- Email: [email protected]
Cu granular films deposited by co-evaporation and cluster beam techniques
ELSEVIER
LMarerials Science and Engineering A217/218 (1996) 326-330
Comparative GMR study of Fe/Cu granular films deposited by co-evaploration and cluster beam techniques K. Wakoh, Institute
T. Hihara, for
Materials
T.J. Konno, Research,
Tohoku
K. Sumiyama,
University,
Sendd
980-77,
K. Suzuki Jupnn
Abstract We produced Fe/Cu thin films by co-evaporation and cluster-beam(CB) deposition, and comparedtheir magnetoresistance (MR) and magneticproperties.Both co-evaporatedand CB-depositedfilms exhibit giant magnetoresistance (GMR), which do not saturate even at high fields: conduction electronssuffer spin-disorderscattering. With increasingFe concentration, MR of the co-evaporatedfilms show a sharp maximum at around 23 at.% Fe owing to percolation of magnetic Fe atoms. MR of the C&depositedfilms, on the other hand, doesnot showsucha peak. The magnetization of co-evaporated films is smaller than that of CB-depositedfilms and they do not saturateeasily.Theseobservationssuggests that the magneticstatesof the former are spin glass, while the latter may contain small ferromagnetic clusters.These results demonstratethat the co-evaporatedfilms are chemicallyand magneticallyhomogeneous,whereasthe CB-depositedfilms are heterogeneoushaving granular natures. Keywords: Fe/Q granular films; Giant magnetoresistance; Co-evaporation; Cluster beam
1. Introduction
An ionized cluster beam (ICB) method has been known as an effective technique to produce optically flat thin films and multilayers [l-3]. In this process, small metallic clusters were formed by adiabatic expansion of vaporized atoms through a crucible’s nozzle (with an aspect ratio of the nozzle length to its diameter being about one) into a vacuum. By ionizing these clusters with electron bombardment and applying a bias voltage between the source and a substrate, the clusters are crashed onto the substrate and surface migration of adatoms is enhanced, resulting in the formation of uniform films. In order to keep weakly-bound atom clusters intact and to form so-called cluster-assembled materials, we deposited neutral clusters by soft landing without ionization and acceleration [4,5]. We call this process a cluster beam (CB) deposition. Using the CB method, we have produced Fe/Ag [6-91 and Fe/Cu [lo] granular materials and found that they show, without any heat treatment, giant magnetoresistance (GMR) as large as ones observed in magnetic metal/noble metal multilayer films [ll-131.
Despite the fact that ICB and CB deposition methods are being used to produce high-quality granular films, however, elemental processes of these deposition methods have not been well understood. This is partly due to the lack of detailed studies on various processing parameters. For example, the best nozzle shape is still under investigation [14]. In the present work, we compare the magnetic properties of Fe,0 thin films produced by a conventional co-evaporation method (without a nozzle) with those produced by the CB deposition. In particular, we describe the characteristics of their GMR, which is very sensitive to both nanometric heterogeneity and the magnetic state at the interface between magnetic clusters and noble metal matrices. Our results show that films produced with conventional co-evaporation are more homogeneous than those produced by the CB method, indicating that the nozzle of a crucible is indeed effective in generating nanometric clusters. 2. Experimental
Fe-cluster-dispersed Cu (Fe/Cu granular) films about 3 000 A in thickness were prepared by co-evaporation 0921-5093/96/%X5.00 0 1996 - Elsevier Science S.A. AH rights reserved PII SO921-5093(96)10334-S
K.
Wakoh et al. 1 Materials Science and Engineering
A217/218
(1996)
326-330
-...... N.
\
--A...,,
16 ---.. ...,..,.I , -..., 11
Magnetic
327
.“. 1 \ at% Fe
Field (kOe)
Fig. 1. Magnetoresistance ratio, Ap/p(O), of Fe/Cu granular films at 4.2 K as a function of an applied magnetic field, H, for (a) co-evaporated films, and (b) CB-deposited films.
and CB methods. The Fe source consists of a zirconia inner crucible and a molybdenum outer crucible, which was heated by eiectron bombardment. For the co-evaporation, Fe was deposited from an open crucible, while, for the CB, a nozzle of 1.4 mm in diameter and 1.4 mm in length was put onto the crucible. (The Cu source has no nozzle for the two deposition methods.) We used polyimide flrns as substrates for both magnetoresistance and magnetic measurements. The base pressure of the chamber was about 10 --5 Pa, which was obtained with an oil-free pumping system. The distance between the substrate and Fe and Cu sources is about 30 cm. The temperature of the substrate was monitored throughout the deposition, being about 35 “C. The deposition rates of Fe and Cu were monitored independently to ensure homogeneity throughout the specimen thickness. The chemical compositions of deposited alloy urns were determined by inductively coupled plasma (ICP) spectrometry. A standard four-probe method was employed to measure the electrical resistivity of specimens at 4.2 K in magnetic fields up to 140 kOe. We used a Quantum Design superconducting quantum interference device (SQUID) magnetometer to measure the magnetic properties. 3. Results
Fig. 1 compares magnetoresistance (MR) ratios, ApIp. as a function of an applied magnetic field, H,
of co-evaporated Fe/Cu films (a) with those of CBdeposited films (b) [lo]. The MR ratios are displayed as Ap/p = [p(H) - p(O)]/p(O), where p(0) is the value in the initial H = 0 state. The composition of the films ranges from about 10 to 40% Fe for the two kinds of the films. All the curves in Fig. l(a) and l(b) exhibit a monotonic decrease, which does not saturate even at H = 140 kOe. The magnitude of the MR ratio depends on the Fe content of the films and fabrication methods. For the co-evaporated films (Fig. l(a)), the MR ratio increases with increasing Fe content until it reaches the maximum at the Fe concentration of 23%. A further increase in the Fe content suppresses the MR ratio, as can be seen for 26%, 29% Fe films, etc. On the other hand, for the CB-deposited films, the magnitudes of the MR ratio gradually decreases with increasing Fe content without showing a prominent rise in their magnitudes (Fig. l(b)). In order to better describe the difference between the MR of co-evaporated films and that of CB-deposited films, we plotted the MR ratio at a particular magnetic field as a function of the Fe content. Fig, 2 shows the MR ratio at H = 140 kOe as a function of the Fe content. It is clear that co-evaporated &ns exhibit a maximum around 23% of Fe. Fig. 3(a) and 3(b) show the magnetization, M, curves (M-H curve), and the thermomagnetic curves (M-T curve) for 36% Fe co-evaporated and CB-deposited films. The former is normalized to the M at H = 50 kOe at constant temperature, T = 5 K, while
328
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I
I
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&+..., L.,,,,,.,-.....,........ ..,....* .. .. ...__... I I ,,..,,,,,_,,,.,_,,,,. r ,,,,_A
30 composition
40 (at%-Fe)
50
the latter is normalized to the .M at T= 5 K at constant field, H= 10 kOe. The M-H curve in Fig. 3(a) shows that magnetizations of ,these samples do not saturate even at H = 50 kOe. It is also clear that this “non-saturating behavior” is more pronounced in the co-evaporated film than in the CB-deposited film, indicating that the magnetic state of the former is nearly a spin glass, while that the latter contain significant amount of ferromagnetic components. As shown in Fig. 3(b) the M-T curve of the co-evaporated film decreases more rapidly with temperature than that of the CB-deposited film. This indicates that the relative amount of temperature-sensitive superparamagnetic and/or paramagnetic components are larger in the co-evaporated film than in the CBdeposited tirns. In order to study, in detail, the magnetic state of the co-evaporated films, we measured a,c. susceptibil-
0’
0.10
g
0.08 0.06
.=*
0.04
za
0.00
‘;s Q
0.12
Q
0.10
3 w
0.08
(1996)
326-330
0.02
0.06 2
0.04
J
I 40 temperature
I 60
26 at.%Fe I I 80 100
(K) WAC=~04
Fig. 4. A.C. susceptibilities of co-evaporated Fe/Cu granular films with applied frequencies of 2, 20, 200, and 1 000 Hz. Composition of the films: (a) 12% Fe, (b) 15% Fe, (c) 23% Fe, and (d) 26% Fe.
r
H (ItOe)
*
60
Fig. 2. MR ratio, Ap/p(O), of Fe/Cu granular films as a function of the Fe concentration of the films at a constant magnetic field of H = 140 kOe at 4.2 K.
A217/218
1 100
,
temperature
i 200
,
1 0.0 300
(5)
Fig. 3. The magnetization curves of Fe/Cu granular films (36% Fe) as a function of (a) an applied magnetic field (M-H curves), and (b) a temperature (M-T curves).
ities. Fig. 4. shows the a.c. susceptibilities of the coevaporated ,Fe/Cu Elms of four different compositions taken with four different frequencies (2, 20, 200, and 1000 Hz). The films with 12 and 15% Fe exhibits clqar peaks in the susceptibility curves, which can be ascribed to a spin glass to (super) paramagnetic transition. We identify the temperature at which the susceptibility reveals the maximum with the blocking temperature, TB. With increasing the Fe content of the films, T, increases and reaches approximately 60 K for the 23% Fe film. Note that (i) the absolute values of the a.c. susceptibility above TB increases with the Fe content; (ii) curves taken at different frequencies do not merge at high temper-
K. Walcoh
et al. 1 Materials
Science
atures, especially for the 23% Fe sample. The former indicates that the size of superparamagnetic clusters increases with increasing Fe content, while the latter suggests that these clusters begin to couple magnetically, i.e. they start to show a ferromagnetic character. For the 26% Fe film, TB decreases to 40 K. The absolute value of a.c. susceptibility at T, for this film is much smaller than that of the 23% Fe film. Note that the differences in the a.c. susceptibility values among the four curves taken at different frequencies for the 26% Fe film remain constant for almost the entire temperature range, in contrast to the other three films. This indicate that strongly developed ferromagnetic coupling among the clusters in the 26% Fe film. Correspondingly, the magnetic state below TB can be better described as cluster glass for this composition. 4. Discussion
We showed that both co-evaporated and CB-deposited films exhibit GMR, which do not saturate at a magnetic field as high as 140 kOe. We also showed that MR ratio of co-evaporated films becomes very large (Ap/p z 45% at H = 140 kOe) for 23% Fe films, in contrast to CB-deposited films, that show a monotonic increase with decreasing Fe content. Magnetization curves of both co-evaporated and CB-deposited films do not saturate at 50 kOe, and this “non-saturating behavior” is more pronounced in the co-evaporated films than CB-deposited films. The magnetic state of the co-evaporated films at low temperatures are spin glass when the Fe content is small and begin to show a ferromagnetic character at around 23-25% of Fe. Fig. 5 shows the magnetic phase diagram for sputter-deposited Fe/Cu films [15], together with the transition temperatures of the co-evaporated films obtained in the present work. As shown here, the magnetic states as a function of temperature and composition of the co-evaporated films are well described by the same magnetic phase diagram obtained for sputtered Fe/Cu films. Since sputtering can generate a film with Fe atoms dispersed homogeneously in a Cu matrix, we speculate that co-evaporated films also possess a similar degree of homogeneity. The evolution of the MR values of co-evaporated films as a function of Fe content is in agreement with the magnetic states of the films. Namely, the MR ratio at 4.2 K increases with increasing Fe content because each Fe moment independently acts as a scattering center to conduction electrons. The scattering probability decreases with alignment of Fe
aizd Engineerbzg
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329
moments along an applied magnetic field, resulting in large negative MR. Once ferromagnetic coupling starts to develop among the Fe moments, however, these moments start to possess a spontaneous magnetization, resulting in the decrease in the net MR value. In the co-evaporated films, this transition takes place at about 23% of Fe. On the other hand, the lack of the maximum in MR ratio of the CB-deposited Fe/Cu i?lms (as shown in Fig. 2) indicates that their magnetic state and structure are different from those of the coevaporated iilms. Even though the magnetization of the CB-deposited 36% Fe film does not saturate at H = 50 kOe, the departure from saturation of the CB-deposited film is significantly smaller than that of the co-evaporated film. This observation suggests that the CB-deposited film contains ferromagnetic (bee) Fe clusters, as well as spin glass and antiferromagnetic components. The temperature dependence of the magnetization, as shown in Fig. 3(b), also supports this view. Therefore, we suggest that CB-deposited films are chemically and magnetically more heterogeneous than co-evaporated films. In addition, a Miissbauer study carried out recently by Hihara et al. [lo] indicated that the CB-deposited 36% Fe film is indeed composed of bee and face centered cubic (fee) Fe. We, therefore, suggest that, in the Fe-Cu system, the difference in the size and mount of Fe clusters in a Cu matrix is crucial in obtaining large MR.
Fig. 5. Magnetic phase diagram of sputtered Fe/& films [lj]. The solid lines should be used as a guide for the eyes. Also shown in the figure are the blocking temperatures observed for co-evaporated films in the present work. FM, ferromagnetism; PM, paramagnetism; CG, cluster glass; and SG, spin glass.
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et ai. 1 Materials
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5. Conclusions
We produced Fe/Cu granular films using conventional co-evaporation and CB methods. The MR ratio, in general, exhibits a monotonous decrease with increasing an applied field, which does not saturate even at 140 kOe. In terms of compositional dependence, the magnitude of the MR ratio rises sharply around 23% of Fe for co-evaporated films, while such a maximum was not observed for CB-deposited films. Magnetizations of co-evaporated flrns are smaller than those of CB-deposited films, and they do not saturate easily. This indicates that the magnetic states of the former is spin glass, while the latter contains small Fe clusters. These results suggest that co-evaporated films are chemically and magnetically more homogeneous than CB-deposited films.
and Engineering
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References PI T. Takagi, I. Yamada and A. Sasaki, J.
Vat. Sci. Technol.,
12
(1975) 1128. 121I. Yamada, H. Usui and T. Takagi, J. PQs. C/xx, 91(1987) 2463. [31 W.L. Brown, M.F. Jarrold, R.L. McEachern, M. Sosnowski, G. Takaoka, H. Usui and I. Yamada, Nzlcl. Inslwm. Met/z., B59/60 (1991) 182. [41 E.W. Becker, K. Bier and W. Henkes, 2. Phys., 146 (1956) 333. [51 T. Hihara, K. Sumiyama, Y. Xu, Y. Wang, T. Kamiyama, K. Wakoh and K. Suzuki, to be published. VI S.A. Makhlouf, K. Sumiyama, T. Kamiyama, K. Wakoh and K. Suzuki, Mater. Sci. Eng., A 179/180 (1994) 483. [71 S.A. Makhlouf, K. Sumiyama, K. Wakoh and K. Suzuki, Jpn. J. Appl. Pfiys., 33 (1994) 1323. PI %A. Makhlouf, K. Sumiyama and K. Suzuki, Jpn. J. Appl. Phys., 33 (1994) 4913. [91 Y. Xu, K. Sumiyama, K. Wakoh and K. Suzuki, J. Magn. Magn. Mater.,
l&(1995)
220.
1101T. Hihara, K. Sumiyama, M. Sakurai, H. Onodera, K. Wakoh and K. Suzuki, to be published.
1111M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen van Dau, F.
Acknowledgements
This work was supported by the New Frontier Program Grant-in-Aid for Scientific Research (No. 07NP0301) given by the Ministry of Education, Science and Culture of Japan. One of the authors (TH) appreciates financial support from the Japan Society for the Promotion of Science.
Petroff, P. Etienne, G. Creuzet, A. Friedrich and J. Chazelas, Piiys. Rev. Lett., 61 (19%) 2472. 1121S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett., 64(1990) 2304. [131 W.P. Pratt, Jr., S.-F. Lee, J.M. Slaughter, R. Loloee and P.A. Schroeder, phys. Rev. Lelt., 66(1991) 3060. u41 S.W. Feng, J.J. Nainaparampil, M.F. Tabet and F.K. Urban III, Thin Solid Films,
253 (1994)
402.
[I51 K. Sumiyama, K. Nishi, Y. Nakamura, V. Manns, B. Scholz, M. Privik, W. Keune, W. Stamm, G. Dumpich and E.F. Wassermann, Magn. Itlater., 96 (1991) 329.
J. Magn.
LMarerials Science and Engineering A217/218 (1996) 326-330
Comparative GMR study of Fe/Cu granular films deposited by co-evaploration and cluster beam techniques K. Wakoh, Institute
T. Hihara, for
Materials
T.J. Konno, Research,
Tohoku
K. Sumiyama,
University,
Sendd
980-77,
K. Suzuki Jupnn
Abstract We produced Fe/Cu thin films by co-evaporation and cluster-beam(CB) deposition, and comparedtheir magnetoresistance (MR) and magneticproperties.Both co-evaporatedand CB-depositedfilms exhibit giant magnetoresistance (GMR), which do not saturate even at high fields: conduction electronssuffer spin-disorderscattering. With increasingFe concentration, MR of the co-evaporatedfilms show a sharp maximum at around 23 at.% Fe owing to percolation of magnetic Fe atoms. MR of the C&depositedfilms, on the other hand, doesnot showsucha peak. The magnetization of co-evaporated films is smaller than that of CB-depositedfilms and they do not saturateeasily.Theseobservationssuggests that the magneticstatesof the former are spin glass, while the latter may contain small ferromagnetic clusters.These results demonstratethat the co-evaporatedfilms are chemicallyand magneticallyhomogeneous,whereasthe CB-depositedfilms are heterogeneoushaving granular natures. Keywords: Fe/Q granular films; Giant magnetoresistance; Co-evaporation; Cluster beam
1. Introduction
An ionized cluster beam (ICB) method has been known as an effective technique to produce optically flat thin films and multilayers [l-3]. In this process, small metallic clusters were formed by adiabatic expansion of vaporized atoms through a crucible’s nozzle (with an aspect ratio of the nozzle length to its diameter being about one) into a vacuum. By ionizing these clusters with electron bombardment and applying a bias voltage between the source and a substrate, the clusters are crashed onto the substrate and surface migration of adatoms is enhanced, resulting in the formation of uniform films. In order to keep weakly-bound atom clusters intact and to form so-called cluster-assembled materials, we deposited neutral clusters by soft landing without ionization and acceleration [4,5]. We call this process a cluster beam (CB) deposition. Using the CB method, we have produced Fe/Ag [6-91 and Fe/Cu [lo] granular materials and found that they show, without any heat treatment, giant magnetoresistance (GMR) as large as ones observed in magnetic metal/noble metal multilayer films [ll-131.
Despite the fact that ICB and CB deposition methods are being used to produce high-quality granular films, however, elemental processes of these deposition methods have not been well understood. This is partly due to the lack of detailed studies on various processing parameters. For example, the best nozzle shape is still under investigation [14]. In the present work, we compare the magnetic properties of Fe,0 thin films produced by a conventional co-evaporation method (without a nozzle) with those produced by the CB deposition. In particular, we describe the characteristics of their GMR, which is very sensitive to both nanometric heterogeneity and the magnetic state at the interface between magnetic clusters and noble metal matrices. Our results show that films produced with conventional co-evaporation are more homogeneous than those produced by the CB method, indicating that the nozzle of a crucible is indeed effective in generating nanometric clusters. 2. Experimental
Fe-cluster-dispersed Cu (Fe/Cu granular) films about 3 000 A in thickness were prepared by co-evaporation 0921-5093/96/%X5.00 0 1996 - Elsevier Science S.A. AH rights reserved PII SO921-5093(96)10334-S
K.
Wakoh et al. 1 Materials Science and Engineering
A217/218
(1996)
326-330
-...... N.
\
--A...,,
16 ---.. ...,..,.I , -..., 11
Magnetic
327
.“. 1 \ at% Fe
Field (kOe)
Fig. 1. Magnetoresistance ratio, Ap/p(O), of Fe/Cu granular films at 4.2 K as a function of an applied magnetic field, H, for (a) co-evaporated films, and (b) CB-deposited films.
and CB methods. The Fe source consists of a zirconia inner crucible and a molybdenum outer crucible, which was heated by eiectron bombardment. For the co-evaporation, Fe was deposited from an open crucible, while, for the CB, a nozzle of 1.4 mm in diameter and 1.4 mm in length was put onto the crucible. (The Cu source has no nozzle for the two deposition methods.) We used polyimide flrns as substrates for both magnetoresistance and magnetic measurements. The base pressure of the chamber was about 10 --5 Pa, which was obtained with an oil-free pumping system. The distance between the substrate and Fe and Cu sources is about 30 cm. The temperature of the substrate was monitored throughout the deposition, being about 35 “C. The deposition rates of Fe and Cu were monitored independently to ensure homogeneity throughout the specimen thickness. The chemical compositions of deposited alloy urns were determined by inductively coupled plasma (ICP) spectrometry. A standard four-probe method was employed to measure the electrical resistivity of specimens at 4.2 K in magnetic fields up to 140 kOe. We used a Quantum Design superconducting quantum interference device (SQUID) magnetometer to measure the magnetic properties. 3. Results
Fig. 1 compares magnetoresistance (MR) ratios, ApIp. as a function of an applied magnetic field, H,
of co-evaporated Fe/Cu films (a) with those of CBdeposited films (b) [lo]. The MR ratios are displayed as Ap/p = [p(H) - p(O)]/p(O), where p(0) is the value in the initial H = 0 state. The composition of the films ranges from about 10 to 40% Fe for the two kinds of the films. All the curves in Fig. l(a) and l(b) exhibit a monotonic decrease, which does not saturate even at H = 140 kOe. The magnitude of the MR ratio depends on the Fe content of the films and fabrication methods. For the co-evaporated films (Fig. l(a)), the MR ratio increases with increasing Fe content until it reaches the maximum at the Fe concentration of 23%. A further increase in the Fe content suppresses the MR ratio, as can be seen for 26%, 29% Fe films, etc. On the other hand, for the CB-deposited films, the magnitudes of the MR ratio gradually decreases with increasing Fe content without showing a prominent rise in their magnitudes (Fig. l(b)). In order to better describe the difference between the MR of co-evaporated films and that of CB-deposited films, we plotted the MR ratio at a particular magnetic field as a function of the Fe content. Fig, 2 shows the MR ratio at H = 140 kOe as a function of the Fe content. It is clear that co-evaporated &ns exhibit a maximum around 23% of Fe. Fig. 3(a) and 3(b) show the magnetization, M, curves (M-H curve), and the thermomagnetic curves (M-T curve) for 36% Fe co-evaporated and CB-deposited films. The former is normalized to the M at H = 50 kOe at constant temperature, T = 5 K, while
328
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0 0
I
I
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20
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and Engineering
&+..., L.,,,,,.,-.....,........ ..,....* .. .. ...__... I I ,,..,,,,,_,,,.,_,,,,. r ,,,,_A
30 composition
40 (at%-Fe)
50
the latter is normalized to the .M at T= 5 K at constant field, H= 10 kOe. The M-H curve in Fig. 3(a) shows that magnetizations of ,these samples do not saturate even at H = 50 kOe. It is also clear that this “non-saturating behavior” is more pronounced in the co-evaporated film than in the CB-deposited film, indicating that the magnetic state of the former is nearly a spin glass, while that the latter contain significant amount of ferromagnetic components. As shown in Fig. 3(b) the M-T curve of the co-evaporated film decreases more rapidly with temperature than that of the CB-deposited film. This indicates that the relative amount of temperature-sensitive superparamagnetic and/or paramagnetic components are larger in the co-evaporated film than in the CBdeposited tirns. In order to study, in detail, the magnetic state of the co-evaporated films, we measured a,c. susceptibil-
0’
0.10
g
0.08 0.06
.=*
0.04
za
0.00
‘;s Q
0.12
Q
0.10
3 w
0.08
(1996)
326-330
0.02
0.06 2
0.04
J
I 40 temperature
I 60
26 at.%Fe I I 80 100
(K) WAC=~04
Fig. 4. A.C. susceptibilities of co-evaporated Fe/Cu granular films with applied frequencies of 2, 20, 200, and 1 000 Hz. Composition of the films: (a) 12% Fe, (b) 15% Fe, (c) 23% Fe, and (d) 26% Fe.
r
H (ItOe)
*
60
Fig. 2. MR ratio, Ap/p(O), of Fe/Cu granular films as a function of the Fe concentration of the films at a constant magnetic field of H = 140 kOe at 4.2 K.
A217/218
1 100
,
temperature
i 200
,
1 0.0 300
(5)
Fig. 3. The magnetization curves of Fe/Cu granular films (36% Fe) as a function of (a) an applied magnetic field (M-H curves), and (b) a temperature (M-T curves).
ities. Fig. 4. shows the a.c. susceptibilities of the coevaporated ,Fe/Cu Elms of four different compositions taken with four different frequencies (2, 20, 200, and 1000 Hz). The films with 12 and 15% Fe exhibits clqar peaks in the susceptibility curves, which can be ascribed to a spin glass to (super) paramagnetic transition. We identify the temperature at which the susceptibility reveals the maximum with the blocking temperature, TB. With increasing the Fe content of the films, T, increases and reaches approximately 60 K for the 23% Fe film. Note that (i) the absolute values of the a.c. susceptibility above TB increases with the Fe content; (ii) curves taken at different frequencies do not merge at high temper-
K. Walcoh
et al. 1 Materials
Science
atures, especially for the 23% Fe sample. The former indicates that the size of superparamagnetic clusters increases with increasing Fe content, while the latter suggests that these clusters begin to couple magnetically, i.e. they start to show a ferromagnetic character. For the 26% Fe film, TB decreases to 40 K. The absolute value of a.c. susceptibility at T, for this film is much smaller than that of the 23% Fe film. Note that the differences in the a.c. susceptibility values among the four curves taken at different frequencies for the 26% Fe film remain constant for almost the entire temperature range, in contrast to the other three films. This indicate that strongly developed ferromagnetic coupling among the clusters in the 26% Fe film. Correspondingly, the magnetic state below TB can be better described as cluster glass for this composition. 4. Discussion
We showed that both co-evaporated and CB-deposited films exhibit GMR, which do not saturate at a magnetic field as high as 140 kOe. We also showed that MR ratio of co-evaporated films becomes very large (Ap/p z 45% at H = 140 kOe) for 23% Fe films, in contrast to CB-deposited films, that show a monotonic increase with decreasing Fe content. Magnetization curves of both co-evaporated and CB-deposited films do not saturate at 50 kOe, and this “non-saturating behavior” is more pronounced in the co-evaporated films than CB-deposited films. The magnetic state of the co-evaporated films at low temperatures are spin glass when the Fe content is small and begin to show a ferromagnetic character at around 23-25% of Fe. Fig. 5 shows the magnetic phase diagram for sputter-deposited Fe/Cu films [15], together with the transition temperatures of the co-evaporated films obtained in the present work. As shown here, the magnetic states as a function of temperature and composition of the co-evaporated films are well described by the same magnetic phase diagram obtained for sputtered Fe/Cu films. Since sputtering can generate a film with Fe atoms dispersed homogeneously in a Cu matrix, we speculate that co-evaporated films also possess a similar degree of homogeneity. The evolution of the MR values of co-evaporated films as a function of Fe content is in agreement with the magnetic states of the films. Namely, the MR ratio at 4.2 K increases with increasing Fe content because each Fe moment independently acts as a scattering center to conduction electrons. The scattering probability decreases with alignment of Fe
aizd Engineerbzg
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(1996)
326-330
329
moments along an applied magnetic field, resulting in large negative MR. Once ferromagnetic coupling starts to develop among the Fe moments, however, these moments start to possess a spontaneous magnetization, resulting in the decrease in the net MR value. In the co-evaporated films, this transition takes place at about 23% of Fe. On the other hand, the lack of the maximum in MR ratio of the CB-deposited Fe/Cu i?lms (as shown in Fig. 2) indicates that their magnetic state and structure are different from those of the coevaporated iilms. Even though the magnetization of the CB-deposited 36% Fe film does not saturate at H = 50 kOe, the departure from saturation of the CB-deposited film is significantly smaller than that of the co-evaporated film. This observation suggests that the CB-deposited film contains ferromagnetic (bee) Fe clusters, as well as spin glass and antiferromagnetic components. The temperature dependence of the magnetization, as shown in Fig. 3(b), also supports this view. Therefore, we suggest that CB-deposited films are chemically and magnetically more heterogeneous than co-evaporated films. In addition, a Miissbauer study carried out recently by Hihara et al. [lo] indicated that the CB-deposited 36% Fe film is indeed composed of bee and face centered cubic (fee) Fe. We, therefore, suggest that, in the Fe-Cu system, the difference in the size and mount of Fe clusters in a Cu matrix is crucial in obtaining large MR.
Fig. 5. Magnetic phase diagram of sputtered Fe/& films [lj]. The solid lines should be used as a guide for the eyes. Also shown in the figure are the blocking temperatures observed for co-evaporated films in the present work. FM, ferromagnetism; PM, paramagnetism; CG, cluster glass; and SG, spin glass.
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et ai. 1 Materials
Science
5. Conclusions
We produced Fe/Cu granular films using conventional co-evaporation and CB methods. The MR ratio, in general, exhibits a monotonous decrease with increasing an applied field, which does not saturate even at 140 kOe. In terms of compositional dependence, the magnitude of the MR ratio rises sharply around 23% of Fe for co-evaporated films, while such a maximum was not observed for CB-deposited films. Magnetizations of co-evaporated flrns are smaller than those of CB-deposited films, and they do not saturate easily. This indicates that the magnetic states of the former is spin glass, while the latter contains small Fe clusters. These results suggest that co-evaporated films are chemically and magnetically more homogeneous than CB-deposited films.
and Engineering
A21 7/218 (1996)
326-330
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Acknowledgements
This work was supported by the New Frontier Program Grant-in-Aid for Scientific Research (No. 07NP0301) given by the Ministry of Education, Science and Culture of Japan. One of the authors (TH) appreciates financial support from the Japan Society for the Promotion of Science.
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