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soft FePt composite films
NanoStmctured Materials, Vol. 12, pp. 1027-1030, 1999 Else&r Science Ltd 0 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 096%9773199/$-see front matter
Pergamon
PI1 SO965-9773(99)00292-S
NANOSTRUCTURE OF EXCHANGE COUPLED HARD/SOFT FePt COMPOSITE FILMS Y. Liu*, J. P. Liu** and D. J. Sellmyer** * Center for Materials Research and Analysis and Department of Mechanical Engineering, ** Center for Materials Research and Analysis and Behlen Laboratory of Physics, University of Nebraska, Lincoln, NE 68588. Abstract -- Multilayers of Fe and Pt were deposited by sputtering. The ratio of the thickness of Fe to Pt was adjusted such that stoichiometric FePt and Fe rich films were formed. These films were either annealed at XWC and above, or processed by rapid thermal annealing (RTA) using a high Ipower lamp. Both the Fe and Pt in the as-deposited films were found to be FCC structure. After heat treatment, Llo phase (hard phase) and disordered FCC phase (soft phase) were identified in Fe-rich films. Small precipitates of 3-8 nm in the matrix which has larger grains of about 50-200 nm are observed. Perfect coherent interface between the hard phase and soft phase is observed. Processing route, microstructure and properties relation will be discussed. 01999 Acta kfetallurgica Inc.
INTRODUCTION In the Fe-Pt system several magnetic phases exist. Among these phases, Fe solid solution and :Fe# are soft phases having high magnetization. The Llo type structure FePt has a layered structure in the [OOl] direction and a very high anisotropy of 6.6~10~ ergs/cc [l]. High energy produ~ctsfrom 15 MGGe to 30 MGGe have been achieved by several groups [2-4]. Recently, using multi-layer sputtering technique combined with post-annealing and rapid thermal annealing, energy products up to 50 MGGe have been achieved [5,6]. Theoretical calculations showed that when the dimension of the soft phase is smaller than two times the domain wall thickness of the hard phase, effective inter-grain coupling can be achieved [7]. In this paper we present phase identification and volume fraction measurement of the hard phase and soft phase. Their relation with magnetic properties will be discussed.
EXPERIMENTAL
PROCEDURE
Multilayers of Fe and Pt were deposited by sputtering. The ratio of the thickness of Fe to Pt was adjusted such that stoichiometric FePt and Fe rich films were formed. One group of these films were then annealed at 500°C and above. Another group of films were processed by rapid thermal annealing (RTA) using a high power lamp. Magnetization loops were measured by a Micromag 2900 alternating gradient force magnetometer. Plan-view transmission electron microscopy (TEM) samples were prepared by dimpling and ion milling process. TEM study was conducted using a JEOL 2010 transmission electron microscope. 1027
FOURTHINTERNATIONAL CONFERENCEON NAMXTRUCTUREDMATERIALS
1028
RESULT AND DISCUSSION Figure 1 compares the selected area diffraction (SAD) patterns (a) from an as-deposited film and (b) from an annealed film. Although pure Fe at room temperature is BCC structure, both the Pt and Fe in the as-deposited FVFe multi-layers have the FCC structure as shown in Figure 1 (a). The pattern of the annealed film in Figure 1 (b) can be matched to the Llo structure. A second phase has been identified by nanodiffraction [8] to be a disordered FCC phase which does not contribute to any additional lines in Figure 1 (b).
100
a
111
30 70
70 0
so 40
b
111
!Jo30-
90
40
loo,
I
60. 3
200
50 40-
I
Figure 1. Comparison of SAD patterns (a) from as-deposited film, (b) from annealed film (No2 listed in Table 1).
Figure 2 (a) shows the microstructure of rapid thermal processed film (film No4 in Table 1). The soft phase has the spherical shape and is distributed in the hard phase. High resolution transmission electron microscopy (HRTEM) image in Figure 2 (b) shows that the soft phase and the hard phase have coherent interphase boundary. The processing route, microstructure and properties relation are summan‘zed in Table 1. Conventional heat treatment results in larger grains of both the soft phase and the hard phase in contrast to the rapid thermal processing. The volume fraction of the soft phase increases with Fe content. However, when the Fe content is close the ratio of FeRt - 2/l, the morphology of the soft phase change from smaller spherical shape to grains with the same dimension of the hard phase.
FOURTH INTERNATIONAL CONFERENCEON NAN~STRUCTUREDMATERIALS
1029
Figure 2 (a) bright field TEM image, (b) HRTEM image. The film is No4 listed in Table Table 1. Summary of processing, microstructure and magnetic properties of Fe-Pt films.
1030
FOURTH INTERNATIONAL CONFERENCEON NAN~STRIJCTUREDMATERIALS
It is known that in the film form several elements exhibit a different structure from the equilibrium phase. Such examples are FCC Co [9], FCC Ti [lo] etc. The perfect match of the FCC Fe and Pt lattices favors the low energy state of the interface boundary. In the equilibrium state, the two phases should be Fe+? and Llo FePt. The appearance of disordered FCC Fe is associated with the as-deposited state of the film, the insufficient annealing time and the low ordering temperature of the Fe3Pt phase. The final morphology of the second phase in the annealed film could be affected by the initial thickness of the Fe layer and Pt layer. It is suggested that initial thinner Fe and Pt layer might facilitate finer grains of the disordered FCC Fe phase. The grain size of the hard phase is affected by the annealing temperature. Lower annealing temperature resulted in smaller gain size of both the hard phase and the soft phase. To increase the energy product, coercivity and magnetization must be adjusted to the optimum. Coercivity is strongly affected by the microstructure. Texture which enh anisotropy of polycrystalline materials is preferred to increase coercivity. The be improved by increasing the volume fraction of the soft phase while the soft phase unchanged.
CONCLUSIONS In this paper, we investigated the relation between film composition, processing, microstructure and magnetic properties relation. Two phases: the Llo FePt phase and disordered FCC Fe phase were identified to be the hard phase and soft phase, respectively. The grain sizes of both the hard phase and soft phase can be controlled by post-thermal processing. The magnetization can be controlled by the Fe content. Maximum energy product was obtained from films with two phases the Llo phase and the disordered FCC phase generated by rapid thermal processing. Effective coupling between soft phase and hard phase is responsible for the high energy product in this alloy system. Acknowledgment: Authors wish to thank Xueli Zhao for preparing the TEM samples. This work was performed at the CMRA Central Facility for Electron Microscopy, and supported by Department of Energy, Grant Number DE-FG-02-86ER45262, References 1. T. Klemmer, D.Hoydick, H.Okumura, B. Zhang, and W.A. Soffa, Ser. Metall. Muter. (1995), 33, 1793. 2. K. Watanabe and H.Masumoto, J. Jpn,. Insfi. Mefuls (1984), 48,930. 3. S. W. Yung, Y.H. Chang, T. J. Lin, and M.P. Hung, J.Mugn. Mugn. Muter. (1992), 116, 411. 4. M. Watanabe and M.Homma, Jpn. J. Appl. Phys., Part 2, (1996), 35, L1264. 5. J. P.Liu, Y. Liu , C. Lou, Z.S. Shan and D.J.Sellmyer, J. Appl. Phys.,( 1997), 81, 5644. 6. J. P. Liu, C.P. Luo, Y. Liu and D.J. Sellmyer, Appl. Phys. Let., 1998, 72, 483. 7. R. Skomski, and J. M. D. Coey, Phys. Rev. B, 1993,48, 15 812. 8. Y. Liu, J. P. Liu and D. J. Sellmyer, Electron Microscopy 1998, Cancun, Mexico, 2, 305. 9. D. Lesley-Pelecky et al. to be published. 10. A.F. Jankowski and M.A. Wall, NunoStructured Materials, 1996, 7, 89.
Pergamon
PI1 SO965-9773(99)00292-S
NANOSTRUCTURE OF EXCHANGE COUPLED HARD/SOFT FePt COMPOSITE FILMS Y. Liu*, J. P. Liu** and D. J. Sellmyer** * Center for Materials Research and Analysis and Department of Mechanical Engineering, ** Center for Materials Research and Analysis and Behlen Laboratory of Physics, University of Nebraska, Lincoln, NE 68588. Abstract -- Multilayers of Fe and Pt were deposited by sputtering. The ratio of the thickness of Fe to Pt was adjusted such that stoichiometric FePt and Fe rich films were formed. These films were either annealed at XWC and above, or processed by rapid thermal annealing (RTA) using a high Ipower lamp. Both the Fe and Pt in the as-deposited films were found to be FCC structure. After heat treatment, Llo phase (hard phase) and disordered FCC phase (soft phase) were identified in Fe-rich films. Small precipitates of 3-8 nm in the matrix which has larger grains of about 50-200 nm are observed. Perfect coherent interface between the hard phase and soft phase is observed. Processing route, microstructure and properties relation will be discussed. 01999 Acta kfetallurgica Inc.
INTRODUCTION In the Fe-Pt system several magnetic phases exist. Among these phases, Fe solid solution and :Fe# are soft phases having high magnetization. The Llo type structure FePt has a layered structure in the [OOl] direction and a very high anisotropy of 6.6~10~ ergs/cc [l]. High energy produ~ctsfrom 15 MGGe to 30 MGGe have been achieved by several groups [2-4]. Recently, using multi-layer sputtering technique combined with post-annealing and rapid thermal annealing, energy products up to 50 MGGe have been achieved [5,6]. Theoretical calculations showed that when the dimension of the soft phase is smaller than two times the domain wall thickness of the hard phase, effective inter-grain coupling can be achieved [7]. In this paper we present phase identification and volume fraction measurement of the hard phase and soft phase. Their relation with magnetic properties will be discussed.
EXPERIMENTAL
PROCEDURE
Multilayers of Fe and Pt were deposited by sputtering. The ratio of the thickness of Fe to Pt was adjusted such that stoichiometric FePt and Fe rich films were formed. One group of these films were then annealed at 500°C and above. Another group of films were processed by rapid thermal annealing (RTA) using a high power lamp. Magnetization loops were measured by a Micromag 2900 alternating gradient force magnetometer. Plan-view transmission electron microscopy (TEM) samples were prepared by dimpling and ion milling process. TEM study was conducted using a JEOL 2010 transmission electron microscope. 1027
FOURTHINTERNATIONAL CONFERENCEON NAMXTRUCTUREDMATERIALS
1028
RESULT AND DISCUSSION Figure 1 compares the selected area diffraction (SAD) patterns (a) from an as-deposited film and (b) from an annealed film. Although pure Fe at room temperature is BCC structure, both the Pt and Fe in the as-deposited FVFe multi-layers have the FCC structure as shown in Figure 1 (a). The pattern of the annealed film in Figure 1 (b) can be matched to the Llo structure. A second phase has been identified by nanodiffraction [8] to be a disordered FCC phase which does not contribute to any additional lines in Figure 1 (b).
100
a
111
30 70
70 0
so 40
b
111
!Jo30-
90
40
loo,
I
60. 3
200
50 40-
I
Figure 1. Comparison of SAD patterns (a) from as-deposited film, (b) from annealed film (No2 listed in Table 1).
Figure 2 (a) shows the microstructure of rapid thermal processed film (film No4 in Table 1). The soft phase has the spherical shape and is distributed in the hard phase. High resolution transmission electron microscopy (HRTEM) image in Figure 2 (b) shows that the soft phase and the hard phase have coherent interphase boundary. The processing route, microstructure and properties relation are summan‘zed in Table 1. Conventional heat treatment results in larger grains of both the soft phase and the hard phase in contrast to the rapid thermal processing. The volume fraction of the soft phase increases with Fe content. However, when the Fe content is close the ratio of FeRt - 2/l, the morphology of the soft phase change from smaller spherical shape to grains with the same dimension of the hard phase.
FOURTH INTERNATIONAL CONFERENCEON NAN~STRUCTUREDMATERIALS
1029
Figure 2 (a) bright field TEM image, (b) HRTEM image. The film is No4 listed in Table Table 1. Summary of processing, microstructure and magnetic properties of Fe-Pt films.
1030
FOURTH INTERNATIONAL CONFERENCEON NAN~STRIJCTUREDMATERIALS
It is known that in the film form several elements exhibit a different structure from the equilibrium phase. Such examples are FCC Co [9], FCC Ti [lo] etc. The perfect match of the FCC Fe and Pt lattices favors the low energy state of the interface boundary. In the equilibrium state, the two phases should be Fe+? and Llo FePt. The appearance of disordered FCC Fe is associated with the as-deposited state of the film, the insufficient annealing time and the low ordering temperature of the Fe3Pt phase. The final morphology of the second phase in the annealed film could be affected by the initial thickness of the Fe layer and Pt layer. It is suggested that initial thinner Fe and Pt layer might facilitate finer grains of the disordered FCC Fe phase. The grain size of the hard phase is affected by the annealing temperature. Lower annealing temperature resulted in smaller gain size of both the hard phase and the soft phase. To increase the energy product, coercivity and magnetization must be adjusted to the optimum. Coercivity is strongly affected by the microstructure. Texture which enh anisotropy of polycrystalline materials is preferred to increase coercivity. The be improved by increasing the volume fraction of the soft phase while the soft phase unchanged.
CONCLUSIONS In this paper, we investigated the relation between film composition, processing, microstructure and magnetic properties relation. Two phases: the Llo FePt phase and disordered FCC Fe phase were identified to be the hard phase and soft phase, respectively. The grain sizes of both the hard phase and soft phase can be controlled by post-thermal processing. The magnetization can be controlled by the Fe content. Maximum energy product was obtained from films with two phases the Llo phase and the disordered FCC phase generated by rapid thermal processing. Effective coupling between soft phase and hard phase is responsible for the high energy product in this alloy system. Acknowledgment: Authors wish to thank Xueli Zhao for preparing the TEM samples. This work was performed at the CMRA Central Facility for Electron Microscopy, and supported by Department of Energy, Grant Number DE-FG-02-86ER45262, References 1. T. Klemmer, D.Hoydick, H.Okumura, B. Zhang, and W.A. Soffa, Ser. Metall. Muter. (1995), 33, 1793. 2. K. Watanabe and H.Masumoto, J. Jpn,. Insfi. Mefuls (1984), 48,930. 3. S. W. Yung, Y.H. Chang, T. J. Lin, and M.P. Hung, J.Mugn. Mugn. Muter. (1992), 116, 411. 4. M. Watanabe and M.Homma, Jpn. J. Appl. Phys., Part 2, (1996), 35, L1264. 5. J. P.Liu, Y. Liu , C. Lou, Z.S. Shan and D.J.Sellmyer, J. Appl. Phys.,( 1997), 81, 5644. 6. J. P. Liu, C.P. Luo, Y. Liu and D.J. Sellmyer, Appl. Phys. Let., 1998, 72, 483. 7. R. Skomski, and J. M. D. Coey, Phys. Rev. B, 1993,48, 15 812. 8. Y. Liu, J. P. Liu and D. J. Sellmyer, Electron Microscopy 1998, Cancun, Mexico, 2, 305. 9. D. Lesley-Pelecky et al. to be published. 10. A.F. Jankowski and M.A. Wall, NunoStructured Materials, 1996, 7, 89.