Rapid Thermal Annealing Induced Metastable Phase in FePt Thin Films

IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011

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Rapid Thermal Annealing Induced Metastable Phase in FePt Thin Films J. K. Mei1;4 , F. T. Yuan2 , W. M. Liao3 , Y. D. Yao5 , H. M. Lin1 , J. H. Hsu2 , and H. Y. Lee3 Department of Materials Engineering, Tatung University, Taipei 104, Taiwan, ROC Department and Institute of Electrical Engineering, Minghsin Univ. Science and Technology, Hsinchu 30401, Taiwan, ROC Institute of Applied Physics & Center for Nanostorage Research, National Taiwan University, Taipei 106, Taiwan, ROC National Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan, ROC Institute of Applied Science and Engineering, Fu Jen Catholic University, Taipei 24205, Taiwan, ROC Room-temperature-deposited FePt thin films with thickness ( ) ranged from 5 to 100 nm treated by rapid-thermal annealing (RTA) were studied. With annealing condition of 900 C for 60 seconds at heating rate of 80 C/sec., a metastable phase of FePt was observed 40 nm. The phase is chemically ordered with a face-centered-cubic (fcc) structure. The lattice parameter of it is in the films with found identical to the planar spacing of 10 (100). The metastable structure is dominant in the film with = 40 nm and gradually replaced by 10 phase with increasing . The fcc phase is soft magnetic with saturation magnetization similar to that of disordered FePt. Drastic changes were also observed in surface morphology. The results infer the connection between the metastable transformation and internal strain. The formation of the fcc structure is sensitive to processing parameters except . Slower heating rate, lower annealing temperature, and longer annealing time, tend to suppress the formation of it. The interesting findings provide additional knowledge for FePt thin films. Index Terms—FePt thin films, metastable phase,

10 , rapid thermal annealing.

I. INTRODUCTION

R

APID THERMAL annealing (RTA), i.e., a unique heat treatment that achieves extremely high heating rate, is designed for various applications including optimizing the phase and microstructure of a thin film material by controlling the nucleation and growth mechanisms, modifying the interface structure between layers or films and a substrate, activating dopants in semiconductors, diffusion control, and densification, as well as removing defects, etc. On hard magnetic thin films, RTA is an effective approach to enhance the magnetic performance. In the case of FePt and CoPt thin films, materials with high magerg/cm ) [1] which netocrystalline anisotropy ( is highly promising for various applications especially in future recording media with an areal density exceeding 1 terabit/inch , RTA treated samples show distinct properties from those annealed by conventional processes. It has been reported that RTA is a highly effective means of controlling the texture and microstructure. Liu et al. demonstrated the feasibility of forming nano-composite Fe Pt-FePt thin films with an energy density higher than 50 MGOe by annealing FePt/Fe multilayers using RTA [2]. Jeong et al. indicated that RTA at 700 C for 10 minutes could induce a strong (001) texture of FePt thin films with a thickness of 5 nm deposited on an oxidized Si substrate. Also, in addition to achieving good perpendicular magnetic anisotropy [3], a related coercive mechanism was investigated [4]. Yan et al. firstly combined RTA with Fe/Pt multilayer deposition with thicknesses of each layer ranging from 0.2 nm to 1.6 nm to form a perfect (001)-texture at reduced temperatures of 550 C [5]. Wu et al. later modified this atomic-scaled multilayer structure Manuscript received February 20, 2011; revised April 23, 2011; accepted April 24, 2011. Date of current version September 23, 2011. Corresponding author: F. T. Yuan (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2011.2148697

by inserting SiO layers into each Fe/Pt pairs. 1 (001) FePt with a nano-granular microstructure was obtained at a reduced ordering temperature of 350 C [6]. Additionally, RTA scheme was adopted to induce the (001) texture of FePt 1 dot array [7], trigger the order transformation of FePt nanoparticles [8], [9], and control the exchange interactions between hard and soft magnetic layers in the exchange-coupled composite media [10]. In this study, effect of RTA at different temperature on FePt single layer films with different thickness is reported. A metastable phase was observed in the films with thickness from 40 nm to 100 nm annealed at 900 C for 60 s and heating rate of 80 C/sec. The phase is chemically ordered similar to 1 structure but with a face-centered-cubic unit cell. Detailed characterizations in structure and magnetism are reported in details. II. EXPERIMENTAL FePt single-layer thin films were deposited by radio frequency (RF) rotational co-sputtering at room temperature torr on (RT) with a background vacuum better than Corning 1737 glass substrates. High purity targets of Fe and of the films ranged from 5 nm Pt were used. Thickness to 100 nm. The chemical composition of the films analyzed by calibrated energy dispersive spectroscopy (EDS) was Fe Pt . Following deposition, the samples were submitted to from 400 to 900 C for RTA treatment at temperatures of 40 and 60 and 120 s with different heating rate 80 C/sec. After annealing, the samples were naturally cooled down to RT in RTA chamber in vacuum. The phase structure . was studied using x-ray diffractometry (XRD) with Cu Surface morphology was characterized by scanning electron microscopy (SEM). Magnetic properties were measured at RT using a vibrating sample magnetometer (VSM). III. RESULTS AND DISCUSSION Fig. 1 shows the XRD patterns for the 100-nm-thick sample annealed at from 400 to 900 C for 120 s at C/sec. The RT-prepared sample exhibits only

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Fig. 1. XRD patterns for the 100-nm-thick RT-deposited film and films an; ; ; ; , and 900 C for 120 seconds. nealed at T

= 400 500 600 700 800

fundamental peaks (111) and (200), indicating disordered 1 phase with a face-centered-cubic (fcc) lattice. Superlattice peaks of (001), (110), and (201) emerge at C, evidencing the formation of face-centered-tetragonal (fct) order phase. The order parameter, , evaluated structure, from the intensities of the slow scan profiles of (001) and (002) peaks, is about 0.81. Intensities of the diffraction peaks are , which is due to significantly enhanced with increasing saturates after reaches 700 C extensive grain growth. with values of about 0.9. The intensity ratio of peak (001) to C, it suggests (111) is enhanced in the sample with that high temperature annealing promotes the development of FePt (001) orientation. Although this typical phase evolution, which is consistent with previous studies [11], [12], is obtained in samples with different film thickness, onset point of ordering changes. With increasing of RT-deposited FePt films decreases from 600 to less than from 5 nm to 100 nm, is well agree with the 500 C. This effect of thickness on literatures [11], [12]. An abnormal structural change was observed in the 900 C-annealed samples with shortened annealing time of 60 s. Normalized slow scan XRD patterns for the samples with different thickness are shown in Fig. 2. For films with , and nm, the presence of strong (001), (111), and (002) peaks indicates highly ordered phase with significant (001) preferred orientation. The values of 2 angles of the peaks increase with the increasing of , which could be a result of accumulation of internal tensile stress/strain induced by intensive densification during high temperature annealing [13]. Dramatic changes in diffraction (001) and (111) peaks, appear profiles, the splitting of nm. Two unknown peaks emerge at in the samples with angle of 23.20 –23.25 and 40.65 –40.73 on the left side of the (001) and (111) peaks, respectively, denoted as S and to S . Interestingly, the splitting of (001) does not cause split. The intensity of S and S is as a function of . They are dominant in the 40-nm-thick film and are replaced gradually by (001) and (111) with increasing . Distinct variation

Fig. 2. Normalized slow scan XRD patterns for the FePt films with different thicknesses ranging from 5 nm to 100 nm annealed at 900 C with heating rate of 80 C/sec. for 60 seconds. The scan rate of x-ray diffraction is 0.5 /min. and step size is 0.05 .

in angles of the diffractions on thickness is also noteworthy. For , and (200) peaks, the values of are independent of ; however, peaks (001), (111), and (002) shift to high angles when is increased. Consistent dependence of correlates the unknown peaks and with (200). The relation of and to the peaks is investigated. and were separated from (001) and (111), respectively, by a pseudo-Voigt function, i.e., a symmetric hybrid function expressed by a weighted sum of Lorentzian and Gaussian functions [14], [15]. Figs. 3(a)–(d) summarize the fitting results and 75 nm. of the (001) and (111) for the films with Detailed fitting parameters are also listed. Extremely high (goodness-of-fitting) values verify the reliability of the fitting results and the information extracted from which angle, and integrated intensity. Planar including linewidth, , and (001), , obtained from angles spacing of nm, in Figs. 3(a), are 3.82 , 3.70 for films with respectively. The values is exactly twice of spacing of (200) and (002) (3.83 and 3.70 ) as shown in Fig. 3(b). Consistent peak relations were also obtained in the sample with [Fig. 3(c) and (d)], 50, and 40 nm. This finding suggests that is a superlattice diffraction of (200) and confirms the metastable phase is a chemically ordered. To examine the lattice structure of the unknown phase, we assume a face-centered-cubic (fcc) and examine with . The unit cell with lattice parameter of planar spacings of obtained from Fig. 3(b) and (d) are 2.21 for both and 75. The value equal to , reveals that the lattice of the ordered metastable phase may be fcc. Figs. 4(a)–(d) show the reduced in-plane and out-of-plane hysteresis loops for samples with different from 20 to 75 nm and 30 nm as annealed at 900 C for 60 s. Films with shown in Fig. 4(a) and (b) are hard magnetic with strong perpendicular anisotropy. The ceorcivity in plane normal direction exceeds 1 tesla. Only minor loops can be obtained. The hard magof highly ordered strucnetism originates from the large ture with (001) texture described previously. A drastic magnetic

MEI et al.: RAPID THERMAL ANNEALING INDUCED METASTABLE PHASE IN FEPT THIN FILMS

Fig. 3. Psudo-Voigt fitting of (a) (001) and (b) (111) peaks for the samples , and (c) (001) and (d) (111) for the samples with t nm with t annealed at 900 C with heating rate of 80 C/sec. for 60 seconds. The open squares represent raw data, green lines are the Pseudo-Voigt fitting curves, and red line denotes the sum of the green lines.

= 100

= 75

Fig. 4. Hysteresis loops for the samples annealed at 900 C for 60 s and R C/sec. with thickness of (a) 20, (b) 30, (c) 40, and (d) 75 nm.

= 80

softening occurs in the film with , corresponding to the sudden change in phase structure. The coercivity in the in-plane and out-of-plane directions decreases to 1 and 2 kOe, respectively [Fig. 4(c)]. Saturation magnetization increases to about 900 emu/cm , which is similar to the value of FePt 1 phase. . The results indicate that the phase change largely reduces Small shoulders appear in the third and first quadrant, signaling phase which has larger coercivity. Enthe small amount of hanced coercivity in the film with is shown in Fig. 4(d). The large coercivity and steps emerged in the second and the fourth quadrant indicate the increment of hard magnetic phase. This is consistent with the results shown in Fig. 2 that phase replaces the fcc phase when is increased. Changes in surface morphology were also observed in the samples with different thickness. SEM images are shown in

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Fig. 5. SEM images for the samples with thickness of (a) 20, (b) 30, (c) 40, C/sec. and (d) 75 nm annealed at 900 C for 60 s with R

= 80

Fig. 5(a)–(d). 20- and 30-nm-thick films exhibit network structure. The diameter of the particles can be considered as grain size since they are well grown grains with distinct orientations with clear boundaries. There exists large difference in grain size. With a 10 nm increase in thickness, the average grain size innm) to 220 nm ( nm). creases from about 50 nm ( nm where the metastable phase apFor the films with pears [Fig. 5(c)], the surface morphology becomes continuous with reduced grain size. With further increasing of , continuous surface remains but it can no longer provide information of grain size. Change of microstructure suggests variation in strain state of the films. Grain growth induces tensile strain by densification [13]. However, the discontinuous morphology relaxes it. The relation between phase structure and surface morphology reveals that the internal strain may be important to the formation of the fcc phase. Moreover, we examine the influence of annealing condition on the formation of metastable phase. Four annealing conditions were applied on the 40-nm-thick FePt films where the fcc phase C for 60 is dominant as described afore: Sample I: of 80 C/sec.; Sample II: C for 120 s with s and of 80 C/sec.; Sample III: C for 120 s of 40 C/sec.; and Sample IV: C for 120 and of 80 C/sec. Figs. 6(a)–(d) show normalized slow s and scan XRD patterns for Sample I, II, III, and IV, respectively. In Sample I, the metastable phase is the majority. With the inphase recreasing of annealing time to 120 s (Sample II), places most of the fcc phase as evidenced by the largely reduced and enhanced (001). This result evidences that the fcc phase has higher free energy as compared to phase. The single sample with lower heating rate (Sample III) exhibits phase as revealed by the symmetric peaks of (001) and (111) to 800 C was also found to suppress [Fig. 6(c)]. Reducing the formation of metastable phase. Films annealed for 60 s (not phase. By summarizing shown) and 120 s [Fig. 6(d)] are of , and above results, it is known that an extremely high

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ACKNOWLEDGMENT This work was supported by the Ministry of Economic Affairs of Taiwan, Republic of China (ROC) under Grant No. 99-EC-17-A-08-S1-006 and National Science Council under Grant No. NSC 99-2112-M-002-020-MY3. REFERENCES [1] D. Weller, A. Moser, L. Folks, M. E. Best, L. Wen, M. F. Toney, M. Schwickert, J.-U. Thiele, and M. F. Doerner, “High K materials approach to 100 Gbits/inch ,” IEEE Trans. Magn., vol. 36, pp. 10–15, 2000. [2] J. P. Liu, C. P. Luo, Y. Liu, and D. J. Sellmyer, “High energy products in rapidly annealed nanoscale Fe/Pt multilayers,” Appl. Phys. Lett., vol. 72, pp. 483–485, 1998. [3] S. Jeong, Y. N. Hsu, M. E. McHenry, and D. E. Laughlin, “Structure and magnetic properties of L CoPt (Ag/MgO, MgO) thin films,” J. Appl. Phys., vol. 87, pp. 6950–6952, 2000. [4] S. Jeong, Y. N. Hsu, D. E. Laughlin, and M. E. McHenry, “Magnetic properties of nanostructured CoPt and FePt thin films,” IEEE Trans. Magn., vol. 36, pp. 2336–2338, 2000. [5] M. L. Yan, X. Z. Li, L. Gao, S. H. Liu, D. J. Sellmyer, R. J. M. Van de Veerdonk, and K. W. Wierman, “Fabrication of nonepitaxially grown double-layered FePt:C/FeCoNi thin films for perpendicular recording,” Appl. Phys. Lett., vol. 83, pp. 3332–3334, 2003. [6] Y. C. Wu, L. W. Wang, and C. H. Lai, “Low-temperature ordering of (001) granular FePt films by inserting ultrathin SiO layers,” Appl. Phys. Lett., vol. 91, p. 072502, 2007. [7] C. Kim, T. Loedding, S. Jang, H. Zeng, Z. Li, Y. Sui, and D. J. Sellmyer, “FePt nanodot arrays with perpendicular easy axis, large coercivity, and extremely high density,” Appl. Phys. Lett., vol. 91, p. 172508, 2007. [8] Y. Ding and S. A. Majetich, “Size dependence, nucleation, and phase transformation of FePt nanoparticles,” Appl. Phys. Lett., vol. 87, p. 022508, 2005. [9] X. W. Wu, C. Liu, L. Li, P. Jones, R. W. Chantrell, and D. Weller, “Nonmagnetic shall in surfactant-coated FePt nanoparticles,” J. Appl. Phys., vol. 95, pp. 6810–6812, 2004. [10] D. Makarov, J. Lee, C. Brombacher, C. Schubert, M. Fuger, D. Suess, J. Fidler, and M. Albrecht, “Perpendicular FePt-based exchange-coupled composite media,” Appl. Phys. Lett., vol. 96, p. 062501, 2010. [11] Y. K. Takahashi and K. Hono, “On low-temperature ordering of FePt films,” Scripta Mater., vol. 53, pp. 403–409, 2005. [12] A. C. Sun, P. C. Kuo, S. C. Chen, C. Y. Chou, H. L. Huang, and J. H. Hsu, “Magnetic properties and microstructure of low ordering temperature L FePt thin films,” J. Appl. Phys., vol. 95, pp. 7264–7266, 2004. [13] S. N. Hsiao, F. T. Yuan, H. W. Chang, H. W. Huang, S. K. Chen, and H. Y. Lee, “Effect of initial stress/strain state on order-disorder transformation of FePt thin films,” Appl. Phys. Lett., vol. 94, 2009, 232505. [14] D. Balzer, “Profile fitting of X-ray diffraction lines and Fourier analysis of broadening,” J. Appl. Crystallogr., vol. 25, pp. 559–570, 1992. [15] D. Balzer and H. Ledbetter, “Voigt-function modeling in Fourier analysis of size- and strain-broadened X-ray diffraction peaks,” J. Appl. Crystallogr., vol. 26, pp. 97–103, 1993.

1

Fig. 6. Normalized slow scan XRD patterns for the 40-nm-thick samples anC/sec.; (b) 900 C for 120 s and nealed at (a) 900 C for 60 s and R C/sec.; (c) 900 C for 120 s and R C/sec.; and (d) R 800 C for 120 s and R C/sec.

= 80

= 80

= 80

= 40

proper annealing time are essential for forming the metastable structure.

IV. CONCLUSION In summary, a metastable structure of FePt was observed in the RTA treated thin films within thickness range from 40 to 100 of 80 C/sec., of nm under annealing condition of high 900 C, and proper annealing time of 60 s. The phase is chemically ordered with fcc lattice. The lattice parameter is found to (100). The fcc phase has identical to the planar spacing of a soft magnetic nature with a saturation magnetization similar FePt, which is unique for a Fe Pt to that of disorder order structure. Abrupt change in surface morphology relates the formation of metastable phase to the variation of internal strain induced by different grain growth mechanism. Several annealing conditions were found to reduce the fcc phase, they are , increased thickness, extended annealing time, decreased and lower .

1