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Magnetoresistance of Co-Based multilayered structures
Journal of Magnetism and Magnetic Materials 93 (1991) 481t-484 North-Holland
Magnetoresistance of Co-Based multilayered structures D.H. M o s c a ~, A. B a r t h e l e m y , F. Petroff, A. F e r t Laboratoire de Physique des' Solides, Unit'ersitt; Paris-Sud, 91405 Orsay, France
P.A. Schroeder 2, W.P. Pratt Jr., R. L a l o e e Department Of Physics and Astronomy, Michigan State Unit'ersity, East Lansing, MI 48824, USA
and
R. C a b a n e l LCR Thomson CSF, 91404 Orsay. France
Magnetoresistance measurements have been made on sputtered multilayerd films of A g / C o and C u / C o . Magnetoresistances as high as 16% at 4.2K have been measured in the field range where the magnetization reverses. The results are ascribed to the partial antiparallel arrangement of the magnetizations in alternate Co layers.
1. Introduction Enhanced magnetoresistance effects have been found in many multilayered structures. The most fascinating example is the magnetoresistance of F e / C r multilayers [1-4] which is associated with the antiparallel alignment of the magnetization of neighbouring Fe layers at zero field. Much work [3-7] is being done to devise other systems with the desired antiparallel arrangements of alternate ferromagnetic layers. Theoretical models have been proposed by Camley and Barnas [8] and Levy and coworkers [9]. In this paper we survey magnetoresistance results for sputtered A g / C o and C u / C o multilayers. The choice of A g / C o and C u / C o was inspired by previous results on the closely related I Permanent address: Universidade Federal do Rio Grande do Sul, 9151/0 Porto Alegre, Brasil. 2Temporary address: Laboratoire de Physique des Solides, Universit~ Paris-Sud, France.
system A u / C o [3, 5, 6] and also by some large magnetoresistance effects already reported on A g / C o [10, 11].
2. Samples and experimental The present samples were prepared in a UHV compatible sputtering system [12] at Michigan State University. With the exception of one sample sputtered on a sapphire substrate, they were deposited on Si substrates held at a temperature close to 0°C where island formation in the A g / C o system is minimal. While X-ray data on the C u / C o system have yet to be obtained, a well resolved satellite system is observed in the A g / C o system [10, 11]. Silver crystallites with A g ( l l l ) faces parallel to the sample surface predominate. X-ray studies also suggest that the Co is predominantly fcc at low thickness and hcp for thicknesses greater than -= 60,~. Nuclear magnetic
113114-8853/91/$113.51/ r~i 1991- Elsevier Science Publishers B.V. (North-Holland)
D.H. Mosca et al. / Magnetoresistance of Co-based rnultilayered structures
resonance measurements [13] confirm a strong hcp component for a A g 6 0 / C o 6 0 sample (in our notation A g x / C o y , x and y are thicknesses given in A) while measurements on C u 3 0 / C o 3 0 showed a strong fcc component. Measurements for a greater range of film thicknesses are continuing. The magnetoresistance (MR) measurements were performed at the Universit6 Paris-Sud at Orsay using an ac system with built-in compensation for zero magnetic field resistance. For all samples measurements were made with H perpendicular to the sample surface, or with H parallel to the surface and in the current direction. For some samples, measurements were made with H parallel to the surface and perpendicular to the current, and for some others, measurements were made as a function of the angle between H and the normal to the surface, either in the plane or perpendicular to the plane of the current I. Magnetization measurements have been performed at LCR Thomson-CSF on several samples using a S Q U I D magnetometer.
481
H T=4.2 K R o = O. 28 -"L
¢n
Ro
iii
0 Z < I(f)
T-=247 K Ro = 0 . 4 5 -n,-
LU
n~ I
NO
=51~ " o
o
I-UJ
Z rj <
~E I
0.15 H
0 Tesla
z
0.15
Fig. 1. Magnetoresistance for a A g 6 0 / C o 6 0 sample prepared on a sapphire substrate. The field is in-plane and perpendicular to the current.
3. Results We have found that for all the samples the magnetization is spontaneously in the plane of the layers, which means that the perpendicular anisotropy is insufficient to overcome the demagnetizing field. In fig. 1 we show typical results for H in the layer plane and perpendicular to I for a A g 6 0 / C o 6 0 sample (the one sample on a sapphire substrate). The MR peaks occur in the vicinity of H = H c the coercive field. In this case there is a sizeable 11.2% MR at 4.2 K (we define the M R % as M R % = 1 0 0 A R / R o where AR is the maximal change in the resistance). We also show the results for the same sample at 247K, and notice that an appreciable part of the decrease in M R % is due to the increase in the zero field resistance R a. In fact 60% of the drop in M R % comes from the increase in R 0 and 40% comes from the drop in AR. This is directly
related to the long mean free path found in the A g / C o system [10, 11]. We believe that the MR arises from the same mechanism already proposed for A u / C o [3, 5], namely to some degree of antiparallel alignment of the magnetizations of neighbouring Co layers during the magnetization reversal. We note that the M R % of one of our sputtered Co films is about 1% and, on this basis, we dismiss domain wall scattering in the Co as a secondary effect. It is interesting to note the dependence of the MR on the thickness of Ag in fig. 2 and table 1. The decrease in M R % as the thickness decreases cannot be accounted for by the relatively small increase in resistivity. It is probably caused by the progressive onset of ferromagnetic interactions between the Co layers across the Ag. Such interactions by coupling the magnetization reversal in neighbouring Co layers are expected to reduce
D.H. Mosca et al. / Magnetoresistance of Co-based multilayered structure,;
482
12 10 8
decrease of the MR for increasing thickness as predicted by theories [8, 9]. The MR of C u x / C o 3 0 exhibits a similar increase with x above 45,~, but at smaller x the MR increases again and exhibits a maximum at o x = 20A. We suggest that this maximum is due to the existence of antiferromagnetic coupling between the Co layers as already found by spin polarized low energy electron diffraction and neutron diffraction in this thickness range [14, 15]. We have also studied the MR of multilayers of the type Agx/Co60/Agx/Coy and Cux/ Co60/Cux/Coy (fig. 3), in which alternate Co layers have different thicknesses 60 and y ,~ (10,~ < y < 60,~). Making such multilayers was inspired by the work of Dupas et al. [5] for the A u / C o system. In C o / A u / C o sandwiches with two different Co thicknesses these authors have obtained magnetization reversals at two well defined different fields. In this way, they were able to produce a more complete antiparallel alignment between these two fields and an enhance-
O
Co60/Agx • Co30/Cux
/
o
/
8
C
~
6
o
4
~
2
/ ~-~-o---7".
0
10
20
. . . . . . .
30
40
50
60
70
Thickness of Ag or Cu layer, x(A)
Fig. 2. The maximum magnetoresistance ratio as a function of the thicknesses of Ag or Cu for the series of Agx/Co60 and C u x / C o 3 0 multilayers. The magnetic field is perpendicular to the film plane. The temperature is 4.2 K. The lines are guides to the eye.
the degree of antiparallel alignment and to lower the MR. Earlier measurements on A g / C o samples with equal thickness layers indicated that the MR% became a maximum for thicknesses = 60.~. This suggests that above 60,~ the interlayer coupling can be neglected and that one recovers the
Table 1 In this table we summarize the magnetoresistance data at 4.2 K for all series of samples studied. Parallel refers to measurements with H in the plane of the sample and parallel to the current. Normal refers to H perpendicular to the sample. The number of periods is given in brackets. The top layer was always silver or copper
normal
(thickness in ,~)
0.04 0.18 1.07 3.10 11.15
0.50 0.14 0.18 1.61 2.70 11.50
(Cu 10/Co30),)o (Cu 15/Co30)a0 (Cu20/Co30)7z (Cu30/Co30)6o (Cu45/Co30)4o (Cu60/Co30)40
(Ag30/Co60/Ag30/Co 15)44
1.50
3.60
(Ag30/Co60/Ag30/Co20)43
3.50
4.00
(Ag30/Co60/Ag30/Co25)41 (Ag30/Co60/Ag30/Co30)40
2.26 2.10
2.88 2.10
(thickness in A)
(Agl0/Co60)s7 (Agl 5/Co60)s3 (Ag20/Co60)s() (Ag30/Co60)44 (Ag40/Co60)40 (Ag60/Co60)42
parallel -
MR(%)
Samples
MR(%)
Samples
(Ag60/Co60/Ag60/Co 15)30 (Ag60/Co60/Ag60/Co30)3o
9.00 5.78
14.55 6.09
(Ag60/Co60/Ag60/Co45)30
4.90
4.83
parallel
normal
0.60 0.37 4.1/0 3.20 1.46 4.40
0.81 0.60 4.93 3.50 1.9() 3.4(I
5.90
6.00
3.37
4.87
(Cu30/Co60/60/ Cu30/Co30)41 )
3.90
4.53
(Cu60/Co60/ Cu60/Co10)40
10.08
8.44
(Cu30/Co60/ Cu30/Co 15)44 (Cu30/Co60/ Cu30/Co20)4~
483
D.H. Mosca et al. /Magnetoresistance of Co-based muhilayered structures 15
Ag30/Co60/Ag30/Coy o Ag60/Co60/Ag60/Coy •
•
~
~
Cu30/Co6OlCu30/Coy
H
[] 0°
Cu60/Co60/Cu60/Coy •
10
Q) O
-
i
i
i
i
I
i
10
20
30
40
50
60
Thickness
of
Co
layer,
70
y(A)
Fig. 3. The maximum magnetoresistance effect as a function of the Co thickness for different series of samples. The magnetic field is perpendicular to the film plane. The temperature is 4.2 K. The lines are guides to the eye.
z O3 O3 UJ
n-
__J
O
ment in the MR. In our multilayers with two different Co thicknesses, although a reversal of the magnetization in two well-defined steps cannot be seen in the rather smooth magnetization loops obtained to date, comparison of the results in figs. 2 and 3 shows that some enhancement in the MR occurs especially for low thicknesses of the variable Co layers. Furthermore, in fig. 3 we see that for all the systems studied the MR b decreases as y increases, which, if our interpretation is correct, suggests a greater degree of parallel alignment for y small. We note again that for both Ag and Cu, the series with x = 60,~, has the largest MR%. In fig. 4 we show the change in the magnetoresistance for A g 6 0 / C o 6 0 / A g 6 0 / C o l 5 as a function of the angle 0 between H and the normal to the sample, in the plane of I and the normal. For 0 = 0 °, we have a typical curve with H perpendicular to the sample. With this field orientation a much larger field is necessary to overcome the demagnetizing field. However, as soon as the field has a component in the plane of the sample, the peak corresponding to the reversal of the in-plane magnetization appears and develops to its final state at 0 = 90 °. In this sample we have found the highest M R % to date, 16% at 4.2K (fig. 5). We note that, surprisingly, this maximum is observed for a field at 45 ° from the normal in the plane
4 5.0
O
Ag60JCo60[Ag601Co15
_S -3.0
I
-2.0
90.0 l
t
0
-tO H
1D
[ 2.0
3.0
Tesla
Fig. 4. Magnetoresistance at 4.2K for A g 6 0 / C o 6 0 / A g 6 0 / Co15 as a function of the angle between H and the normal to the surface of the sample in the plane of the current and the normal. The curves are given for decreasing fields only.
perpendicular to the current. We have no explanation why this maximum should occur in a tilted direction. A large MR, 10% at 4.2K, was also found in the similar system C u 6 0 / C o 6 0 / C u 6 0 / Col0.
4. C o n c l u s i o n s
We have studied the MR of A g / C o and C u / C o multilayers and we have found significant MR peaks in the field range where the magnetization is reversed. The maximum observed M R % is 16% at 4.2K and this change occurs in a field range of the order of magnitude of 100Oe.
484
D.H. Mosca et al. /,14ag* etore~is'tal ce 0[ Co-hased multiho'cred ,~tructure.s
Acknowledgement
~67.T: 4 . 2 K
h45" I
Ag60JCo601Ag601Co15
t~
This work was supported in part by the National Science Foundation under grant no. DMR11-13287. The Laboratoirc de Physique des Solides is Laboratoire Associd du Centre National de la Recherche Scicntifique.
r, o
References
E
03.O
0
H
3.0
Tesla
Fig. 5. Magnetoresistance curves of a A g 6 0 / C o f O / A g 6 0 / (7o15 multilayer at 4.2K. The ctuwes fire for a magnetic field at 45 ° from the normal to the film plane in a plane perpendicular to the current (see inset).
We believe that the MR is similar in nature to that already observed in the A u / C o system and ascribed to partial alignment of the magnetizations of neighbouring Co layers during the magnetization reversal. The MR is enhanced tor multilayers in which the alternate Co layers have different thicknesses. The strong increase in MR as the Ag layer increases in thickness indicates that for significant MR it is necessary to avoid ferromagnetic coupling between neighbouring Co layers, while maintaining a long electron mean free path. Above a thickness of Ag of about 60,~ we anticipate that the MR will decrease in agreement with the conventional scaling of the MR with the ratio of the thickness to the mean free path. The only difference for C u / C o is a maximum of the MR for a Cu thickness = 20A, which should be due to the existence of antiferromagnetic interlayer coupling.
[1] M.N. Baibich, JIM. Broto, A. Fert. F. Nguyen Van l)au, F. Petroff, P. Etienne, G. Creuzct, A. Friedel'ich and J. Chazelas, Phys. Rev. Len. 61 (1989) 2472. A. Barth61emy, A. Fert. M. N. Baibich, P. Etienne, S. Lequien and R. Cabanel, J. Appl. Phys. 67 (1990) 59{)8. [2] G. Binash, P. Grrmberg, F. Saurenbach and W. Zinm Phys. Rev. B 39 (1989) 4828. [3] J. Barnas, A. Fuss, R.E, Camley, P. Griinberg and W. Zinn, submined. [4] S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lcn. 64 (19911) 2304. [5] C. Dupas, P. Beauvillain, C. Chappert, J.P. Renard, F. Trigui, E. Velu and D. Renard, submitted. [6] T. Takahata, S. Aruki and T. Shinjo, J. Magn. Magn. Mat. 82 (1989) 287. [7] B. Dieny, V.S. Speriosu, B.A. Gurney, S.S.P. Parkin, D.R. WilhoiL K.P. Roche, S. Metin. D.T. Peterson and S. Nadimi. J. Magn. Magn. Mat. 93 (1991) 101. [8] R.E. Camley and J. Barnas. Phys. Rev. Lctt. 63 (1989) 664. [9] P.M. Lc'<~,', K. Ounadiela, S. Zhang, Y. Wang, ('.B. Sommers and A. Fert, J. Appl. Phys. 67 (1990) 5914. P.M. Levy and S. Zhang, J. Magn. Magn. Mat. 93 (1991) 67. P.M. Levy, S. Z h a n g and A. Ferl, Phys. Rev. l,ett, 65 (199(I) 1643. [11)] J.M. Slaughter, P h . D . Thesis. Michigan State University (1987). [11] 1|. Sato, P.A. Schrocdcr. J. Slaughter, W.P. Pratt Jr. and W. Abdul-Razzag, Supcrluttices Microstructurcs 4 (1987) 45. [12] J.M. Slaughter, W.P. Pratt and P.A, Schrocdcr, Rcv. Sci. Inst. 60 (1989) 127. [13] Collaboration with K. kc Dang and P. Vcillct, to hc published. [14] 1). Pescia. D. Kerkmann, F. Schumann and W. Gudat, Z. Phys. t3 78 (1990) 475. [15] A. Cebollada, J . L Martinez, J.M. Gallego, J.J. dc Miguel. R. Miranda, S. Fcrrcr, F. Batallan, G. Fillion and J.P. Rehouillat. Phys. Rcv. 39 (1989)9726.
Magnetoresistance of Co-Based multilayered structures D.H. M o s c a ~, A. B a r t h e l e m y , F. Petroff, A. F e r t Laboratoire de Physique des' Solides, Unit'ersitt; Paris-Sud, 91405 Orsay, France
P.A. Schroeder 2, W.P. Pratt Jr., R. L a l o e e Department Of Physics and Astronomy, Michigan State Unit'ersity, East Lansing, MI 48824, USA
and
R. C a b a n e l LCR Thomson CSF, 91404 Orsay. France
Magnetoresistance measurements have been made on sputtered multilayerd films of A g / C o and C u / C o . Magnetoresistances as high as 16% at 4.2K have been measured in the field range where the magnetization reverses. The results are ascribed to the partial antiparallel arrangement of the magnetizations in alternate Co layers.
1. Introduction Enhanced magnetoresistance effects have been found in many multilayered structures. The most fascinating example is the magnetoresistance of F e / C r multilayers [1-4] which is associated with the antiparallel alignment of the magnetization of neighbouring Fe layers at zero field. Much work [3-7] is being done to devise other systems with the desired antiparallel arrangements of alternate ferromagnetic layers. Theoretical models have been proposed by Camley and Barnas [8] and Levy and coworkers [9]. In this paper we survey magnetoresistance results for sputtered A g / C o and C u / C o multilayers. The choice of A g / C o and C u / C o was inspired by previous results on the closely related I Permanent address: Universidade Federal do Rio Grande do Sul, 9151/0 Porto Alegre, Brasil. 2Temporary address: Laboratoire de Physique des Solides, Universit~ Paris-Sud, France.
system A u / C o [3, 5, 6] and also by some large magnetoresistance effects already reported on A g / C o [10, 11].
2. Samples and experimental The present samples were prepared in a UHV compatible sputtering system [12] at Michigan State University. With the exception of one sample sputtered on a sapphire substrate, they were deposited on Si substrates held at a temperature close to 0°C where island formation in the A g / C o system is minimal. While X-ray data on the C u / C o system have yet to be obtained, a well resolved satellite system is observed in the A g / C o system [10, 11]. Silver crystallites with A g ( l l l ) faces parallel to the sample surface predominate. X-ray studies also suggest that the Co is predominantly fcc at low thickness and hcp for thicknesses greater than -= 60,~. Nuclear magnetic
113114-8853/91/$113.51/ r~i 1991- Elsevier Science Publishers B.V. (North-Holland)
D.H. Mosca et al. / Magnetoresistance of Co-based rnultilayered structures
resonance measurements [13] confirm a strong hcp component for a A g 6 0 / C o 6 0 sample (in our notation A g x / C o y , x and y are thicknesses given in A) while measurements on C u 3 0 / C o 3 0 showed a strong fcc component. Measurements for a greater range of film thicknesses are continuing. The magnetoresistance (MR) measurements were performed at the Universit6 Paris-Sud at Orsay using an ac system with built-in compensation for zero magnetic field resistance. For all samples measurements were made with H perpendicular to the sample surface, or with H parallel to the surface and in the current direction. For some samples, measurements were made with H parallel to the surface and perpendicular to the current, and for some others, measurements were made as a function of the angle between H and the normal to the surface, either in the plane or perpendicular to the plane of the current I. Magnetization measurements have been performed at LCR Thomson-CSF on several samples using a S Q U I D magnetometer.
481
H T=4.2 K R o = O. 28 -"L
¢n
Ro
iii
0 Z < I(f)
T-=247 K Ro = 0 . 4 5 -n,-
LU
n~ I
NO
=51~ " o
o
I-UJ
Z rj <
~E I
0.15 H
0 Tesla
z
0.15
Fig. 1. Magnetoresistance for a A g 6 0 / C o 6 0 sample prepared on a sapphire substrate. The field is in-plane and perpendicular to the current.
3. Results We have found that for all the samples the magnetization is spontaneously in the plane of the layers, which means that the perpendicular anisotropy is insufficient to overcome the demagnetizing field. In fig. 1 we show typical results for H in the layer plane and perpendicular to I for a A g 6 0 / C o 6 0 sample (the one sample on a sapphire substrate). The MR peaks occur in the vicinity of H = H c the coercive field. In this case there is a sizeable 11.2% MR at 4.2 K (we define the M R % as M R % = 1 0 0 A R / R o where AR is the maximal change in the resistance). We also show the results for the same sample at 247K, and notice that an appreciable part of the decrease in M R % is due to the increase in the zero field resistance R a. In fact 60% of the drop in M R % comes from the increase in R 0 and 40% comes from the drop in AR. This is directly
related to the long mean free path found in the A g / C o system [10, 11]. We believe that the MR arises from the same mechanism already proposed for A u / C o [3, 5], namely to some degree of antiparallel alignment of the magnetizations of neighbouring Co layers during the magnetization reversal. We note that the M R % of one of our sputtered Co films is about 1% and, on this basis, we dismiss domain wall scattering in the Co as a secondary effect. It is interesting to note the dependence of the MR on the thickness of Ag in fig. 2 and table 1. The decrease in M R % as the thickness decreases cannot be accounted for by the relatively small increase in resistivity. It is probably caused by the progressive onset of ferromagnetic interactions between the Co layers across the Ag. Such interactions by coupling the magnetization reversal in neighbouring Co layers are expected to reduce
D.H. Mosca et al. / Magnetoresistance of Co-based multilayered structure,;
482
12 10 8
decrease of the MR for increasing thickness as predicted by theories [8, 9]. The MR of C u x / C o 3 0 exhibits a similar increase with x above 45,~, but at smaller x the MR increases again and exhibits a maximum at o x = 20A. We suggest that this maximum is due to the existence of antiferromagnetic coupling between the Co layers as already found by spin polarized low energy electron diffraction and neutron diffraction in this thickness range [14, 15]. We have also studied the MR of multilayers of the type Agx/Co60/Agx/Coy and Cux/ Co60/Cux/Coy (fig. 3), in which alternate Co layers have different thicknesses 60 and y ,~ (10,~ < y < 60,~). Making such multilayers was inspired by the work of Dupas et al. [5] for the A u / C o system. In C o / A u / C o sandwiches with two different Co thicknesses these authors have obtained magnetization reversals at two well defined different fields. In this way, they were able to produce a more complete antiparallel alignment between these two fields and an enhance-
O
Co60/Agx • Co30/Cux
/
o
/
8
C
~
6
o
4
~
2
/ ~-~-o---7".
0
10
20
. . . . . . .
30
40
50
60
70
Thickness of Ag or Cu layer, x(A)
Fig. 2. The maximum magnetoresistance ratio as a function of the thicknesses of Ag or Cu for the series of Agx/Co60 and C u x / C o 3 0 multilayers. The magnetic field is perpendicular to the film plane. The temperature is 4.2 K. The lines are guides to the eye.
the degree of antiparallel alignment and to lower the MR. Earlier measurements on A g / C o samples with equal thickness layers indicated that the MR% became a maximum for thicknesses = 60.~. This suggests that above 60,~ the interlayer coupling can be neglected and that one recovers the
Table 1 In this table we summarize the magnetoresistance data at 4.2 K for all series of samples studied. Parallel refers to measurements with H in the plane of the sample and parallel to the current. Normal refers to H perpendicular to the sample. The number of periods is given in brackets. The top layer was always silver or copper
normal
(thickness in ,~)
0.04 0.18 1.07 3.10 11.15
0.50 0.14 0.18 1.61 2.70 11.50
(Cu 10/Co30),)o (Cu 15/Co30)a0 (Cu20/Co30)7z (Cu30/Co30)6o (Cu45/Co30)4o (Cu60/Co30)40
(Ag30/Co60/Ag30/Co 15)44
1.50
3.60
(Ag30/Co60/Ag30/Co20)43
3.50
4.00
(Ag30/Co60/Ag30/Co25)41 (Ag30/Co60/Ag30/Co30)40
2.26 2.10
2.88 2.10
(thickness in A)
(Agl0/Co60)s7 (Agl 5/Co60)s3 (Ag20/Co60)s() (Ag30/Co60)44 (Ag40/Co60)40 (Ag60/Co60)42
parallel -
MR(%)
Samples
MR(%)
Samples
(Ag60/Co60/Ag60/Co 15)30 (Ag60/Co60/Ag60/Co30)3o
9.00 5.78
14.55 6.09
(Ag60/Co60/Ag60/Co45)30
4.90
4.83
parallel
normal
0.60 0.37 4.1/0 3.20 1.46 4.40
0.81 0.60 4.93 3.50 1.9() 3.4(I
5.90
6.00
3.37
4.87
(Cu30/Co60/60/ Cu30/Co30)41 )
3.90
4.53
(Cu60/Co60/ Cu60/Co10)40
10.08
8.44
(Cu30/Co60/ Cu30/Co 15)44 (Cu30/Co60/ Cu30/Co20)4~
483
D.H. Mosca et al. /Magnetoresistance of Co-based muhilayered structures 15
Ag30/Co60/Ag30/Coy o Ag60/Co60/Ag60/Coy •
•
~
~
Cu30/Co6OlCu30/Coy
H
[] 0°
Cu60/Co60/Cu60/Coy •
10
Q) O
-
i
i
i
i
I
i
10
20
30
40
50
60
Thickness
of
Co
layer,
70
y(A)
Fig. 3. The maximum magnetoresistance effect as a function of the Co thickness for different series of samples. The magnetic field is perpendicular to the film plane. The temperature is 4.2 K. The lines are guides to the eye.
z O3 O3 UJ
n-
__J
O
ment in the MR. In our multilayers with two different Co thicknesses, although a reversal of the magnetization in two well-defined steps cannot be seen in the rather smooth magnetization loops obtained to date, comparison of the results in figs. 2 and 3 shows that some enhancement in the MR occurs especially for low thicknesses of the variable Co layers. Furthermore, in fig. 3 we see that for all the systems studied the MR b decreases as y increases, which, if our interpretation is correct, suggests a greater degree of parallel alignment for y small. We note again that for both Ag and Cu, the series with x = 60,~, has the largest MR%. In fig. 4 we show the change in the magnetoresistance for A g 6 0 / C o 6 0 / A g 6 0 / C o l 5 as a function of the angle 0 between H and the normal to the sample, in the plane of I and the normal. For 0 = 0 °, we have a typical curve with H perpendicular to the sample. With this field orientation a much larger field is necessary to overcome the demagnetizing field. However, as soon as the field has a component in the plane of the sample, the peak corresponding to the reversal of the in-plane magnetization appears and develops to its final state at 0 = 90 °. In this sample we have found the highest M R % to date, 16% at 4.2K (fig. 5). We note that, surprisingly, this maximum is observed for a field at 45 ° from the normal in the plane
4 5.0
O
Ag60JCo60[Ag601Co15
_S -3.0
I
-2.0
90.0 l
t
0
-tO H
1D
[ 2.0
3.0
Tesla
Fig. 4. Magnetoresistance at 4.2K for A g 6 0 / C o 6 0 / A g 6 0 / Co15 as a function of the angle between H and the normal to the surface of the sample in the plane of the current and the normal. The curves are given for decreasing fields only.
perpendicular to the current. We have no explanation why this maximum should occur in a tilted direction. A large MR, 10% at 4.2K, was also found in the similar system C u 6 0 / C o 6 0 / C u 6 0 / Col0.
4. C o n c l u s i o n s
We have studied the MR of A g / C o and C u / C o multilayers and we have found significant MR peaks in the field range where the magnetization is reversed. The maximum observed M R % is 16% at 4.2K and this change occurs in a field range of the order of magnitude of 100Oe.
484
D.H. Mosca et al. /,14ag* etore~is'tal ce 0[ Co-hased multiho'cred ,~tructure.s
Acknowledgement
~67.T: 4 . 2 K
h45" I
Ag60JCo601Ag601Co15
t~
This work was supported in part by the National Science Foundation under grant no. DMR11-13287. The Laboratoirc de Physique des Solides is Laboratoire Associd du Centre National de la Recherche Scicntifique.
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References
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Fig. 5. Magnetoresistance curves of a A g 6 0 / C o f O / A g 6 0 / (7o15 multilayer at 4.2K. The ctuwes fire for a magnetic field at 45 ° from the normal to the film plane in a plane perpendicular to the current (see inset).
We believe that the MR is similar in nature to that already observed in the A u / C o system and ascribed to partial alignment of the magnetizations of neighbouring Co layers during the magnetization reversal. The MR is enhanced tor multilayers in which the alternate Co layers have different thicknesses. The strong increase in MR as the Ag layer increases in thickness indicates that for significant MR it is necessary to avoid ferromagnetic coupling between neighbouring Co layers, while maintaining a long electron mean free path. Above a thickness of Ag of about 60,~ we anticipate that the MR will decrease in agreement with the conventional scaling of the MR with the ratio of the thickness to the mean free path. The only difference for C u / C o is a maximum of the MR for a Cu thickness = 20A, which should be due to the existence of antiferromagnetic interlayer coupling.
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