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Ag granular materials studied by EXAFS
ELSEVIER
Physica B 208&209 (1995) 614-616
Nanometric structure of Fe clusters in Fe/Cu and Fe/Ag granular materials studied by EXAFS Masaki Sakurai*, Salah A. Makhlouf, Takehiko Hihara, Kenji Sumiyama, Kimio Wakoh, Kenji Suzuki Institute for Materials Research, Tohoku Universi~, Katahira 2-1-1, Aoba-Ku, Sendai 980, Japan
Abstract The local structure around Fe in Fe/Cu and Fe/Ag granular materials has been studied by means of extended X-ray absorption fine structure (EXAFS). In the Cu matrix, the structure of the Fe cluster is FCC for Fe concentrations less than 22 at%, while it is BCC for more than 35 at% Fe. In the Ag matrix, on the other hand, the structure of the Fe cluster is BCC even for Fe concentrations down to 12 at%.
1. Introduction
2. Experimental
Granular materials comprising small clusters embedded in metallic or ceramic matrices have been attracting great interest because their structure and physical properties may be designed and controlled on a nanometric scale [1]. Fe/Ag and Fe/Cu granular films produced by cluster beams (CB) reveal a giant magnetoresistance (GMR) effect without any thermal treatment [2]. The local structure of Fe clusters predominates over their magnetic and electrical properties, however, conventional X-ray diffraction and transmission electron microscopy (TEM) studies are not effective to understand their nanometric structures due to the small differences from the homogeneous systems. The present paper describes the local structure of nanometric Fe clusters embedded in Cu and Ag matrices and discuss their relation to GMR.
Fe clusters embedded Fe/Ag and Fe/Cu granular films of 2000 4000/~ in thickness were prepared by simultaneous codeposition of Fe clusters and Ag or Cu atoms on polyimide film substrates. The Fe K-edge X-ray absorption spectra were measured at the beam line 7C of the Photon Factory in the National Laboratory for High Energy Physics (KEK) with a Si(1 1 1) double crystal monochromator and double mirror. The storage ring energy is 2.5 GeV and positron current is 250-350 mA. Fluorescence detection mode was used for Fe/Ag films using an ionization chamber for detection of fluorescence X-rays from the sample. A Mn filter and soller slits were inserted between the sample and the detector to minimize the intensity of scattering X-rays. Transmission mode was used for Fe/Cu films whose thickness was adjusted by the number of layers. All measurements were done at room temperature. The program EXAFS2 [3] was applied for EXAFS data analyses. More details are described elsewhere
* Corresponding author.
[4].
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 7 0 - 5
615
M. Sakurai et al./Physica B 208&209 (1995) 614-616
3. Results and discussion
Ag - Fe ICB deposited
Fig. 1 shows the k3-weighted Fourier transform of Fe K-edge EXAFS for Fe/Cu granular films. The atomic pair correlation (APC) can be detected clearly up to 6 ~, (5th nearest neighbour), corresponding to the same FCC structure as pure Cu for Fe content less than 22 at%. The APC corresponds to the BCC structure for Fe content more than 35 at%, in contrast to the FCC structure of the Fe¢sCuss alloy made by co-sputtering [5], implying that BCC Fe clusters are preferably formed down to a lower Fe concentration range in the Cu matrix by CB deposition. Fig. 2 shows the ka-weighted Fourier transform of Fe K-edge EXAFS for Fe/Ag granular films. All IF (r)l are similar to that of the BCC structure of Fe in the Fe/Ag fillms, although their peaks are small and their widths broad. The marked difference in the structure of Fe clusters between Fe/Ag and Fe/Cu films with low Fe contents is ascribable to the structure and size of the surrounding matrices, as briefly explained below. In the Fe/Cu system, the size-mismatch between the radii of Fe and Cu is rather small, and Fe and Cu show a few at% miscibility in the solid state and a wide miscibility in the liquid state. In the Fe/Ag system, on the other hand, the size-mismatch between the radii of Fe and Ag is very large, and Fe and Ag show almost no miscibility in the solid- and liquid-states. Since the size
,•
B v II
Fe - Cu ICB deposited Fe K-edge
,9.o g) ¢...
.D .
.
.
.
0 1.1_
"6
Fe K-edge
U.
E o
(d) Pure Fe 20
~ 0 0
~
5
"0 "3
~r-- 5 ~ 0 5 0 1
2
3
Distance
4
5
r / A
Fig. 2. k3-weighted Fourier transform of Fe K-edge EXAFS, IF(r)l for Fe/Ag granular film. The k range for the Fourier transform is 3.5 13.5~, 1.
distribution of the initial clusters emitted from the CB source is not different between the F e / C u and Fe/Ag films under similar deposition conditions, the size and structure of the deposited clusters are determined during the deposition process. Fe clusters aggregate with each other on the substrate. When covered by a Cu matrix, they are intermixed at the interface, where the Fe clusters are FCC as long as their size is so small to keep the lattice coherency with the FCC Cu matrix. Fe clusters in the Ag matrix, on the contrary, are not well intermixed at the interface, and aggregation of Fe clusters is accelerated due to the immiscibility of Fe with Ag. Owing to the large size mismatch between Fe and Ag, it is difficult to maintain the lattice coherency between the Fe cluster and Ag matrix. In order to understand the detail of evolution mechanisms in Fe/Ag and Fe/Cu granular materials, we are studying EXAFS spectra of Ag K- and Cu K-edges and their XANES spectra.
.x2_ ¢-
:~
2o
Acknowledgements
10 0
I
I
I
I
I
I
1
2 3 4 Distance r / A
5
6
Fig. 1. ka-weighted Fourier transform of Fe K-edge EXAFS IF(r)[ for Fe/Cu granular films. The k range for the Fourier transfrom is 3.5 15.5~, 1.
This work was performed under the approval of the Photon Factory Program Advisory Committee (accepted No. 93G017). This work was supported by the Grant-inaid for Scientific Research (No. 04750600) and the New Frontier Program Grant-in-aid for Scientific Research
616
M. Sakurai et aL /Physica B 208&209 (1995) 614-616
(No.05NP0501) from the Ministry of Education, Science and Culture.
References [1] G. Xiao and C.L. Chien, Appl. Phys. Lett. 51 (1987) 1280. I-2] S.A. Makhlouf, K. Sumiyama, T. Kamiyama, K. Wakoh and K. Suzuki, to be published in Mater. Sci. Eng.
[3] N. Kosugi and H. Kuroda, Program EXAFS2/V3: Res. Cent. Spectrochem., Fac. Sci., Univ. Tokyo, 1987. [4] M. Sakurai, S.A. Makhlouf, K. Sumiyama, K. Wakoh and K. Suzuki, Jpn. J. Appl. Phys. 33 (1994) 4090. [5] K. Sumiyama, K. Nishi, H. Yasuda, T. Tanaka and S. Yoshida, Phys. Stat. Sol. (a) 132 (1992) 269.
Physica B 208&209 (1995) 614-616
Nanometric structure of Fe clusters in Fe/Cu and Fe/Ag granular materials studied by EXAFS Masaki Sakurai*, Salah A. Makhlouf, Takehiko Hihara, Kenji Sumiyama, Kimio Wakoh, Kenji Suzuki Institute for Materials Research, Tohoku Universi~, Katahira 2-1-1, Aoba-Ku, Sendai 980, Japan
Abstract The local structure around Fe in Fe/Cu and Fe/Ag granular materials has been studied by means of extended X-ray absorption fine structure (EXAFS). In the Cu matrix, the structure of the Fe cluster is FCC for Fe concentrations less than 22 at%, while it is BCC for more than 35 at% Fe. In the Ag matrix, on the other hand, the structure of the Fe cluster is BCC even for Fe concentrations down to 12 at%.
1. Introduction
2. Experimental
Granular materials comprising small clusters embedded in metallic or ceramic matrices have been attracting great interest because their structure and physical properties may be designed and controlled on a nanometric scale [1]. Fe/Ag and Fe/Cu granular films produced by cluster beams (CB) reveal a giant magnetoresistance (GMR) effect without any thermal treatment [2]. The local structure of Fe clusters predominates over their magnetic and electrical properties, however, conventional X-ray diffraction and transmission electron microscopy (TEM) studies are not effective to understand their nanometric structures due to the small differences from the homogeneous systems. The present paper describes the local structure of nanometric Fe clusters embedded in Cu and Ag matrices and discuss their relation to GMR.
Fe clusters embedded Fe/Ag and Fe/Cu granular films of 2000 4000/~ in thickness were prepared by simultaneous codeposition of Fe clusters and Ag or Cu atoms on polyimide film substrates. The Fe K-edge X-ray absorption spectra were measured at the beam line 7C of the Photon Factory in the National Laboratory for High Energy Physics (KEK) with a Si(1 1 1) double crystal monochromator and double mirror. The storage ring energy is 2.5 GeV and positron current is 250-350 mA. Fluorescence detection mode was used for Fe/Ag films using an ionization chamber for detection of fluorescence X-rays from the sample. A Mn filter and soller slits were inserted between the sample and the detector to minimize the intensity of scattering X-rays. Transmission mode was used for Fe/Cu films whose thickness was adjusted by the number of layers. All measurements were done at room temperature. The program EXAFS2 [3] was applied for EXAFS data analyses. More details are described elsewhere
* Corresponding author.
[4].
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 7 0 - 5
615
M. Sakurai et al./Physica B 208&209 (1995) 614-616
3. Results and discussion
Ag - Fe ICB deposited
Fig. 1 shows the k3-weighted Fourier transform of Fe K-edge EXAFS for Fe/Cu granular films. The atomic pair correlation (APC) can be detected clearly up to 6 ~, (5th nearest neighbour), corresponding to the same FCC structure as pure Cu for Fe content less than 22 at%. The APC corresponds to the BCC structure for Fe content more than 35 at%, in contrast to the FCC structure of the Fe¢sCuss alloy made by co-sputtering [5], implying that BCC Fe clusters are preferably formed down to a lower Fe concentration range in the Cu matrix by CB deposition. Fig. 2 shows the ka-weighted Fourier transform of Fe K-edge EXAFS for Fe/Ag granular films. All IF (r)l are similar to that of the BCC structure of Fe in the Fe/Ag fillms, although their peaks are small and their widths broad. The marked difference in the structure of Fe clusters between Fe/Ag and Fe/Cu films with low Fe contents is ascribable to the structure and size of the surrounding matrices, as briefly explained below. In the Fe/Cu system, the size-mismatch between the radii of Fe and Cu is rather small, and Fe and Cu show a few at% miscibility in the solid state and a wide miscibility in the liquid state. In the Fe/Ag system, on the other hand, the size-mismatch between the radii of Fe and Ag is very large, and Fe and Ag show almost no miscibility in the solid- and liquid-states. Since the size
,•
B v II
Fe - Cu ICB deposited Fe K-edge
,9.o g) ¢...
.D .
.
.
.
0 1.1_
"6
Fe K-edge
U.
E o
(d) Pure Fe 20
~ 0 0
~
5
"0 "3
~r-- 5 ~ 0 5 0 1
2
3
Distance
4
5
r / A
Fig. 2. k3-weighted Fourier transform of Fe K-edge EXAFS, IF(r)l for Fe/Ag granular film. The k range for the Fourier transform is 3.5 13.5~, 1.
distribution of the initial clusters emitted from the CB source is not different between the F e / C u and Fe/Ag films under similar deposition conditions, the size and structure of the deposited clusters are determined during the deposition process. Fe clusters aggregate with each other on the substrate. When covered by a Cu matrix, they are intermixed at the interface, where the Fe clusters are FCC as long as their size is so small to keep the lattice coherency with the FCC Cu matrix. Fe clusters in the Ag matrix, on the contrary, are not well intermixed at the interface, and aggregation of Fe clusters is accelerated due to the immiscibility of Fe with Ag. Owing to the large size mismatch between Fe and Ag, it is difficult to maintain the lattice coherency between the Fe cluster and Ag matrix. In order to understand the detail of evolution mechanisms in Fe/Ag and Fe/Cu granular materials, we are studying EXAFS spectra of Ag K- and Cu K-edges and their XANES spectra.
.x2_ ¢-
:~
2o
Acknowledgements
10 0
I
I
I
I
I
I
1
2 3 4 Distance r / A
5
6
Fig. 1. ka-weighted Fourier transform of Fe K-edge EXAFS IF(r)[ for Fe/Cu granular films. The k range for the Fourier transfrom is 3.5 15.5~, 1.
This work was performed under the approval of the Photon Factory Program Advisory Committee (accepted No. 93G017). This work was supported by the Grant-inaid for Scientific Research (No. 04750600) and the New Frontier Program Grant-in-aid for Scientific Research
616
M. Sakurai et aL /Physica B 208&209 (1995) 614-616
(No.05NP0501) from the Ministry of Education, Science and Culture.
References [1] G. Xiao and C.L. Chien, Appl. Phys. Lett. 51 (1987) 1280. I-2] S.A. Makhlouf, K. Sumiyama, T. Kamiyama, K. Wakoh and K. Suzuki, to be published in Mater. Sci. Eng.
[3] N. Kosugi and H. Kuroda, Program EXAFS2/V3: Res. Cent. Spectrochem., Fac. Sci., Univ. Tokyo, 1987. [4] M. Sakurai, S.A. Makhlouf, K. Sumiyama, K. Wakoh and K. Suzuki, Jpn. J. Appl. Phys. 33 (1994) 4090. [5] K. Sumiyama, K. Nishi, H. Yasuda, T. Tanaka and S. Yoshida, Phys. Stat. Sol. (a) 132 (1992) 269.