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Structural and magnetic properties of FeAg granular material
Materials Science and Engineering, A 179/A 180 (1994) 483-486
483
Structural and magnetic properties of Fe-Ag granular material Salah A. Makhlouf*, K. Sumiyama, T. Kamiyama, K. Wakoh and K. Suzuki Institute for Materials Research, Tohoku University, Sendai 980 (Japan)
Abstract The structural and magnetic properties, particularly the giant magnetoresistance (GMR), of Fe-cluster-dispersed Ag films fabricated by an ion cluster beam (1CB) technique are reported. The specimens were characterized by X-ray diffraction, small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). SAXS measurements show that the strong scattered intensities vary in shape and the radii of gyration are in the range 7-16 nm depending on the composition. TEM observations also show a grain size of about 20-30 nm. Giant magnetoresistance of 40% was observed without any heat treatment of the specimens. The magnetoresistance curves do not saturate even at 140 kOe although the magnetization saturates at a very low magnetic field.
1. Introduction There has been much interest in the preparation, characterization and physical properties of granular materials in which very small magnetic particles are dispersed in metallic and non-metallic matrices [1-3]. Fe and Co magnetic clusters have been precipitated in noble metal (NM) matrices such as Ag, Cu and Au [4]. Since this possibility is limited by the very narrow equilibrium solubility region [5], non-equilibrium homogeneous alloys prepared by vapor quenching are better candidates as precursory material for the preparation of highly dispersed granular materials [1]. The giant magnetoresistance (GMR) in magnetic multilayers [6-8] has received considerable attention as a consequence of the discovery of GMR in the antiferromagnetically coupled Fe/Cr multilayer system [6]. Recent reports, however, have explored the salient demonstration of GMR in granular materials, in which ferromagnetic particles are precipitated by annealing in non-magnetic matrices. Typical examples are the observation of GMR in immiscible type Co-Cu and Co-Ag films [9, 10]. The GMR effect has been ascribed to spin-dependent scattering mainly at the interface between the ferromagnetic particle and the non-magnetic matrix and, to lesser extent, to spindependent scattering within the particle [11]. The electron mean free paths, the ratio of spin-dependent and spin-independent scattering potentials and the particle size are the key parameters in this analysis. *Permanent address: Department of Physics, Faculty of Science, Assiut University, Assiut, Egypt. 0921-5093/94/$7.00 SSD1 0921-5093(93)05559-8
Recently we dispersed Fe clusters in an Ag matrix using an ion-cluster-beam (ICB) technique [12]. The M6ssbauer spectra at 4.2 K display a singlet component superimposed on ferromagnetic sextets regardless of the cluster size and the Fe content, although it has not been detected in the homogeneous f.c.c. Fe-Ag alloys [13] and Fe/Ag multilayers [14]. This paramagnetic component may be attributed to fluctuation in the Fe atom moments at the interface between Fe and Ag phases. In this context, the formation of Fe clusters in an Ag matrix using the ICB technique seems to be promising for GMR [15]. In this study we used X-ray diffraction, transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) to characterize the structure and morphology, and the magnetoresistance was measured at high fields to shed light on the origin of the GMR in granular materials.
2. Experimental procedures Granular materials a few thousand fingstr6ms in thickness, of Fe clusters embedded in an f.c.c. Ag matrix were prepared by simultaneous codeposition. In the cluster source, electron bombardment is used to heat and evaporate Fe in a zirconia crucible. Clusters are formed by Fe atoms colliding with each other during adiabatic expansion of the vaporized metal through the crucible's nozzle (1.4 mm in diameter and 1 mm in length) into a high vacuum of about 10 -6 Torr. Simultaneously, Ag atoms are thermally evaporated into the same region by heating the crucible. The chemical composition is estimated from energy-disper© 1994 - Elsevier Sequoia. All rights reserved
S. A. Makhlouf et al.
484
/
sive X-ray spectra using a detector installed in a transmission electron microscope. SAXS measurements were taken using a Cu Kct point focus X-ray beam 0.5 mm x 0.5 mm in dimension. Here, the count rate was corrected for the background with blank A1 foil as a reference specimen. The scattering intensity is estimated as a function of the scattering vector, h = 2~ sin 0/2, where ,t is the wavelength of the radiation and 20 is the scattering angle. Magnetoresistance measurements were taken using a conventional d.c. four-point probe method in magnetic fields up to 140 kOe at 4.2 K.
3. E x p e r i m e n t a l
results
As illustrated in Fig. 1, the X-ray diffraction patterns of the as-deposited Fe/Ag films indicate f.c.c, peaks. The Bragg peaks of Ag and Fe are too close to examine the mutual solubilities of Fe and Ag and the crystal structure of the Fe clusters. Moreover, the asymmetric and broad diffraction peaks are ascribed to compositional fluctuation, particle size distribution and the retained strain. For the compositional ratio [Fe]/ lAg]<50/50, several f.c.c, diffraction peaks are detected and the ( 111 ) line is predominant, indicating a preferred orientation of the grain growth. Figure 2 shows a typical microstructure and the corresponding electron diffraction patterns of sample D ([Fe]/[Ag]=45/55) obtained by TEM. The TEM image confirms the granular nature of the material, in the range 20-40 nm. Figure 3 illustrates the SAXS intensities I(h) presented on a log-log type plot as a function of the scattering vector h for the specimens with [Fe]/ [Ag] -- 20/80, 33/67 and 50/50. The obtained curves
Fe-Ag granular material
show the following features. (1) The higher scattering intensity corresponds to the composition ratio [Fe]/ tag] = 50/50. (2) A broad peak is observed for all compositions corresponding to h = 0.07 A-]. (3) A sharp increase in I(h) is observed at h < 0 . 0 4 A J. (4) Another peak at about 0.023 A-J is observed for [Fe]/ tAg] = 33/67. These features indicate the presence of two kinds of scatter in the specimens. The dimension of the smaller scatterer could not be determined owing to overlap of the high h peak with the main scattering intensity. Here, the peak position indicates that the interparticle distance is about 10 nm. The peak at 0.023 A-i for the [Fe]/[Ag] = 33/67 specimen corresponds to a scatterer with a gyration radius Rg -= 7 nm, separated by an interparticle distance of the order of 18 nm. The presence of more than one type of scatterer and the
100nrn D : Fe/Ag = 4 5 / 5 5
Fig. 2. TEM image and electron diffraction patterns of the [Fe]/ tAg] = 45/55 specimen.
10 s
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Fe/Ag = 20/80 Fe/Ag = 33/67 Fe/Ag=50/50
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~~ f / A ~ ~ "'~ ~.:12at.%Fe
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C: 33 at.% Fe
D: 45 at.% Fe
p-q
30
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40
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50
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70
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20 Fig. 1. X-ray diffraction patterns of Fe/Ag granular materials.
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~ ~ ~ ~
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h (~. t) Fig. 3. Scattered intensities for the Fe/Ag granular materials.
S. A. Makhloufet al. / Fe-Aggranularmaterial intense background due to fluorescence X-rays from Fe atoms are manifested in the high h tail in all of the I(h) curves, making it difficult to check the validity of the Porod plot (I(h)- h-4). However, the higher I(h) is observed for the [Fe]/[Ag] = 50/50 specimen, indicating enhancement of the difference in electron densities between the Fe clusters and surrounding Ag matrix. The gyration radius Rgof the larger scatterer is roughly estimated to be about 15 nm. This value is consistent with the TEM results. Figure 4 shows the magnetoresistance curves measured for the present samples at 4.2 K. The ordinate represents a percentage of the magnetoresistance ratio, Ap/p, as a function of an applied magnetic field H u p to 140 kOe. Here Ap/p is defined by the formula
Ap/p = [p(H) -lo(O)]/p(Hmax) where HmaX= 140 kOe. In sample A which has the lowest Fe content, Ap/p does not saturate. It increases monotonically with increasing H even at H = 140 kOe. Similar features are observed for sample D. In samples B and C, however, Ap/p first increases sharply with increasing H up to H = 2 0 kOe. With further increasing H, Ap/p increases linearly and is about 25% and 40% at H = 140 kOe for samples B and C respectively. As [Fe]/[Ag] approaches 50/50, A p / p sharply decreases.
4. Discussion The Fe-cluster dispersed Fe-Ag granular materials display a considerable non-saturation trend in the
- 10
~,~ - 20
t
- ' u O O o o o o ~
I
°~m~l b
485
magnetoresistance (MR) curves, particularly at high fields, whereas the corresponding magnetization easily saturates at low fields [15]. These features can be described as follows. The magnetization depends mainly on the magnetic moment at the core Fe atoms and to lesser extent on the Fe atoms at the interface of ferromagnetic clusters. MR, however, arises from spindependent scattering at the interface between the Fe cluster and Ag matrix and, to lesser extent, is due to the spin-dependent scattering within Fe clusters. The demagnetization factor leads to anisotropy in the GMR of multilayer systems, whereas the GMR has been found to be isotropic in granular systems [9, 10]. The latter results could be ascribed to the rotation of magnetization under an applied field. The resistance approaches its minimum value with the complete alignment of the magnetic moments of the ferromagnetic entities. However, both saturation- and non-saturationtype MR were observed in the present specimens, whose magnetization curves show almost the same saturation behavior. The rapid increase in Ap/p for samples B and C up to 20 kOe may be explained by the rotation of magnetization. However, the monotonic increase in Ap/p at H > 2 0 kOe for all specimens cannot be explained by this simple mechanism. The paramagnetic or superparamagnetic component observed in our Mrssbauer spectra even at 4.2 K may make a considerable contribution to the GMR in high fields.
5. Conclusion Fe-cluster-dispersed Fe-Ag granular materials were prepared using an ion cluster beam deposition technique. TEM and SAXS measurements proved the granular nature of the as-deposited specimens. Spindependent scattering at the Fe cluster surfaces gives rise to GMR. The specimens display unique nonsaturation behavior of GMR.
~
Q.
Acknowledgments
-30
-40 0
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20
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40
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I
80
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100
120
140
H (kOe) Fig. 4. Dependence on field of the magnetoresistance Ap/p obtained at 4.2 K of Fe/Ag granular materials. &, A, 12 at.% Fe; m, B, 26at.%Fe; ~, C, 33at.%Fe; e, D 45at.%Fe; ~g, E, 50 at.% Fe.
The authors wish to thank Mr. T. Hihara for his support in the MR experiment. They are also indebted for the MR measurements to the High Field Laboratory for Superconducting Materials in Tohoku University. This work was supported by the New Frontier Program Grant-in-aid for Scientific Research (No. 04NP0501) given by the Ministry of Education, Science and Culture of Japan. S. A. Makhlouf appreciates financial support from the Egyptian Ministry of Higher Education.
486
S. A. Makhlouf et al.
/
Fe-Ag granular material
References 1 J.R. Childress and C. L. Chien, Appl. Phys. Lett., 56 (1990) 95. 2 J. R. Childress and C. L. Chien, J. Appl. Phys., 70 (1991) 5885. 3 G. Xiao and C. L. Chien, Appl. Phys. Lett., 51 (1987) 1280. 4 G. Abersfelder, K. Naock, K. Stierstadt, J. Schelten and W. Schmatz, Philos. Mag. B, 41 (1980) 519. 5 T. B. Massalski, H. Okamoto, P. R. Subramanian and L. Kacprzak (eds.), Binary Alloy Phase Diagrams, ASM International, Metals Park, OH, 1990, 2nd edn. 6 M.N. Baibich, J. M. Broto, A. Fert, E N. Van Dau, E Petroff, P. Eitenne, G. Creuzet, A. Friederich and J. Chazelas, Phys. Rev. Lett., 61 (1988) 2472. 7 S. S. P. Parkin, N. More and K. P. Roche, Phys. Rev. Lett., 64 (1990) 2304.
8 S. Zhang and R M. Levy, J. Appl. Phys., 69 ( 1991 ) 4786. 9 A. E. Berkowitz, J. R. Mitchell, M. J. Carey, A. E Young, S. Zhang, F. E. Spada, F. T. Parker, A. Hutten and G. Thomas, Phys. Rev. Lett., 68(1992) 3745. 10 J. Q. Xiao, J. S. Jiang and C. L. Chien, Phys. Rev. B, 46 (1992) 9266. 11 S. Zhang, Appl. Phys. Lett., 61 (1992) 1855. 12 S.A. Makhlouf, K. Sumiyama, H. Onodera, K. Wakoh and K. Suzuki, Nuc. lnstrum. Methods B, 76(1993) 197. 13 N. Kataoka, K. Sumiyama and Y. Nakamura, Acta Metall., 37 (1989) 1135. 14 J. Q. Xiao, A. Gavrin, G. Xiao, J. R. Childress, W. A. Bryden and C. L. Chien, J. Appl. Phys., 67(1990) 5388. 15 S. A. Makhlouf, K. Sumiyama, K. Wakoh, K. Suzuki, K. Takanashi and H. Fujimori, J. Magn. Magn. Mater., 126 (1993)485.
483
Structural and magnetic properties of Fe-Ag granular material Salah A. Makhlouf*, K. Sumiyama, T. Kamiyama, K. Wakoh and K. Suzuki Institute for Materials Research, Tohoku University, Sendai 980 (Japan)
Abstract The structural and magnetic properties, particularly the giant magnetoresistance (GMR), of Fe-cluster-dispersed Ag films fabricated by an ion cluster beam (1CB) technique are reported. The specimens were characterized by X-ray diffraction, small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). SAXS measurements show that the strong scattered intensities vary in shape and the radii of gyration are in the range 7-16 nm depending on the composition. TEM observations also show a grain size of about 20-30 nm. Giant magnetoresistance of 40% was observed without any heat treatment of the specimens. The magnetoresistance curves do not saturate even at 140 kOe although the magnetization saturates at a very low magnetic field.
1. Introduction There has been much interest in the preparation, characterization and physical properties of granular materials in which very small magnetic particles are dispersed in metallic and non-metallic matrices [1-3]. Fe and Co magnetic clusters have been precipitated in noble metal (NM) matrices such as Ag, Cu and Au [4]. Since this possibility is limited by the very narrow equilibrium solubility region [5], non-equilibrium homogeneous alloys prepared by vapor quenching are better candidates as precursory material for the preparation of highly dispersed granular materials [1]. The giant magnetoresistance (GMR) in magnetic multilayers [6-8] has received considerable attention as a consequence of the discovery of GMR in the antiferromagnetically coupled Fe/Cr multilayer system [6]. Recent reports, however, have explored the salient demonstration of GMR in granular materials, in which ferromagnetic particles are precipitated by annealing in non-magnetic matrices. Typical examples are the observation of GMR in immiscible type Co-Cu and Co-Ag films [9, 10]. The GMR effect has been ascribed to spin-dependent scattering mainly at the interface between the ferromagnetic particle and the non-magnetic matrix and, to lesser extent, to spindependent scattering within the particle [11]. The electron mean free paths, the ratio of spin-dependent and spin-independent scattering potentials and the particle size are the key parameters in this analysis. *Permanent address: Department of Physics, Faculty of Science, Assiut University, Assiut, Egypt. 0921-5093/94/$7.00 SSD1 0921-5093(93)05559-8
Recently we dispersed Fe clusters in an Ag matrix using an ion-cluster-beam (ICB) technique [12]. The M6ssbauer spectra at 4.2 K display a singlet component superimposed on ferromagnetic sextets regardless of the cluster size and the Fe content, although it has not been detected in the homogeneous f.c.c. Fe-Ag alloys [13] and Fe/Ag multilayers [14]. This paramagnetic component may be attributed to fluctuation in the Fe atom moments at the interface between Fe and Ag phases. In this context, the formation of Fe clusters in an Ag matrix using the ICB technique seems to be promising for GMR [15]. In this study we used X-ray diffraction, transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) to characterize the structure and morphology, and the magnetoresistance was measured at high fields to shed light on the origin of the GMR in granular materials.
2. Experimental procedures Granular materials a few thousand fingstr6ms in thickness, of Fe clusters embedded in an f.c.c. Ag matrix were prepared by simultaneous codeposition. In the cluster source, electron bombardment is used to heat and evaporate Fe in a zirconia crucible. Clusters are formed by Fe atoms colliding with each other during adiabatic expansion of the vaporized metal through the crucible's nozzle (1.4 mm in diameter and 1 mm in length) into a high vacuum of about 10 -6 Torr. Simultaneously, Ag atoms are thermally evaporated into the same region by heating the crucible. The chemical composition is estimated from energy-disper© 1994 - Elsevier Sequoia. All rights reserved
S. A. Makhlouf et al.
484
/
sive X-ray spectra using a detector installed in a transmission electron microscope. SAXS measurements were taken using a Cu Kct point focus X-ray beam 0.5 mm x 0.5 mm in dimension. Here, the count rate was corrected for the background with blank A1 foil as a reference specimen. The scattering intensity is estimated as a function of the scattering vector, h = 2~ sin 0/2, where ,t is the wavelength of the radiation and 20 is the scattering angle. Magnetoresistance measurements were taken using a conventional d.c. four-point probe method in magnetic fields up to 140 kOe at 4.2 K.
3. E x p e r i m e n t a l
results
As illustrated in Fig. 1, the X-ray diffraction patterns of the as-deposited Fe/Ag films indicate f.c.c, peaks. The Bragg peaks of Ag and Fe are too close to examine the mutual solubilities of Fe and Ag and the crystal structure of the Fe clusters. Moreover, the asymmetric and broad diffraction peaks are ascribed to compositional fluctuation, particle size distribution and the retained strain. For the compositional ratio [Fe]/ lAg]<50/50, several f.c.c, diffraction peaks are detected and the ( 111 ) line is predominant, indicating a preferred orientation of the grain growth. Figure 2 shows a typical microstructure and the corresponding electron diffraction patterns of sample D ([Fe]/[Ag]=45/55) obtained by TEM. The TEM image confirms the granular nature of the material, in the range 20-40 nm. Figure 3 illustrates the SAXS intensities I(h) presented on a log-log type plot as a function of the scattering vector h for the specimens with [Fe]/ [Ag] -- 20/80, 33/67 and 50/50. The obtained curves
Fe-Ag granular material
show the following features. (1) The higher scattering intensity corresponds to the composition ratio [Fe]/ tag] = 50/50. (2) A broad peak is observed for all compositions corresponding to h = 0.07 A-]. (3) A sharp increase in I(h) is observed at h < 0 . 0 4 A J. (4) Another peak at about 0.023 A-J is observed for [Fe]/ tAg] = 33/67. These features indicate the presence of two kinds of scatter in the specimens. The dimension of the smaller scatterer could not be determined owing to overlap of the high h peak with the main scattering intensity. Here, the peak position indicates that the interparticle distance is about 10 nm. The peak at 0.023 A-i for the [Fe]/[Ag] = 33/67 specimen corresponds to a scatterer with a gyration radius Rg -= 7 nm, separated by an interparticle distance of the order of 18 nm. The presence of more than one type of scatterer and the
100nrn D : Fe/Ag = 4 5 / 5 5
Fig. 2. TEM image and electron diffraction patterns of the [Fe]/ tAg] = 45/55 specimen.
10 s
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i
i
i
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i
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Fe/Ag = 20/80 Fe/Ag = 33/67 Fe/Ag=50/50
°.~
=s °~
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~~ f / A ~ ~ "'~ ~.:12at.%Fe
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C: 33 at.% Fe
D: 45 at.% Fe
p-q
30
I
40
!
50
' 610
70
80
90
20 Fig. 1. X-ray diffraction patterns of Fe/Ag granular materials.
I
I
100
10 ~ 0.01
!
~
~
~ ~ ~ ~
0.1
h (~. t) Fig. 3. Scattered intensities for the Fe/Ag granular materials.
S. A. Makhloufet al. / Fe-Aggranularmaterial intense background due to fluorescence X-rays from Fe atoms are manifested in the high h tail in all of the I(h) curves, making it difficult to check the validity of the Porod plot (I(h)- h-4). However, the higher I(h) is observed for the [Fe]/[Ag] = 50/50 specimen, indicating enhancement of the difference in electron densities between the Fe clusters and surrounding Ag matrix. The gyration radius Rgof the larger scatterer is roughly estimated to be about 15 nm. This value is consistent with the TEM results. Figure 4 shows the magnetoresistance curves measured for the present samples at 4.2 K. The ordinate represents a percentage of the magnetoresistance ratio, Ap/p, as a function of an applied magnetic field H u p to 140 kOe. Here Ap/p is defined by the formula
Ap/p = [p(H) -lo(O)]/p(Hmax) where HmaX= 140 kOe. In sample A which has the lowest Fe content, Ap/p does not saturate. It increases monotonically with increasing H even at H = 140 kOe. Similar features are observed for sample D. In samples B and C, however, Ap/p first increases sharply with increasing H up to H = 2 0 kOe. With further increasing H, Ap/p increases linearly and is about 25% and 40% at H = 140 kOe for samples B and C respectively. As [Fe]/[Ag] approaches 50/50, A p / p sharply decreases.
4. Discussion The Fe-cluster dispersed Fe-Ag granular materials display a considerable non-saturation trend in the
- 10
~,~ - 20
t
- ' u O O o o o o ~
I
°~m~l b
485
magnetoresistance (MR) curves, particularly at high fields, whereas the corresponding magnetization easily saturates at low fields [15]. These features can be described as follows. The magnetization depends mainly on the magnetic moment at the core Fe atoms and to lesser extent on the Fe atoms at the interface of ferromagnetic clusters. MR, however, arises from spindependent scattering at the interface between the Fe cluster and Ag matrix and, to lesser extent, is due to the spin-dependent scattering within Fe clusters. The demagnetization factor leads to anisotropy in the GMR of multilayer systems, whereas the GMR has been found to be isotropic in granular systems [9, 10]. The latter results could be ascribed to the rotation of magnetization under an applied field. The resistance approaches its minimum value with the complete alignment of the magnetic moments of the ferromagnetic entities. However, both saturation- and non-saturationtype MR were observed in the present specimens, whose magnetization curves show almost the same saturation behavior. The rapid increase in Ap/p for samples B and C up to 20 kOe may be explained by the rotation of magnetization. However, the monotonic increase in Ap/p at H > 2 0 kOe for all specimens cannot be explained by this simple mechanism. The paramagnetic or superparamagnetic component observed in our Mrssbauer spectra even at 4.2 K may make a considerable contribution to the GMR in high fields.
5. Conclusion Fe-cluster-dispersed Fe-Ag granular materials were prepared using an ion cluster beam deposition technique. TEM and SAXS measurements proved the granular nature of the as-deposited specimens. Spindependent scattering at the Fe cluster surfaces gives rise to GMR. The specimens display unique nonsaturation behavior of GMR.
~
Q.
Acknowledgments
-30
-40 0
I
20
I
40
I
60
I
80
I
I
100
120
140
H (kOe) Fig. 4. Dependence on field of the magnetoresistance Ap/p obtained at 4.2 K of Fe/Ag granular materials. &, A, 12 at.% Fe; m, B, 26at.%Fe; ~, C, 33at.%Fe; e, D 45at.%Fe; ~g, E, 50 at.% Fe.
The authors wish to thank Mr. T. Hihara for his support in the MR experiment. They are also indebted for the MR measurements to the High Field Laboratory for Superconducting Materials in Tohoku University. This work was supported by the New Frontier Program Grant-in-aid for Scientific Research (No. 04NP0501) given by the Ministry of Education, Science and Culture of Japan. S. A. Makhlouf appreciates financial support from the Egyptian Ministry of Higher Education.
486
S. A. Makhlouf et al.
/
Fe-Ag granular material
References 1 J.R. Childress and C. L. Chien, Appl. Phys. Lett., 56 (1990) 95. 2 J. R. Childress and C. L. Chien, J. Appl. Phys., 70 (1991) 5885. 3 G. Xiao and C. L. Chien, Appl. Phys. Lett., 51 (1987) 1280. 4 G. Abersfelder, K. Naock, K. Stierstadt, J. Schelten and W. Schmatz, Philos. Mag. B, 41 (1980) 519. 5 T. B. Massalski, H. Okamoto, P. R. Subramanian and L. Kacprzak (eds.), Binary Alloy Phase Diagrams, ASM International, Metals Park, OH, 1990, 2nd edn. 6 M.N. Baibich, J. M. Broto, A. Fert, E N. Van Dau, E Petroff, P. Eitenne, G. Creuzet, A. Friederich and J. Chazelas, Phys. Rev. Lett., 61 (1988) 2472. 7 S. S. P. Parkin, N. More and K. P. Roche, Phys. Rev. Lett., 64 (1990) 2304.
8 S. Zhang and R M. Levy, J. Appl. Phys., 69 ( 1991 ) 4786. 9 A. E. Berkowitz, J. R. Mitchell, M. J. Carey, A. E Young, S. Zhang, F. E. Spada, F. T. Parker, A. Hutten and G. Thomas, Phys. Rev. Lett., 68(1992) 3745. 10 J. Q. Xiao, J. S. Jiang and C. L. Chien, Phys. Rev. B, 46 (1992) 9266. 11 S. Zhang, Appl. Phys. Lett., 61 (1992) 1855. 12 S.A. Makhlouf, K. Sumiyama, H. Onodera, K. Wakoh and K. Suzuki, Nuc. lnstrum. Methods B, 76(1993) 197. 13 N. Kataoka, K. Sumiyama and Y. Nakamura, Acta Metall., 37 (1989) 1135. 14 J. Q. Xiao, A. Gavrin, G. Xiao, J. R. Childress, W. A. Bryden and C. L. Chien, J. Appl. Phys., 67(1990) 5388. 15 S. A. Makhlouf, K. Sumiyama, K. Wakoh, K. Suzuki, K. Takanashi and H. Fujimori, J. Magn. Magn. Mater., 126 (1993)485.