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A simple contactless apparatus for determination of critical transition temperature of high-Tc superconducting films
PII: S0011-2275(98)00088-5
A simple contactless apparatus for determination of critical transition temperature of high-Tc superconducting films S.S. Tinchev* Institute of Electronics, Bulgarian Academy of Sciences, Tzarigradsko Chaussee 72, Sofia 1784, Bulgaria
Received 15 June 1998; revised 18 August 1998 The design and operation of a simple measurement system for Tc determination of high Tc superconducting films has been described in the present design note. 1998 Elsevier Science Ltd. All rights reserved
Keywords: high-Tc films; critical temperature measurement; contactless methods Many techniques can be used for measurement of the transition temperature (Tc ) of the high-Tc superconducting materials. Among them contactless methods are very often applied, because they measure a two-dimensional superconductivity and they do not need electrical contacts. Several contactless methods for characterization of superconductors have been developed. In the mutual inductance technique1,2 the sample is placed between two coils and the change of its mutual inductance is measured during the superconducting transition. For such measurements one needs thin samples, which can be placed between the coils. Another group of measurement systems3 uses a single coil, where no restrictions exist on sample geometry. In these methods the sample, placed in the vicinity of the coil, changes its self-inductance. This change may be used in different ways. The system, which is described here is based on a LC-oscillator, similar to4,5. The change of the self-inductance of the sensing coil modifies the amplitude and the frequency of the oscillations. It is well *Tel: + 359-2-7431-678; Fax: + 359-2-9753-201; E-mail: [email protected]
Cryogenics 38 (1998) 1039–1040 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0011-2275/98/$ - see front matter
known, that both responses contain information for the magnetically coupled sample. The amplitude variation of the signal is rarely used in contrast to variation of frequency versus temperature, which is usually monitored. The same principle is used in the present system, which we have had in operation in our laboratory for many years. A block diagram of the system for a Tc-measurement is shown in Figure 1. According to this diagram, the variation of the tank circuit resonant frequency produces changes of the oscillator frequency, which is converted by a frequencyvoltage converter in an analog voltage. This output voltage can be used further to record a superconducting transition during a sample temperature variation. The detailed electronic circuit is shown in Figure 2. In the circuit the oscillator made by the transistor T1 produces a sinusoidal output voltage with frequency of about 20 MHz and peak to peak amplitude of about 5 volts. The amplitude of the oscillations is enough to induce sufficiently high shielding currents in the measured high-Tc film. This is useful, because such measurement also gives information concerning critical current density of the sample. In the present design the shielding current is about 5 × 104 A/cm2 in 200 nm thick films. This value is chosen in order to make difference between ‘good’ and ‘bad’ films in sense of their critical current density. Frequency to voltage conversion is made by the PLLcircuit NE564. The voltage at its output is changed during a superconducting transition with about 100 mV. Further this voltage is inverted and expanded × 10 by a DC amplifier. The output voltage offset is adjustable by means of a variable resistance VR1. This voltage can be fed directly to an X–Y recorder or to analog-digital converter and personal computer. A typical measurement result is shown in Figure 3, where a superconducting transition is recorded during the deposition of a YBCO layer on both sides of a LaAlO3 substrate. Curve 1 is a transition measured after a deposition of YBCO film on one side of the substrate only. In the next step the substrate is turned and the second side is
Figure 1 The block diagram of the apparatus. The coil has 7 turns of Cu wire 0.15 mm O.D
Cryogenics 1998 Volume 38, Number 10 1039
Contactless apparatus for determination of critical transition temperature: S.S. Tinchev
Figure 2 A schematic diagram of the electronic circuit
one superconducting transition (the higher one) is seen. In the third measurement the sample is turned back so that the coil is again next to the first side of the sample. In this position the superconducting transitions of both sides are measurable as curve 3 shows. A difference in the signal height in the measurements is caused by differences in the films lateral position over the coil. In curve 1 the whole film covered the sensing coil, whereas in the cases 2 and 3 only a part of it. The described system has been used extensively and found to be extremely useful during optimization of highTc film deposition and also during a high-Tc film ionbeam modification.
Acknowledgement This work was supported by the National Science Fund of Ministry of Education, Science, and Technology of the Bulgaria, Project ⌽-561. Figure 3 Plot of the output voltage vs. temperature for YBCO films deposited on both sides on LaAlO3 substrate. Curve 1— first side, curve 2—second side, curve 3—again first side, after the deposition of the second side
also coated. Curve 2 represents its superconducting transition, measured with this side next the sensing coil. Obviously the second side has higher Tc then the first one. It perfectly shielded the film on the first side, therefore only
1040
Cryogenics 1998 Volume 38, Number 10
References 1. Fiory, A.T., Hebard, A.F., Mankiewich, P.M. and Howard, R.E., Appl. Phys. Lett, 1988, 52, 2165–2167. 2. Pena, O., Meas. Sci. Technol, 1991, 2, 470–474. 3. Xenikos, D.G. and Lemberger, T.R., Rev. Sci. Insrum, 1989, 60, 831–834. 4. Elliot, J.G. and Liang, W.Y., Meas. Sci. Technol, 1990, 1, 1351–1355. 5. Doss, J.D., Cooke, D.W., Arendt, P.N., Nastasi, M., Muenchausen, R.E. and Tesmer, J.R., Supercond. Sci. Technol, 1989, 2, 63–65.
A simple contactless apparatus for determination of critical transition temperature of high-Tc superconducting films S.S. Tinchev* Institute of Electronics, Bulgarian Academy of Sciences, Tzarigradsko Chaussee 72, Sofia 1784, Bulgaria
Received 15 June 1998; revised 18 August 1998 The design and operation of a simple measurement system for Tc determination of high Tc superconducting films has been described in the present design note. 1998 Elsevier Science Ltd. All rights reserved
Keywords: high-Tc films; critical temperature measurement; contactless methods Many techniques can be used for measurement of the transition temperature (Tc ) of the high-Tc superconducting materials. Among them contactless methods are very often applied, because they measure a two-dimensional superconductivity and they do not need electrical contacts. Several contactless methods for characterization of superconductors have been developed. In the mutual inductance technique1,2 the sample is placed between two coils and the change of its mutual inductance is measured during the superconducting transition. For such measurements one needs thin samples, which can be placed between the coils. Another group of measurement systems3 uses a single coil, where no restrictions exist on sample geometry. In these methods the sample, placed in the vicinity of the coil, changes its self-inductance. This change may be used in different ways. The system, which is described here is based on a LC-oscillator, similar to4,5. The change of the self-inductance of the sensing coil modifies the amplitude and the frequency of the oscillations. It is well *Tel: + 359-2-7431-678; Fax: + 359-2-9753-201; E-mail: [email protected]
Cryogenics 38 (1998) 1039–1040 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0011-2275/98/$ - see front matter
known, that both responses contain information for the magnetically coupled sample. The amplitude variation of the signal is rarely used in contrast to variation of frequency versus temperature, which is usually monitored. The same principle is used in the present system, which we have had in operation in our laboratory for many years. A block diagram of the system for a Tc-measurement is shown in Figure 1. According to this diagram, the variation of the tank circuit resonant frequency produces changes of the oscillator frequency, which is converted by a frequencyvoltage converter in an analog voltage. This output voltage can be used further to record a superconducting transition during a sample temperature variation. The detailed electronic circuit is shown in Figure 2. In the circuit the oscillator made by the transistor T1 produces a sinusoidal output voltage with frequency of about 20 MHz and peak to peak amplitude of about 5 volts. The amplitude of the oscillations is enough to induce sufficiently high shielding currents in the measured high-Tc film. This is useful, because such measurement also gives information concerning critical current density of the sample. In the present design the shielding current is about 5 × 104 A/cm2 in 200 nm thick films. This value is chosen in order to make difference between ‘good’ and ‘bad’ films in sense of their critical current density. Frequency to voltage conversion is made by the PLLcircuit NE564. The voltage at its output is changed during a superconducting transition with about 100 mV. Further this voltage is inverted and expanded × 10 by a DC amplifier. The output voltage offset is adjustable by means of a variable resistance VR1. This voltage can be fed directly to an X–Y recorder or to analog-digital converter and personal computer. A typical measurement result is shown in Figure 3, where a superconducting transition is recorded during the deposition of a YBCO layer on both sides of a LaAlO3 substrate. Curve 1 is a transition measured after a deposition of YBCO film on one side of the substrate only. In the next step the substrate is turned and the second side is
Figure 1 The block diagram of the apparatus. The coil has 7 turns of Cu wire 0.15 mm O.D
Cryogenics 1998 Volume 38, Number 10 1039
Contactless apparatus for determination of critical transition temperature: S.S. Tinchev
Figure 2 A schematic diagram of the electronic circuit
one superconducting transition (the higher one) is seen. In the third measurement the sample is turned back so that the coil is again next to the first side of the sample. In this position the superconducting transitions of both sides are measurable as curve 3 shows. A difference in the signal height in the measurements is caused by differences in the films lateral position over the coil. In curve 1 the whole film covered the sensing coil, whereas in the cases 2 and 3 only a part of it. The described system has been used extensively and found to be extremely useful during optimization of highTc film deposition and also during a high-Tc film ionbeam modification.
Acknowledgement This work was supported by the National Science Fund of Ministry of Education, Science, and Technology of the Bulgaria, Project ⌽-561. Figure 3 Plot of the output voltage vs. temperature for YBCO films deposited on both sides on LaAlO3 substrate. Curve 1— first side, curve 2—second side, curve 3—again first side, after the deposition of the second side
also coated. Curve 2 represents its superconducting transition, measured with this side next the sensing coil. Obviously the second side has higher Tc then the first one. It perfectly shielded the film on the first side, therefore only
1040
Cryogenics 1998 Volume 38, Number 10
References 1. Fiory, A.T., Hebard, A.F., Mankiewich, P.M. and Howard, R.E., Appl. Phys. Lett, 1988, 52, 2165–2167. 2. Pena, O., Meas. Sci. Technol, 1991, 2, 470–474. 3. Xenikos, D.G. and Lemberger, T.R., Rev. Sci. Insrum, 1989, 60, 831–834. 4. Elliot, J.G. and Liang, W.Y., Meas. Sci. Technol, 1990, 1, 1351–1355. 5. Doss, J.D., Cooke, D.W., Arendt, P.N., Nastasi, M., Muenchausen, R.E. and Tesmer, J.R., Supercond. Sci. Technol, 1989, 2, 63–65.