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Strongly in-plane magnetized FePt thin film on ultrathin Fe underlayer
Physica B 406 (2011) 2646–2649
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Strongly in-plane magnetized FePt thin film on ultrathin Fe underlayer S.H. Lee a,n, S.H. Eun b a b
Department of Materials Science and Engineering, UCLA, 410 Westwood Plaza, 3111 Engineering V, Los Angeles, CA 90095-1595, USA Department of Materials Science and Engineering, KAIST, 373-1 Guseong-dong, Yuseong-gu, 305-701 Daejeon, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2010 Received in revised form 22 January 2011 Accepted 2 April 2011 Available online 13 April 2011
The effect of ultrathin Fe underlayer on the strong in-plane magnetization of FePt magnetic thin film was investigated. This FePt thin film could be attained using the ultrathin Fe underlayer with 1 nm thickness. The in-plane coercivity of FePt film with 20 nm thickness grown on ultrathin Fe underlayer was high up to 7400 Oe. However, its out-of-plane coercivity was extremely low to 350 Oe compared to those of FePt thin films in other conventional studies. This result indicates that FePt thin film was strongly in-plane magnetized by ultrathin Fe underlayer. The strong ordering phase transformation kinetics and the high texturing to in-plane direction of the FePt thin film by ultrathin Fe underlayer were confirmed by Kinetics Monte Carlo (KMC) simulation and XRD measurement result, respectively. It is also supposed that they are associated with the reduction of an interface free energy between the film and the substrate with an introduction of ultrathin underlayer. & 2011 Elsevier B.V. All rights reserved.
Keywords: FePt Ultrathin Fe underlayer In-plane magnetization Coercivity Ordering phase transformation kinetics Texturing
1. Introduction The ordered (L10) FePt-based thin films are known to be a promising candidate for ultra high density magnetic recording media because of its high magnetocrystalline anisotropy energy [1–7]. Generally, the FePt magnetic thin films synthesized by sputtering tend to grow with either random orientations or (1 1 1) texture that place the c-axis of grains 371 out of the film plane [4,6]. Therefore, it is required to control the crystallographic orientation in FePt thin films in order to make them suitable for the various applications. The in-plane magnetized FePt thin films are required for a novel application such as bias magnets and exchange spring magnets [8] whereas the perpendicular magnetized FePt thin films are required as perpendicular magnetic recording media [9]. Recently, there have been a few conventional studies [10–14] on the in-plane magnetization of FePt thin film on glass substrate [10], quartz substrate [11], MgO (1 1 0) single crystal substrate [12,13], and CrRu underlayer [14]. Even though the in-plane magnetization of the fct–FePt thin films was presented in a few conventional studies, it was difficult to obtain strong in-plane magnetization of FePt thin films because the outof-plane coercivity of FePt thin films was not extremely low below 1000 Oe. Therefore, in this study, we used an ultrathin Fe underlayer with (2 0 0) texturing in the film plane to induce the strong
n
Corresponding author. Tel.: þ1 310 806 2982; fax: þ1 310 206 7353. E-mail address: [email protected] (S.H. Lee).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.04.007
in-plane magnetization of FePt thin film. The Fe layers deposited at relatively high deposition rate exhibited (1 1 0) texture as seen in the general case of Fe films. However, a lower deposition rate ˚ for Fe layers caused (2 0 0) texture [15]. It was also below 0.6 A/s reported that the ultrathin ( o5 nm) underlayer results in the high texturing of thin films [16]. Thus, it is expected that highly in-plane textured FePt thin films are attained on ultrathin Fe underlayer. We explained the magnetic properties such as the magnetization and the magnetic anisotropy of FePt thin films to the microstructural characterizations such as the L10-ordering phase transformation kinetics of FePt thin films and the grain texturing and orientation of FePt thin films. We also investigated the effect of ultrathin Fe underlayer on the microstructural characterizations of FePt thin films by the Kinetics Monte Carlo (KMC) simulation and the XRD measurement.
2. Experimental 20 nm-thick equiatomic FePt thin films were cosputtered both Fe and Pt targets on the SiO2/Si substrates using dc magnetron sputter at the substrate temperature heated up to 600 1C by a heater with halogen lamp under 5 mTorr of Ar in a sputter chamber of 4.0 10 7 Torr. The ultrathin Fe or Pt underlayer with the film thickness of 1 nm was deposited using the Fe or Pt ˚ target. The deposition rate of Fe layer was 0.3 A/s and the ˚ deposition rate of Pt layer was 0.5 A/s. Specially, to investigate the film thickness effect of Fe underlayer on the in-plane
S.H. Lee, S.H. Eun / Physica B 406 (2011) 2646–2649
magnetization of FePt thin films, the film thickness of Fe underlayer was increased up to 5 nm. The chemical compositions of FePt thin films were determined by Energy Dispersive X-ray microanalysis (EDX). Magnetic characterizations were carried out by the Vibrating Sample Magnetometer (VSM) with an applied field of up to 710 kOe. The KMC simulation implemented with vacancy atomic-jump mechanism [17] was carried out to study the effect of the L10-ordering phase transformation kinetics of FePt thin films on Fe or Pt underlayer. The simulated sample consisted of 50 50 50 unit cell and atomic ratio of Fe and Pt was 50:50. The thin layers limited by free surfaces were modeled by removing periodic boundary conditions in Z-axis direction, whereas the periodic boundary conditions were retained in X and Y-axis direction. The ordering kinetics in FePt systems was determined by Bragg–Williams-type Long Range Ordering (LRO) parameter [17]. Then this LRO parameter can be calculated as follows:
Z ¼ 12
Fe NPt , NPt
Fe is the number of Fe atoms on the Pt sublattice where NPt (antisites), NPt is the number of Pt sites. The crystal structure of the thin films was characterized by Rigaku D/MAX-RC and RB X-ray diffractometer using CuKa radiation.
3. Results and discussion The magnetic properties of the 20 nm-thick FePt thin films that are deposited at the substrate temperature of 600 1C were investigated on SiO2/Si substrates (Fig. 1a) and ultrathin Fe (Fig. 1b) and ultrathin Pt (Fig. 1c) underlayer with the film thickness of 1 nm, as shown in Fig. 1. The effect of SiO2/Si substrate and ultrathin Pt underlayer on the magnetic properties and the structural characterization analysis of the FePt thin films were also investigated as the reference data for the effect of
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ultrathin Fe underlayer. Specially, the reason that the Pt underlayer is used as the reference data is because it has a possibility as a function to enhance the perpendicular magnetization and the perpendicular texturing of FePt thin films [18]. The summary of coercivity results indicated that the in-plane coercivity of FePt thin film on ultrathin Fe underlayer was the largest but its out-ofplane coercivity was the smallest compared to those of FePt thin films on SiO2/Si substrate and ultrathin Pt underlayer. The values of in-plane coercivity and out-of-plane coercivity were high above 7400 Oe and extremely low below 350 Oe, respectively. This means that the strong in-plane magnetization of ordered FePt thin film on ultrathin Fe underlayer was induced with in-plane preferred texturing of FePt magnetic grains. Specially, as indicated in Fig. 1d, to investigate the effect of the film thickness of Fe underlayer on the in-plane magnetization of FePt thin films, the film thickness of Fe underlayer was increased. As the film thickness of Fe underlayer was thicker to 5 nm, the out-of-plane coercivity value of FePt thin film was significantly increased above 4000 Oe and thus in-plane magnetization of FePt thin film was prominently decreased. From this result, it is supposed that thinner underlayer has a good and positive effect with controlling in the strong magnetization and the high texturing of FePt thin films. Table 1 shows the in-plane coercivity (HcJ), out-of-plane coercivity (Hc?), coercivity ratio (HcJ/Hc?), and in-plane coercivity squareness (SnJ ) for FePt thin films with underlayer type. As shown in Table 1, the coercivity ratio of the FePt thin film with ultrathin Fe underlayer was significantly large above 21.1 compared to those of the FePt thin films with SiO2/Si substrate and ultrathin Pt underlayer. This means that the in-plane magnetization of the FePt thin film was prominently large on ultrathin Fe underlayer. The in-plane coercivity squareness of the FePt thin film also was large above 0.76 on ultrathin Fe underlayer. It is also considered that this value for coercivity squareness is suitable for an application of ultra high density magnetic recording media.
Fig. 1. Magnetization hysteresis curves of FePt thin films on (a) SiO2/Si substrates, (b) 1 nm ultrathin Fe underlayer, (c) 1 nm ultrathin Pt underlayer, and (d) 5 nm Fe underlayer.
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S.H. Lee, S.H. Eun / Physica B 406 (2011) 2646–2649
In order to explain the magnetization of FePt thin films to the microstructural characterizations such as the L10-ordering phase transformation kinetics and the texturing of FePt thin films, we investigated the effect of ultrathin Fe underlayer on the microstructural characterizations of the FePt thin films using the KMC simulation and the XRD measurement. Fig. 2 shows the Long Range Ordering (LRO) parameters of FePt thin films on SiO2/Si substrates (Fig. 2a) and ultrathin Fe (Fig. 2b) and Pt (Fig. 2c) underlayer with KMC simulation. As indicated in Fig. 2b, the LRO parameter for the Y-axis with the in-plane direction of FePt thin film with an increase of the Monte Carlo Steps (MCS) that means ‘‘order–order’’ processing time [17] for Fe and Pt atoms in the FePt thin films was significantly increased compared to that for the Z-axis with the out-of-plane direction, on ultrathin Fe underlayer. Also, the LRO parameter for the Y-axis of the FePt thin film with MCS on ultrathin Fe underlayer was significantly enhanced compared to those of FePt thin films with MCS on SiO2/Si substrate and ultrathin Pt underlayer. This crucial KMC simulation result is an indication of the definite preference imposed by the Fe surface with the higher stability of the longitudinal (X or Y-axis) L10-superstructure variants with a c-axis parallel to FePt film surface [17]. This means that the ordering phase transformation kinetics of FePt thin films was prominently enhanced to in-plane direction using ultrathin Fe underlayer (Fig. 2b), which was well
corresponded with the strong in-plane magnetization of FePt thin film on ultrathin Fe underlayer (Fig. 1b). The present result thus suggested that the degree of ordering phase transformation kinetics to the in-plane direction of FePt thin films was a critical factor to determine the in-plane magnetization of FePt thin films, which seems worth considering when designing longitudinal magnetic recording media associated with this intermetallic compound. Fig. 3 shows the XRD patterns of FePt thin films on SiO2/Si substrates (Fig. 3a) and ultrathin Fe (Fig. 3b) and Pt (Fig. 3c) underlayer. As shown in Fig. 3b, the intensity for a (2 0 0) peak with the in-plane direction of FePt thin film on ultrathin Fe underlayer was significantly large compared to those of FePt thin films on SiO2/Si substrate and ultrathin Pt underlayer. Also, a
Table 1 In-plane coercivity (HcJ), out-of-plane coercivity (Hc?), coercivity ratio (HcJ/Hc?), and in-plane coercivity squareness (SnJ ) for FePt thin films with underlayer type. Underlayer type
HcJ (Oe)
Hc? (Oe)
HcJ/Hc?
SnJ
SiO2/Si substrate Ultrathin Fe underlayer Ultrathin Pt underlayer
5958 7400 4898
2998 350 5584
2.0 21.1 0.9
0.70 0.76 0.42
Fig. 3. XRD patterns of FePt thin films on (a) SiO2/Si substrates, (b) 1 nm ultrathin Fe and, (c) 1 nm ultrathin Pt underlayer.
Fig. 2. Long Range Ordering (LRO) parameters of FePt thin films with MCS on (a) SiO2/Si substrates, (b) 1 nm ultrathin Fe and, (c) 1 nm ultrathin Pt underlayer with Kinetics Monte Carlo (KMC) simulation.
S.H. Lee, S.H. Eun / Physica B 406 (2011) 2646–2649
(1 1 1) peak with an isotropic of FePt thin films was not observed on ultrathin Fe underlayer. This means that the in-plane texturing of FePt thin films was highly developed using ultrathin Fe underlayer. It had been reported [16] that ultrathin underlayer plays a catalytic role in decreasing interface free energy between thin films and substrates. Thus, it is supposed that the FePt thin films might be strongly adhered to the surface of SiO2/Si substrates due to a reduction of interfacial energy, resulting in the high in-plane (2 0 0) texturing development of the FePt thin films with an introduction of ultrathin Fe underlayer. The strong development for the in-plane texturing of FePt thin film (Fig. 3b) was closely accorded with the strong in-plane magnetization of FePt thin film on ultrathin Fe underlayer (Fig. 1b). Therefore, it is suggested that the in-plane magnetization of FePt thin films was strongly dependent on the in-plane texturing of FePt thin films.
Acknowledgment The authors are grateful to the Center for Nanostructured Materials Technology under the 21st Century Frontier R&D Programs of the Ministry of Science and Technology, Korea for their financial support of this research through the Grant number 07K1501-01210. References [1] [2] [3] [4] [5] [6] [7]
4. Conclusions In conclusions, the strong in-plane magnetization of FePt thin films could be obtained with the high in-plane coercivity of 7400 Oe and the extremely low out-of-plane coercivity of 350 Oe on an ultrathin Fe underlayer with 1 nm thickness. The ratio of the in-plane coercivity and out-of-plane coercivity of FePt thin film on ultrathin Fe underlayer was also significantly large above 21.1 compared to those of FePt films in other conventional studies. The strong in-plane magnetization of the FePt thin film on ultrathin Fe underlayer was closely associated with strong ordering phase transformation kinetics and high texturing to in-plane direction of the FePt thin film.
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[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
O.A. Ivanov, L.V. Solina, V.A. Demshina, Phys. Met. Metallogr. 35 (1973) 81. B.M. Lairson, B.M. Clemens, Appl. Phys. Lett. 63 (1993) 1438. M.R. Visokay, R. Sinclair, Appl. Phys. Lett. 66 (1995) 1692. C.P. Luo, S.H. Liou, L. Gao, Y. Liu, D.J. Sellmyer, Appl. Phys. Lett. 77 (2000) 2225. Y.-N. Hsu, S. Jeong, D.N. Lambeth, D.E. Laughlin, IEEE Trans. Magn. 36 (2000) 2945. V. Karanasos, I. Panagiotopoulos, D. Niarchos, H. Okumura, G.C. Hadjipanayis, Appl. Phys. Lett. 79 (2001) 1255. T. Suzuki, T. Kiya, N. Honda, K. Ouchi, J. Magn. Magn. Mater. 235 (2001) 312. T. Seki, T. Shima, K. Takanashi, J. Magn. Magn. Mater. 272–276 (2004) 2182. M.L. Yan, H. Zeng, N. Powers, D.J. Sellmyer, J. Appl. Phys. 91 (10) (2002) 8471. S.-Y. Bae, K.-H. Shin, J.-Y. Jeong, J.-G. Kim, J. Appl. Phys. 87 (2000) 6953. S.K. Chen, F.T. Yuan, S.N. Hsiao, H.C. Chang, C.Y. Liou, W.C. Chang, J. Magn. Magn. Mater. 282 (2004) 198. T. Seki, T. Shima, K. Takanashi, J. Magn. Magn. Mater. 272–276 (2004) 2182. R.F.C. Farrow, D. Weller, R.F. Marks, M.F. Toney, D.J. Smith, M.R. McCartney, J. Appl. Phys. 84 (1998) 934. S.C. Chen, P.C. Kuo, A.C. Sun, C.Y. Chou, Y.H. Fang, C.T. Lee, J. Magn. Magn. Mater. 310 (2007) e921. S. Nakagawa, T. Kamiki, J. Magn. Magn. Mater. 287 (2005) 204. A. Kamijo, T. Mitsuzuka, J. Appl. Phys. 77 (8) (1995) 3799. M. Koz"owski, R. Kozubski, V. Pierron-Bohnes, W. Pfeiler, Comput. Mater. Sci. 33 (2005) 287. J.U. Thiele, L. Folks, M.F. Toney, D.K. Weller, J. Appl. Phys. 84 (1998) 5686.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Strongly in-plane magnetized FePt thin film on ultrathin Fe underlayer S.H. Lee a,n, S.H. Eun b a b
Department of Materials Science and Engineering, UCLA, 410 Westwood Plaza, 3111 Engineering V, Los Angeles, CA 90095-1595, USA Department of Materials Science and Engineering, KAIST, 373-1 Guseong-dong, Yuseong-gu, 305-701 Daejeon, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2010 Received in revised form 22 January 2011 Accepted 2 April 2011 Available online 13 April 2011
The effect of ultrathin Fe underlayer on the strong in-plane magnetization of FePt magnetic thin film was investigated. This FePt thin film could be attained using the ultrathin Fe underlayer with 1 nm thickness. The in-plane coercivity of FePt film with 20 nm thickness grown on ultrathin Fe underlayer was high up to 7400 Oe. However, its out-of-plane coercivity was extremely low to 350 Oe compared to those of FePt thin films in other conventional studies. This result indicates that FePt thin film was strongly in-plane magnetized by ultrathin Fe underlayer. The strong ordering phase transformation kinetics and the high texturing to in-plane direction of the FePt thin film by ultrathin Fe underlayer were confirmed by Kinetics Monte Carlo (KMC) simulation and XRD measurement result, respectively. It is also supposed that they are associated with the reduction of an interface free energy between the film and the substrate with an introduction of ultrathin underlayer. & 2011 Elsevier B.V. All rights reserved.
Keywords: FePt Ultrathin Fe underlayer In-plane magnetization Coercivity Ordering phase transformation kinetics Texturing
1. Introduction The ordered (L10) FePt-based thin films are known to be a promising candidate for ultra high density magnetic recording media because of its high magnetocrystalline anisotropy energy [1–7]. Generally, the FePt magnetic thin films synthesized by sputtering tend to grow with either random orientations or (1 1 1) texture that place the c-axis of grains 371 out of the film plane [4,6]. Therefore, it is required to control the crystallographic orientation in FePt thin films in order to make them suitable for the various applications. The in-plane magnetized FePt thin films are required for a novel application such as bias magnets and exchange spring magnets [8] whereas the perpendicular magnetized FePt thin films are required as perpendicular magnetic recording media [9]. Recently, there have been a few conventional studies [10–14] on the in-plane magnetization of FePt thin film on glass substrate [10], quartz substrate [11], MgO (1 1 0) single crystal substrate [12,13], and CrRu underlayer [14]. Even though the in-plane magnetization of the fct–FePt thin films was presented in a few conventional studies, it was difficult to obtain strong in-plane magnetization of FePt thin films because the outof-plane coercivity of FePt thin films was not extremely low below 1000 Oe. Therefore, in this study, we used an ultrathin Fe underlayer with (2 0 0) texturing in the film plane to induce the strong
n
Corresponding author. Tel.: þ1 310 806 2982; fax: þ1 310 206 7353. E-mail address: [email protected] (S.H. Lee).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.04.007
in-plane magnetization of FePt thin film. The Fe layers deposited at relatively high deposition rate exhibited (1 1 0) texture as seen in the general case of Fe films. However, a lower deposition rate ˚ for Fe layers caused (2 0 0) texture [15]. It was also below 0.6 A/s reported that the ultrathin ( o5 nm) underlayer results in the high texturing of thin films [16]. Thus, it is expected that highly in-plane textured FePt thin films are attained on ultrathin Fe underlayer. We explained the magnetic properties such as the magnetization and the magnetic anisotropy of FePt thin films to the microstructural characterizations such as the L10-ordering phase transformation kinetics of FePt thin films and the grain texturing and orientation of FePt thin films. We also investigated the effect of ultrathin Fe underlayer on the microstructural characterizations of FePt thin films by the Kinetics Monte Carlo (KMC) simulation and the XRD measurement.
2. Experimental 20 nm-thick equiatomic FePt thin films were cosputtered both Fe and Pt targets on the SiO2/Si substrates using dc magnetron sputter at the substrate temperature heated up to 600 1C by a heater with halogen lamp under 5 mTorr of Ar in a sputter chamber of 4.0 10 7 Torr. The ultrathin Fe or Pt underlayer with the film thickness of 1 nm was deposited using the Fe or Pt ˚ target. The deposition rate of Fe layer was 0.3 A/s and the ˚ deposition rate of Pt layer was 0.5 A/s. Specially, to investigate the film thickness effect of Fe underlayer on the in-plane
S.H. Lee, S.H. Eun / Physica B 406 (2011) 2646–2649
magnetization of FePt thin films, the film thickness of Fe underlayer was increased up to 5 nm. The chemical compositions of FePt thin films were determined by Energy Dispersive X-ray microanalysis (EDX). Magnetic characterizations were carried out by the Vibrating Sample Magnetometer (VSM) with an applied field of up to 710 kOe. The KMC simulation implemented with vacancy atomic-jump mechanism [17] was carried out to study the effect of the L10-ordering phase transformation kinetics of FePt thin films on Fe or Pt underlayer. The simulated sample consisted of 50 50 50 unit cell and atomic ratio of Fe and Pt was 50:50. The thin layers limited by free surfaces were modeled by removing periodic boundary conditions in Z-axis direction, whereas the periodic boundary conditions were retained in X and Y-axis direction. The ordering kinetics in FePt systems was determined by Bragg–Williams-type Long Range Ordering (LRO) parameter [17]. Then this LRO parameter can be calculated as follows:
Z ¼ 12
Fe NPt , NPt
Fe is the number of Fe atoms on the Pt sublattice where NPt (antisites), NPt is the number of Pt sites. The crystal structure of the thin films was characterized by Rigaku D/MAX-RC and RB X-ray diffractometer using CuKa radiation.
3. Results and discussion The magnetic properties of the 20 nm-thick FePt thin films that are deposited at the substrate temperature of 600 1C were investigated on SiO2/Si substrates (Fig. 1a) and ultrathin Fe (Fig. 1b) and ultrathin Pt (Fig. 1c) underlayer with the film thickness of 1 nm, as shown in Fig. 1. The effect of SiO2/Si substrate and ultrathin Pt underlayer on the magnetic properties and the structural characterization analysis of the FePt thin films were also investigated as the reference data for the effect of
2647
ultrathin Fe underlayer. Specially, the reason that the Pt underlayer is used as the reference data is because it has a possibility as a function to enhance the perpendicular magnetization and the perpendicular texturing of FePt thin films [18]. The summary of coercivity results indicated that the in-plane coercivity of FePt thin film on ultrathin Fe underlayer was the largest but its out-ofplane coercivity was the smallest compared to those of FePt thin films on SiO2/Si substrate and ultrathin Pt underlayer. The values of in-plane coercivity and out-of-plane coercivity were high above 7400 Oe and extremely low below 350 Oe, respectively. This means that the strong in-plane magnetization of ordered FePt thin film on ultrathin Fe underlayer was induced with in-plane preferred texturing of FePt magnetic grains. Specially, as indicated in Fig. 1d, to investigate the effect of the film thickness of Fe underlayer on the in-plane magnetization of FePt thin films, the film thickness of Fe underlayer was increased. As the film thickness of Fe underlayer was thicker to 5 nm, the out-of-plane coercivity value of FePt thin film was significantly increased above 4000 Oe and thus in-plane magnetization of FePt thin film was prominently decreased. From this result, it is supposed that thinner underlayer has a good and positive effect with controlling in the strong magnetization and the high texturing of FePt thin films. Table 1 shows the in-plane coercivity (HcJ), out-of-plane coercivity (Hc?), coercivity ratio (HcJ/Hc?), and in-plane coercivity squareness (SnJ ) for FePt thin films with underlayer type. As shown in Table 1, the coercivity ratio of the FePt thin film with ultrathin Fe underlayer was significantly large above 21.1 compared to those of the FePt thin films with SiO2/Si substrate and ultrathin Pt underlayer. This means that the in-plane magnetization of the FePt thin film was prominently large on ultrathin Fe underlayer. The in-plane coercivity squareness of the FePt thin film also was large above 0.76 on ultrathin Fe underlayer. It is also considered that this value for coercivity squareness is suitable for an application of ultra high density magnetic recording media.
Fig. 1. Magnetization hysteresis curves of FePt thin films on (a) SiO2/Si substrates, (b) 1 nm ultrathin Fe underlayer, (c) 1 nm ultrathin Pt underlayer, and (d) 5 nm Fe underlayer.
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S.H. Lee, S.H. Eun / Physica B 406 (2011) 2646–2649
In order to explain the magnetization of FePt thin films to the microstructural characterizations such as the L10-ordering phase transformation kinetics and the texturing of FePt thin films, we investigated the effect of ultrathin Fe underlayer on the microstructural characterizations of the FePt thin films using the KMC simulation and the XRD measurement. Fig. 2 shows the Long Range Ordering (LRO) parameters of FePt thin films on SiO2/Si substrates (Fig. 2a) and ultrathin Fe (Fig. 2b) and Pt (Fig. 2c) underlayer with KMC simulation. As indicated in Fig. 2b, the LRO parameter for the Y-axis with the in-plane direction of FePt thin film with an increase of the Monte Carlo Steps (MCS) that means ‘‘order–order’’ processing time [17] for Fe and Pt atoms in the FePt thin films was significantly increased compared to that for the Z-axis with the out-of-plane direction, on ultrathin Fe underlayer. Also, the LRO parameter for the Y-axis of the FePt thin film with MCS on ultrathin Fe underlayer was significantly enhanced compared to those of FePt thin films with MCS on SiO2/Si substrate and ultrathin Pt underlayer. This crucial KMC simulation result is an indication of the definite preference imposed by the Fe surface with the higher stability of the longitudinal (X or Y-axis) L10-superstructure variants with a c-axis parallel to FePt film surface [17]. This means that the ordering phase transformation kinetics of FePt thin films was prominently enhanced to in-plane direction using ultrathin Fe underlayer (Fig. 2b), which was well
corresponded with the strong in-plane magnetization of FePt thin film on ultrathin Fe underlayer (Fig. 1b). The present result thus suggested that the degree of ordering phase transformation kinetics to the in-plane direction of FePt thin films was a critical factor to determine the in-plane magnetization of FePt thin films, which seems worth considering when designing longitudinal magnetic recording media associated with this intermetallic compound. Fig. 3 shows the XRD patterns of FePt thin films on SiO2/Si substrates (Fig. 3a) and ultrathin Fe (Fig. 3b) and Pt (Fig. 3c) underlayer. As shown in Fig. 3b, the intensity for a (2 0 0) peak with the in-plane direction of FePt thin film on ultrathin Fe underlayer was significantly large compared to those of FePt thin films on SiO2/Si substrate and ultrathin Pt underlayer. Also, a
Table 1 In-plane coercivity (HcJ), out-of-plane coercivity (Hc?), coercivity ratio (HcJ/Hc?), and in-plane coercivity squareness (SnJ ) for FePt thin films with underlayer type. Underlayer type
HcJ (Oe)
Hc? (Oe)
HcJ/Hc?
SnJ
SiO2/Si substrate Ultrathin Fe underlayer Ultrathin Pt underlayer
5958 7400 4898
2998 350 5584
2.0 21.1 0.9
0.70 0.76 0.42
Fig. 3. XRD patterns of FePt thin films on (a) SiO2/Si substrates, (b) 1 nm ultrathin Fe and, (c) 1 nm ultrathin Pt underlayer.
Fig. 2. Long Range Ordering (LRO) parameters of FePt thin films with MCS on (a) SiO2/Si substrates, (b) 1 nm ultrathin Fe and, (c) 1 nm ultrathin Pt underlayer with Kinetics Monte Carlo (KMC) simulation.
S.H. Lee, S.H. Eun / Physica B 406 (2011) 2646–2649
(1 1 1) peak with an isotropic of FePt thin films was not observed on ultrathin Fe underlayer. This means that the in-plane texturing of FePt thin films was highly developed using ultrathin Fe underlayer. It had been reported [16] that ultrathin underlayer plays a catalytic role in decreasing interface free energy between thin films and substrates. Thus, it is supposed that the FePt thin films might be strongly adhered to the surface of SiO2/Si substrates due to a reduction of interfacial energy, resulting in the high in-plane (2 0 0) texturing development of the FePt thin films with an introduction of ultrathin Fe underlayer. The strong development for the in-plane texturing of FePt thin film (Fig. 3b) was closely accorded with the strong in-plane magnetization of FePt thin film on ultrathin Fe underlayer (Fig. 1b). Therefore, it is suggested that the in-plane magnetization of FePt thin films was strongly dependent on the in-plane texturing of FePt thin films.
Acknowledgment The authors are grateful to the Center for Nanostructured Materials Technology under the 21st Century Frontier R&D Programs of the Ministry of Science and Technology, Korea for their financial support of this research through the Grant number 07K1501-01210. References [1] [2] [3] [4] [5] [6] [7]
4. Conclusions In conclusions, the strong in-plane magnetization of FePt thin films could be obtained with the high in-plane coercivity of 7400 Oe and the extremely low out-of-plane coercivity of 350 Oe on an ultrathin Fe underlayer with 1 nm thickness. The ratio of the in-plane coercivity and out-of-plane coercivity of FePt thin film on ultrathin Fe underlayer was also significantly large above 21.1 compared to those of FePt films in other conventional studies. The strong in-plane magnetization of the FePt thin film on ultrathin Fe underlayer was closely associated with strong ordering phase transformation kinetics and high texturing to in-plane direction of the FePt thin film.
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[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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