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Superconducting proximity effect in Cu-clad Nb wires doped with Mn, Fe, Co and Ni
PflYSICAI
Physica B 186-188 (1993) 1053-1055 North-Holland
Superconducting proximity effect in Cu-clad Nb wires doped with Mn, Fe, Co and Ni Y. Oda a, A. Sumiyama a, H. T o y o d a b and K. A s a y a m a b "Faculty of Science, Himeji Institute of Technology, Hyogo, Japan bDepartment of Material Physics, Osaka University, Toyonaka, Japan The superconducting proximity effect in Cu-clad Nb wires, doped with dilute magnetic impurities, has been studied in relation to the Kondo effect. The spin flip scattering by Mn is found to be rapidly suppressed below 0.5 K, probably due to the Mn-Mn interaction, while that by Fe, Co and Ni decreases linearly with temperature below 1 K.
The coherence length e N is given as
1. Introduction It is known as the superconducting proximity effect that a normal metal in good electrical contact with a superconducting material exhibits superconducting properties [1-4]. If magnetic impurities are doped into a normal metal, it is well known to show the dilute Kondo effect [5,6]. The spin flip scattering should cause the pair breaking of the induced superconductivity in the normal metal [7-9]. The thickness, p, of the induced Meissner region of Cu-clad Nb wires doped with magnetic impurities was measured over a temperature range between 9 K and 50 mK. From these measurements, it has been investigated how the magnetic impurities affect the proximity effect. As for Fe, Co and Ni impurities in Cu, of which the Kondo temperatures T K are about 10, 103 and 105 K, respectively, we have already studied the impurity effect [7]. In this paper, we have mainly studied the pair breaking effect by Mn impurities in Cu, of which T K is below 10 i K. It is noted that the use of the proximity effect is a new experimental technique to study the pair breaking effect in typical Kondo alloys such as noble metals. The theoretical derivation of p was first made by de Gennes and his coworkers [1]: p = e N[Ln(eN/A(0)) - 0.116]
(1)
where e N is the coherence length and A(0)/e N = K(0) is the Ginzburg-Landau parameter at the interface in the normal side.
Correspondence to: Y. Oda, Faculty of Science, Himeji Institute of Technology, Kamigori-cho, Ako-gun, Hyogo 678-12, Japan.
eN = (hVFiN/6,trkn)l/2(T + ha/aXkB) ,/2
(2)
in the case of the dirty limit (e N >> IN) of the normal metal which includes magnetic impurities [1,4]. Here, a is the probability of the spin flip scattering by magnetic impurities. Thus, a = 0 in the normal metal which does not include magnetic impurities. And rE, l N and T are the Fermi velocity, the electronic mean free path and temperature, respectively. Recently, Narikiyo and Fukuyama have made a theoretical calculation of p, taking account of the pair breaking effect and the effective repulsive interaction due to magnetic impurities [9]. Our results are discussed compared with their theory [7-8].
2. Experimental Seven Cu-clad Nb samples, pure Cu, Mn (26, 42 and 91ppm), Fe (68ppm), Co (183ppm) and Ni (508 ppm) were prepared. Hereafter, they are called pure Cu, Mn-26, Mn-42, and Mn-91, Fe-68, Co-183 and Ni-508 samples, respectively. For the measurement of the resistivity, another wire of Cu doped with Mn 91 ppm without a Nb core was also prepared. The method of sample preparation and the dimension of the pure, Fe, Co and Ni samples are presented in refs. [7,8], together with the measuring technique. The radius of the Mn sample wires is about 56 i~m and that of Nb core is 22 ~m. The thickness of Cu, thus, is about 34 ~m. In order to control the electronic mean free path l N, the samples were annealed at several temperatures below 600°C. The concentration was
0921-4526/93/$06.00 © 1993- Elsevier Science Publishers B.V. All rights reserved
1054
Y. Oda et al. / Proximity effect in Cu-Nb with magnetic impurities
confirmed to be unchanged before and after the annealing. The main impurities which are included in the original Cu material were F e 6 p p m , N i 6 p p m , and Mn 0.5 ppm (atomic ratio).
oo
oo
25
o o
Mn 26
o°
oo oo oo o
20
g g g g oo
3. Results and discussion
3.1. Fe, Co and Ni impurities The temperature dependence of the Meissner length p of the annealed Fe-68, Co-183, Ni-508 and pure Cu is summarized here [7]. All p's are linear functions of T 1/2 below 1 K. No anomaly was observed even in the case of the F-68 sample of which Kondo temperature, TK, is nearly equal to T c of Nb. This indicates that a is proportional to temperature (or negligibly small). As for nonannealed samples, similar temperature dependence of p is obtained, while the absolute value is smaller. Our results are reproduced by calculation according to the theory of Narikiyo and F u k u y a m a qualitatively, adopting the K o n d o temperatures of 1 0 K for Fe, 10 3 K for Co and 105 K for Ni. In case of Fe sample, however, the calculated p value becomes considerably larger, if T K is assumed to be oc (nonmagnetic). This means that the a term in eq. (2) cannot be neglected in comparison with T even below 1 K, although a decreases linearly to T.
o_
15
o
g g o o
oo g g
10 8
g
g
oo
~o o
o
_J 1
Fig. 1. Temperature dependence of p of Mn-26. temperature. It was found that p sample (long IN) shows the up-turn tures as shown in figs. 1 and 2, but nealed one (short lN) seems to be temperatures.
of the annealed at low temperap of the nonansaturated at low
3.2. Mn impurity 12,
Figures 1 and 2 show the temperature dependence of p of Mn-26, Mn-42 and Mn-91, which were all annealed. As for Mn-26 in fig. 1, the upturn of p against T - 1 / 2 is observed around about 5 0 0 m K ( T 1/2 = 1.4) with decreasing temperature. Similar behaviors are also observed in fig. 2. This result suggests that a term of Mn impurities rapidly decreases below 500 mK. Below about 7 0 m K ( T ~/2 = 3.8), p in fig. 1 seems to b e c o m e saturated. This is because p becomes comparable with the thickness of Cu, and is not an intrinsic property. The value of p rapidly decreases with increasing concentration of Mn impurities. The values of the electronic mean free path, IN, of Mn-26, Mn-42 and Mn-91 are 4.3, 3.2 and 3.0 ~m, respectively. F r o m eq. (2), the change of l N from 4.3 to 3.2 should cause only a 14% decrease in p. Therefore, the rapid decrease of p with Mn concentration cannot be attributed to the change of l N , but is attributed to the change of a, which depends on Mn concentration. The temperature dependence of p was also studied as for Mn samples with a different electronic mean free path, which was controlled by the annealing
o o
10
g
Mn 42
o
o
o. o*
oo o
o o o
o o
#
y'.
0
4,~A A
,
1
,
2 3 T_v.z(K4,2 )
4
Fig. 2. Temperature dependence of p of Mn-42 and Mn-91.
Y. Oda et al. / Proximity effect in Cu-Nb with magnetic impurities
Narikiyo and Fukuyama pointed out that an up-turn should be observed around T K, when the temperature is decreased beyond T K. However, it may be difficult that the up-turn in figs. 1 and 2 are attributed to decreasing of the spin flip scattering by single impurities (dilute Kondo effect), because the temperature of 500 mK is too high compared with the reported T K (less than 100 mK) of Mn in Cu. Furthermore, the tendency of saturation of p of the nonannealed samples could not be explained by the theory. Therefore, our result suggests another origin of decreasing of the spin flip scattering. Figure 3 shows the resistivity of Cu wires doped with Mn 9 1 p p m without Nb core, of which one is annealed (/N = 3.2 Ixm) and the other is nonannealed (l N = 0.67). The resistivity becomes minimum around 17K. In the present case, however, the resistivity maximum is also observed. The temperatures of the maximum are about 450 and 500 mK for the nonannealed and the annealed wires, respectively. The minimum resistivities, Rmin, of the nonannealed and the annealed wires are 99.9 and 20.7 n12 cm, respectively. The temperature of the resistivity maximum of Mn34ppm pieces was about 150mK (not shown). Thus, the temperature of the maximum decreases with decreasing concentration of Mn impurity. The temperatures of the resistivity maximum are almost equal to those of the departure from the Curie-Weiss law in susceptibility, which is explained by the Mn-Mn interaction [10]. Thus, the temperature dependence of the resistivity may be understood through the R K K Y interaction between Mn impurities. This interaction may depend on the electronic
10
'
1
I
oo*° a ° ° ° * o%
o°
o
°°oonOn annealed
• • " o
E
%
o°
%
n," II I!
o o
4
o
o o
o oo
2
~ A m
A A~'~A~
*%.
annealed A
o
%
A~ AA~
~A
~ AAAA~ A ~
Z
~ ° ° %
o i
0.05
I
0,1
I
I
0.5
I
T(K) 5
10
50
Fig. 3. Temperature dependence of the resistivity of nonanhealed and annealed Mn 91 ppm wires.
1055
mean free path, l N. If l N is long, the increasing rate of the resistivity should be much suppressed because the R K K Y interaction becomes stronger. As for the l N dependence of the RKKY interaction, Heeger et al. have made a Cu NMR study of C u - M n dilute alloys doped with various amounts of A1 as nonmagnetic scatterers [11]. The line width of Cu N M R becomes narrow with increasing the concentration of Al. This is explained by the RKKY interaction which depends o n [ N. The A1 impurities dec r e a s e /N, weaken the R K K Y interaction, and narrow the line width. The up-turns of p, as shown in figs. 1 and 2, may be explained by the R K K Y interaction. When l N is long, the spin fluctuation of Mn impurities is slow or almost stops, because this interaction is strong. So the probability of spin flip scattering decreases and p increases. If l N is short, the spin fluctuation is not suppressed, and then p decreases. The authors are grateful to Professor H. Fukuyama and Dr. O. Narikiyo, and Professor T. Matuura for valuable discussions. Appreciation is also expressed to Dr. M. Nagata of Sumitomo Electric Industries, Ltd., for valuable discussions and cooperation for making the wire samples.
References
[1] G. Deutscher and P.G.de Gennes, Superconductivity, ed. R.D. Parks, Vol. 2 (Marcel Dekker, New York, 1969) p. 1005. [2] Y. Oda and H. Nagano, Solid State Commun. 35 (1980) 631. [3] Y. Oda, A. Sumiyama and H. Nagano, Jpn. J. Appl. Phys. 22 (1983) 464. [4] Th. Bergmann, K.H. Kuhl, B. Schroder, M. Jutzler and F. Pobell, J. Low. Temp. Phys. 66 (1987) 209. [5] M.D. Daybell and W.A. Steyert, Phys. Rev. Lett. 18 (1967) 398; Phys. Rev. 167 (1968) 536. [6] T. Matsuura, S. Ichinose and Y. Nagaoka, Prog. Theor. Phys. 57 (1977) 713. [7] H. Toyoda, A. Sumiyama, Y. Oda and K. Asayama, J. Phys. Soc. Jpn. 59 (1990) 4215. [8] H. Toyoda, A. Sumiyama, Y. Oda and K. Asayama, J. Phys. Soc. Jpn. 62 (1993) 672. [9] O. Narikiyo and H. Fukuyama, J. Phys. Soc. Jpn. 58 (1989) 4557. [10] E.C. Hirschoff, O.G. Symko and J.C. Wheatley, Phys. Lett. A 33 (1970) 19. [11] A.J. Heeger, A.P. Klein and P. Tu, Phys. Rev. Lett. 17 (1966) 803.
Physica B 186-188 (1993) 1053-1055 North-Holland
Superconducting proximity effect in Cu-clad Nb wires doped with Mn, Fe, Co and Ni Y. Oda a, A. Sumiyama a, H. T o y o d a b and K. A s a y a m a b "Faculty of Science, Himeji Institute of Technology, Hyogo, Japan bDepartment of Material Physics, Osaka University, Toyonaka, Japan The superconducting proximity effect in Cu-clad Nb wires, doped with dilute magnetic impurities, has been studied in relation to the Kondo effect. The spin flip scattering by Mn is found to be rapidly suppressed below 0.5 K, probably due to the Mn-Mn interaction, while that by Fe, Co and Ni decreases linearly with temperature below 1 K.
The coherence length e N is given as
1. Introduction It is known as the superconducting proximity effect that a normal metal in good electrical contact with a superconducting material exhibits superconducting properties [1-4]. If magnetic impurities are doped into a normal metal, it is well known to show the dilute Kondo effect [5,6]. The spin flip scattering should cause the pair breaking of the induced superconductivity in the normal metal [7-9]. The thickness, p, of the induced Meissner region of Cu-clad Nb wires doped with magnetic impurities was measured over a temperature range between 9 K and 50 mK. From these measurements, it has been investigated how the magnetic impurities affect the proximity effect. As for Fe, Co and Ni impurities in Cu, of which the Kondo temperatures T K are about 10, 103 and 105 K, respectively, we have already studied the impurity effect [7]. In this paper, we have mainly studied the pair breaking effect by Mn impurities in Cu, of which T K is below 10 i K. It is noted that the use of the proximity effect is a new experimental technique to study the pair breaking effect in typical Kondo alloys such as noble metals. The theoretical derivation of p was first made by de Gennes and his coworkers [1]: p = e N[Ln(eN/A(0)) - 0.116]
(1)
where e N is the coherence length and A(0)/e N = K(0) is the Ginzburg-Landau parameter at the interface in the normal side.
Correspondence to: Y. Oda, Faculty of Science, Himeji Institute of Technology, Kamigori-cho, Ako-gun, Hyogo 678-12, Japan.
eN = (hVFiN/6,trkn)l/2(T + ha/aXkB) ,/2
(2)
in the case of the dirty limit (e N >> IN) of the normal metal which includes magnetic impurities [1,4]. Here, a is the probability of the spin flip scattering by magnetic impurities. Thus, a = 0 in the normal metal which does not include magnetic impurities. And rE, l N and T are the Fermi velocity, the electronic mean free path and temperature, respectively. Recently, Narikiyo and Fukuyama have made a theoretical calculation of p, taking account of the pair breaking effect and the effective repulsive interaction due to magnetic impurities [9]. Our results are discussed compared with their theory [7-8].
2. Experimental Seven Cu-clad Nb samples, pure Cu, Mn (26, 42 and 91ppm), Fe (68ppm), Co (183ppm) and Ni (508 ppm) were prepared. Hereafter, they are called pure Cu, Mn-26, Mn-42, and Mn-91, Fe-68, Co-183 and Ni-508 samples, respectively. For the measurement of the resistivity, another wire of Cu doped with Mn 91 ppm without a Nb core was also prepared. The method of sample preparation and the dimension of the pure, Fe, Co and Ni samples are presented in refs. [7,8], together with the measuring technique. The radius of the Mn sample wires is about 56 i~m and that of Nb core is 22 ~m. The thickness of Cu, thus, is about 34 ~m. In order to control the electronic mean free path l N, the samples were annealed at several temperatures below 600°C. The concentration was
0921-4526/93/$06.00 © 1993- Elsevier Science Publishers B.V. All rights reserved
1054
Y. Oda et al. / Proximity effect in Cu-Nb with magnetic impurities
confirmed to be unchanged before and after the annealing. The main impurities which are included in the original Cu material were F e 6 p p m , N i 6 p p m , and Mn 0.5 ppm (atomic ratio).
oo
oo
25
o o
Mn 26
o°
oo oo oo o
20
g g g g oo
3. Results and discussion
3.1. Fe, Co and Ni impurities The temperature dependence of the Meissner length p of the annealed Fe-68, Co-183, Ni-508 and pure Cu is summarized here [7]. All p's are linear functions of T 1/2 below 1 K. No anomaly was observed even in the case of the F-68 sample of which Kondo temperature, TK, is nearly equal to T c of Nb. This indicates that a is proportional to temperature (or negligibly small). As for nonannealed samples, similar temperature dependence of p is obtained, while the absolute value is smaller. Our results are reproduced by calculation according to the theory of Narikiyo and F u k u y a m a qualitatively, adopting the K o n d o temperatures of 1 0 K for Fe, 10 3 K for Co and 105 K for Ni. In case of Fe sample, however, the calculated p value becomes considerably larger, if T K is assumed to be oc (nonmagnetic). This means that the a term in eq. (2) cannot be neglected in comparison with T even below 1 K, although a decreases linearly to T.
o_
15
o
g g o o
oo g g
10 8
g
g
oo
~o o
o
_J 1
Fig. 1. Temperature dependence of p of Mn-26. temperature. It was found that p sample (long IN) shows the up-turn tures as shown in figs. 1 and 2, but nealed one (short lN) seems to be temperatures.
of the annealed at low temperap of the nonansaturated at low
3.2. Mn impurity 12,
Figures 1 and 2 show the temperature dependence of p of Mn-26, Mn-42 and Mn-91, which were all annealed. As for Mn-26 in fig. 1, the upturn of p against T - 1 / 2 is observed around about 5 0 0 m K ( T 1/2 = 1.4) with decreasing temperature. Similar behaviors are also observed in fig. 2. This result suggests that a term of Mn impurities rapidly decreases below 500 mK. Below about 7 0 m K ( T ~/2 = 3.8), p in fig. 1 seems to b e c o m e saturated. This is because p becomes comparable with the thickness of Cu, and is not an intrinsic property. The value of p rapidly decreases with increasing concentration of Mn impurities. The values of the electronic mean free path, IN, of Mn-26, Mn-42 and Mn-91 are 4.3, 3.2 and 3.0 ~m, respectively. F r o m eq. (2), the change of l N from 4.3 to 3.2 should cause only a 14% decrease in p. Therefore, the rapid decrease of p with Mn concentration cannot be attributed to the change of l N , but is attributed to the change of a, which depends on Mn concentration. The temperature dependence of p was also studied as for Mn samples with a different electronic mean free path, which was controlled by the annealing
o o
10
g
Mn 42
o
o
o. o*
oo o
o o o
o o
#
y'.
0
4,~A A
,
1
,
2 3 T_v.z(K4,2 )
4
Fig. 2. Temperature dependence of p of Mn-42 and Mn-91.
Y. Oda et al. / Proximity effect in Cu-Nb with magnetic impurities
Narikiyo and Fukuyama pointed out that an up-turn should be observed around T K, when the temperature is decreased beyond T K. However, it may be difficult that the up-turn in figs. 1 and 2 are attributed to decreasing of the spin flip scattering by single impurities (dilute Kondo effect), because the temperature of 500 mK is too high compared with the reported T K (less than 100 mK) of Mn in Cu. Furthermore, the tendency of saturation of p of the nonannealed samples could not be explained by the theory. Therefore, our result suggests another origin of decreasing of the spin flip scattering. Figure 3 shows the resistivity of Cu wires doped with Mn 9 1 p p m without Nb core, of which one is annealed (/N = 3.2 Ixm) and the other is nonannealed (l N = 0.67). The resistivity becomes minimum around 17K. In the present case, however, the resistivity maximum is also observed. The temperatures of the maximum are about 450 and 500 mK for the nonannealed and the annealed wires, respectively. The minimum resistivities, Rmin, of the nonannealed and the annealed wires are 99.9 and 20.7 n12 cm, respectively. The temperature of the resistivity maximum of Mn34ppm pieces was about 150mK (not shown). Thus, the temperature of the maximum decreases with decreasing concentration of Mn impurity. The temperatures of the resistivity maximum are almost equal to those of the departure from the Curie-Weiss law in susceptibility, which is explained by the Mn-Mn interaction [10]. Thus, the temperature dependence of the resistivity may be understood through the R K K Y interaction between Mn impurities. This interaction may depend on the electronic
10
'
1
I
oo*° a ° ° ° * o%
o°
o
°°oonOn annealed
• • " o
E
%
o°
%
n," II I!
o o
4
o
o o
o oo
2
~ A m
A A~'~A~
*%.
annealed A
o
%
A~ AA~
~A
~ AAAA~ A ~
Z
~ ° ° %
o i
0.05
I
0,1
I
I
0.5
I
T(K) 5
10
50
Fig. 3. Temperature dependence of the resistivity of nonanhealed and annealed Mn 91 ppm wires.
1055
mean free path, l N. If l N is long, the increasing rate of the resistivity should be much suppressed because the R K K Y interaction becomes stronger. As for the l N dependence of the RKKY interaction, Heeger et al. have made a Cu NMR study of C u - M n dilute alloys doped with various amounts of A1 as nonmagnetic scatterers [11]. The line width of Cu N M R becomes narrow with increasing the concentration of Al. This is explained by the RKKY interaction which depends o n [ N. The A1 impurities dec r e a s e /N, weaken the R K K Y interaction, and narrow the line width. The up-turns of p, as shown in figs. 1 and 2, may be explained by the R K K Y interaction. When l N is long, the spin fluctuation of Mn impurities is slow or almost stops, because this interaction is strong. So the probability of spin flip scattering decreases and p increases. If l N is short, the spin fluctuation is not suppressed, and then p decreases. The authors are grateful to Professor H. Fukuyama and Dr. O. Narikiyo, and Professor T. Matuura for valuable discussions. Appreciation is also expressed to Dr. M. Nagata of Sumitomo Electric Industries, Ltd., for valuable discussions and cooperation for making the wire samples.
References
[1] G. Deutscher and P.G.de Gennes, Superconductivity, ed. R.D. Parks, Vol. 2 (Marcel Dekker, New York, 1969) p. 1005. [2] Y. Oda and H. Nagano, Solid State Commun. 35 (1980) 631. [3] Y. Oda, A. Sumiyama and H. Nagano, Jpn. J. Appl. Phys. 22 (1983) 464. [4] Th. Bergmann, K.H. Kuhl, B. Schroder, M. Jutzler and F. Pobell, J. Low. Temp. Phys. 66 (1987) 209. [5] M.D. Daybell and W.A. Steyert, Phys. Rev. Lett. 18 (1967) 398; Phys. Rev. 167 (1968) 536. [6] T. Matsuura, S. Ichinose and Y. Nagaoka, Prog. Theor. Phys. 57 (1977) 713. [7] H. Toyoda, A. Sumiyama, Y. Oda and K. Asayama, J. Phys. Soc. Jpn. 59 (1990) 4215. [8] H. Toyoda, A. Sumiyama, Y. Oda and K. Asayama, J. Phys. Soc. Jpn. 62 (1993) 672. [9] O. Narikiyo and H. Fukuyama, J. Phys. Soc. Jpn. 58 (1989) 4557. [10] E.C. Hirschoff, O.G. Symko and J.C. Wheatley, Phys. Lett. A 33 (1970) 19. [11] A.J. Heeger, A.P. Klein and P. Tu, Phys. Rev. Lett. 17 (1966) 803.