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Porous anodic Al2O3 ayers for superconducting films
ICEC 15 Proceedings
Porous Anodic Al203 Layers For Superconducting Films
Serghej L. Prischepa I Leonid M. Lynkov ~ Alexander N. Lykov 2 and Valentin I. Dedyu 3 University of Computer Science and RadioElectronics, P.Brovka str., 6, Minsk 220600, Belarus. 2 P.N.Lebedev Physical Institute, Leninsky Prospect, 53, Moscow 117924, Russia. 3 Institute of Applied Physics, Academiya str., 5, Kishinev 277028, Moldova. Porous anodic aluminium oxide layers have been applied as substrates for superconducting niobium films. Pores in aluminium oxide layers at certain conditions serve as ordered pinning structure. The degree of its ordering is estimated. Properties of superconductors on porous substrates are investigated. Correlation is observed between the period of pinning potential modulation obtained from technological regimes of aluminium development and that obtained from magnetic measurements.
In real superconductors different inhomogeneities always exist preventing vortex flowing. This role may be played by dislocations, grain boundaries, structure, phase and thickness inhomogeneities, etc. The inhomogeneities lead to position dependence of vortex energy in superconductor, i.e. to appearance of effective potential wells. Their dimensions is determined by inhomogeneities dimension a or by temperature dependent coherence length ~(T). The flux line energy for thin film depends on magnetic penetration depth, coherence length and film thickness [ 1]. Series of methods, allowing the creation of artificial ordinary arranged pinning centres, is well known today [2]. Originally this kind of structure was realised by method of electron beam lithography [3 ]. New method of the creation of artificial pinning centres is proposed in this work. It is well known, that porous anodic Al~O3layers, formed in electrolytes, morphology may be presented as closely packed hexagonal collection of elementary cell structures of hexagonal form with cylindrical holes in the central part of each [4]. Hence, the ideal porous arrangement must be triangular lattice. The characteristic sell dimensions is of order of about 0.1...0.5 micrometers and depend on conditions of electrochemical anodization process, nature of electrolyte and its concentration, anodic current density, quality of initial surface. It seems to be reasonable to tentatively assume porous anodic A1203 layers as substrate for superconducting films. In this case one may hope to create planar pinning structure with sub micrometer dimensions. As superconducting film is deposited on porous substrate, the network structure is formed there. Its regularity is determined by the regularity of porous arrangement. In Fig. 1 the typical view of Nb surface on porous anodic A1203layer is shown. It is worth mentioning here, that porous structure in Nb thin film would be present until Nb thickness exceeds pore diameter in Al203. About 1 micrometer thick A1 films were prepared for anodization by vacuum deposition at a pressure of about 10-5 Tor onto cleaned polycrystal A1203 substrates. The anodization were performed in an electrolyte consisting of dilute solution of phosphoric acid. During growth the cell voltage was slowly increased while the current was held constant until the desired voltage (forming voltage Vf) was attained and the voltage was then switched off Current densities in the range 0.5...5.0 m A × c m -2 and Vf = 160 V were used in the present study. The distribution histogram of distances between pores for sample from Fig. 1 is shown in Fig.2. This kind of histogram is typical for samples under investigation in
Cryogenics 1994 Vol 34 ICEC Supplement
851
ICEC 15 Proceedings
this work. 653 points (each of them corresponds to one pore) had been analysed; the medium obtained distance between pores being 153 nm. 0.11 micrometers thick Nb film have been prepared by dc magnetron sputtering in Ar atmosphere (P = 2...5 roTor). The substrates were not specially heated and Tc(R=O) exceeded 9.0 K. Investigations o f l c versus B dependence displayed no peculiarities that show us the absence of long-range order in our structures. At the same time short-range order is definitely present (see Fig.2) and has to influence on perpendicular He2 versus T dependence. For usual superconductors Hc2(T) is proportional to (Tc - 7"). In our experiment the deviation from linear dependence on He2 versus T curves was observed. In Fig.3 one can see the peculiarities at the magnetic field quantities values 0.9 kOe and 1.8 kOe. These values correspond to Bo//t o and 2Bo//to, (where/t 0 is magnetic permeability, B o = 2 ~ o/~a2o, where a 0 is the spacing of the pinning array), i.e. Bo is equal to matching field B,, when the period of pinning centres lattice is determined by the distance among pores. This experiment reveals the influence of substrate microstructure on the Nb films properties. The similar dependence was observed in [5] for thin films with artificial pinning lattice fabricated by electron beam lithography. It can be seen that the upper critical magnetic field increases at the matched fields nB m. At these fields, the superposition of the superconducting order parameter by the external magnetic field is slight: For the one-dimensional thickness-modulated films this effect was predicted by Kuzii [6]. The correct explanation of the nonmonotonic H-T phase diagram of the perforated films is likely to be based on the approach of this work. , In conclusion, we have carried out the investigation of the magnetic properties of Nb thin films deposited onto porous anodic AI~O3 layers. The elaborated technology allowed us to create the shortrange ordered planar pinning structure with characteristic dimensions < 0.1/tin. References 1. Pearl, J., Current distribution in superconducting films carrying quantized fluxoids. Appl. Phvs. Lett. (1964) 5 65-66. 2. Lykov, A.N., The superconducting mixed state of artificial microstructures. Adv. Phvs. (1993), 162 263-342. 3. Martinoli, P., Daldini, O., Leeman, C., and Van den Brandt, B., Josephson oscillation of a moving vortex lattice. ~ (1976) 36 382-385. 4. Keller, F., Hunter, M.S., and Robinson, D.L., Structural features of oxides coatings on aluminium. Electrochem. Soc. (1953) 100 411-419. 5. Lykov, A.N., Pinning in superconducting films with triangular lattice of holes. Solid St. Commun. (1993)86 531-533. 6. Kuzii, V.V., Superconducting corrugated film in a transverse magnetic field near Hoe. S.o__v_~L_Id~ Temp. Phvs. (1982)8 89-92.
Figure 1. An optical micrograph of Nb thin film 0.11 micrometers thick on porous anodic A1203..Pores diameter is 0.15 micrometers. 852
Cryogenics1994Vol 341CECSupplement
ICEC 15 Proceedings
af o,o.q
O,ot
~J
a,p :J~ Figure 2. Distribution histogram of distances between porous ap for sample shown in Fig. 1.
3
2 r~
0
O,g5
T/Tc Figure 3. The perpendicular Hc2 versus T for sample shown in Fig. I.
Cryogenics 1994 Vo134 ICEC Supplement 853
Porous Anodic Al203 Layers For Superconducting Films
Serghej L. Prischepa I Leonid M. Lynkov ~ Alexander N. Lykov 2 and Valentin I. Dedyu 3 University of Computer Science and RadioElectronics, P.Brovka str., 6, Minsk 220600, Belarus. 2 P.N.Lebedev Physical Institute, Leninsky Prospect, 53, Moscow 117924, Russia. 3 Institute of Applied Physics, Academiya str., 5, Kishinev 277028, Moldova. Porous anodic aluminium oxide layers have been applied as substrates for superconducting niobium films. Pores in aluminium oxide layers at certain conditions serve as ordered pinning structure. The degree of its ordering is estimated. Properties of superconductors on porous substrates are investigated. Correlation is observed between the period of pinning potential modulation obtained from technological regimes of aluminium development and that obtained from magnetic measurements.
In real superconductors different inhomogeneities always exist preventing vortex flowing. This role may be played by dislocations, grain boundaries, structure, phase and thickness inhomogeneities, etc. The inhomogeneities lead to position dependence of vortex energy in superconductor, i.e. to appearance of effective potential wells. Their dimensions is determined by inhomogeneities dimension a or by temperature dependent coherence length ~(T). The flux line energy for thin film depends on magnetic penetration depth, coherence length and film thickness [ 1]. Series of methods, allowing the creation of artificial ordinary arranged pinning centres, is well known today [2]. Originally this kind of structure was realised by method of electron beam lithography [3 ]. New method of the creation of artificial pinning centres is proposed in this work. It is well known, that porous anodic Al~O3layers, formed in electrolytes, morphology may be presented as closely packed hexagonal collection of elementary cell structures of hexagonal form with cylindrical holes in the central part of each [4]. Hence, the ideal porous arrangement must be triangular lattice. The characteristic sell dimensions is of order of about 0.1...0.5 micrometers and depend on conditions of electrochemical anodization process, nature of electrolyte and its concentration, anodic current density, quality of initial surface. It seems to be reasonable to tentatively assume porous anodic A1203 layers as substrate for superconducting films. In this case one may hope to create planar pinning structure with sub micrometer dimensions. As superconducting film is deposited on porous substrate, the network structure is formed there. Its regularity is determined by the regularity of porous arrangement. In Fig. 1 the typical view of Nb surface on porous anodic A1203layer is shown. It is worth mentioning here, that porous structure in Nb thin film would be present until Nb thickness exceeds pore diameter in Al203. About 1 micrometer thick A1 films were prepared for anodization by vacuum deposition at a pressure of about 10-5 Tor onto cleaned polycrystal A1203 substrates. The anodization were performed in an electrolyte consisting of dilute solution of phosphoric acid. During growth the cell voltage was slowly increased while the current was held constant until the desired voltage (forming voltage Vf) was attained and the voltage was then switched off Current densities in the range 0.5...5.0 m A × c m -2 and Vf = 160 V were used in the present study. The distribution histogram of distances between pores for sample from Fig. 1 is shown in Fig.2. This kind of histogram is typical for samples under investigation in
Cryogenics 1994 Vol 34 ICEC Supplement
851
ICEC 15 Proceedings
this work. 653 points (each of them corresponds to one pore) had been analysed; the medium obtained distance between pores being 153 nm. 0.11 micrometers thick Nb film have been prepared by dc magnetron sputtering in Ar atmosphere (P = 2...5 roTor). The substrates were not specially heated and Tc(R=O) exceeded 9.0 K. Investigations o f l c versus B dependence displayed no peculiarities that show us the absence of long-range order in our structures. At the same time short-range order is definitely present (see Fig.2) and has to influence on perpendicular He2 versus T dependence. For usual superconductors Hc2(T) is proportional to (Tc - 7"). In our experiment the deviation from linear dependence on He2 versus T curves was observed. In Fig.3 one can see the peculiarities at the magnetic field quantities values 0.9 kOe and 1.8 kOe. These values correspond to Bo//t o and 2Bo//to, (where/t 0 is magnetic permeability, B o = 2 ~ o/~a2o, where a 0 is the spacing of the pinning array), i.e. Bo is equal to matching field B,, when the period of pinning centres lattice is determined by the distance among pores. This experiment reveals the influence of substrate microstructure on the Nb films properties. The similar dependence was observed in [5] for thin films with artificial pinning lattice fabricated by electron beam lithography. It can be seen that the upper critical magnetic field increases at the matched fields nB m. At these fields, the superposition of the superconducting order parameter by the external magnetic field is slight: For the one-dimensional thickness-modulated films this effect was predicted by Kuzii [6]. The correct explanation of the nonmonotonic H-T phase diagram of the perforated films is likely to be based on the approach of this work. , In conclusion, we have carried out the investigation of the magnetic properties of Nb thin films deposited onto porous anodic AI~O3 layers. The elaborated technology allowed us to create the shortrange ordered planar pinning structure with characteristic dimensions < 0.1/tin. References 1. Pearl, J., Current distribution in superconducting films carrying quantized fluxoids. Appl. Phvs. Lett. (1964) 5 65-66. 2. Lykov, A.N., The superconducting mixed state of artificial microstructures. Adv. Phvs. (1993), 162 263-342. 3. Martinoli, P., Daldini, O., Leeman, C., and Van den Brandt, B., Josephson oscillation of a moving vortex lattice. ~ (1976) 36 382-385. 4. Keller, F., Hunter, M.S., and Robinson, D.L., Structural features of oxides coatings on aluminium. Electrochem. Soc. (1953) 100 411-419. 5. Lykov, A.N., Pinning in superconducting films with triangular lattice of holes. Solid St. Commun. (1993)86 531-533. 6. Kuzii, V.V., Superconducting corrugated film in a transverse magnetic field near Hoe. S.o__v_~L_Id~ Temp. Phvs. (1982)8 89-92.
Figure 1. An optical micrograph of Nb thin film 0.11 micrometers thick on porous anodic A1203..Pores diameter is 0.15 micrometers. 852
Cryogenics1994Vol 341CECSupplement
ICEC 15 Proceedings
af o,o.q
O,ot
~J
a,p :J~ Figure 2. Distribution histogram of distances between porous ap for sample shown in Fig. 1.
3
2 r~
0
O,g5
T/Tc Figure 3. The perpendicular Hc2 versus T for sample shown in Fig. I.
Cryogenics 1994 Vo134 ICEC Supplement 853