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Co) superlattices
Journal of Magnetism and Magnetic Materials 121 (1993) 374-377 North-Holland
Oscillations in low-field giant magnetoresistance in Ni-Fe-Co/Cu(/Co) superlattices Hiroshi Sakakima and Mitsuo Satomi Magnetic Devices Research Lab., Matsushita Electric Industrial Co., Ltd., Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan N i - F e - C o / C u ( / C o ) , N i - C o / C u ( / C o ) superlattices were prepared by rf sputtering. The MR ratio oscillates with the Cu layer thickness (tcu) with maximum values of 24 and 15 % for tcu = 0.9 and 2 nm at room temperature in applied field of 0.5 kOe. tcu modulated films were prepared to investigate whether the GMR depends on the artificial periodicity of the films or not.
1. Introduction
Since the discovery of giant magnetoresistance (GMR) in F e / C r superlattices by Baibich et al. [1], many studies have been conducted on the phenomenon and the origin is attributed to interlayer antiferromagnetic coupling between the magnetic layers across the nonmagnetic layers. The antiferromagnetic coupling was found to show RKKY-like oscillation with nonmagnetic layer thickness in C o / R u , Co/Cr, F e / C r and N i - F e / C u by Parkin et al. [2,3]. Different types of GMR were proposed by Dieny et al. [4] using the spin-valve effect such as N i - F e / C u / N i - C o and by Shinjo et al. [5] using superlattices composed of noncoupled magnetic layers having different H c (coercivities) such as N i - F e / C u / C o . A considerably large MR ratio (= 10%) was obtained for the evaporated films with tcu = 3-7 nm at RT with application of 3 kOe. It is desirable that the superlattices show a low-field GMR at RT from the applied point of view. We have prepared N i - F e - C o / C u / C o superlattices by sputtering and reported that the films with tc~ -- 2 nm (2nd peak) showed a large MR ratio (15%) at RT with application of 0.5 kOe ( = 4 × 10 4 A / m ) and oscillates with tcu [6]. In this paper, we would like to report on the MR
Correspondence to: Dr. H. Sakakima, Magnetic Devices Research Lab., Matsushita Electric Industrial Co., Ltd., 3-1-1 Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan. Tel: + 816-9064840; telefax: + 81-6-9064589.
characteristics of N i - F e - C o / C u ( / C o ) superlattices with tcu -- 0.9 nm (lst peak) and on those of tcu modulated films to investigate whether the GMR depends on the artificial periodicity or not. 2. Experimental
The films were prepared by sputtering Nis0Fe20_xCo x (or Ni80Co20), Cu and Co targets directly onto glass substrates are fixed on a watercooled rotating holder. The sputtering Ar gas pressure was kept at 1.1 Pa (8 mTorr) and the deposition rates of Ni-Fe-Co, Cu and Co were 29, 40 and 26 nm/min, respectively. The MR (magnetoresistance) was measured using the four-point probe method with application of a 0.5 or 2.5 kOe magnetic field in the sample plane. The magnetization curves were measured with a VSM (Vibrating Sample Magnetometer). 3. Results and discussion
Fig. 1 shows the cross-sectional TEM (Transmission Electron Microscopy) image of the NiFe-Co(3 nm)/Cu(0.9 nm)/Co(3 nm)/Cu(0.9 nm) superlattice which exhibits low-field GMR at RT. Samples were prepared without any buffer layers with considerably high deposition rates, but they showed a clear and continuous layered structure. All the samples were confirmed to have artificial periodicities by small angle X-ray diffraction [7]. Fig. 2 shows the tcu dependence of the MR ratio of the Ni-Fe-Co(3 nm)/Cu/Co(3 nm) superlattices. The MR ratio takes the maximum
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
375
H. Sakakima, M. Satomi / Oscillations in GMR in Ni-Fe-Co/Cu( /Co)
u =0.9nm
1Onto
Fig. 1. TEM image of [Ni-Fe-Co(3 nm)/Cu(0.9 nm)/Co(3 nm)/Cu(0.9 nm)].
value 24% (with 0.5 kOe and 28% with 2.5 kOe) for tc, = 0.9 nm and oscillates with tcu with a period of 1.1 nm for our sputtered samples and this was not observed in prior work on evaporated N i - F e / C u / C o superlattices [5]. This implies that the G M R found in our superlattices originates in the interlayer exchange coupling between the magnetic layers across the Cu layers rather than in the difference in coercivities between the noncoupled magnetic layers. G M R and the RKKY-Iike oscillation were also found in our samples of N i - F e - C o / C u , N i - C o / C u / C o and N i - C o / C u superlattices. The results on the M R ratios are shown in table 1. Fig. 3 shows the M R curves on the N i - F e C o / C u / C o superlattices with tcu = 0.9 nm (a) and tcu-2 nm (b). The M R curve of the sample
Table 1 MR ratio of superlattices Superlattices
MR ratio (%)
Ni-Fe-Co/Cu/Co Ni-Fe-Co/Cu Ni-Co/Cu/Co Ni-Co/Cu
tcu = 0.9 28 20 25 19
tcu = 2.0 15 9 13 9
with tcu = 2 nm saturates with very low field compared with those of exchange-coupled-type superlattices such as C o / R u [2] or compared with those of noncoupled-type superlattices such as N i - F e / C u / C o [5]. N i - F e - C o / C u , NiC o / C u / C o and N i - C o / C u superlattices of the 25
2O o
15
"
I0
nr~E
5
¢x=
0 I
I
I
I
|
I
20 15
(b)
°_
10
rr IE
5 0
~
20
15
20
(c)
a=
tr 5
c~
Q=
10
0 I
-0,
0
"-" Cu Layer Thickness (n•)
Fig. 2. Cu layer thickness dependence of the MR ratio of [Ni-Fe-Co(3 nm)/Cu(tcu)/Co(3 nm)/Cu(tc~)].
i
O. 5
H (k0n)
i 4
|
0
Fig. 3. MR
curve of (a) [Ni-Fe-Co(3
nm)/Cu(0.9 rim)/
Co(3 nm)/Cu(0.9 nm)], (b) [Ni-Fe-Co(3 nm)/Cu(2 nm)/ Co(3 nm)/Cu(2 nm)], (c) [Ni-Fe-Co(3 nm)/Cu(0.9 nm)/ Co(3 nm)/Cu(0.9 nm)/Ni-Fe-Co(3 nm)/Cu(2 nm)/ Co(3 nm)/Cu(2 nm)].
H. Sakakirna, M. Satomi / Oscillations in GMR in Ni-Fe-Co/Cu( /Co)
376
present study show M R curves similar to N i - F e C o / C u / C o . MR curve of the N i - F e - C o / C u superlattices with tcu = 2 nm saturates with the lowest field ( = 0.05 kOe) among the superlattices of the present study, although the M R ratio is about 10% and less than that of N i - F e C o / C u / C o . Our N i - F e - C o / C u ( / C o ) superlattices, at present, are probably the softest published magnetic materials that have MR ratios larger than 10% at RT. Fig. 3(c) shows the M R curve of N i - F e Co/Cu(0.9 nm)/Co/Cu(0.9 nm)/Ni-FeC o / C u ( 2 n m ) / C o / C u ( 2 nm). The curve (c) seems to be formed simply superimposing curve (b) on (a). This implies that no interference effect occurs between the tcu = 0.9 and 2 nm blocks. MR
t cu=O" 9 i
v
tc~=1.4
j
tc==2" 0
-0.5
0 App
I i e d
F i e I d
0.5 (kOe)
Fig. 5. Magnetization curves of [ N i - F e - C o ( 3 nm)/Cu(tcu)]
Y
with tc~ = 0.9, 1.4, 2 nm. in our samples is, therefore, rather independent on the artificial periodicity of the layers. Figs. 4 and 5 show magnetization curves of the Ni-Fe-Co(3 nm)/Cu(tcu)/Co(3 nm) and N i F e - C o ( 3 n m ) / C u ( t c u ) superlattices. The saturation magnetic field, H s, seems to oscillate with tcu and the interlayer exchange coupling is considered to be ferromagnetic for the samples with tcu = 1.4 nm and antiferromagnetic for the samples with tcu = 0.9 and 2.0 nm, where spin orientations of the magnetic layers are considered to be almost antiparallel at zero field and the remanence in the fig. 4 is mainly due to the magnetization difference between N i - F e - C o and Co. It is possible to simulate the magnetization curves [7] by minimizing the energy of the system,
tcu=0.9
J
V 3 v
tcu=l.4
E = - ~ _ , M i n cos 0 i -- ~ _ , g i cos h i ( O i - f ~ i ) Jr EJsi,i+l cos(/91-01+1),
tcu=2.0
-0.5
0 App
I ied
F i e I d
0.5 (kOe)
Fig. 4. Magnetization curves of [Ni-Fe-Co(3 nm)/Cu (tcu)/Co(3 nm)/Cu(tcu)] with tcu = 0.9, 1.4, 2.0 nm.
(1)
where M i is the magnetization of the ith magnetic layer and 0i is the direction. K i is the magnetocrystalline anisotropy with a symmetry of A i (for sixfold, A = 6) and 4)i is the easy axis direction, Jsi,÷l is the interlayer exchange coupling energy ~etween i and the (i + 1)th magnetic layer, Js is assumed to oscillate with Tc~ as
H. Sakakima, M. S atomi / Oscillations in G MR in Ni-Fe-Co / Cu ( / Co )
R K K Y oscillation. In order to obtain low-field G M R , K and Js should be small, but Js not be smaller than K in order to realize the antiparallel spin orientation. The interlayer exchange coupling estimated from H s is = 1-2 x 10 -2 e r g / c m 2 for our sample with tcu = 2 nm, which is very small compared with the previous results on exchange coupled type such as J = 5 e r g / c m 2 for C o / R u [2]. The estimated K and Js values of our samples are very small and this probably makes it possible to realize the low-field GMR.
4. Conclusions
N i - F e - C o / C u ( / C o ) , N i - C o / C u ( / C o ) superlattices were prepared by rf sputtering. The superlattices showed G M R (24% at R T with 0.5 kOe, 28% with 2.5 kOe) with RKKY-Iike oscillations, tcu modulated samples were prepared to prove the M R is independent on the artificial periodicity. The estimated values of magnetic anisotropy and exchange coupling energy of our
377
samples were very small, which probably facilitates the realization of low-field GMR. The authors wish to thank Mr. T. Kohsaki, Matsushita Techno-Research for obtaining crosssectional T E M images of our samples.
References [1] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich and J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. [2] S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [3] S.S.P. Parkin, Appl. Phys. Lett. 60 (1992) 512. [4] B. Dieny, V.S. Speriosu, B.A. Gurney, S.S.P. Parkin, D.R. Wilhoit, K.P. Roche, S. Metin, D.T. Peterson and S. Nadimi, J. Magn. Magn. Mater. 93 (1991) 101. [5] T. Shinjo and H. Yamamoto, J. Phys. Soc. Jpn. 59 (1990) 3061. [6] M. Satomi and H. Sakakima, IEICE Tech. Rep. (1991) MR91-9. [7] H. Sakakima and M. Satomi, Jpn. J. Appl. Phys. 31 (1992) 1484.
Oscillations in low-field giant magnetoresistance in Ni-Fe-Co/Cu(/Co) superlattices Hiroshi Sakakima and Mitsuo Satomi Magnetic Devices Research Lab., Matsushita Electric Industrial Co., Ltd., Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan N i - F e - C o / C u ( / C o ) , N i - C o / C u ( / C o ) superlattices were prepared by rf sputtering. The MR ratio oscillates with the Cu layer thickness (tcu) with maximum values of 24 and 15 % for tcu = 0.9 and 2 nm at room temperature in applied field of 0.5 kOe. tcu modulated films were prepared to investigate whether the GMR depends on the artificial periodicity of the films or not.
1. Introduction
Since the discovery of giant magnetoresistance (GMR) in F e / C r superlattices by Baibich et al. [1], many studies have been conducted on the phenomenon and the origin is attributed to interlayer antiferromagnetic coupling between the magnetic layers across the nonmagnetic layers. The antiferromagnetic coupling was found to show RKKY-like oscillation with nonmagnetic layer thickness in C o / R u , Co/Cr, F e / C r and N i - F e / C u by Parkin et al. [2,3]. Different types of GMR were proposed by Dieny et al. [4] using the spin-valve effect such as N i - F e / C u / N i - C o and by Shinjo et al. [5] using superlattices composed of noncoupled magnetic layers having different H c (coercivities) such as N i - F e / C u / C o . A considerably large MR ratio (= 10%) was obtained for the evaporated films with tcu = 3-7 nm at RT with application of 3 kOe. It is desirable that the superlattices show a low-field GMR at RT from the applied point of view. We have prepared N i - F e - C o / C u / C o superlattices by sputtering and reported that the films with tc~ -- 2 nm (2nd peak) showed a large MR ratio (15%) at RT with application of 0.5 kOe ( = 4 × 10 4 A / m ) and oscillates with tcu [6]. In this paper, we would like to report on the MR
Correspondence to: Dr. H. Sakakima, Magnetic Devices Research Lab., Matsushita Electric Industrial Co., Ltd., 3-1-1 Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan. Tel: + 816-9064840; telefax: + 81-6-9064589.
characteristics of N i - F e - C o / C u ( / C o ) superlattices with tcu -- 0.9 nm (lst peak) and on those of tcu modulated films to investigate whether the GMR depends on the artificial periodicity or not. 2. Experimental
The films were prepared by sputtering Nis0Fe20_xCo x (or Ni80Co20), Cu and Co targets directly onto glass substrates are fixed on a watercooled rotating holder. The sputtering Ar gas pressure was kept at 1.1 Pa (8 mTorr) and the deposition rates of Ni-Fe-Co, Cu and Co were 29, 40 and 26 nm/min, respectively. The MR (magnetoresistance) was measured using the four-point probe method with application of a 0.5 or 2.5 kOe magnetic field in the sample plane. The magnetization curves were measured with a VSM (Vibrating Sample Magnetometer). 3. Results and discussion
Fig. 1 shows the cross-sectional TEM (Transmission Electron Microscopy) image of the NiFe-Co(3 nm)/Cu(0.9 nm)/Co(3 nm)/Cu(0.9 nm) superlattice which exhibits low-field GMR at RT. Samples were prepared without any buffer layers with considerably high deposition rates, but they showed a clear and continuous layered structure. All the samples were confirmed to have artificial periodicities by small angle X-ray diffraction [7]. Fig. 2 shows the tcu dependence of the MR ratio of the Ni-Fe-Co(3 nm)/Cu/Co(3 nm) superlattices. The MR ratio takes the maximum
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
375
H. Sakakima, M. Satomi / Oscillations in GMR in Ni-Fe-Co/Cu( /Co)
u =0.9nm
1Onto
Fig. 1. TEM image of [Ni-Fe-Co(3 nm)/Cu(0.9 nm)/Co(3 nm)/Cu(0.9 nm)].
value 24% (with 0.5 kOe and 28% with 2.5 kOe) for tc, = 0.9 nm and oscillates with tcu with a period of 1.1 nm for our sputtered samples and this was not observed in prior work on evaporated N i - F e / C u / C o superlattices [5]. This implies that the G M R found in our superlattices originates in the interlayer exchange coupling between the magnetic layers across the Cu layers rather than in the difference in coercivities between the noncoupled magnetic layers. G M R and the RKKY-Iike oscillation were also found in our samples of N i - F e - C o / C u , N i - C o / C u / C o and N i - C o / C u superlattices. The results on the M R ratios are shown in table 1. Fig. 3 shows the M R curves on the N i - F e C o / C u / C o superlattices with tcu = 0.9 nm (a) and tcu-2 nm (b). The M R curve of the sample
Table 1 MR ratio of superlattices Superlattices
MR ratio (%)
Ni-Fe-Co/Cu/Co Ni-Fe-Co/Cu Ni-Co/Cu/Co Ni-Co/Cu
tcu = 0.9 28 20 25 19
tcu = 2.0 15 9 13 9
with tcu = 2 nm saturates with very low field compared with those of exchange-coupled-type superlattices such as C o / R u [2] or compared with those of noncoupled-type superlattices such as N i - F e / C u / C o [5]. N i - F e - C o / C u , NiC o / C u / C o and N i - C o / C u superlattices of the 25
2O o
15
"
I0
nr~E
5
¢x=
0 I
I
I
I
|
I
20 15
(b)
°_
10
rr IE
5 0
~
20
15
20
(c)
a=
tr 5
c~
Q=
10
0 I
-0,
0
"-" Cu Layer Thickness (n•)
Fig. 2. Cu layer thickness dependence of the MR ratio of [Ni-Fe-Co(3 nm)/Cu(tcu)/Co(3 nm)/Cu(tc~)].
i
O. 5
H (k0n)
i 4
|
0
Fig. 3. MR
curve of (a) [Ni-Fe-Co(3
nm)/Cu(0.9 rim)/
Co(3 nm)/Cu(0.9 nm)], (b) [Ni-Fe-Co(3 nm)/Cu(2 nm)/ Co(3 nm)/Cu(2 nm)], (c) [Ni-Fe-Co(3 nm)/Cu(0.9 nm)/ Co(3 nm)/Cu(0.9 nm)/Ni-Fe-Co(3 nm)/Cu(2 nm)/ Co(3 nm)/Cu(2 nm)].
H. Sakakirna, M. Satomi / Oscillations in GMR in Ni-Fe-Co/Cu( /Co)
376
present study show M R curves similar to N i - F e C o / C u / C o . MR curve of the N i - F e - C o / C u superlattices with tcu = 2 nm saturates with the lowest field ( = 0.05 kOe) among the superlattices of the present study, although the M R ratio is about 10% and less than that of N i - F e C o / C u / C o . Our N i - F e - C o / C u ( / C o ) superlattices, at present, are probably the softest published magnetic materials that have MR ratios larger than 10% at RT. Fig. 3(c) shows the M R curve of N i - F e Co/Cu(0.9 nm)/Co/Cu(0.9 nm)/Ni-FeC o / C u ( 2 n m ) / C o / C u ( 2 nm). The curve (c) seems to be formed simply superimposing curve (b) on (a). This implies that no interference effect occurs between the tcu = 0.9 and 2 nm blocks. MR
t cu=O" 9 i
v
tc~=1.4
j
tc==2" 0
-0.5
0 App
I i e d
F i e I d
0.5 (kOe)
Fig. 5. Magnetization curves of [ N i - F e - C o ( 3 nm)/Cu(tcu)]
Y
with tc~ = 0.9, 1.4, 2 nm. in our samples is, therefore, rather independent on the artificial periodicity of the layers. Figs. 4 and 5 show magnetization curves of the Ni-Fe-Co(3 nm)/Cu(tcu)/Co(3 nm) and N i F e - C o ( 3 n m ) / C u ( t c u ) superlattices. The saturation magnetic field, H s, seems to oscillate with tcu and the interlayer exchange coupling is considered to be ferromagnetic for the samples with tcu = 1.4 nm and antiferromagnetic for the samples with tcu = 0.9 and 2.0 nm, where spin orientations of the magnetic layers are considered to be almost antiparallel at zero field and the remanence in the fig. 4 is mainly due to the magnetization difference between N i - F e - C o and Co. It is possible to simulate the magnetization curves [7] by minimizing the energy of the system,
tcu=0.9
J
V 3 v
tcu=l.4
E = - ~ _ , M i n cos 0 i -- ~ _ , g i cos h i ( O i - f ~ i ) Jr EJsi,i+l cos(/91-01+1),
tcu=2.0
-0.5
0 App
I ied
F i e I d
0.5 (kOe)
Fig. 4. Magnetization curves of [Ni-Fe-Co(3 nm)/Cu (tcu)/Co(3 nm)/Cu(tcu)] with tcu = 0.9, 1.4, 2.0 nm.
(1)
where M i is the magnetization of the ith magnetic layer and 0i is the direction. K i is the magnetocrystalline anisotropy with a symmetry of A i (for sixfold, A = 6) and 4)i is the easy axis direction, Jsi,÷l is the interlayer exchange coupling energy ~etween i and the (i + 1)th magnetic layer, Js is assumed to oscillate with Tc~ as
H. Sakakima, M. S atomi / Oscillations in G MR in Ni-Fe-Co / Cu ( / Co )
R K K Y oscillation. In order to obtain low-field G M R , K and Js should be small, but Js not be smaller than K in order to realize the antiparallel spin orientation. The interlayer exchange coupling estimated from H s is = 1-2 x 10 -2 e r g / c m 2 for our sample with tcu = 2 nm, which is very small compared with the previous results on exchange coupled type such as J = 5 e r g / c m 2 for C o / R u [2]. The estimated K and Js values of our samples are very small and this probably makes it possible to realize the low-field GMR.
4. Conclusions
N i - F e - C o / C u ( / C o ) , N i - C o / C u ( / C o ) superlattices were prepared by rf sputtering. The superlattices showed G M R (24% at R T with 0.5 kOe, 28% with 2.5 kOe) with RKKY-Iike oscillations, tcu modulated samples were prepared to prove the M R is independent on the artificial periodicity. The estimated values of magnetic anisotropy and exchange coupling energy of our
377
samples were very small, which probably facilitates the realization of low-field GMR. The authors wish to thank Mr. T. Kohsaki, Matsushita Techno-Research for obtaining crosssectional T E M images of our samples.
References [1] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich and J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. [2] S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [3] S.S.P. Parkin, Appl. Phys. Lett. 60 (1992) 512. [4] B. Dieny, V.S. Speriosu, B.A. Gurney, S.S.P. Parkin, D.R. Wilhoit, K.P. Roche, S. Metin, D.T. Peterson and S. Nadimi, J. Magn. Magn. Mater. 93 (1991) 101. [5] T. Shinjo and H. Yamamoto, J. Phys. Soc. Jpn. 59 (1990) 3061. [6] M. Satomi and H. Sakakima, IEICE Tech. Rep. (1991) MR91-9. [7] H. Sakakima and M. Satomi, Jpn. J. Appl. Phys. 31 (1992) 1484.