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Ge sandwiches
PHYSICAl',
Physica B 194-196 (1994) 2373-2374 North-Holland
Influence of the interface r o u g h n e s s on 3c of G e / P b / G e wiches
sand-
K. Temst, M. Baert, E. R.osseel, M. Dhall6*, C.D. Potter, V.V. Moshchalkov, C. Van Haesendonck, and Y. Bruynseraede Laboratorium voor Vaste Stof-Fysika en Magnetisme, Katholieke Universiteit Leuven, B-3001 Leuyen, Belgium V. V. Metlushko and G. G/intherodt 2. Physikalisches Institut, RWTH Aachen, W-5100 Aachen, Germany L. Hellemans, J. Snauwaert Laboratorium voor Chemische en Biologische Dynamica, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium " now at Blackett Laboratory, Imperial College, London SW7 2BZ, UK Critical current densities Jr for fields applied perpendicular to MBE prepared G e / P b / G e sandwiches and bare Pb films have been obtained from magnetization measurements. The single Pb film shows smooth magnetization loops at all temperatures and a monotonous decrease of Jr with rising temperature. The G e / P b / G e sandwiches reveal peaks in the magnetization curves at lower temperatures and become smooth only in the vicinity of T¢ ('-- 7K). 3c(T) for the sandwiches shows a well defined maximum at approximately 5K. As a result, the Jr values of this sandwich are increased by a factor of 3 in the temperature range 5-7K. This different behaviour may be due to enhanced pinning at the Ge-Pb interfaces, which were studied using Atomic Force Microscopy (AFM). 1. PREPARATION Superconducting critical parameters of artificially prepared N-S-N (normal/superconductor/normal) sandwiches may be strongly modified by the interface roughness which can eventually act as a collection of different pinning centers to confine the flux lines. To study this effect we have prepared G e ( 5 0 h ) / P b ( 2 2 0 0 ] t ) / G e ( 5 0 0 . ~ ) sandwiches and reference bare P b ( 2 2 0 0 ~ ) films. The samples were prepared by electron beam evaporation in a MBE apparatus with a base pressure of 2 x 10 -9 Torr. The evaporation rates, controlled by a quadrupole mass spectrometer, are 15 /s for Pb and 4 / ~ / s for Ge. In order to obtain continuous thin Pb films the SiO2 substrates are held at liquid nitrogen temperature during preparation. This low substrate temperature leads to crystalline Pb (111 textured) layers while the Ge is amorphous. The structure and thickness of the crystalline Pb layers was checked using X-ray Diffraction. Both Pb films show the same atomic orientation and texture. 2. RESULTS The chosen thickness of the Pb films is below the critical thickness of about 4000 ~ which corre-
E
4
• ~(soA)/pb(22ooA)/G~(sooA)
O
3 O
v
O
321 , p ~ 0
Tempera(K) ture
Fig. 1: Temperature dependence of the zero field critical current for a Ge/Pb/Ge sandwich ( + ) and a bare Pb film (Q). sponds to the crossover from type II to type I superconductivity in Pb films [1]. We have studied temperature and field dependences of the magnetization M(T,H±) using an MPMS2 Q u a n t u m Design SQUID magnetometer which has a field homogeneity better than 0.05 % . The irreversible contribution to M ( T , H x ) has been used to calcu-
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1819-8
/ \
2374
late the critical current values. Temperature dependences of the critical current Jc(T) determined from the zero field width of the M(T,Hj_) hysteresis loops are shown in Fig. 1. These Jc(T) dependences demonstrate a clear difference between the N-S-N sandwich and a reference S-sample with the same thickness of the superconducting layer. The Jc(T) curve for the N-S-N sandwiches shows a maximum at T_~ 5K (Fig. 1). In the temperature range 5-7K the Jc(T) values for the sandwich are systematically larger than the J r ( T ) values for the reference bare Pb film by a factor of 3. Below 4K both Jc(T) curves are characterized by approximately the same value of the critical current. The enhancement of Jc(T) at T = 5 - 7 K can be related to the presence of additional pinning centers at the P b / G e interfaces. This tentative interpretation is supported by the appearance of anomalous peaks in the magnetization curves of the N-S-N sandwiches at low temperatures (Fig. 2). While the bare Pb film shows a smooth regular field dependence (Fig. 2 (a)), in a field H± < 0.3 kG, the Ge-Pb-Ge sandwich shows several M(Hx) maxima and minima, thus forming a characteristic irregular low field M(Hx) pattern, which is symmetric with respect to H± and fully reproducible. We want to stress that these types of M(H) curves have been observed in several other similar sandwiches and multilayers as well. This anomalous M(H±) pattern may result from the matching between the flux lattice period and M(emu) 0.03 (a)
T=4K
0.02 0.01
Xf
0 -0.01 -0.02
M(emu)
0.03
00,°°2'°' 001
-0.02 -0.03 -
. . . . . . . . . . . . . . . . . . --0.5 0 0.5
Hl(kG) Fig. 2(b): Magnetization at 4K for the Ge/Pb/Ge sandwich. The error in M is smaller than the markers. the available characteristic length scales of the pinning potential U(x,y) at the P b / G e interfaces in the N-S-N sandwiches. Following Larkin and Ovchinnikov [2], the M(H±) maxima (Fig. 2 (b)) may be related to the successive coincidence of the flux lattice period a~(H±) with the peaks in the Fourier transform of the pinning potential. As there is a correlation between roughness and deposition rate, one may assume that the pinning potential is mainly created by the variations in Pb film thickness, which can be checked by AFM. Preliminary AFM studies of both the Pb and Ge surfaces (measured after each step in the deposition process) revealed the presence of several maxima, ranging from 2000 to 8000/~ in the Fourier transform. This roughness length scale is larger than the lateral AFM resolution ( + 50 nm) due to the tip radius and has been used to estimate the expected positions of the M(H±) maxima. These positions are in a reasonable agreement with the positions of the anomalous M(H±) peaks shown in Fig.2b.
Work supported by the Belgian High Temperature Superconductivity Incentive and Concerted Action Programs. REFERENCES
-0.03
Fig. film.
2(a):
-0.5
0 0.5 H~(kG) Magnetization at 4K for the bare Pb
[1] G.J. Dolan, J. Low Temp. Phys. 15 (1974) 133 [2] A.I. Larkin and Yu. N. Ovchinnikov, J. Low Temp. Phys. 34 (1979) 409
Physica B 194-196 (1994) 2373-2374 North-Holland
Influence of the interface r o u g h n e s s on 3c of G e / P b / G e wiches
sand-
K. Temst, M. Baert, E. R.osseel, M. Dhall6*, C.D. Potter, V.V. Moshchalkov, C. Van Haesendonck, and Y. Bruynseraede Laboratorium voor Vaste Stof-Fysika en Magnetisme, Katholieke Universiteit Leuven, B-3001 Leuyen, Belgium V. V. Metlushko and G. G/intherodt 2. Physikalisches Institut, RWTH Aachen, W-5100 Aachen, Germany L. Hellemans, J. Snauwaert Laboratorium voor Chemische en Biologische Dynamica, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium " now at Blackett Laboratory, Imperial College, London SW7 2BZ, UK Critical current densities Jr for fields applied perpendicular to MBE prepared G e / P b / G e sandwiches and bare Pb films have been obtained from magnetization measurements. The single Pb film shows smooth magnetization loops at all temperatures and a monotonous decrease of Jr with rising temperature. The G e / P b / G e sandwiches reveal peaks in the magnetization curves at lower temperatures and become smooth only in the vicinity of T¢ ('-- 7K). 3c(T) for the sandwiches shows a well defined maximum at approximately 5K. As a result, the Jr values of this sandwich are increased by a factor of 3 in the temperature range 5-7K. This different behaviour may be due to enhanced pinning at the Ge-Pb interfaces, which were studied using Atomic Force Microscopy (AFM). 1. PREPARATION Superconducting critical parameters of artificially prepared N-S-N (normal/superconductor/normal) sandwiches may be strongly modified by the interface roughness which can eventually act as a collection of different pinning centers to confine the flux lines. To study this effect we have prepared G e ( 5 0 h ) / P b ( 2 2 0 0 ] t ) / G e ( 5 0 0 . ~ ) sandwiches and reference bare P b ( 2 2 0 0 ~ ) films. The samples were prepared by electron beam evaporation in a MBE apparatus with a base pressure of 2 x 10 -9 Torr. The evaporation rates, controlled by a quadrupole mass spectrometer, are 15 /s for Pb and 4 / ~ / s for Ge. In order to obtain continuous thin Pb films the SiO2 substrates are held at liquid nitrogen temperature during preparation. This low substrate temperature leads to crystalline Pb (111 textured) layers while the Ge is amorphous. The structure and thickness of the crystalline Pb layers was checked using X-ray Diffraction. Both Pb films show the same atomic orientation and texture. 2. RESULTS The chosen thickness of the Pb films is below the critical thickness of about 4000 ~ which corre-
E
4
• ~(soA)/pb(22ooA)/G~(sooA)
O
3 O
v
O
321 , p ~ 0
Tempera(K) ture
Fig. 1: Temperature dependence of the zero field critical current for a Ge/Pb/Ge sandwich ( + ) and a bare Pb film (Q). sponds to the crossover from type II to type I superconductivity in Pb films [1]. We have studied temperature and field dependences of the magnetization M(T,H±) using an MPMS2 Q u a n t u m Design SQUID magnetometer which has a field homogeneity better than 0.05 % . The irreversible contribution to M ( T , H x ) has been used to calcu-
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1819-8
/ \
2374
late the critical current values. Temperature dependences of the critical current Jc(T) determined from the zero field width of the M(T,Hj_) hysteresis loops are shown in Fig. 1. These Jc(T) dependences demonstrate a clear difference between the N-S-N sandwich and a reference S-sample with the same thickness of the superconducting layer. The Jc(T) curve for the N-S-N sandwiches shows a maximum at T_~ 5K (Fig. 1). In the temperature range 5-7K the Jc(T) values for the sandwich are systematically larger than the J r ( T ) values for the reference bare Pb film by a factor of 3. Below 4K both Jc(T) curves are characterized by approximately the same value of the critical current. The enhancement of Jc(T) at T = 5 - 7 K can be related to the presence of additional pinning centers at the P b / G e interfaces. This tentative interpretation is supported by the appearance of anomalous peaks in the magnetization curves of the N-S-N sandwiches at low temperatures (Fig. 2). While the bare Pb film shows a smooth regular field dependence (Fig. 2 (a)), in a field H± < 0.3 kG, the Ge-Pb-Ge sandwich shows several M(Hx) maxima and minima, thus forming a characteristic irregular low field M(Hx) pattern, which is symmetric with respect to H± and fully reproducible. We want to stress that these types of M(H) curves have been observed in several other similar sandwiches and multilayers as well. This anomalous M(H±) pattern may result from the matching between the flux lattice period and M(emu) 0.03 (a)
T=4K
0.02 0.01
Xf
0 -0.01 -0.02
M(emu)
0.03
00,°°2'°' 001
-0.02 -0.03 -
. . . . . . . . . . . . . . . . . . --0.5 0 0.5
Hl(kG) Fig. 2(b): Magnetization at 4K for the Ge/Pb/Ge sandwich. The error in M is smaller than the markers. the available characteristic length scales of the pinning potential U(x,y) at the P b / G e interfaces in the N-S-N sandwiches. Following Larkin and Ovchinnikov [2], the M(H±) maxima (Fig. 2 (b)) may be related to the successive coincidence of the flux lattice period a~(H±) with the peaks in the Fourier transform of the pinning potential. As there is a correlation between roughness and deposition rate, one may assume that the pinning potential is mainly created by the variations in Pb film thickness, which can be checked by AFM. Preliminary AFM studies of both the Pb and Ge surfaces (measured after each step in the deposition process) revealed the presence of several maxima, ranging from 2000 to 8000/~ in the Fourier transform. This roughness length scale is larger than the lateral AFM resolution ( + 50 nm) due to the tip radius and has been used to estimate the expected positions of the M(H±) maxima. These positions are in a reasonable agreement with the positions of the anomalous M(H±) peaks shown in Fig.2b.
Work supported by the Belgian High Temperature Superconductivity Incentive and Concerted Action Programs. REFERENCES
-0.03
Fig. film.
2(a):
-0.5
0 0.5 H~(kG) Magnetization at 4K for the bare Pb
[1] G.J. Dolan, J. Low Temp. Phys. 15 (1974) 133 [2] A.I. Larkin and Yu. N. Ovchinnikov, J. Low Temp. Phys. 34 (1979) 409