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The growth of thick YBa2Cu3O7−x films by DC magnetron sputtering
Physica C 338 Ž2000. 246–250 www.elsevier.nlrlocaterphysc
The growth of thick YBa 2 Cu 3 O 7yx films by DC magnetron sputtering E.K. Hollmann, S.V. Razumov, A.V. Tumarkin ) St. Petersburg Electrotechnical UniÕersity, 197376, 5 prof. PopoÕ str., St. Petersburg, Russia Received 3 March 2000; accepted 27 March 2000
Abstract The results of the study of growth of thick Ž0.1–3.6 mm. YBCO films are presented. The orientation of c-phase is improving and mechanical stress is decreasing up to critical thickness, which is due to the deposition rate, as was shown by X-ray diffraction and Rutherford backscattering ŽRBS. analysis. The electrical properties Žcritical current density and surface resistance at 77 K, 8.3 GHz. follow the same tendency. The dielectric inclusions were observed to promote the structure quality improvement working as drains for defects and mechanical stresses. The maximum possible ion-plasma deposition rate of high quality YBCO thick films is estimated. q 2000 Elsevier Science B.V. All rights reserved. Keywords: High-temperature superconductors; YBa 2 Cu 3 O 7yx
1. Introduction In a short period of time, YBa 2 Cu 3 O 7yx films of thickness more than 0.5 mm have become promising materials for high-power electronics and microwave device applications. Thickness of several London penetration depth, l L , is required in microwave devices to minimise the microwave signal losses w1x. As for high power applications, increase of YBCO film thickness allows us to increase the value of critical current. This means that thick Ž) 0.5 mm. films of satisfactory quality are needed. However, the growth of thick films meets significant difficulties like deterioration of film structure quality and ) Corresponding author. Tel.: q7-812-234-5980; fax: q7-812234-4809. E-mail address: [email protected] ŽA.V. Tumarkin..
change of preferred growth orientation. Moreover, different from the primary phase, some islands may appear onto the substrate during the initial stages of YBCO films deposition w2,3x. Later on, this leads to the origin of pores situated above the islands w4x. Today there is another difficulty with YBCO thick films preparation — depending on the deposition rate, it takes from 10 to 40 h to make high quality YBCO film of thickness ; 1 mm by ion-plasma sputtering w5x. In this case, the connection between the growth rate and structure and electrical properties of the film is very important. In our previous works reported earlier w5,6x, we were studying the growth of YBCO superconducting films of thickness up to 2.6 mm. Formation of dielectric inclusions and holes in this superconducting films was observed. The quality of our films evaluated by X-ray spectroscopy and Rutherford
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 2 8 9 - 6
E.K. Hollmann et al.r Physica C 338 (2000) 246–250
backscattering ŽRBS. techniques did not deteriorate with thickness in contrast to most results reported in the literature. Instead, the content of the a-oriented phase was decreasing with thickness and alignment of c-oriented grains was better in the thicker films. We proposed that this results from the presence of dielectric inclusions and holes and it can improve electrical characteristics of our films as well w7x. In the present work, we are studying the structure and electrical characteristics of YBa 2 Cu 3 O 7yx films of 0.1–3.6 mm thick. These films were obtained at higher growth rates in comparison with Ref. w5x and deposition conditions provided the formation of dielectric inclusions and holes during the film growth. We are also trying to determine the maximum possible ion-plasma deposition rate of high quality YBCO thick films for MW and DC applications.
2. Experiment Films were deposited in planar DC magnetron system in atmosphere of pure O 2 Žpressure 1 Torr.. We used the sapphire Al 2 O 3 w1102x with a thin 200 ˚ buffer CeO 2 layer of mixed Ž001.rŽ111. orientaA tion as the substrate. Films were deposited at discharge current of 600 mA. The discharge current was increased for the first 30–60 min of growth and then
247
was not changed. Temperature of substrate measured under the substrate holder was kept at 8548C and was not increased during the film growth. Deposition time varied from 1 to 40 h. The thickness of deposited films measured with a DEKTAK 3030 profilometer varied from 0.1 to 3.6 mm. Deposition rate estimated from the thickness ˚ Ž"5%.. was near 15 Armin The surface morphology of our films was studied with the Scanning Electron Microscope JSM-35 with ˚ Deposition of additional resolution of about 70 A. conducting Žgold. layer was not needed as conductance of all films was high enough. The structure quality of films was studied by an XRD technique using the Gagerflex diffractometer Rigaku Dmax. FWHM of Ž005. peak Ž v-scan. was used for measurements of film alignment along the c-axis. Then the RBS technique was applied. The RBS ˚ and the provided a depth resolution of about 5 A information on the film quality along the c-direction. So the structure quality at the fixed depth can be estimated w8x. For the comparison of electrical properties of the films, we chose the surface microwave resistance R s and the critical current density jc — the most sensitive to the structure quality and orientation parameters.
˚ Fig. 1. SEM images of films deposited at 15 Armin deposition rate Ža. film of thickness 1.26 mm, Žb. film of thickness 1.8 mm.
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E.K. Hollmann et al.r Physica C 338 (2000) 246–250
3. Results and discussion 3.1. Structure measurements Scanning Electron Microscopy of the film surfaces showed two types of macrodefects — holes and second-phase inclusions. The first prevails in the thicker films, the second — in thinner ones; the presence of slower growing inclusions on the bottom of the holes is also likely. Total concentration of macrodefects remains at the level of about 5 = 10 8 cmy2 . From Fig. 1a,b, one can see the surface relief evolution, i.e. hole dimensions and area occupied by holes increase with the film thickness, while the inclusions concentration remains invariable. XRD analysis shows that there are all allowed Ž00 l . reflections in the diffraction patterns of all ˚ films deposited at 15 Armin rate. It means perfect c-oriented structure of the films. The presence of a-oriented grains in the films thicker than 0.5 mm was not observed. At the deposition rate of 15 ˚ Armin, we observed the same decrease in FWHM of Ž005. peak Ž v-scan. with film thickness up to 1.26 mm Žsee Fig. 2. as we did for the lower deposition rates w5x. This decrease can be connected with recrystallization of ‘‘a’’-oriented grains in the atmosphere of pure oxygen and in the presence of Y, Ba, and Cu flow on the substrate. The increase of the width of the Ž005. peak for the films thicker than 1.26 mm is obviously due to the well-developed surface of these films.
Fig. 2. FWHM of Ž005. YBCO peak of thick films deposited at different deposition rates.
Fig. 3. c-parameter dependence vs. thickness calculated from Ž0011. peak for films deposited at different deposition rates.
Fig. 3 shows the c-parameter dependence vs. thickness calculated from Ž0011. peak for films de˚ posited at 3, 10 w5x and 15 Armin. The c-parameter ˚ for the ideal crystal. Common is equal to 11.682 A tendency of the c-parameter increase allows us to talk about mechanical stress decrease when the film thickness increases. So we can conclude that dielectric inclusions promote the structure quality improvement working as drains for defects and mechanical stresses. Minimum in the FWHM vs. thickness dependence ˚ for films deposited at 15 Armin rate Žsee Fig. 2. and the SEM images of the film surface Žsee Fig. 1a,b. allows us to talk about at least two concurrent processes, which influence the film quality. The first process is a slow growth of dielectric inclusions on the bottom of holes and decrease of defects quantity and stresses connected to it. The second one is an extension of the area occupied by holes, which determines the film quality deterioration. It is an interesting fact that the holes area of the film of thickness ˚ 2.6 mm deposited at 10 Armin rate is less than holes area of the film of thickness 1.26 mm prepared at 15 ˚ Armin one. It means the change of the film growth mechanism from ‘‘layer by layer’’ to ‘‘pyramidal’’ with the deposition rate increase. At the ‘‘pyramidal’’ growth mechanism new islands arise onto the film surface, providing the increase of pores dimensions. We suppose the deposition rate is responsible for the structure quality of the film. We applied the RBS analysis for the evaluation of surface structure quality of films. We used the values
E.K. Hollmann et al.r Physica C 338 (2000) 246–250
of the dechanneling coefficient, x Ž50., proportional to the ratio of backscattered signals in the aligned and random positions at the distance of 50 nm from the film surface for the comparison. The dechanneling x is higher for the films with larger concentration of defects, interfaces and lattice distortions as well as larger grain disorientation along the surfaceperpendicular channels. Results of comparison for ˚ the films deposited at 3, 10 w5,8x and 15 Armin are shown in the Fig. 4. The x value decreases when the thickness in˚ w5x creases for all films prepared at 3 and 10 Armin ˚ and up to 1.26 mm for films deposited at 15 Armin. Apparently well-developed surface of the films of ˚ . deterthickness more than 1.26 mm Ž15 Armin mines the increase of the x value.
249
Fig. 5. Microwave surface resistance Ž R s . of YBCO films de˚ posited at 15 Armin deposition rate vs. thickness.
The microwave surface resistance Ž R s . of all YBCO films of thickness up to 3.6 mm measured at frequency of 60 GHz was less than 50 m V. Additionally, R s of our films was measured in a dielectric Žrutile. resonator at a frequency of 8.3 GHz. From Fig. 5, one can see that there is some deterioration in surface resistance for films of thickness more than 1.26 mm. Obviously, it is connected to the large quantity of holes and their area. Fig. 6 shows the dependence of the average critical current density from the film thickness. Results of critical current measurements show that the average critical current density Jc up to thickness of
about 1 mm is independent from both film thickness and bridge width. On the SEM images of film surface one can see that the film surface is very inhomogeneous and porous. It is visible at films presented on the pictures, the ratio between areas of holes and superconducting channel is constant. Thin superconducting channel structure on the film surface seems to determine the homogeneous current distribution along the width of film, which, in turn, determines the linear dependence of critical current on film width. Therefor we calculate the average critical current density Jc s Icrwh, Žwhere w is the film width, h is the film thickness. and take it as a criteria to compare the films characteristics. It is necessary to note that all films have a critical temperature TC about 90 K.
Fig. 4. Comparison of the RBS spectra of thick films deposited at different deposition rates: dechanneling at 50 nm.
Fig. 6. Average critical current density of YBCO films deposited ˚ at 15 Armin deposition rate vs. thickness.
3.2. Electrical measurements
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E.K. Hollmann et al.r Physica C 338 (2000) 246–250
An obvious advantage of films with thickness of about 1–2 mm for high-power DC applications, is that their critical currents are three times higher in comparison to films of another thickness. For the films with thickness of about 3.6 mm, critical current is lower than for thinner films. This may result from the bad surface quality of thick films Žone can see higher roughness and larger holes on SEM images. that leads to comparatively high value of surface resistance R s .
and a-oriented grains in favor of c-phase. After our last deposition rates of thick YBCO films research, we can conclude that there is a maximum possible deposition rate of high quality films, which is determined by the growth mechanism; and in our case, it ˚ is less than 15 Armin. Electrical measurements show that it is possible to use YBCO films of thickness up to 2 mm deposited ˚ at 15 Armin rate for MW and DC applications. On the other hand, high quality YBCO films of thickness more than 2 mm can be prepared by using smaller deposition rates.
4. Conclusion Results of our structure characteristics and YBCO films growth Žreported in this article and previous ones w5–7x. studies, show that the structure quality of our films, being relatively low for the 0.1–0.3 mm film, is improving with thickness. Films of 1–2.6 mm show structure characteristics close to the best results obtained in the processes optimized for the growth of thin single-phase YBCO films. The content of the a-phase reduces with thickness. Among the possible reasons for such behavior, only the presence of macrodefects is evident. They are certainly acting as defect and overstoichiometric atom drains. These macrodefects may work also as prevailing nucleation sites for the a-phase, inhibiting their growth and shifting the ratio of growth of the c-
References w1x F. Vassenden, G. Linker, J. Geerk, Physica C 175 Ž1991. 566. w2x E.K. Hollmann, D.A. Plotkin et al., Tech. Phys. Lett. 22 Ž1996. 942. w3x R. Ramesh, A. Inam et al., Appl. Phys. Lett. 58 Ž1991. 1557. w4x A.H. Carim, S.N. Basu, R.E. Muenchausen, Appl. Phys. Lett. 58 Ž1991. 871. w5x E.K. Hollmann, D.A. Plotkin, S.V. Razumov, A.V. Tumarkin, Physica C 296 Ž1998. 37. w6x E.K. Hollmann, D.A. Plotkin, S.V. Razumov, A.V. Tumarkin, Tech. Phys. Lett. 24 Ž10. Ž1998. 786. w7x E.K. Hollmann, D.A. Plotkin, S.V. Razumov, A.V. Tumarkin, Tech. Phys. 44 Ž1. Ž1999. 85. w8x V.V. Afrosimov, E.K. Hollmann, R.N. Il’in, M.N. Panov, D.A. Plotkin, S.V. Razumov, V.I. Sakharov, I.T. Serenkov, A.V. Tumarkin, Tech. Phys. Lett. 24 Ž1. Ž1998. 41.
The growth of thick YBa 2 Cu 3 O 7yx films by DC magnetron sputtering E.K. Hollmann, S.V. Razumov, A.V. Tumarkin ) St. Petersburg Electrotechnical UniÕersity, 197376, 5 prof. PopoÕ str., St. Petersburg, Russia Received 3 March 2000; accepted 27 March 2000
Abstract The results of the study of growth of thick Ž0.1–3.6 mm. YBCO films are presented. The orientation of c-phase is improving and mechanical stress is decreasing up to critical thickness, which is due to the deposition rate, as was shown by X-ray diffraction and Rutherford backscattering ŽRBS. analysis. The electrical properties Žcritical current density and surface resistance at 77 K, 8.3 GHz. follow the same tendency. The dielectric inclusions were observed to promote the structure quality improvement working as drains for defects and mechanical stresses. The maximum possible ion-plasma deposition rate of high quality YBCO thick films is estimated. q 2000 Elsevier Science B.V. All rights reserved. Keywords: High-temperature superconductors; YBa 2 Cu 3 O 7yx
1. Introduction In a short period of time, YBa 2 Cu 3 O 7yx films of thickness more than 0.5 mm have become promising materials for high-power electronics and microwave device applications. Thickness of several London penetration depth, l L , is required in microwave devices to minimise the microwave signal losses w1x. As for high power applications, increase of YBCO film thickness allows us to increase the value of critical current. This means that thick Ž) 0.5 mm. films of satisfactory quality are needed. However, the growth of thick films meets significant difficulties like deterioration of film structure quality and ) Corresponding author. Tel.: q7-812-234-5980; fax: q7-812234-4809. E-mail address: [email protected] ŽA.V. Tumarkin..
change of preferred growth orientation. Moreover, different from the primary phase, some islands may appear onto the substrate during the initial stages of YBCO films deposition w2,3x. Later on, this leads to the origin of pores situated above the islands w4x. Today there is another difficulty with YBCO thick films preparation — depending on the deposition rate, it takes from 10 to 40 h to make high quality YBCO film of thickness ; 1 mm by ion-plasma sputtering w5x. In this case, the connection between the growth rate and structure and electrical properties of the film is very important. In our previous works reported earlier w5,6x, we were studying the growth of YBCO superconducting films of thickness up to 2.6 mm. Formation of dielectric inclusions and holes in this superconducting films was observed. The quality of our films evaluated by X-ray spectroscopy and Rutherford
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 2 8 9 - 6
E.K. Hollmann et al.r Physica C 338 (2000) 246–250
backscattering ŽRBS. techniques did not deteriorate with thickness in contrast to most results reported in the literature. Instead, the content of the a-oriented phase was decreasing with thickness and alignment of c-oriented grains was better in the thicker films. We proposed that this results from the presence of dielectric inclusions and holes and it can improve electrical characteristics of our films as well w7x. In the present work, we are studying the structure and electrical characteristics of YBa 2 Cu 3 O 7yx films of 0.1–3.6 mm thick. These films were obtained at higher growth rates in comparison with Ref. w5x and deposition conditions provided the formation of dielectric inclusions and holes during the film growth. We are also trying to determine the maximum possible ion-plasma deposition rate of high quality YBCO thick films for MW and DC applications.
2. Experiment Films were deposited in planar DC magnetron system in atmosphere of pure O 2 Žpressure 1 Torr.. We used the sapphire Al 2 O 3 w1102x with a thin 200 ˚ buffer CeO 2 layer of mixed Ž001.rŽ111. orientaA tion as the substrate. Films were deposited at discharge current of 600 mA. The discharge current was increased for the first 30–60 min of growth and then
247
was not changed. Temperature of substrate measured under the substrate holder was kept at 8548C and was not increased during the film growth. Deposition time varied from 1 to 40 h. The thickness of deposited films measured with a DEKTAK 3030 profilometer varied from 0.1 to 3.6 mm. Deposition rate estimated from the thickness ˚ Ž"5%.. was near 15 Armin The surface morphology of our films was studied with the Scanning Electron Microscope JSM-35 with ˚ Deposition of additional resolution of about 70 A. conducting Žgold. layer was not needed as conductance of all films was high enough. The structure quality of films was studied by an XRD technique using the Gagerflex diffractometer Rigaku Dmax. FWHM of Ž005. peak Ž v-scan. was used for measurements of film alignment along the c-axis. Then the RBS technique was applied. The RBS ˚ and the provided a depth resolution of about 5 A information on the film quality along the c-direction. So the structure quality at the fixed depth can be estimated w8x. For the comparison of electrical properties of the films, we chose the surface microwave resistance R s and the critical current density jc — the most sensitive to the structure quality and orientation parameters.
˚ Fig. 1. SEM images of films deposited at 15 Armin deposition rate Ža. film of thickness 1.26 mm, Žb. film of thickness 1.8 mm.
248
E.K. Hollmann et al.r Physica C 338 (2000) 246–250
3. Results and discussion 3.1. Structure measurements Scanning Electron Microscopy of the film surfaces showed two types of macrodefects — holes and second-phase inclusions. The first prevails in the thicker films, the second — in thinner ones; the presence of slower growing inclusions on the bottom of the holes is also likely. Total concentration of macrodefects remains at the level of about 5 = 10 8 cmy2 . From Fig. 1a,b, one can see the surface relief evolution, i.e. hole dimensions and area occupied by holes increase with the film thickness, while the inclusions concentration remains invariable. XRD analysis shows that there are all allowed Ž00 l . reflections in the diffraction patterns of all ˚ films deposited at 15 Armin rate. It means perfect c-oriented structure of the films. The presence of a-oriented grains in the films thicker than 0.5 mm was not observed. At the deposition rate of 15 ˚ Armin, we observed the same decrease in FWHM of Ž005. peak Ž v-scan. with film thickness up to 1.26 mm Žsee Fig. 2. as we did for the lower deposition rates w5x. This decrease can be connected with recrystallization of ‘‘a’’-oriented grains in the atmosphere of pure oxygen and in the presence of Y, Ba, and Cu flow on the substrate. The increase of the width of the Ž005. peak for the films thicker than 1.26 mm is obviously due to the well-developed surface of these films.
Fig. 2. FWHM of Ž005. YBCO peak of thick films deposited at different deposition rates.
Fig. 3. c-parameter dependence vs. thickness calculated from Ž0011. peak for films deposited at different deposition rates.
Fig. 3 shows the c-parameter dependence vs. thickness calculated from Ž0011. peak for films de˚ posited at 3, 10 w5x and 15 Armin. The c-parameter ˚ for the ideal crystal. Common is equal to 11.682 A tendency of the c-parameter increase allows us to talk about mechanical stress decrease when the film thickness increases. So we can conclude that dielectric inclusions promote the structure quality improvement working as drains for defects and mechanical stresses. Minimum in the FWHM vs. thickness dependence ˚ for films deposited at 15 Armin rate Žsee Fig. 2. and the SEM images of the film surface Žsee Fig. 1a,b. allows us to talk about at least two concurrent processes, which influence the film quality. The first process is a slow growth of dielectric inclusions on the bottom of holes and decrease of defects quantity and stresses connected to it. The second one is an extension of the area occupied by holes, which determines the film quality deterioration. It is an interesting fact that the holes area of the film of thickness ˚ 2.6 mm deposited at 10 Armin rate is less than holes area of the film of thickness 1.26 mm prepared at 15 ˚ Armin one. It means the change of the film growth mechanism from ‘‘layer by layer’’ to ‘‘pyramidal’’ with the deposition rate increase. At the ‘‘pyramidal’’ growth mechanism new islands arise onto the film surface, providing the increase of pores dimensions. We suppose the deposition rate is responsible for the structure quality of the film. We applied the RBS analysis for the evaluation of surface structure quality of films. We used the values
E.K. Hollmann et al.r Physica C 338 (2000) 246–250
of the dechanneling coefficient, x Ž50., proportional to the ratio of backscattered signals in the aligned and random positions at the distance of 50 nm from the film surface for the comparison. The dechanneling x is higher for the films with larger concentration of defects, interfaces and lattice distortions as well as larger grain disorientation along the surfaceperpendicular channels. Results of comparison for ˚ the films deposited at 3, 10 w5,8x and 15 Armin are shown in the Fig. 4. The x value decreases when the thickness in˚ w5x creases for all films prepared at 3 and 10 Armin ˚ and up to 1.26 mm for films deposited at 15 Armin. Apparently well-developed surface of the films of ˚ . deterthickness more than 1.26 mm Ž15 Armin mines the increase of the x value.
249
Fig. 5. Microwave surface resistance Ž R s . of YBCO films de˚ posited at 15 Armin deposition rate vs. thickness.
The microwave surface resistance Ž R s . of all YBCO films of thickness up to 3.6 mm measured at frequency of 60 GHz was less than 50 m V. Additionally, R s of our films was measured in a dielectric Žrutile. resonator at a frequency of 8.3 GHz. From Fig. 5, one can see that there is some deterioration in surface resistance for films of thickness more than 1.26 mm. Obviously, it is connected to the large quantity of holes and their area. Fig. 6 shows the dependence of the average critical current density from the film thickness. Results of critical current measurements show that the average critical current density Jc up to thickness of
about 1 mm is independent from both film thickness and bridge width. On the SEM images of film surface one can see that the film surface is very inhomogeneous and porous. It is visible at films presented on the pictures, the ratio between areas of holes and superconducting channel is constant. Thin superconducting channel structure on the film surface seems to determine the homogeneous current distribution along the width of film, which, in turn, determines the linear dependence of critical current on film width. Therefor we calculate the average critical current density Jc s Icrwh, Žwhere w is the film width, h is the film thickness. and take it as a criteria to compare the films characteristics. It is necessary to note that all films have a critical temperature TC about 90 K.
Fig. 4. Comparison of the RBS spectra of thick films deposited at different deposition rates: dechanneling at 50 nm.
Fig. 6. Average critical current density of YBCO films deposited ˚ at 15 Armin deposition rate vs. thickness.
3.2. Electrical measurements
250
E.K. Hollmann et al.r Physica C 338 (2000) 246–250
An obvious advantage of films with thickness of about 1–2 mm for high-power DC applications, is that their critical currents are three times higher in comparison to films of another thickness. For the films with thickness of about 3.6 mm, critical current is lower than for thinner films. This may result from the bad surface quality of thick films Žone can see higher roughness and larger holes on SEM images. that leads to comparatively high value of surface resistance R s .
and a-oriented grains in favor of c-phase. After our last deposition rates of thick YBCO films research, we can conclude that there is a maximum possible deposition rate of high quality films, which is determined by the growth mechanism; and in our case, it ˚ is less than 15 Armin. Electrical measurements show that it is possible to use YBCO films of thickness up to 2 mm deposited ˚ at 15 Armin rate for MW and DC applications. On the other hand, high quality YBCO films of thickness more than 2 mm can be prepared by using smaller deposition rates.
4. Conclusion Results of our structure characteristics and YBCO films growth Žreported in this article and previous ones w5–7x. studies, show that the structure quality of our films, being relatively low for the 0.1–0.3 mm film, is improving with thickness. Films of 1–2.6 mm show structure characteristics close to the best results obtained in the processes optimized for the growth of thin single-phase YBCO films. The content of the a-phase reduces with thickness. Among the possible reasons for such behavior, only the presence of macrodefects is evident. They are certainly acting as defect and overstoichiometric atom drains. These macrodefects may work also as prevailing nucleation sites for the a-phase, inhibiting their growth and shifting the ratio of growth of the c-
References w1x F. Vassenden, G. Linker, J. Geerk, Physica C 175 Ž1991. 566. w2x E.K. Hollmann, D.A. Plotkin et al., Tech. Phys. Lett. 22 Ž1996. 942. w3x R. Ramesh, A. Inam et al., Appl. Phys. Lett. 58 Ž1991. 1557. w4x A.H. Carim, S.N. Basu, R.E. Muenchausen, Appl. Phys. Lett. 58 Ž1991. 871. w5x E.K. Hollmann, D.A. Plotkin, S.V. Razumov, A.V. Tumarkin, Physica C 296 Ž1998. 37. w6x E.K. Hollmann, D.A. Plotkin, S.V. Razumov, A.V. Tumarkin, Tech. Phys. Lett. 24 Ž10. Ž1998. 786. w7x E.K. Hollmann, D.A. Plotkin, S.V. Razumov, A.V. Tumarkin, Tech. Phys. 44 Ž1. Ž1999. 85. w8x V.V. Afrosimov, E.K. Hollmann, R.N. Il’in, M.N. Panov, D.A. Plotkin, S.V. Razumov, V.I. Sakharov, I.T. Serenkov, A.V. Tumarkin, Tech. Phys. Lett. 24 Ž1. Ž1998. 41.