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Synthesis of epitaxial Y-type magnetoplumbite thin films by quick optimization with combinatorial pulsed laser deposition
Journal of Crystal Growth 247 (2003) 105–109
Synthesis of epitaxial Y-type magnetoplumbite thin films by quick optimization with combinatorial pulsed laser deposition I. Ohkuboa,*, Y. Matsumotoa, K. Uenob, T. Chikyowc, M. Kawasakic,d, H. Koinumaa,c a
Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan b Central Technology Laboratory, ASAHI KASEI CORPORATION, Fuji, 416-8501, Japan c Combinatorial Materials Exploration and Technology, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan d Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 13 August 2002; accepted 11 September 2002 Communicated by M. Schieber
Abstract ( Y-type magnetoplumbite (Ba2Co2Fe12O22:Co2Y) epitaxial thin films with such a huge lattice parameter as 43.5 A have been synthesized for the first time. Combinatorial thin film technology was successfully employed to eliminate an impurity phase in the film by quickly optimizing such reaction parameters as the deposition temperature and the thickness of pre-deposition of CoO layer. The coupling of combinatorial pulsed laser deposition and subsequent concurrent X-ray diffraction, both of which we have developed, is a promising way to high throughput optimization of thin film growth and properties. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55; 75.70; 81.15.F Keywords: A1. Phase equilibria; A3. Laser epitaxy; B1. Oxides; B2. Magnetic materials
1. Introduction A group of Y-type magnetoplumbite (Ba2Co2Fe12O22:Co2Y) attract much interest as core material for thin film inductor and other microwave devices operated at GHz frequency range. Its natural ferromagnetic resonance in the bulk *Corresponding author. Present address: Solid State Division, Oak Ridge National Laboratory, Bethel Valley Road, MS 6030, P.O. Box 2008, Oak Ridge, TN 37831-6030, USA. Tel.: +1-865-576-8676; fax: +1-865-574-4143. E-mail address: [email protected] (I. Ohkubo).
polycrystalline form was reported to exist in a frequency range above 1 GHz [1]. Y-type magnetoplumbite has a unit cell composed of S block (CoxFe3xO4) and T block (BaCoxFe2xO4) which can be described by the close-packed structure depicted in Fig. 1. Because of this huge and complex lattice, single crystal phase of Y-type magnetoplumbite is difficult to be synthesized in bulk and even more in thin films. In fact, no previous reports have appeared on the synthesis of this complex ceramic materials in thin film form. From both viewpoints of thin film growth control and device application, we have challenged the
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 7 9 3 - 1
106
I. Ohkubo et al. / Journal of Crystal Growth 247 (2003) 105–109
Fig. 1. Schematic illustration of Y-type magnetoplumbite (Ba2Co2Fe12O22) crystal structure. It is composed of an alternating stack of S-block (CoFe2O4: spinel structure) and T-block (BaFe4O7)[* indicates the rotation by 180 (around the c-axis)]. S-block have face centered close-packed structure (fcc, ABCABC—stacking sequence composed of oxygen atoms) and T-block have hexagonal close-packed structure (hcp, ABAB— stacking sequence composed of oxygen and barium atoms). Therefore, Y-type magnetoplimbite structure can be described as ABCBCBCAB.
epitaxial thin film growth of a single phase Y-type magnetoplumbite, which would presumably set a record of the high lattice parameter in epitaxial thin films. In order to overcome the problem of long time experimental efforts anticipated for optimizing the growth conditions, by the traditional one-by-one process, we employed the combinatorial approach which we had been developing [2]. In this paper, we report on the successful epitaxial growth of Ytype magnetoplumbite by taking the full advantage of combinatorial thin film technology.
2. Experimental procedure The synthesis of Co2Y thin films was performed by the combinatorial pulsed laser deposition
(PLD) which was installed with a Nd:YAG laser substrate heating system and moving mask [3]. A MgAl2O4(1 1 1) single crystal was employed as a substrate for the growth of Co2Y film due to its relatively small (2.5%) lattice mismatch. A sintered stoichiometric Co2Y ceramics target was ablated by KrF excimer laser pulses (l ¼ 248 nm, 5 Hz, 20 ns) with a fluence of 1 J/cm2 to deposit ( films at a rate of 0.7 A/pulse under a typical the oxygen pressure of 200 mTorr. A sliding mask action enabled the production of thin film library with continuously varying thickness, while the anisotropic Nd–YAG laser heating did the temperature gradient of substrate for quick optimization of deposition temperature [4]. The crystal structure of film library was quickly identified by means of the concurrent X-ray diffractiometer [5]. Scanning electron microscope (SEM) and microAuger electron spectroscopy (micro-AES) measurements were used for the surface structural analyses. Interface structural analysis between the deposited thin films and substrate were done by transmission electron microscope (TEM).
3. Results and discussion The formation of Co2Y phase was not detected until the deposition temperature was increased above 10001C. A typical X-ray diffraction (XRD) pattern and SEM photograph of the film fabricated at 11401C in 200 mTorr oxygen are shown in Figs. 1(a) and (b). The film exhibited X-ray diffraction corresponding to Co2Y and its chemical composition was very close to that of Ba2Co2Fe12O22 (Co2Y). However, the film contained impurity phases mainly of BaFe2O4 (Fig. 2(a)) and many precipitates of about 3 mm size on the film surface (Fig. 2(b)). The micro-AES analysis indicates that Co atoms exist in the precipitates and not in the base film as shown in Fig. 2(c). Co2Y phase apparently segregated from the base flat film, which could be identified as BaFe2O4. In fact, by polishing this film surface to remove the precipitates, XRD peaks of Co2Y disappeared completely. According to the differential thermal analysis (DTA) experiment, Co2Y melts incongruently to
CoO layer thickness [A]
BaFe2O4
Co2Y(00018)
Co2Y(00012)
S
107
800
850
900
950
1000
Ba2Co2Fe12O22 (Co2Y) [40000A ] 15
(a)
BaFe2O4
Intensity [arb. units]
Co2Y(0009)
S
Co2Y(00015)
I. Ohkubo et al. / Journal of Crystal Growth 247 (2003) 105–109
20
25
30 35 2 [degree]
40
45
CoO layer MgAl2O4 (111) substrate (a)
2theta [deg.]
44 43 42 41 (b) 2theta [deg.]
(b) 3µm
Intensity of Co Signal
(c) 3µm Fig. 2. XRD pattern, SEM and AES mapping images of the thin film deposited at 11401C in 200 mTorr. (a) XRD pattern of 2y–y scan. MgAl2O4 substrate peaks indicated by S. (b) SEM images. (c) AES intensity mapping image of Co atom signal.
decompose into BaFe2O4 and Co2Y, being consistent with the phase separation in the film. It is well known that YBa2Cu3O7 superconductor is
32
30 (c)
Fig. 3. Schematic experimental configurations of CoO underlayer with thickness gradient on a substrate. (a) The deposition sequence profiles. (b),(c) Concurrent XRD patterns of BaFe2O4 (2y=41.31) and unknown compound (2y=431) (b) and Co2Y (2y=31.81) (c).
also incongruently melt and the primary phase field (PPF) of YBa2Cu3O7 is in the Y deficient BaCuOx area [6]. We extended this thermodynamics to the growth of single crystal thin films of RBa2Cu3O7 (R=Y, Nd) by the tri-phase epitaxy [7]. From the analogous to the case of YBa2Cu3O7, BaFe2O4 was presumed to be a Co deficient impurity phase and its formation would be suppressed by feeding CoO in excess. This hypothesis prompted us to deposit thin films by the laser ablation of a stoichiometric Co2Y
I. Ohkubo et al. / Journal of Crystal Growth 247 (2003) 105–109
15
(a)
20
25
Substrate
Co2Y(00018)
Co2Y(00015)
Co2Y(00012)
Substrate
Co2Y(0009)
Intensity (arb. units)
target on a substrate pre-deposited with CoO layer. In order to optimize the thickness of underlying CoO layer promptly, Co2Y films were deposited on the CoO films with their thicknesses ( by partially systematically varied from 0 to 2000 A shielding the deposition area with a linearly moving mask. The layered film library structure and its concurrent X-ray diffraction patterns are schematically depicted in Fig. 3 for the CoO layer ( Figs. 3(b) thickness range between 800 and 1000 A. and (c) show the concurrent XRD patterns for the peaks of impurity phase and Co2Y, respectively. The impurity peaks completely disappeared and Co2Y peak intensity is strongly intensified when ( reaching the thickness of CoO layer exceed 850 A, ( the maximum at a 950 A thick CoO thickness. ( Fig. 4(a) is an XRD pattern of Co2Y (40,000 A) ( film deposited on CoO layer (950 A) under the conditions optimized of growth temperature (11401C) and of oxygen pressure (200 mTorr). All the diffraction peaks of (0 0 0 3 n) [n ¼ 1; 2; 3y:] for c-axis oriented Co2Y film are observed without any impurity peaks such as of BaFe2O4 and CoO.
30
35
2θ [degree]
3
x10 2
Magnetization [G/4 Ms]
108
1
0 Single phase thin film -1 Polycrystal -2 -2
-1
0
1
2
Magnetic Field [T] Fig. 5. Magnetization curves of single phase epitaxial Co2Y thin film and polycrystalline sample.
The cross-sectional TEM image of the films exhibited the sharp interface and no sign of remained CoO underlayer at the interface between Co2Y and the substrate (Fig. 4(b)). The fact that extra CoO was favorable for the growth of Co2Y in single epitaxial phase indicates the crystallization of Co2Y from the melt primary phase field under a quasi-equilibrium condition. The saturated magnetization (Ms ) of obtained epitaxial Co2Y thin films were evaluated to be about 2000 G (Fig. 5). The Ms of thin films was equivalent to bulk polycrystalline sample (Ms ¼ 200022100 G). The details are reported elsewhere [8].
4. Conclusion
(b)
50nm
Fig. 4. XRD pattern (a) and TEM image (b) of single phase epitaxial Co2Y thin film.
In conclusion, a schematic consideration on crystal growth and combinatorial technology has guided us to the successful synthesis of epitaxial thin film of Y-type magnetoplumbite, ( which has such a huge lattice parameter as 43.5 A. Magnetic properties attractive for high frequency application.
I. Ohkubo et al. / Journal of Crystal Growth 247 (2003) 105–109
References [1] G.H. Jonker, H.P.J. Wijn, P.B. Braun, Philips Techn. Rev. 18 (1956) 145. [2] H. Koinuma, H.N. Aiyer, Y. Matsumoto, Sci. Tech. Adv. Mater. 1 (2000) 1. [3] T. Fukumura, Y. Okimoto, M. Ohtani, T. Kageyama, T. Koida, M. Kawasaki, T. Hasegawa, Y. Tokura, H. Koinuma, Appl. Phys. Lett. 77 (2000) 3426. [4] T. Koida, D. Komiyama, H. Koinuma, M. Ohtani, M. Lippmaa, M. Kawasaki, Appl. Phys Lett. 80 (2002) 565.
109
[5] M. Ohtani, T. Fukumura, M. Kawasaki, K. Omote, T. Kikuchi, J. Harada, A. Ohtomo, M. Lippmaa, T. Ohnishi, D. Komiyama, R. Takahashi, Y. Matsumoto, H. Koinuma, Appl. Phys Lett. 79 (2001) 3594. [6] H.J. Scheel, F. Licci, Thermochim. Acta. 174 (1991) 115. [7] K.S. Yun, B.D. Choi, Y. Matsumoto, J.H. Song, N. Kanda, T. Itoh, M. Kawasaki, T. Chikyow, P. Ahmet, H. Koinuma, Appl. Phys Lett. 80 (2002) 61. [8] I. Ohkubo, Y. Matsumoto, K. Ueno, N. Okazaki, H. Hasegawa, M. Kawasaki, H. Koinuma, in preparation.
Synthesis of epitaxial Y-type magnetoplumbite thin films by quick optimization with combinatorial pulsed laser deposition I. Ohkuboa,*, Y. Matsumotoa, K. Uenob, T. Chikyowc, M. Kawasakic,d, H. Koinumaa,c a
Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan b Central Technology Laboratory, ASAHI KASEI CORPORATION, Fuji, 416-8501, Japan c Combinatorial Materials Exploration and Technology, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan d Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 13 August 2002; accepted 11 September 2002 Communicated by M. Schieber
Abstract ( Y-type magnetoplumbite (Ba2Co2Fe12O22:Co2Y) epitaxial thin films with such a huge lattice parameter as 43.5 A have been synthesized for the first time. Combinatorial thin film technology was successfully employed to eliminate an impurity phase in the film by quickly optimizing such reaction parameters as the deposition temperature and the thickness of pre-deposition of CoO layer. The coupling of combinatorial pulsed laser deposition and subsequent concurrent X-ray diffraction, both of which we have developed, is a promising way to high throughput optimization of thin film growth and properties. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55; 75.70; 81.15.F Keywords: A1. Phase equilibria; A3. Laser epitaxy; B1. Oxides; B2. Magnetic materials
1. Introduction A group of Y-type magnetoplumbite (Ba2Co2Fe12O22:Co2Y) attract much interest as core material for thin film inductor and other microwave devices operated at GHz frequency range. Its natural ferromagnetic resonance in the bulk *Corresponding author. Present address: Solid State Division, Oak Ridge National Laboratory, Bethel Valley Road, MS 6030, P.O. Box 2008, Oak Ridge, TN 37831-6030, USA. Tel.: +1-865-576-8676; fax: +1-865-574-4143. E-mail address: [email protected] (I. Ohkubo).
polycrystalline form was reported to exist in a frequency range above 1 GHz [1]. Y-type magnetoplumbite has a unit cell composed of S block (CoxFe3xO4) and T block (BaCoxFe2xO4) which can be described by the close-packed structure depicted in Fig. 1. Because of this huge and complex lattice, single crystal phase of Y-type magnetoplumbite is difficult to be synthesized in bulk and even more in thin films. In fact, no previous reports have appeared on the synthesis of this complex ceramic materials in thin film form. From both viewpoints of thin film growth control and device application, we have challenged the
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 7 9 3 - 1
106
I. Ohkubo et al. / Journal of Crystal Growth 247 (2003) 105–109
Fig. 1. Schematic illustration of Y-type magnetoplumbite (Ba2Co2Fe12O22) crystal structure. It is composed of an alternating stack of S-block (CoFe2O4: spinel structure) and T-block (BaFe4O7)[* indicates the rotation by 180 (around the c-axis)]. S-block have face centered close-packed structure (fcc, ABCABC—stacking sequence composed of oxygen atoms) and T-block have hexagonal close-packed structure (hcp, ABAB— stacking sequence composed of oxygen and barium atoms). Therefore, Y-type magnetoplimbite structure can be described as ABCBCBCAB.
epitaxial thin film growth of a single phase Y-type magnetoplumbite, which would presumably set a record of the high lattice parameter in epitaxial thin films. In order to overcome the problem of long time experimental efforts anticipated for optimizing the growth conditions, by the traditional one-by-one process, we employed the combinatorial approach which we had been developing [2]. In this paper, we report on the successful epitaxial growth of Ytype magnetoplumbite by taking the full advantage of combinatorial thin film technology.
2. Experimental procedure The synthesis of Co2Y thin films was performed by the combinatorial pulsed laser deposition
(PLD) which was installed with a Nd:YAG laser substrate heating system and moving mask [3]. A MgAl2O4(1 1 1) single crystal was employed as a substrate for the growth of Co2Y film due to its relatively small (2.5%) lattice mismatch. A sintered stoichiometric Co2Y ceramics target was ablated by KrF excimer laser pulses (l ¼ 248 nm, 5 Hz, 20 ns) with a fluence of 1 J/cm2 to deposit ( films at a rate of 0.7 A/pulse under a typical the oxygen pressure of 200 mTorr. A sliding mask action enabled the production of thin film library with continuously varying thickness, while the anisotropic Nd–YAG laser heating did the temperature gradient of substrate for quick optimization of deposition temperature [4]. The crystal structure of film library was quickly identified by means of the concurrent X-ray diffractiometer [5]. Scanning electron microscope (SEM) and microAuger electron spectroscopy (micro-AES) measurements were used for the surface structural analyses. Interface structural analysis between the deposited thin films and substrate were done by transmission electron microscope (TEM).
3. Results and discussion The formation of Co2Y phase was not detected until the deposition temperature was increased above 10001C. A typical X-ray diffraction (XRD) pattern and SEM photograph of the film fabricated at 11401C in 200 mTorr oxygen are shown in Figs. 1(a) and (b). The film exhibited X-ray diffraction corresponding to Co2Y and its chemical composition was very close to that of Ba2Co2Fe12O22 (Co2Y). However, the film contained impurity phases mainly of BaFe2O4 (Fig. 2(a)) and many precipitates of about 3 mm size on the film surface (Fig. 2(b)). The micro-AES analysis indicates that Co atoms exist in the precipitates and not in the base film as shown in Fig. 2(c). Co2Y phase apparently segregated from the base flat film, which could be identified as BaFe2O4. In fact, by polishing this film surface to remove the precipitates, XRD peaks of Co2Y disappeared completely. According to the differential thermal analysis (DTA) experiment, Co2Y melts incongruently to
CoO layer thickness [A]
BaFe2O4
Co2Y(00018)
Co2Y(00012)
S
107
800
850
900
950
1000
Ba2Co2Fe12O22 (Co2Y) [40000A ] 15
(a)
BaFe2O4
Intensity [arb. units]
Co2Y(0009)
S
Co2Y(00015)
I. Ohkubo et al. / Journal of Crystal Growth 247 (2003) 105–109
20
25
30 35 2 [degree]
40
45
CoO layer MgAl2O4 (111) substrate (a)
2theta [deg.]
44 43 42 41 (b) 2theta [deg.]
(b) 3µm
Intensity of Co Signal
(c) 3µm Fig. 2. XRD pattern, SEM and AES mapping images of the thin film deposited at 11401C in 200 mTorr. (a) XRD pattern of 2y–y scan. MgAl2O4 substrate peaks indicated by S. (b) SEM images. (c) AES intensity mapping image of Co atom signal.
decompose into BaFe2O4 and Co2Y, being consistent with the phase separation in the film. It is well known that YBa2Cu3O7 superconductor is
32
30 (c)
Fig. 3. Schematic experimental configurations of CoO underlayer with thickness gradient on a substrate. (a) The deposition sequence profiles. (b),(c) Concurrent XRD patterns of BaFe2O4 (2y=41.31) and unknown compound (2y=431) (b) and Co2Y (2y=31.81) (c).
also incongruently melt and the primary phase field (PPF) of YBa2Cu3O7 is in the Y deficient BaCuOx area [6]. We extended this thermodynamics to the growth of single crystal thin films of RBa2Cu3O7 (R=Y, Nd) by the tri-phase epitaxy [7]. From the analogous to the case of YBa2Cu3O7, BaFe2O4 was presumed to be a Co deficient impurity phase and its formation would be suppressed by feeding CoO in excess. This hypothesis prompted us to deposit thin films by the laser ablation of a stoichiometric Co2Y
I. Ohkubo et al. / Journal of Crystal Growth 247 (2003) 105–109
15
(a)
20
25
Substrate
Co2Y(00018)
Co2Y(00015)
Co2Y(00012)
Substrate
Co2Y(0009)
Intensity (arb. units)
target on a substrate pre-deposited with CoO layer. In order to optimize the thickness of underlying CoO layer promptly, Co2Y films were deposited on the CoO films with their thicknesses ( by partially systematically varied from 0 to 2000 A shielding the deposition area with a linearly moving mask. The layered film library structure and its concurrent X-ray diffraction patterns are schematically depicted in Fig. 3 for the CoO layer ( Figs. 3(b) thickness range between 800 and 1000 A. and (c) show the concurrent XRD patterns for the peaks of impurity phase and Co2Y, respectively. The impurity peaks completely disappeared and Co2Y peak intensity is strongly intensified when ( reaching the thickness of CoO layer exceed 850 A, ( the maximum at a 950 A thick CoO thickness. ( Fig. 4(a) is an XRD pattern of Co2Y (40,000 A) ( film deposited on CoO layer (950 A) under the conditions optimized of growth temperature (11401C) and of oxygen pressure (200 mTorr). All the diffraction peaks of (0 0 0 3 n) [n ¼ 1; 2; 3y:] for c-axis oriented Co2Y film are observed without any impurity peaks such as of BaFe2O4 and CoO.
30
35
2θ [degree]
3
x10 2
Magnetization [G/4 Ms]
108
1
0 Single phase thin film -1 Polycrystal -2 -2
-1
0
1
2
Magnetic Field [T] Fig. 5. Magnetization curves of single phase epitaxial Co2Y thin film and polycrystalline sample.
The cross-sectional TEM image of the films exhibited the sharp interface and no sign of remained CoO underlayer at the interface between Co2Y and the substrate (Fig. 4(b)). The fact that extra CoO was favorable for the growth of Co2Y in single epitaxial phase indicates the crystallization of Co2Y from the melt primary phase field under a quasi-equilibrium condition. The saturated magnetization (Ms ) of obtained epitaxial Co2Y thin films were evaluated to be about 2000 G (Fig. 5). The Ms of thin films was equivalent to bulk polycrystalline sample (Ms ¼ 200022100 G). The details are reported elsewhere [8].
4. Conclusion
(b)
50nm
Fig. 4. XRD pattern (a) and TEM image (b) of single phase epitaxial Co2Y thin film.
In conclusion, a schematic consideration on crystal growth and combinatorial technology has guided us to the successful synthesis of epitaxial thin film of Y-type magnetoplumbite, ( which has such a huge lattice parameter as 43.5 A. Magnetic properties attractive for high frequency application.
I. Ohkubo et al. / Journal of Crystal Growth 247 (2003) 105–109
References [1] G.H. Jonker, H.P.J. Wijn, P.B. Braun, Philips Techn. Rev. 18 (1956) 145. [2] H. Koinuma, H.N. Aiyer, Y. Matsumoto, Sci. Tech. Adv. Mater. 1 (2000) 1. [3] T. Fukumura, Y. Okimoto, M. Ohtani, T. Kageyama, T. Koida, M. Kawasaki, T. Hasegawa, Y. Tokura, H. Koinuma, Appl. Phys. Lett. 77 (2000) 3426. [4] T. Koida, D. Komiyama, H. Koinuma, M. Ohtani, M. Lippmaa, M. Kawasaki, Appl. Phys Lett. 80 (2002) 565.
109
[5] M. Ohtani, T. Fukumura, M. Kawasaki, K. Omote, T. Kikuchi, J. Harada, A. Ohtomo, M. Lippmaa, T. Ohnishi, D. Komiyama, R. Takahashi, Y. Matsumoto, H. Koinuma, Appl. Phys Lett. 79 (2001) 3594. [6] H.J. Scheel, F. Licci, Thermochim. Acta. 174 (1991) 115. [7] K.S. Yun, B.D. Choi, Y. Matsumoto, J.H. Song, N. Kanda, T. Itoh, M. Kawasaki, T. Chikyow, P. Ahmet, H. Koinuma, Appl. Phys Lett. 80 (2002) 61. [8] I. Ohkubo, Y. Matsumoto, K. Ueno, N. Okazaki, H. Hasegawa, M. Kawasaki, H. Koinuma, in preparation.