Appl Phys A (2010) 101: 609–613 DOI 10.1007/s00339-010-5937-0
Oriented growth of CoPt nanoparticles by pulsed laser deposition Z.Y. Pan · R.S. Rawat · J.J. Lin · S. Mahmood · R.V. Ramanujan · P. Lee · S.V. Springham · T.L. Tan
Received: 22 November 2009 / Accepted: 15 June 2010 / Published online: 22 July 2010 © Springer-Verlag 2010
Abstract The magnetic nanoparticles of Fe/FeCo/FePt, in the past, in a PLD system were grown by us using argon ambient gas pressure of about 0.1–75.0 mbar, as the ambient gas pressure can be used to tune the energy of the incident plasma plume species, the expansion volume, the growth duration, etc. which can control the particle size. In present paper, we report the direct synthesis of small-sized nanoparticles even when no ambient gas was used, with the experiments being done in higher vacuum of about 10−5 mbar in PLD chamber. The deposition rate under vacuum condition is significantly higher than the deposition rate at high ambient pressure. The study of inplane and outplane magnetic properties, along with XRD results, confirmed that the asdeposited CoPt nanoparticles thin film has oriented growth. The as-deposited CoPt nanoparticles are in magnetically soft fcc phase and a post deposition annealing at 600°C resulted in phase transition to magnetically hard fct phase. Keywords Oriented growth · CoPt nanoparticles · Pulsed laser deposition
Z.Y. Pan · R.S. Rawat () · S. Mahmood · P. Lee · S.V. Springham · T.L. Tan NSSE, National Institute of Education, Nanyang Technological University, 1 Nanyang walk, 637616 Singapore, Singapore e-mail:
[email protected] J.J. Lin Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, 117574 Singapore, Singapore R.V. Ramanujan School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, 639798 Singapore, Singapore
1 Introduction CoPt alloys have been attractive owing to their potential application in ultrahigh density magnetic recording media because the ordered fct phase L10 structured CoPt nanoparticles could remain ferromagnetic with high thermal stability in very small particle size due to their large magnetoanisotropy constant (approximately Ku ≈ 4.9 × 107 erg/cc) [1], which is able to avoid the superparamagnetic behavior. It is expected that the storage density of CoPt nanoparticles can be greater than 1 Tb/in2 if the particle size is reduced to about 4 nm. Therefore, the synthesis of small-sized CoPt nanoparticles with preferred orientation is of technological interest. Many techniques have been employed to synthesize CoPt nanoparticles based on chemical methods [2] and thin films deposition methods such as sputtering [3], plasma focus device [4, 5], and pulsed laser deposition (PLD) [6, 7]. In this article, we describe the oriented growth and characterization of CoPt nanoparticles films synthesized by heating assisted PLD system. PLD has been used for the synthesis of magnetic nanoparticles of Fe/FeCo/FePt [8, 9] by our group in ambient gas such as argon with gas pressure ranging from 0.1 to 75.0 mbar. It has been reported that the changing ambient gas pressure can be used to tune the energy of the incident plasma plume species, the expansion volume, the growth duration, etc., which can control the particle size. However, it is expected that with the increase in ambient gas pressure the amount of scattering of the ablated material will be higher due to its interaction with ambient gas species. This will result in smaller synthesis rate of nanoparticles. According to Lin et al. [10], the typical thickness for Fe nanoparticle thin film synthesized at argon ambient gas pressure of 5 mbar is about 20 nm for 3000 laser pulses. Moreover, the energy of the impinging ablated species at substrate will be much
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lower at such higher ambient gas pressure due to loss of energy of the ablated species owing to their increasingly significant interaction with background gas species, which may affect the crystalline structure of the as-deposited nanoparticles. Hence, it will be interesting to directly synthesize CoPt nanoparticles, using thermal nanostructuring, under higher vacuum conditions with an aim to achieve higher deposition rate and improved structural properties. In this paper, the direct synthesis of CoPt nanoparticles is achieved with the help of suitable substrate type and substrate temperature in a higher vacuum of about 10−5 mbar.
2 Experiment The experimental setup is shown in Fig. 1 where a Continuum Nd:YAG laser (10 ns, 532 nm) was focused on a high-purity (50:50 at%; Kurt J. Lesker, 99.99%) solid CoPt target to synthesize the CoPt nanoparticles. The laser pulse energy, measured on the target by the calibrated laser energy/power meter, was estimated to be about 79.3 ± 4.4 mJ. Hence the energy density of the laser at the target surface was estimated to be about 49 J/cm2 , which is more than enough to ablate the CoPt due to their intermediate ablation threshold of 1–2 J/cm2 [11]. Target is rotated by magnetic motors for achieving more uniform ablation and deposition. The MgO(100) wafer is used as substrate, which was heated and maintained at 350◦ C during deposition. The separation between the target disc and substrates was fixed at 60 mm. The PLD chamber was evacuated to a vacuum better than 5×10−5 mbar using a system of rotary and turbo molecular pumps. The total number deposition shots were fixed at 3000 with a firing frequency of laser pulse at 10 Hz. Samples were consequently annealed at temperatures of 400 and 600◦ C in a vacuum furnace for 1 hour duration to investigate the effects of annealing on the structural and magnetic properties of the deposited CoPt thin films. Fig. 1 Schematic of heating assisted pulsed laser deposition
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The morphology of CoPt nanoparticle thin films was studied using a JOEL JSM-6700F scanning electron microscope. The structural properties of the CoPt nanoparticle thin films were obtained by a Siemens D5005 X-ray diffractometer with CuKα radiation. A LakeShore 7400 vibrating sample magnetometer (VSM) was used to measure the magnetic properties of the samples.
3 Results and discussions Figure 2 shows the SEM images of the morphology and cross sectional view of the as-deposited sample synthesized in vacuum by heating assisted PLD on a heated substrate kept at a temperature of 350◦ C. It can be noted, in Fig. 2(a), that the small and narrow size distributed CoPt nanoparticles were directly formed in high vacuum condition. Normally, a smooth and uniform films (without any observable features) are expected to grow, as reported [8, 9], in vacuum or in low ambient gas by PLD due to sufficiently high surface mobility of the impinging ablated species at the substrate surface. As reported by Happy et al. [8], the kinetic energy of ablated species in vacuum remains high due to fewer collision among ablated species as well as with ambient gas, hence these high energy impinging ablated species have enough surface mobility to form a smooth thin film at room temperature. In present case, the direct formation of CoPt nanoparticles is mainly due to the thermal diffusion induced by heat transferred from the heated substrate, since the activation energies for the self-diffusion of Pt and β-Co are about 2.91 and 2.89 eV, respectively. The thermal heating can activate surface diffusion by providing enough thermal energy and lead to the self-ordering processes in thin films to form nanostructures [12]. The average particle size of the CoPt nanoparticles of as-deposited sample, as measured by the Smile View©2000 JEOL software on the magnified SEM image, is of the order
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Fig. 3 Time-resolved plume images (a) in vacuum and (b) at a filling gas pressure of 4 mbar. The white line shows the target position
Fig. 2 (a) Plane view and (b) cross-sectional view of SEM images of the as-deposited CoPt nanoparticles on MgO(100) substrate
of 9.9 ± 1.4 nm. The small and narrow sized distribution of the as-deposited nanoparticles is on account of the fast propagation rate and less possibility of the collision and agglomerates formation in vacuum during the plasma plume expansion phase, which could be evidently verified by studying the plume expansion dynamics as discussed later. The thickness of as-deposited sample, as measured from the cross sectional view of SEM image in Fig. 2(b), is about 120 ± 5.0 nm for 3000 laser shots. Hence, the deposition rate is found to be 0.40 Å/shot, which are expected to be higher than deposition rates normally obtained in ambient gas. The significant increase in the deposition rate under higher vacuum conditions, used in the present experiment, can be attributed to the fact that the depositions done at high vacuum do not suffer from the large back flux of the ablated plume species, which normally happen at higher pressures due to relatively stronger interaction of ablated species
with ambient gas species. It has been confirmed that the confinement of the ablated plume increases with the increase in ambient gas pressure, as seen in Fig. 3. Figure 3 shows the imaging sequence of the plume dynamics in vacuum and ambient gas environment that were captured using the time-resolved gated ICCD camera. Suitable delays were adjusted on the programmable pulse generator to capture these time resolved images. In vacuum, as shown in Fig. 3(a), the plasma plume propagates very fast and almost disappeared shortly after t = 1000 ns. In contrast, in ambient gas at about 4 mbar pressure, shown in Fig. 3(b), the plume propagation is slowed down due to the interaction/compression of the ablated species with/by the ambient gas and therefore the plume stays in front of the target surface for a much longer duration as strong luminous plasma can still be seen at t = 3000 ns in Fig. 3(b). The increased confinement of the ablated plasma plume at higher ambient gas pressure confirms that the backscattering will increase with the increasing ambient gas pressure. In other words, in vacuum, the forward flux of the ablated plume species will be maximized, therefore, the deposition rate will be higher than what it is in ambient gas. Figure 4 shows that XRD patterns of substrate and CoPt thin films samples in (i) large (for overall view) and (ii) small (for magnified view) y-axis scale. In these two different y-axis scaled figures, the XRD patterns are designates as (a) MgO(100) substrate, (b) as-deposited CoPt nanoparticles, (c) the CoPt sample after annealed at 400◦ C and (d) the CoPt sample after annealed at 600◦ C. It is evidently shown in Fig. 4(i)(b-d) that the as-deposited CoPt nanoparticles and the samples after annealing are in highly (200) oriented fcc phase. The degree of the crystallinity is enhanced after annealing and also increases with the increase of annealing temperature as noticed through the enhancement of the intensity of CoPt (200) peak at about 47.65◦ . In the magnified, small y-axis scale Fig. 4(ii), in addition to (200) peak, a very weak CoPt (111) peaks is found in the XRD pattern of the as-deposited and annealed CoPt samples. A superlattice CoPt (001) peak at 24.45◦ is, however, clearly observed only in the XRD pattern of the sample annealed at 600◦ C in Fig. 4(ii)(d), indicating the transformation to fct phase.
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Fig. 4 XRD patterns of MgO(100) substrate, as-deposited CoPt sample and samples after annealing in (1): large (2): small scale. For both scales: (a) MgO(100) substrate, (b) as-deposited CoPt nanoparticles, (c) the sample after annealed at 400◦ C and (d) the sample after annealed at 600◦ C
As the intensity of (001) peak is relatively low, we conclude that this sample is only partially transformed to fct phase. Figure 5 shows the inplane and outplane hysteresis loops for the as-deposited sample and the samples annealed at 600◦ C. The inplane hysteresis loop of as-deposited sample seen in Fig. 5(a) shows relatively hard magnetic property with a bigger coercivity of 665 Oe, while the soft magnetic property was observed in the outplane hysteresis loop with the weak coercivity of 120 Oe. The different coercivity for inplane and outplane measurements indicate that the as-deposited sample itself has oriented growth, which could be also verified by XRD results in Fig. 4(i)(b) that shows strong CoPt (200) peak in the as-deposited sample. Two weak kinks are observed in the inplane hysteresis loop of the as-deposited sample as the curve is reaching the saturation magnetization. The kinks in the hysteresis curves are attributed to the exchange coupling between the grains of the hard fct and soft fcc phases [13, 14]. This suggests the existence of composite phase consisting of small amount of hard fct phase in as-deposited fcc phase CoPt sample. Figure 5(b) shows the inplane and outplane hysteresis loops of the sample annealed at 600◦ C. It is obvious to find the large enhancement of the inplane coercivity for the annealed sample. The inplane coercivity was estimated to be about 3737 Oe, which is 5 times more as compared with that of as-deposited sample, which may due to a certain degree phase transi-
tion from low Ku fcc to high Ku fct after annealing and was also confirmed by the presence of (001) superlattice peak of ordered L10 fct-structured CoPt in XRD pattern. The outplane coercivity of the annealed sample almost remains at the same level as that of as-deposited sample with a value estimated to be about 203 Oe, which is one order of magnitude less than the inplane coercivity of the annealed sample. This indicates that the orientation degree of the sample was significantly enhanced by annealing although it was not along the c-axis. It is also noted that the magnitude of the kinks have been significantly enhanced in the inplane hysteresis loop of the annealed sample indicating the larger exchange coupling between hard fct and soft fcc phase existing as a result of higher ordering degree, that is, higher percentage of hard fct phase after annealing.
4 Conclusions Oriented CoPt nanoparticles are successfully grown in vacuum by heating assisted PLD on MgO(100) substrate. The direct synthesis of the small-sized CoPt nanoparticles of 9.9 ± 1.4 nm is successfully achieved on heated substrate in vacuum. The vacuum environment is favorable to minimize the particle size increase in the gas expansion phase. Furthermore, synthesis at higher vacuum condition is also
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plane and outplane measurements of VSM results, we could further confirm the oriented growth as well as demonstrate that post deposition annealing at 600◦ C leads to significant degrees of phase transition from low Ku fcc for as-deposited sample to high Ku fct phase after annealing. Acknowledgements The authors are grateful to the National Institute of Education/Nanyang Technological University, Singapore, for providing the AcRF grant RI 7/08RSR. One of the authors, ZYP would like to thank NIE/NTU for providing the research scholarship.
References
Fig. 5 Inplane and outplane hysteresis loops of (a) the as-deposited CoPt sample and (b) the sample after annealed at 600◦ C
in favor of maximizing the forward deposition rate due to the least ablation back flux, which was confirmed from the increase in deposition rate by one order of magnitude for high vacuum depositions as opposed to that of deposition reported under ambient gas pressure of few mbar. The heated MgO(100) substrate helps in growth of highly oriented (200) CoPt nanoparticles, as verified by XRD patterns. With in-
1. D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J.U. Thiele, M.F. Doerner, IEEE Trans. Magn. 36, 10 (2000) 2. M. Mizuno, Y. Sasaki, M. Inoue, C.N. Chinnasamy, B. Jeyadevan, D. Hasegawa, T. Ogawa, M. Takahashi, K. Tohji, K. Sato, S. Hisano, J. Appl. Phys. 97, 10J301 (2005) 3. J.P. Hu, P. Lin, IEEE Trans. Magn. 32, 4096 (1996) 4. Z.Y. Pan, R.S. Rawat, J.J. Lin, R. Verma, M.V. Roshan, P. Lee, S.V. Springham, T.L. Tan, J. Plasma Fusion Res. Ser. 8, 0504 (2009) 5. Z.Y. Pan, R.S. Rawat, M.V. Roshan, J.J. Lin, R. Verma, P. Lee, T.L. Tan, S.V. Springham, J. Phys. D, Appl. Phys. 42, 175001 (2008) 6. R.K. Rakshit, S.K. Bose, R. Sharma, R.C. Budhani, T. Vijaykumar, S.J. Neena, G.U. Kulkarni, J. Appl. Phys. 103, 023915 (2008) 7. E. Agostinelli, S. Laureti, G. Varvaro, A. Generosi, B. Paci, V. Rossi-Albertini, G. Scavia, A.M. Testa, Mater. Sci. Eng. C, Biomim. Supramol. Syst. 27, 1466 (2007) 8. Happy, S.R. Mohanty, P. Lee, T.L. Tan, S.V. Springham, A. Patran, R.V. Ramanujan, R.S. Rawat, Appl. Surf. Sci. 252, 2806 (2006) 9. J.J. Lin, S. Mahmood, T. Zhang, S.M. Hassan, T. White, R.V. Ramanujan, P. Lee, R.S. Rawat, J. Phys. D, Appl. Phys. 40, 2548 (2007) 10. J.J. Lin, S. Mahmood, T.L. Tan, S.V. Springham, P. Lee, R.S. Rawat, Nanotechnology 18, 115617 (2007) 11. J. Shen, Z. Gai, J. Kirschner, Surf. Sci. Rep. 52, 163 (2004) 12. M. Alexe, D. Hesse, J. Mater. Sci. 41, 1 (2006) 13. J.P. Liu, Y. Liu, C.P. Luo, Z.S. Shan, D.J. Sellmyer, J. Appl. Phys. 81, 5644 (1997) 14. Y. Tamada, Y. Morimoto, S. Yamamoto, N. Hayashi, M. Takano, S. Nasu, T. Ono, Jpn. J. Appl. Phys. 45, L1232 (2006)
