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Zr multilayers
Journal of Magnetism and Magnetic Materials 126 (1993) 128-130 North-Holland
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Annealing effect on structure of F e / Z r multilayers K. Y a m a m o t o a, T. N a k a y a m a ", H. Satoh ~, T.J. K o n n o t, and R. Sinclair b Materials Research Laboratory, Kobe Steel Ltd, Hyogo 651-22, Japan b Department of Materials Science and Engineering, Stanford Unit'ersitv Stanford, CA 94305. USA
F e / Z r multilayers with various modulation wavelenghts (A) were synthesized by sputter deposition. A structure change due to solid state amorphization and crystallization was studied using X-ray diffraction, HRTEM and RBS techniques, h was found that the structure change depends significantly on the annealing temperature and the modulation wavelength. 1. Introduction
Solid state amorphization p h e n o m e n a have been reported in many multilayer systems [1-3]. It attracts a great deal of attention as a new method to synthesize amorphous materials. F e / Z r multilayers is also one of these systems of interest. Previously we reported the effect of the modulation wavelength on the structure of the F e / Z r multilaycrs [4]. It was found in the previous study that for modulation wavelengths smaller than 40 ,~, F e / Z r multilayers were in amorphous state, and for longer modulation wavelengths the samples were in crystalline state with amorphous interfaces. The dependence of the amorphization behavior on the modulation wavelength is also important. In this study, solid state amorphization and subsequent crystallization of F e / Z r multilayers with various modulation wavelengths are investigated using X-ray diffraction, H R T E M and RBS techniques. 2. Experimental
A series of F e / Z r multilayers were deposited onto glass substrates by dc magnetron sputtering. The modulation wavelength of the multilayers was varied from 8 to 920 A. Details of the sputering process are described elsewhere [5]. The composition of the samples was measured by inductively coupled plasma mass spectrometry and varied from 40 to 51 at% Zr. These samples were annealed at 350-600 ° C for 2 h in a high vacuum furnace at a pressure of better than 3 x 10 ~ Torr. For all samples both high- and low-angle X-ray diffraction were carried out to characterize the structure of F e / Z r multilayers in as-deposited and annealed states. On some of these samples high-resolu-
Correspondence to. K. Yamamoto, Materials Research Laboratory, Kobe Steel Ltd, 5-5, Takatukadai, 1-Chome, Nishiku, Kobe, Hyogo 651-22, Japan. Fax: 078-992-5512.
tion transmission electron microscopy ( H R T E M ) and Rutherford backscattering (RBS) were also performed to examine the change of the modulation structure. 3. Results and discussion
Fig. l(a) shows an example of high-angle X-ray diffraction patterns of as-deposited samples. The samples with modulation wavelengths of 8 and 16 ,~ reveal a single broad maximum typical of an amorphous material. For the longer A, Zr(002) and Fe(110) peaks are observed in the high angle region indicating that these samples are crystalline. The crystalline bcc iron phase is strongly textured in the (110) orientation, while the hcp Zr has an (002) orientation with a small amount of (101) component. In low-angle X-ray diffraction patterns, superlattice lines due to composition modulation were observed for all samples. Fig. l(b) shows some of high-angle X-ray diffraction pattern of the samples annealed at 400°C for 2 h. F e ( l l 0 ) and Zr(002) peak,s disappeared for the samples with A - 9 0 and 180 A and a single broad peak is observed. It indicates the transformation from crystalline Fe and Zr to an amorphous F e / Z r alloy. For the sample with ~1 = 18I) A, however, a weak Fe(110) peak is seen, indicating that a small amount of crystalline Fe still remains. For the samples with ,'1 = 920 ,~, a F e Z r 4 crystalline compound was recognized. This suggests a multilayer with wellseparated interface directly transforms to a crystalline F e / Z r alloy. For the samples with A smaller than 9(1 A no significant variation is sccn in the X-ray diffraction patterns. For the sample with A = 18(1 A the Fe(11()) peak disappeared upon annealing at 500 o C for 2 h, indicating the completion of amorphization. No significant change was secn for other samples annealed at 500 ° C for 2 h. Fig. l(c) shows some high-angle X-ray diffraction patterns of the samples annealed al 600 ° C tk)r 2 h. For the samples with A > 40 A,, a mixture of crystalline F e / Z r compounds is observed as the major phases in the diffraction pattern. These phases arc
0304-8853/93/$06.(10 © 1993 - Elsevier Science Publishers B.V. (North-Holland)
K. Yamamoto et al. /Annealing effect on structure o f Fe / Zr '
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OFeZr3 II FeZr 2 •Fe2Zr
.=
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40
50
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30
40
20/CuK~
20/CuK~
20/CuK~t
(a)
(b)
(e)
50
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Fig. 1. Result of high-angle X-ray diffraction. (a) As-deposited, (b) annealed at 400°C for 2 h, and (c) annealed at 600°C for 2 h. identified as FeZr 3, FeZr 2 and Fe2Zr. For the samples with A < 40 A, diffraction from crystalline F e / Z r compounds was very weak, indicating that they are a mixture of an amorphous phase and a small amount of crystalline F e / Z r compound. Figs. 2(a)-(c) show cross-sectional HRTEM micrographs of the sample with A = 90 A. These micrographs are as-deposited, annealed at 400 and 600°C, respectively. In fig. 2(a), we can see a clear composition modulation of Fe and Zr. Grains of Fe and Zr are visible, indicating each layer is crystalline. This agrees with the results of X-ray diffraction. At the interface of the Fe and Zr layers, a small amount of amorphous phase is observed. As shown in fig. 2(b), for the sample annealed at 400°C, the modulation structure is lost completely and, as confirmed by X-ray diffraction, and only an amorphous phase is seen. This amorphous
phase was thought to transform to crystalline F e / Z r compound during annealing at 600°C. Large grains of crystalline F e / Z r compound are clearly seen in fig. 2(c). A small amount of crystalline phase is also seen in the remaining amorphous matrix. From electron diffraction patterns, we confirmed that these grains have a strong texture. In fig. 3 the annealing effect on the RBS spectra of the sample with A = 920/k is shown as a function of annealing temperature. A composition modulation is clearly observed for the spectrum of the as-deposited sample. Because the thicknesses of the Fe and Zr layers are nearly identical for this sample, peaks from Fe and Zr layers are almost superpose& On increasing the annealing temperature, the composition modulation gradually disappears due to the interdiffusion of the layers. For the sample annealed at 500°C, only a
130
K. Yamamoto et al. /Annealing effect on structure of Fe / Zr
Fig. 2. Cross-sectional HRTEM micrograph of F e / Z r multilayer with A = 90 A. (a) As-deposited, (b) annealed at 400°C for 2 h, and (c) annealed at 600°C for 2 h. peak of the o u t e r m o s t Z r layer is distinguishable. This suggests that no m o d u l a t i o n structure exists after annealing at 500°C for 2 h.
4. Conclusions In this study, we observed the a n n e a l i n g effect on the structure of F e / Z r multilayers. F e / Z r multilayers
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with A = 90 a n d 180 A exhibited a crystalline to amorp h o u s to crystalline transition upon increasing the annealing t e m p e r a t u r e . T h e sample with A = 920 showed a direct crystalline to crystalline transition at o 400°C. For the samples with A < 90 A, an a m o r p h o u s p h a s e was stable over the whole t e m p e r a t u r e range tested. For more complete u n d e r s t a n d i n g , considerations of the activation energy for the a m o r p h i z a t i o n and crystallization processes in c o n n e c t i o n with their m o d u l a t i o n wavelengths are required.
Acknowledgement: Support from the U S National Science F o u n d a t i o n (grant No. DMR-8902232) is much a p p r e c i a t e d (TJKRS).
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References
Annealed at 400 °C
~ ~ / X ~ ~ ~
(b)
N ~ 0
(e) 1 O0
200
300
400
Channel Fig. 3. RBS spectra for the F e / Z r multilayers with A = 920 ,~,. (a) As-deposited, (b) annealed at 400°C for 2 h, and (c) annealed at 500°C for 2 h.
[1] R.B. Schwartz, K.L. Wong, W.L. Johnson and B.M. Clemens, J. Non-Cryst. Solids 61&62 (1984) 129. [2] B.M. Clemens and M.J. Suchoski, Appl. Phys. Lett. 47 (1986) 943. [3] R.B. Schwartz and W.L. Johnson, Phys. Rev. Len. 51 (1983) 415. [4] Nakayama et al. Proc. 38th Jpn. Corrosion Conf., c-107 (1991) p. 193. [5] T.W. Barbee and D.L. Keith, in: Synthesis of Metastable Materials by Sputter Deposition Techniques (Metallugical Society of AIME, 1993) p. 113.
Ai la
Annealing effect on structure of F e / Z r multilayers K. Y a m a m o t o a, T. N a k a y a m a ", H. Satoh ~, T.J. K o n n o t, and R. Sinclair b Materials Research Laboratory, Kobe Steel Ltd, Hyogo 651-22, Japan b Department of Materials Science and Engineering, Stanford Unit'ersitv Stanford, CA 94305. USA
F e / Z r multilayers with various modulation wavelenghts (A) were synthesized by sputter deposition. A structure change due to solid state amorphization and crystallization was studied using X-ray diffraction, HRTEM and RBS techniques, h was found that the structure change depends significantly on the annealing temperature and the modulation wavelength. 1. Introduction
Solid state amorphization p h e n o m e n a have been reported in many multilayer systems [1-3]. It attracts a great deal of attention as a new method to synthesize amorphous materials. F e / Z r multilayers is also one of these systems of interest. Previously we reported the effect of the modulation wavelength on the structure of the F e / Z r multilaycrs [4]. It was found in the previous study that for modulation wavelengths smaller than 40 ,~, F e / Z r multilayers were in amorphous state, and for longer modulation wavelengths the samples were in crystalline state with amorphous interfaces. The dependence of the amorphization behavior on the modulation wavelength is also important. In this study, solid state amorphization and subsequent crystallization of F e / Z r multilayers with various modulation wavelengths are investigated using X-ray diffraction, H R T E M and RBS techniques. 2. Experimental
A series of F e / Z r multilayers were deposited onto glass substrates by dc magnetron sputtering. The modulation wavelength of the multilayers was varied from 8 to 920 A. Details of the sputering process are described elsewhere [5]. The composition of the samples was measured by inductively coupled plasma mass spectrometry and varied from 40 to 51 at% Zr. These samples were annealed at 350-600 ° C for 2 h in a high vacuum furnace at a pressure of better than 3 x 10 ~ Torr. For all samples both high- and low-angle X-ray diffraction were carried out to characterize the structure of F e / Z r multilayers in as-deposited and annealed states. On some of these samples high-resolu-
Correspondence to. K. Yamamoto, Materials Research Laboratory, Kobe Steel Ltd, 5-5, Takatukadai, 1-Chome, Nishiku, Kobe, Hyogo 651-22, Japan. Fax: 078-992-5512.
tion transmission electron microscopy ( H R T E M ) and Rutherford backscattering (RBS) were also performed to examine the change of the modulation structure. 3. Results and discussion
Fig. l(a) shows an example of high-angle X-ray diffraction patterns of as-deposited samples. The samples with modulation wavelengths of 8 and 16 ,~ reveal a single broad maximum typical of an amorphous material. For the longer A, Zr(002) and Fe(110) peaks are observed in the high angle region indicating that these samples are crystalline. The crystalline bcc iron phase is strongly textured in the (110) orientation, while the hcp Zr has an (002) orientation with a small amount of (101) component. In low-angle X-ray diffraction patterns, superlattice lines due to composition modulation were observed for all samples. Fig. l(b) shows some of high-angle X-ray diffraction pattern of the samples annealed at 400°C for 2 h. F e ( l l 0 ) and Zr(002) peak,s disappeared for the samples with A - 9 0 and 180 A and a single broad peak is observed. It indicates the transformation from crystalline Fe and Zr to an amorphous F e / Z r alloy. For the sample with ~1 = 18I) A, however, a weak Fe(110) peak is seen, indicating that a small amount of crystalline Fe still remains. For the samples with ,'1 = 920 ,~, a F e Z r 4 crystalline compound was recognized. This suggests a multilayer with wellseparated interface directly transforms to a crystalline F e / Z r alloy. For the samples with A smaller than 9(1 A no significant variation is sccn in the X-ray diffraction patterns. For the sample with A = 18(1 A the Fe(11()) peak disappeared upon annealing at 500 o C for 2 h, indicating the completion of amorphization. No significant change was secn for other samples annealed at 500 ° C for 2 h. Fig. l(c) shows some high-angle X-ray diffraction patterns of the samples annealed al 600 ° C tk)r 2 h. For the samples with A > 40 A,, a mixture of crystalline F e / Z r compounds is observed as the major phases in the diffraction pattern. These phases arc
0304-8853/93/$06.(10 © 1993 - Elsevier Science Publishers B.V. (North-Holland)
K. Yamamoto et al. /Annealing effect on structure o f Fe / Zr '
I
'
I
'
I
'
I
'
I
i~
I
129
'
'
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°i i~" la.
N
20
30
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]
FeZr3 iI OFeZr2 I 0
A=920A
s-
._./
vm
N
A=920A
•=
'
~ I/
•
~" tl °l~e , . ,
.~
L_. 1 8 o A
OFeZr3 II FeZr 2 •Fe2Zr
.=
~
40
50
60
20
30
40
50
60
20
30
40
20/CuK~
20/CuK~
20/CuK~t
(a)
(b)
(e)
50
60
Fig. 1. Result of high-angle X-ray diffraction. (a) As-deposited, (b) annealed at 400°C for 2 h, and (c) annealed at 600°C for 2 h. identified as FeZr 3, FeZr 2 and Fe2Zr. For the samples with A < 40 A, diffraction from crystalline F e / Z r compounds was very weak, indicating that they are a mixture of an amorphous phase and a small amount of crystalline F e / Z r compound. Figs. 2(a)-(c) show cross-sectional HRTEM micrographs of the sample with A = 90 A. These micrographs are as-deposited, annealed at 400 and 600°C, respectively. In fig. 2(a), we can see a clear composition modulation of Fe and Zr. Grains of Fe and Zr are visible, indicating each layer is crystalline. This agrees with the results of X-ray diffraction. At the interface of the Fe and Zr layers, a small amount of amorphous phase is observed. As shown in fig. 2(b), for the sample annealed at 400°C, the modulation structure is lost completely and, as confirmed by X-ray diffraction, and only an amorphous phase is seen. This amorphous
phase was thought to transform to crystalline F e / Z r compound during annealing at 600°C. Large grains of crystalline F e / Z r compound are clearly seen in fig. 2(c). A small amount of crystalline phase is also seen in the remaining amorphous matrix. From electron diffraction patterns, we confirmed that these grains have a strong texture. In fig. 3 the annealing effect on the RBS spectra of the sample with A = 920/k is shown as a function of annealing temperature. A composition modulation is clearly observed for the spectrum of the as-deposited sample. Because the thicknesses of the Fe and Zr layers are nearly identical for this sample, peaks from Fe and Zr layers are almost superpose& On increasing the annealing temperature, the composition modulation gradually disappears due to the interdiffusion of the layers. For the sample annealed at 500°C, only a
130
K. Yamamoto et al. /Annealing effect on structure of Fe / Zr
Fig. 2. Cross-sectional HRTEM micrograph of F e / Z r multilayer with A = 90 A. (a) As-deposited, (b) annealed at 400°C for 2 h, and (c) annealed at 600°C for 2 h. peak of the o u t e r m o s t Z r layer is distinguishable. This suggests that no m o d u l a t i o n structure exists after annealing at 500°C for 2 h.
4. Conclusions In this study, we observed the a n n e a l i n g effect on the structure of F e / Z r multilayers. F e / Z r multilayers
.-'.. ~ .: ] ~" ~ ;
"7-
i.-"
*
:A
As-Deposited
" .." "
~
Surface
.~
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....
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with A = 90 a n d 180 A exhibited a crystalline to amorp h o u s to crystalline transition upon increasing the annealing t e m p e r a t u r e . T h e sample with A = 920 showed a direct crystalline to crystalline transition at o 400°C. For the samples with A < 90 A, an a m o r p h o u s p h a s e was stable over the whole t e m p e r a t u r e range tested. For more complete u n d e r s t a n d i n g , considerations of the activation energy for the a m o r p h i z a t i o n and crystallization processes in c o n n e c t i o n with their m o d u l a t i o n wavelengths are required.
Acknowledgement: Support from the U S National Science F o u n d a t i o n (grant No. DMR-8902232) is much a p p r e c i a t e d (TJKRS).
,& \
~.~t~ / ~
~.
References
Annealed at 400 °C
~ ~ / X ~ ~ ~
(b)
N ~ 0
(e) 1 O0
200
300
400
Channel Fig. 3. RBS spectra for the F e / Z r multilayers with A = 920 ,~,. (a) As-deposited, (b) annealed at 400°C for 2 h, and (c) annealed at 500°C for 2 h.
[1] R.B. Schwartz, K.L. Wong, W.L. Johnson and B.M. Clemens, J. Non-Cryst. Solids 61&62 (1984) 129. [2] B.M. Clemens and M.J. Suchoski, Appl. Phys. Lett. 47 (1986) 943. [3] R.B. Schwartz and W.L. Johnson, Phys. Rev. Len. 51 (1983) 415. [4] Nakayama et al. Proc. 38th Jpn. Corrosion Conf., c-107 (1991) p. 193. [5] T.W. Barbee and D.L. Keith, in: Synthesis of Metastable Materials by Sputter Deposition Techniques (Metallugical Society of AIME, 1993) p. 113.