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Diffusion and magnetic properties of compositionally modulated films
86
Journal of Magnetism and Magnetic Materials 35 (1983) 86-88 North-Holland Publishing Company DIFFUSION AND MAGNETIC PROPERTIES OF COMPOSITIONALLY MODULATED FILMS N.S. K A Z A M A a n d H. F U J I M O R I The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai 980, Japan
Magnetization and interdiffusion measurements have been carried out on the compositionally modulated films Fe8oC20/Si. The magnetization and the Curie temperature have been significantlyenhanced by the diffusion of Fe from FeC to Si for the films with a shorter modulation wavelength, on the contrary, for the longer modulation wavelength film, the ferromagnetism has been eliminated through the diffusion of Fe from Si to FeC.
1. Introduction The study of artificially layered materials has recently attracted much attention. This has provided a strong motivation for the development of unique semiconductor superlattices, compositionally modulated magnetic substances and layered superconductors with a high critical current [1-6]. This paper presents magnetization and interdiffusion properties for compositionally modulated materials consisting of ferromagnetic amorphous Fes0C20 and amorphous semiconductor Si. The observed phenomena suggest the importance of the exchange interaction between magnetic layers through non-magnetic Si layers. 2. Experimental procedure The layered composite films were prepared by a multi-target high-rate sputtering apparatus which consists of two components of de-triode and rf-planar magnetron sputterings. Substrates made of copper, quartz, silicon wafer or Fotoceram were placed on a water-cooled turntable type holder. The individual sputter-depositions by the well-isolated dc and magnetron sources were performed alternately. A shield was used to provide a good isolation of the sputtering plasmas from each other. Both the individual sputtering rate and the table rotation speed could be accurately controlled. This apparatus offered us the flexibility necessary to produce samples with individual layer thickness ranging from 10 A to 104 A for metals and insulators. We prepared amorphous films (Fes0C20)50/Si~0 (about 5000 layers) with modulation wavelengths of 17.0 A and 48.3 A by the sequential sputterings of F e - C target and a Si target. The Ar pressure was maintained at 2 x 10 -2 Torr. A protective coating of 100 A of Si was deposited on the last F e - C layer put down. A complementary coating was also deposited on the each substrate prior to deposition of the layers. He + ion backscattering has detected a considerable 0304-8853/83/0000-0000/$03.00 © North-Holland
amount of Ar in the amorphous Si layers. Thermal anealings up to 600°C did not reduce the Ar content at all [7]. X-ray measurements were made with a standard symmetrical diffractometer using Fe s a radiation. The layered amorphous composites exhibited no sharp Bragg reflections, but the satellites of the (000) reflection did. The modulation wavelength was determined from the position of the satellite peak. The interdiffusivity was determined by monitoring the decay in the satellite peak intensity by annealing. 3. Results and discussion It can be said from the X-ray analysis that the present layered films show a sinusoidally modulated composition profile along the normal to the film plane, since the first order satellite peaks are by far the most intense. Figs. la-1 and a-2 represent schematically the structures analysized concerning the Fe distribution. The magnetization curves for (FesoC2o)s.o/Si50 films with a wavelength of (X) 17.0 A and 48.3 A are shown in figs. lb-1 and b-2. The sohd curves represent the as-deposited samples, while the dashed curves indicate the samples annealed at 500 K. In all cases the magnetizing field ( H ) was applied in the plane of the sample. The obtained magnetizations are normalized to the magnetization of the film with h = 48.3 A at 4.2 K and H = 17.5 kOe. It is found that the magnetization ( M ) approaching saturation can be well expressed by the formula dM/dH
= a o M J H 2 + X0,
(1)
where a 0 and X0 are the constants, and M s the saturation magnetization. The first term may arise from a certain inhomogeneity, and the Xo term implies a paramagnetic contribution. The value of X0 is extraordinarily large for the films with X = 17.0 A and almost independent of the temperature, while for the film with
N.S. Kazama, 1-1,Fujimori / Properties of compositionally modulatedfilms
87
( Fe80C20)50/ Sic
A
U c''O 0'---
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~
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Tc
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200
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xlo 3.
/
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Tc = 295.=1K Tc= 80-" 0 5K 5 0 (,/ 0
5 "
i 50
I 100
I 150
((~emu) . , 0 200 HIM 0
~ 50
100
150
(OWemu) 200 HIM
Fig. I. (a) Plot of the ICe fraction along x, which is the direction along the film normal. The hatched part indicates the magnetization (M) at 0 K for compositions along x; (b) the magnetization versus H for (Fes0Czo)5o/Siso films with modulation wavelength (X) 17.0 (I) and 48.3 A (2). AII the magnetizations have been normalized to the value of the films with X = 48.3 A at 4.2 K and H = 17.5 kOe. The dashed curves show these films annealed at 500 K; (c) the solid curve show the temperature dependence of the magnetization at H = 6.0 kOe for the films as deposited. The dashed curves show the temperature dependence after annealings at 500 K; (d) Arrot plot of the compositionally modulated films with ?~= 17.0 A (1) and 48.3 A (2). X = 48.3 A Xo is small, suggesting the n o r m a l ferromagnetic state. T h e magnetization (at 6 kOe) versus t e m p e r a t u r e curves are s h o w n in figs. lc-I a n d c-2. I n the low t e m p e r a t u r e range, the magnetization decreases almost linearly in b o t h the films. As is s h o w n in the t e m p e r a t u r e d e p e n d e n c e of the
magnetization, the curve for the compositionally modulated materials graudally; approaches the Curie temper° ature (T~) as c o m p a r e d to the steeper a p p r o a c h to T~ for the h o m o g e n e o u s materials. Therefore, we used the usual m e t h o d by d e t e r m i n i n g Tc b y plotting M 2 versus H / M with t e m p e r a t u r e as a parameter. Figs. 1, d-I a n d d-2 show A r r o t t plots near T~ in the field range from 2 kOe
N.S. Kazama, H. Fujimori / Properties of compositionally modulatedfilms
88 I
I
I
I
0.6
I
I
o
1st. order (48.3A ) OA
2
~ T = 2 0 0 , 1
~0.~ ¢-
-
0
~-
/" / "
%
/
st. order (170A) o--
T=250°C
.
-0.~
~,
:
:
.
2 nd order
- 0.4
- 0£ I
0
I
I
I
I
I
20 40 60 80 100 120 Annealing Time ( rain ) t
Fig. 2. Changes of satellite intensities for X = 17.0.~ and 48.3 films on annealing at 250°C and 200°C respectively.
to 20 kOe. We found Tc to be 80 K and 295 K for the samples with k = 17.0 A and 48.3 A, respectively. After keeping these samples for 40 min at 500 K, the magnetization can be enhanced for the shorter wavelength film. On the contrary, the ferromagnetism disappears for the longer wavelength film. This peculiar behaviour on annealing can obviously be attributed to a certain diffusion effect, but cannot simply be explained only in terms of the possible diffusion between Fe and Si, since such a diffusion would give rise to only a monotonic decrease in magnetization. In order to study the unusual magnetization changes due to annealing in more detail, the interdiffusivity was examined by measuring the change of the satellite peak intensity upon annealing. In the case of a sinusoidal composition modulation, the intensity of the first order satellite peak is proportional to the square of the modulation amplitude. Consequently, the effective interdiffusivity,/)x, for a modulation wavelength X is given by the relative change of the satellite intensity I(t) against the time (t) as J ~ k --'--
h2 d l n [ I ( t ) / l o ] 8~r2
(2)
dt
where I 0 is the initial intensity [8]. Since the X-ray atomic scattering factor of Fe in the amorphous F e - C film is much larger than that of C, the interdiffusivity that can be measured in our samples is primarily concerned with Si and Fe. Diffusion anneals at various temperatures were performed on these films. The observed changes in the first order satellite intensity for both the films are shown in fig. 2 as well as the second order satellite intensity of the films with h = 48.3 A. According to eq. (2), the initial gradients of l n [ I ( t ) / I o]
versus t in fig. 2 give rise to Dx = + 10 -20 cm2/s and -- 10 -ls cm2//s for the films with h = 17.0 ,~ and 48.3 A, respectively. In other words, the first satellite intensity of the film with )~ = 17.0 :k decreases, implying the positive diffusion of Fe from F e - C to Si. While the steep increase in the first satellite with X = 48.3 ,~ implies an incease in the modulation amplitude, i.e., the negative diffusion of Fe from the Si layer side to the F e - C layer side. For a reasonable interpretation of the relationship between the diffusion characteristics and the magnetization changes found in figs. Ic-I and c-2, we recognize the importance of the exchange interaction between F e - C / F e - C magnetic layers through their interfaced Si layers. That is, in the as-deposited state, the F e - C / F e - C layers are magnetically interacted through the Fe atoms which are inevitably introduced into the Si layers by the sputtering process, resulting in the totally ferromagnetic layered film irrespective of the value of k. However, if the films are annealed, the interaction between the F e - C / F e - C layers becomes strong in the film with )~ = 17.0 ,~ due to the increase in the Fe atoms in Si layers (the positive diffusion of Fe from F e - C to Si), while it becomes weak in the film with k = 48.3 ,~ due to the decrease in the Fe atoms in Si layers (the negative diffusion of Fe from Si to Fe-C). As a result, the diffusion anneals can enhance the magnetization of the shorter wavelength film, but eliminates the ferromagnetism of the longer wavelength film, similarly to the observed changes in figs. lc-1 and c-2. The reason for the different diffusions of Fe between the films with )~ = 17.0 A and 48.3 A has not been made clear through the present work. It seems that the exchange interaction between the F e - C / F e - C layers arises from a certain conduction electron contribution, such as a quantum-mechanical tunneling effect.
References
[1] Barry J. Thalder, J.B. Ketterson and J.F. Hilliard, Phys. Rev. Lett. 41 (1978) 336. [2] E.M. Gyorgy, J.F. Dillon, Jr., D.B. McWhan, L.W. Rupp, Jr., L.R. Testardi and P.J. Flanders, Phys. Rev. Lett. 45 (1980) 57. [3] J.R. Zhen& C.M. Falco, J.R. Ketterson and Ivan K. Schuller, Appl. Phys. Lett. 38 (1981) 424. [4] A.C. Gossard, Thin Solid Films 57 (1979) 3. [5] R.E. Howard, M.R. Beasley, T.H. Geballe, C.N. King, R.H. Hammond, R.H. Norton, J.R. Salem and R.B. Zubeck, IEEE Trans. Magn. MAG-13 (1977) 138. [6] S. Ruggiero and T.W. Barbee, Jr. and M.R. Beasley, Phys. Rev. Lett. 45 (1980) 1299. [7] N.S. Kazama, H. Fujimori, S. Yamaguchi and Y. Fujino, to be published. [8] M.P. Roscnblum, F. Spaepcn and D. Turnbuil, Appl. Phys. Lett. 37 (! 980) i 84.
Journal of Magnetism and Magnetic Materials 35 (1983) 86-88 North-Holland Publishing Company DIFFUSION AND MAGNETIC PROPERTIES OF COMPOSITIONALLY MODULATED FILMS N.S. K A Z A M A a n d H. F U J I M O R I The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai 980, Japan
Magnetization and interdiffusion measurements have been carried out on the compositionally modulated films Fe8oC20/Si. The magnetization and the Curie temperature have been significantlyenhanced by the diffusion of Fe from FeC to Si for the films with a shorter modulation wavelength, on the contrary, for the longer modulation wavelength film, the ferromagnetism has been eliminated through the diffusion of Fe from Si to FeC.
1. Introduction The study of artificially layered materials has recently attracted much attention. This has provided a strong motivation for the development of unique semiconductor superlattices, compositionally modulated magnetic substances and layered superconductors with a high critical current [1-6]. This paper presents magnetization and interdiffusion properties for compositionally modulated materials consisting of ferromagnetic amorphous Fes0C20 and amorphous semiconductor Si. The observed phenomena suggest the importance of the exchange interaction between magnetic layers through non-magnetic Si layers. 2. Experimental procedure The layered composite films were prepared by a multi-target high-rate sputtering apparatus which consists of two components of de-triode and rf-planar magnetron sputterings. Substrates made of copper, quartz, silicon wafer or Fotoceram were placed on a water-cooled turntable type holder. The individual sputter-depositions by the well-isolated dc and magnetron sources were performed alternately. A shield was used to provide a good isolation of the sputtering plasmas from each other. Both the individual sputtering rate and the table rotation speed could be accurately controlled. This apparatus offered us the flexibility necessary to produce samples with individual layer thickness ranging from 10 A to 104 A for metals and insulators. We prepared amorphous films (Fes0C20)50/Si~0 (about 5000 layers) with modulation wavelengths of 17.0 A and 48.3 A by the sequential sputterings of F e - C target and a Si target. The Ar pressure was maintained at 2 x 10 -2 Torr. A protective coating of 100 A of Si was deposited on the last F e - C layer put down. A complementary coating was also deposited on the each substrate prior to deposition of the layers. He + ion backscattering has detected a considerable 0304-8853/83/0000-0000/$03.00 © North-Holland
amount of Ar in the amorphous Si layers. Thermal anealings up to 600°C did not reduce the Ar content at all [7]. X-ray measurements were made with a standard symmetrical diffractometer using Fe s a radiation. The layered amorphous composites exhibited no sharp Bragg reflections, but the satellites of the (000) reflection did. The modulation wavelength was determined from the position of the satellite peak. The interdiffusivity was determined by monitoring the decay in the satellite peak intensity by annealing. 3. Results and discussion It can be said from the X-ray analysis that the present layered films show a sinusoidally modulated composition profile along the normal to the film plane, since the first order satellite peaks are by far the most intense. Figs. la-1 and a-2 represent schematically the structures analysized concerning the Fe distribution. The magnetization curves for (FesoC2o)s.o/Si50 films with a wavelength of (X) 17.0 A and 48.3 A are shown in figs. lb-1 and b-2. The sohd curves represent the as-deposited samples, while the dashed curves indicate the samples annealed at 500 K. In all cases the magnetizing field ( H ) was applied in the plane of the sample. The obtained magnetizations are normalized to the magnetization of the film with h = 48.3 A at 4.2 K and H = 17.5 kOe. It is found that the magnetization ( M ) approaching saturation can be well expressed by the formula dM/dH
= a o M J H 2 + X0,
(1)
where a 0 and X0 are the constants, and M s the saturation magnetization. The first term may arise from a certain inhomogeneity, and the Xo term implies a paramagnetic contribution. The value of X0 is extraordinarily large for the films with X = 17.0 A and almost independent of the temperature, while for the film with
N.S. Kazama, 1-1,Fujimori / Properties of compositionally modulatedfilms
87
( Fe80C20)50/ Sic
A
U c''O 0'---
Ii
~
I
0
I
I
I
I
.... - . . . . .
I
0 I
,x
I
I
77K
1.0
1.C
~.
.
. . . .
,,_._.,..__-,~-
---
-
'=
4.2K
. . . . .
77K
E O O
N 0.5
0.5
E
0
- --q
"
=i"
5
0
10
300K ' I ~ ~-'1 H 15 20(kOe)
1"5 f . . . . - . ~.~ 1.0 l \
H =6.0 k~~' ]
Tc
H=6.0 k0e
"'~.
1.0
0.5
00
"""~' l 400 T ( K)
200
0
0
xlo
o--~-. . . . . ~. . . . . . ~. . . . . ~----~=,._z. 200 400 T (K)
xlo 3.
/
78 K
E ~15
~
82.6K
~
15
]E
Tc = 295.=1K Tc= 80-" 0 5K 5 0 (,/ 0
5 "
i 50
I 100
I 150
((~emu) . , 0 200 HIM 0
~ 50
100
150
(OWemu) 200 HIM
Fig. I. (a) Plot of the ICe fraction along x, which is the direction along the film normal. The hatched part indicates the magnetization (M) at 0 K for compositions along x; (b) the magnetization versus H for (Fes0Czo)5o/Siso films with modulation wavelength (X) 17.0 (I) and 48.3 A (2). AII the magnetizations have been normalized to the value of the films with X = 48.3 A at 4.2 K and H = 17.5 kOe. The dashed curves show these films annealed at 500 K; (c) the solid curve show the temperature dependence of the magnetization at H = 6.0 kOe for the films as deposited. The dashed curves show the temperature dependence after annealings at 500 K; (d) Arrot plot of the compositionally modulated films with ?~= 17.0 A (1) and 48.3 A (2). X = 48.3 A Xo is small, suggesting the n o r m a l ferromagnetic state. T h e magnetization (at 6 kOe) versus t e m p e r a t u r e curves are s h o w n in figs. lc-I a n d c-2. I n the low t e m p e r a t u r e range, the magnetization decreases almost linearly in b o t h the films. As is s h o w n in the t e m p e r a t u r e d e p e n d e n c e of the
magnetization, the curve for the compositionally modulated materials graudally; approaches the Curie temper° ature (T~) as c o m p a r e d to the steeper a p p r o a c h to T~ for the h o m o g e n e o u s materials. Therefore, we used the usual m e t h o d by d e t e r m i n i n g Tc b y plotting M 2 versus H / M with t e m p e r a t u r e as a parameter. Figs. 1, d-I a n d d-2 show A r r o t t plots near T~ in the field range from 2 kOe
N.S. Kazama, H. Fujimori / Properties of compositionally modulatedfilms
88 I
I
I
I
0.6
I
I
o
1st. order (48.3A ) OA
2
~ T = 2 0 0 , 1
~0.~ ¢-
-
0
~-
/" / "
%
/
st. order (170A) o--
T=250°C
.
-0.~
~,
:
:
.
2 nd order
- 0.4
- 0£ I
0
I
I
I
I
I
20 40 60 80 100 120 Annealing Time ( rain ) t
Fig. 2. Changes of satellite intensities for X = 17.0.~ and 48.3 films on annealing at 250°C and 200°C respectively.
to 20 kOe. We found Tc to be 80 K and 295 K for the samples with k = 17.0 A and 48.3 A, respectively. After keeping these samples for 40 min at 500 K, the magnetization can be enhanced for the shorter wavelength film. On the contrary, the ferromagnetism disappears for the longer wavelength film. This peculiar behaviour on annealing can obviously be attributed to a certain diffusion effect, but cannot simply be explained only in terms of the possible diffusion between Fe and Si, since such a diffusion would give rise to only a monotonic decrease in magnetization. In order to study the unusual magnetization changes due to annealing in more detail, the interdiffusivity was examined by measuring the change of the satellite peak intensity upon annealing. In the case of a sinusoidal composition modulation, the intensity of the first order satellite peak is proportional to the square of the modulation amplitude. Consequently, the effective interdiffusivity,/)x, for a modulation wavelength X is given by the relative change of the satellite intensity I(t) against the time (t) as J ~ k --'--
h2 d l n [ I ( t ) / l o ] 8~r2
(2)
dt
where I 0 is the initial intensity [8]. Since the X-ray atomic scattering factor of Fe in the amorphous F e - C film is much larger than that of C, the interdiffusivity that can be measured in our samples is primarily concerned with Si and Fe. Diffusion anneals at various temperatures were performed on these films. The observed changes in the first order satellite intensity for both the films are shown in fig. 2 as well as the second order satellite intensity of the films with h = 48.3 A. According to eq. (2), the initial gradients of l n [ I ( t ) / I o]
versus t in fig. 2 give rise to Dx = + 10 -20 cm2/s and -- 10 -ls cm2//s for the films with h = 17.0 ,~ and 48.3 A, respectively. In other words, the first satellite intensity of the film with )~ = 17.0 :k decreases, implying the positive diffusion of Fe from F e - C to Si. While the steep increase in the first satellite with X = 48.3 ,~ implies an incease in the modulation amplitude, i.e., the negative diffusion of Fe from the Si layer side to the F e - C layer side. For a reasonable interpretation of the relationship between the diffusion characteristics and the magnetization changes found in figs. Ic-I and c-2, we recognize the importance of the exchange interaction between F e - C / F e - C magnetic layers through their interfaced Si layers. That is, in the as-deposited state, the F e - C / F e - C layers are magnetically interacted through the Fe atoms which are inevitably introduced into the Si layers by the sputtering process, resulting in the totally ferromagnetic layered film irrespective of the value of k. However, if the films are annealed, the interaction between the F e - C / F e - C layers becomes strong in the film with )~ = 17.0 ,~ due to the increase in the Fe atoms in Si layers (the positive diffusion of Fe from F e - C to Si), while it becomes weak in the film with k = 48.3 ,~ due to the decrease in the Fe atoms in Si layers (the negative diffusion of Fe from Si to Fe-C). As a result, the diffusion anneals can enhance the magnetization of the shorter wavelength film, but eliminates the ferromagnetism of the longer wavelength film, similarly to the observed changes in figs. lc-1 and c-2. The reason for the different diffusions of Fe between the films with )~ = 17.0 A and 48.3 A has not been made clear through the present work. It seems that the exchange interaction between the F e - C / F e - C layers arises from a certain conduction electron contribution, such as a quantum-mechanical tunneling effect.
References
[1] Barry J. Thalder, J.B. Ketterson and J.F. Hilliard, Phys. Rev. Lett. 41 (1978) 336. [2] E.M. Gyorgy, J.F. Dillon, Jr., D.B. McWhan, L.W. Rupp, Jr., L.R. Testardi and P.J. Flanders, Phys. Rev. Lett. 45 (1980) 57. [3] J.R. Zhen& C.M. Falco, J.R. Ketterson and Ivan K. Schuller, Appl. Phys. Lett. 38 (1981) 424. [4] A.C. Gossard, Thin Solid Films 57 (1979) 3. [5] R.E. Howard, M.R. Beasley, T.H. Geballe, C.N. King, R.H. Hammond, R.H. Norton, J.R. Salem and R.B. Zubeck, IEEE Trans. Magn. MAG-13 (1977) 138. [6] S. Ruggiero and T.W. Barbee, Jr. and M.R. Beasley, Phys. Rev. Lett. 45 (1980) 1299. [7] N.S. Kazama, H. Fujimori, S. Yamaguchi and Y. Fujino, to be published. [8] M.P. Roscnblum, F. Spaepcn and D. Turnbuil, Appl. Phys. Lett. 37 (! 980) i 84.