPt film was decreased slightly to 11.1 koe and H c// value was increased slightly to 12.5 koe, respectively. The thicker film would lead to grain growth and coalescence between grains, and finally lead to multi-domain grains. The decrease of H c may be due to the increase of grain size [17]. Thus the reduction in H c value above 7.5 nm might be due to the formation of continuous clustered CoPt islands, and the magnetization reversal mechanism may change from domain rotation to domain-wall motion. On the other hand, the increase in H c// value above 7.5 nm might be due to the more amount of fct-copt and fcc-copt (111), which is consistent with the XRD results of Fig. 1.

4 C. L. Shen et al. / Physics Procedia 32 ( 2012 ) Fig. 3. Variations of coercivity with film thickness of CoPt films. Figure 4 shows the variations among out-of-plane squareness (S ), in-plane squareness (S // ) and CoPt film thickness. When the film thickness was 7.5 nm, the S and S // values were 0.84 and 0.55, respectively, which indicated that the CoPt grains inclined toward out-of-plane magnetic anisotropy. However, the S and S // values were 0.80 and 0.77, respectively, as the film thickness was increased to 20 nm, which revealed that the magnetic easy axes of the film was towards random orientation. Fig. 4. Variations of squareness with film thickness of CoPt films. 4. Conclusion The morphology of CoPt thin film would change from a discontinuous nano-size CoPt islands into a continuous film gradually as the film thickness was increased from 1 to 20 nm. The formation mechanism of the CoPt islands may be due to the surface energy difference between the glass substrate and CoPt alloy. The 7.5 nm-thick CoPt film with H c value of 12.0 koe and S value of 0.84 was obtained after annealing at 700 C for 30 min. This CoPt film presents a candidate for ultra-high density magnetic recording media.

5 416 C. L. Shen et al. / Physics Procedia 32 ( 2012 ) Acknowledgements This work was supported by the National Science Council and Ministry of Economic Affairs of Taiwan through grant Nos. NSC E MY3 and 98-EC-17-A-08-S1-006, respectively. References [1] R. A. MaCurrie, P. Gaunt, Philos. Mag. 13 (1966) 567. [2] O. Kitakami, Y. Shimada, K. Oikawa, H. Daimon, K. Fukamichi, Appl. Phys. Lett. 78 (2001) [3] N. I. Vlasova, G. S. Kandaurova, N. N. Shchegoleva, J. Magn. Magn. Mater. 222 (2000) 138. [4] E. Manios, V. Karanasos, D. Niarchos, I. Panagiotopoulos, J. Magn. Magn. Mater (2004) [5] X. H. Xu, Z. G. Yang, H. S. Wu, J. Magn. Magn. Mater. 295 (2005) 106. [6] H. Wang, S. X. Xue, F. J. Yang, H. B. Wang, X. Cao, J. A. Wang, Y. Gao, Z. B. Huang, C. P. Yang, W. Y. Cheung, S. P. Wong, Q. Li, Z Li, Thin Solid Films 505 (2006) 77. [7] J. H. Judy, J. Magn. Magn. Mater. 235 (2001) 235. [8] N. Honda, S. Takahashi, K. Ouchi, J. Magn. Magn. Mater. 320 (2008) [9] H. J. Richter, A. Y. Dobin, O. Heinonen, K. Z. Gao, R. J. M. van de Veerdonk, R. T. Lynch, J. Xue, D. Weller, P. Asselin, M. F. Erden, R. M. Brockie, IEEE Trans. Magn. 42 (2006) [10] B. D. Cullity, S. R. Stock, Elements of X-ray Diffraction third edition, Prentice Hall, [11] H. Zeng, M. L. Yan, N. Powers, D. J. Sellmyer, Appl. Phys. Lett. 80/13 (2002) [12] Y. C. Wu, L. W. Wang, C. H. Laia, Appl. Phys. Lett. 91 (2007) [13] L. Castaldi1, K. Giannakopoulos, A. Travlos, D. Niarchos, S. Boukari, E. Beaurepaire, Nanotechnology 19 (2008) [14] Y. H. Fang, P. C. Kuo, P. L. Lin, S. C. Chen, C. T. Kuo, G. P. Lin, J. Magn. Magn. Mater. 320 (2008) [15] D. Weller, A. Moser, L. Folks, M. E. Best, W. Lee, Mike F. Toney, M. Scgwickert, J. U. Thiele, M. F. Doerner, IEEE Trans. Magn. 36 (2000) 10. [16] T. Shima, K. Takanashi, Y. K. Takehashi, K. Hono, Appl. Phys. Lett. 81 (2002) [17] D. Y. Oh, J. K. Park, J. Appl. Phys. 97 (2005) 10N105.

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