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Noise properties of (Hg,Re)-1212 dc SQUIDs
Physica C 357±360 (2001) 1459±1461
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Noise properties of (Hg,Re)-1212 dc SQUIDs A. Tsukamoto a,*, N. Inoue b, T. Sugano b, S. Adachi b, K. Tanabe b, K. Takagi a b
a Advanced Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185-8601, Japan Superconductivity Research Laboratory, ISTEC, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan
Received 16 October 2000; accepted 7 November 2000
Abstract We have fabricated Hg0:9 Re0:1 Ba2 CaCu2 Oy ((Hg,Re)-1212) dc superconducting quantum interference devices (SQUIDs) using grain boundary junctions on SrTiO3 bicrystal substrates and evaluated their ¯ux noise. The large 1=f noise observed in a dc bias scheme was dramatically decreased by using an ac bias scheme, and the white ¯ux noise below 20 lU0 /Hz1=2 was obtained. This noise is comparable to that for SQUIDs made from high-quality YBa2 Cu3 Oy thin ®lms. However, some devices showed a substantial residual low-frequency noise, indicating the contribution of the noise from vortices motion in the superconducting ®lms and/or junctions. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 85.25.D; 73.50.T Keywords: SQUID; Flux noise; Hg0:9 Re0:1 Ba2 CaCu2 Oy
1. Introduction One of the promising applications of high-Tc superconductors is the superconducting quantum interference devices (SQUIDs) containing only one or two Josephson junctions. High-Tc SQUIDs are usually fabricated using YBa2 Cu3 Oy (YBCO) as a superconductor because high-quality epitaxial thin ®lms can be obtained routinely. However, the Ic Rn products of YBCO at 77 K are 10% or lower of those obtained at 4.2 K because the critical temperature Tc of YBCO (90 K) is close to 77 K, that
* Corresponding author. Tel.: +81-42-323-1111; fax: +81-42327-7722. E-mail address: [email protected] (A. Tsukamoto).
is, the normalized temperature T =Tc is close to 1. The use of superconductors with a higher Tc is an apparently promising way of overcoming these problems. We previously reported the electrical properties of (Hg,Re)-1212 bicrystal Josephson junctions and dc SQUIDs [1,2]. The Josephson junctions showed extremely large Ic Rn products of 400±460 V at 77 K, although the junction parameters had a rather large spread. Fabricated dc SQUIDs showed a ®eld-induced voltage modulation up to 111 K and exhibited a large 1=f noise with a ¯ux noise of 10 3 U0 /Hz1=2 at 1 Hz and at 77 K measured by using dc bias ¯uxlocked loop (FLL) electronics. In this study, the ¯ux noise for several Hg-SQUIDs was measured by using an ac bias scheme to decrease the lowfrequency ¯ux noise.
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 5 8 5 - 8
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A. Tsukamoto et al. / Physica C 357±360 (2001) 1459±1461
2. Experimental (Hg,Re)-1212 thin ®lms were prepared by a twostep process [3,4]. As a precursor, an approximately 100-nm-thick Hg-free Re0:1 Ba2 CaCu2 Oy ®lm was prepared on a 36.8° tilt bicrystal SrTiO3 (1 0 0) substrate (10 5 0:5 mm3 ) by a laser ablation technique at room temperature in vacuum. A 25nm-thick HgO cap layer was also deposited on top of the Re0:1 Ba2 CaCu2 Oy ®lm to protect it from moisture and CO2 . The precursor ®lm was sealed in a quartz tube together with an appropriate amount of pressed pellets of unreacted monoxide mixture with the composition of Hg:Ba:Ca:Cu 1:2:1:2 and 0:2:1:2, and was annealed at 700±775°C to crystallize the superconducting phase in a mercury vapor atmosphere. The ®lm thickness of the annealed ®lm was about 80 nm. The as-grown ®lms with a typical TcZERO of 113± 117 K were patterned by electron beam lithography and ion beam etching. We fabricated stripline-type SQUIDs [1] with a directly coupled feedback coil and washer-type SQUIDs [2] with a magnetically coupled feedback coil. The stripline-type SQUIDs had a 4 lm line width. The outer dimensions of the stripline-type SQUIDs were 10 60 or 10 90 lm2 , corresponding to the SQUID inductance of 65 and 100 pH, respectively. The washer-type SQUIDs had a 150 150 lm2 square washer with an inductance of 45 pH. Electrical measurements were performed in a ®ber reinforced plastic cryostat surrounded by four layers of mu-metal shield in an RF shielded room. The noise properties were measured at 77 K by using FLL electronics with ¯ux modulation. A dc bias current or a 128 kHz ac bias current was used. 3. Results and discussion We measured the ¯ux noise of Hg-SQUIDs with dc bias and ac bias schemes. We found that the low-frequency noise was strongly suppressed in some devices, but almost unchanged in some devices. Fig. 1 shows the ¯ux noise spectra for two typical SQUIDs, sample A and B, measured at 77 K with a dc and ac bias. The ¯ux noise of our YBCO magnetometer with a 10 10 mm2 pickup
Fig. 1. Flux noise spectra of two washer-type (Hg,Re)-1212 dc SQUIDs measured at 77 K. (a) In sample A, the low-frequency noise is reduced by the ac bias current scheme. For comparison, the ¯ux noise of our YBCO magnetometer is plotted as shown by the dotted line. The increase in the ¯ux noise for the YBCO magnetometer below 10 Hz and several strong peaks are ascribed to an environmental noise because of its high ®eld sensitivity. (b) In sample B, the reduction of the low-frequency noise brought about by using the ac bias current is signi®cantly small.
coil measured with the ac bias current is also shown in Fig. 1(a) for comparison [5]. The steep increase of ¯ux noise below 5 Hz and several strong peaks observed in the noise spectrum of the YBCO magnetometer were due to the environ-
A. Tsukamoto et al. / Physica C 357±360 (2001) 1459±1461
mental noise because the YBCO magnetometer had a high ®eld sensitivity. The low-frequency noise in sample A was dramatically reduced by employing the ac bias current scheme, resulting in white noise of less than 20 lU0 /Hz1=2 , which is comparable to that of the SQUIDs made from high-quality epitaxial YBCO thin ®lms. In sample B, on the other hand, the ¯ux noise measured with the ac bias current was almost the same as that measured with the dc bias current and remained at a higher level. These dierent noise behaviors were observed in both SQUID designs, and even in SQUIDs with the same design fabricated on the same substrates, in greater or lesser degrees. There are generally two causes of 1=f noise. One is ¯uctuations in the junction critical current and the other is the thermally activated motion of ¯ux vortices in the superconducting ®lm that forms the SQUID loop. The 1=f noise caused by the critical current ¯uctuations can be eliminated by ¯ux modulation and the ac bias current scheme, while the 1=f noise from the motion of ¯ux vortices cannot be reduced by any technique. The experimental results indicate that the noise from the vortices motion is apparently dominant in some devices. A large 1=f noise from the vortices motion was also observed as a major component in early YBCO SQUIDs, but has been strongly decreased by using better quality YBCO ®lms in recent years, as reported by other groups [6,7]. However, it is still a serious problem for multilayered-type SQUIDs because the quality of the top YBCO layer is usually worse compared to that of the bottom layer [8]. Recent (Hg,Re)-1212 ®lms showed a high critical current density close to 1 107 A/cm2 at 77 K, which is higher than that of the high-quality epitaxial YBCO ®lm [4]. However, some defect structures with a rather high density, such as outgrowths and precipitates, were found in the ®lms and especially at the grain boundaries by SEM and TEM observations [9,10], suggesting these defects as a possible noise source. In the case of strip-line type SQUID, although the line width of SQUID loop is narrow enough to prevent the ¯ux entry into the ®lm, the feedback coil which was directly connected with the SQUID loop had about 100±200 lm width, resulting in the noise generation from the motion of the trapped ¯ux.
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4. Conclusions We have fabricated (Hg,Re)-1212 dc SQUIDs and investigated their noise properties. The large 1=f noise observed in the dc bias scheme was dramatically decreased by using the ac bias scheme and white ¯ux noise of below 20 lU0 /Hz1=2 was obtained, which was comparable to that for highquality YBCO SQUIDs. However, some devices showed a large residual low-frequency noise, indicating a substantial contribution of the noise from the vortices motion in the superconducting ®lms. Recent (Hg,Re)-1212 ®lms show a high critical current density, however, some defect structures with a rather higher density, such as outgrowths and precipitates, were found by SEM and TEM observations, suggesting these defects as a possible noise source.
Acknowledgements This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO).
References [1] A. Tsukamoto, K. Takagi, Y. Moriwaki, T. Sugano, S. Adachi, K. Tanabe, Appl. Phys. Lett. 73 (1998) 990. [2] A. Tsukamoto, K. Takagi, Y. Moriwaki, T. Sugano, S. Adachi, K. Tanabe, A. Kandori, IEEE Trans. Appl. Supercond. 9 (1999) 3093. [3] Y. Moriwaki, T. Sugano, S. Adachi, K. Tanabe, A. Tsukamoto, IEEE Trans. Appl. Supercond. 9 (1999) 2390. [4] N. Inoue, T. Sugano, A. Tsukamoto, T. Utagawa, S. Adachi, K. Tanabe, Physica C 330 (2000) 94. [5] A. Tsukamoto, T. Fukazawa, K. Takagi, unpublished data. [6] M.J. Ferrari, M. Johnson, F.C. Wellstood, J. Clark, P.A. Rothensal, R.H. Hammond, M.R. Beasley, Appl. Phys. Lett. 53 (1988) 695. [7] Y. Huang, K.L. Merkle, L.P. Lee, M. Teepe, K. Char, Appl. Phys. Lett. 71 (1997) 3703. [8] R. Scharnweber, N. Dieckman, M. Schilling, Appl. Phys. Lett. 70 (1997) 2189. [9] X.-J. Wu, N. Inoue, T. Sugano, K. Tanabe, Physica C, in press. [10] Y. Wu, N. Inoue, J.G. Wen, T. Sugano, S. Adachi, K. Tanabe, Physica 357±360 (2001), these Proceedings.
www.elsevier.com/locate/physc
Noise properties of (Hg,Re)-1212 dc SQUIDs A. Tsukamoto a,*, N. Inoue b, T. Sugano b, S. Adachi b, K. Tanabe b, K. Takagi a b
a Advanced Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185-8601, Japan Superconductivity Research Laboratory, ISTEC, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan
Received 16 October 2000; accepted 7 November 2000
Abstract We have fabricated Hg0:9 Re0:1 Ba2 CaCu2 Oy ((Hg,Re)-1212) dc superconducting quantum interference devices (SQUIDs) using grain boundary junctions on SrTiO3 bicrystal substrates and evaluated their ¯ux noise. The large 1=f noise observed in a dc bias scheme was dramatically decreased by using an ac bias scheme, and the white ¯ux noise below 20 lU0 /Hz1=2 was obtained. This noise is comparable to that for SQUIDs made from high-quality YBa2 Cu3 Oy thin ®lms. However, some devices showed a substantial residual low-frequency noise, indicating the contribution of the noise from vortices motion in the superconducting ®lms and/or junctions. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 85.25.D; 73.50.T Keywords: SQUID; Flux noise; Hg0:9 Re0:1 Ba2 CaCu2 Oy
1. Introduction One of the promising applications of high-Tc superconductors is the superconducting quantum interference devices (SQUIDs) containing only one or two Josephson junctions. High-Tc SQUIDs are usually fabricated using YBa2 Cu3 Oy (YBCO) as a superconductor because high-quality epitaxial thin ®lms can be obtained routinely. However, the Ic Rn products of YBCO at 77 K are 10% or lower of those obtained at 4.2 K because the critical temperature Tc of YBCO (90 K) is close to 77 K, that
* Corresponding author. Tel.: +81-42-323-1111; fax: +81-42327-7722. E-mail address: [email protected] (A. Tsukamoto).
is, the normalized temperature T =Tc is close to 1. The use of superconductors with a higher Tc is an apparently promising way of overcoming these problems. We previously reported the electrical properties of (Hg,Re)-1212 bicrystal Josephson junctions and dc SQUIDs [1,2]. The Josephson junctions showed extremely large Ic Rn products of 400±460 V at 77 K, although the junction parameters had a rather large spread. Fabricated dc SQUIDs showed a ®eld-induced voltage modulation up to 111 K and exhibited a large 1=f noise with a ¯ux noise of 10 3 U0 /Hz1=2 at 1 Hz and at 77 K measured by using dc bias ¯uxlocked loop (FLL) electronics. In this study, the ¯ux noise for several Hg-SQUIDs was measured by using an ac bias scheme to decrease the lowfrequency ¯ux noise.
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 5 8 5 - 8
1460
A. Tsukamoto et al. / Physica C 357±360 (2001) 1459±1461
2. Experimental (Hg,Re)-1212 thin ®lms were prepared by a twostep process [3,4]. As a precursor, an approximately 100-nm-thick Hg-free Re0:1 Ba2 CaCu2 Oy ®lm was prepared on a 36.8° tilt bicrystal SrTiO3 (1 0 0) substrate (10 5 0:5 mm3 ) by a laser ablation technique at room temperature in vacuum. A 25nm-thick HgO cap layer was also deposited on top of the Re0:1 Ba2 CaCu2 Oy ®lm to protect it from moisture and CO2 . The precursor ®lm was sealed in a quartz tube together with an appropriate amount of pressed pellets of unreacted monoxide mixture with the composition of Hg:Ba:Ca:Cu 1:2:1:2 and 0:2:1:2, and was annealed at 700±775°C to crystallize the superconducting phase in a mercury vapor atmosphere. The ®lm thickness of the annealed ®lm was about 80 nm. The as-grown ®lms with a typical TcZERO of 113± 117 K were patterned by electron beam lithography and ion beam etching. We fabricated stripline-type SQUIDs [1] with a directly coupled feedback coil and washer-type SQUIDs [2] with a magnetically coupled feedback coil. The stripline-type SQUIDs had a 4 lm line width. The outer dimensions of the stripline-type SQUIDs were 10 60 or 10 90 lm2 , corresponding to the SQUID inductance of 65 and 100 pH, respectively. The washer-type SQUIDs had a 150 150 lm2 square washer with an inductance of 45 pH. Electrical measurements were performed in a ®ber reinforced plastic cryostat surrounded by four layers of mu-metal shield in an RF shielded room. The noise properties were measured at 77 K by using FLL electronics with ¯ux modulation. A dc bias current or a 128 kHz ac bias current was used. 3. Results and discussion We measured the ¯ux noise of Hg-SQUIDs with dc bias and ac bias schemes. We found that the low-frequency noise was strongly suppressed in some devices, but almost unchanged in some devices. Fig. 1 shows the ¯ux noise spectra for two typical SQUIDs, sample A and B, measured at 77 K with a dc and ac bias. The ¯ux noise of our YBCO magnetometer with a 10 10 mm2 pickup
Fig. 1. Flux noise spectra of two washer-type (Hg,Re)-1212 dc SQUIDs measured at 77 K. (a) In sample A, the low-frequency noise is reduced by the ac bias current scheme. For comparison, the ¯ux noise of our YBCO magnetometer is plotted as shown by the dotted line. The increase in the ¯ux noise for the YBCO magnetometer below 10 Hz and several strong peaks are ascribed to an environmental noise because of its high ®eld sensitivity. (b) In sample B, the reduction of the low-frequency noise brought about by using the ac bias current is signi®cantly small.
coil measured with the ac bias current is also shown in Fig. 1(a) for comparison [5]. The steep increase of ¯ux noise below 5 Hz and several strong peaks observed in the noise spectrum of the YBCO magnetometer were due to the environ-
A. Tsukamoto et al. / Physica C 357±360 (2001) 1459±1461
mental noise because the YBCO magnetometer had a high ®eld sensitivity. The low-frequency noise in sample A was dramatically reduced by employing the ac bias current scheme, resulting in white noise of less than 20 lU0 /Hz1=2 , which is comparable to that of the SQUIDs made from high-quality epitaxial YBCO thin ®lms. In sample B, on the other hand, the ¯ux noise measured with the ac bias current was almost the same as that measured with the dc bias current and remained at a higher level. These dierent noise behaviors were observed in both SQUID designs, and even in SQUIDs with the same design fabricated on the same substrates, in greater or lesser degrees. There are generally two causes of 1=f noise. One is ¯uctuations in the junction critical current and the other is the thermally activated motion of ¯ux vortices in the superconducting ®lm that forms the SQUID loop. The 1=f noise caused by the critical current ¯uctuations can be eliminated by ¯ux modulation and the ac bias current scheme, while the 1=f noise from the motion of ¯ux vortices cannot be reduced by any technique. The experimental results indicate that the noise from the vortices motion is apparently dominant in some devices. A large 1=f noise from the vortices motion was also observed as a major component in early YBCO SQUIDs, but has been strongly decreased by using better quality YBCO ®lms in recent years, as reported by other groups [6,7]. However, it is still a serious problem for multilayered-type SQUIDs because the quality of the top YBCO layer is usually worse compared to that of the bottom layer [8]. Recent (Hg,Re)-1212 ®lms showed a high critical current density close to 1 107 A/cm2 at 77 K, which is higher than that of the high-quality epitaxial YBCO ®lm [4]. However, some defect structures with a rather high density, such as outgrowths and precipitates, were found in the ®lms and especially at the grain boundaries by SEM and TEM observations [9,10], suggesting these defects as a possible noise source. In the case of strip-line type SQUID, although the line width of SQUID loop is narrow enough to prevent the ¯ux entry into the ®lm, the feedback coil which was directly connected with the SQUID loop had about 100±200 lm width, resulting in the noise generation from the motion of the trapped ¯ux.
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4. Conclusions We have fabricated (Hg,Re)-1212 dc SQUIDs and investigated their noise properties. The large 1=f noise observed in the dc bias scheme was dramatically decreased by using the ac bias scheme and white ¯ux noise of below 20 lU0 /Hz1=2 was obtained, which was comparable to that for highquality YBCO SQUIDs. However, some devices showed a large residual low-frequency noise, indicating a substantial contribution of the noise from the vortices motion in the superconducting ®lms. Recent (Hg,Re)-1212 ®lms show a high critical current density, however, some defect structures with a rather higher density, such as outgrowths and precipitates, were found by SEM and TEM observations, suggesting these defects as a possible noise source.
Acknowledgements This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO).
References [1] A. Tsukamoto, K. Takagi, Y. Moriwaki, T. Sugano, S. Adachi, K. Tanabe, Appl. Phys. Lett. 73 (1998) 990. [2] A. Tsukamoto, K. Takagi, Y. Moriwaki, T. Sugano, S. Adachi, K. Tanabe, A. Kandori, IEEE Trans. Appl. Supercond. 9 (1999) 3093. [3] Y. Moriwaki, T. Sugano, S. Adachi, K. Tanabe, A. Tsukamoto, IEEE Trans. Appl. Supercond. 9 (1999) 2390. [4] N. Inoue, T. Sugano, A. Tsukamoto, T. Utagawa, S. Adachi, K. Tanabe, Physica C 330 (2000) 94. [5] A. Tsukamoto, T. Fukazawa, K. Takagi, unpublished data. [6] M.J. Ferrari, M. Johnson, F.C. Wellstood, J. Clark, P.A. Rothensal, R.H. Hammond, M.R. Beasley, Appl. Phys. Lett. 53 (1988) 695. [7] Y. Huang, K.L. Merkle, L.P. Lee, M. Teepe, K. Char, Appl. Phys. Lett. 71 (1997) 3703. [8] R. Scharnweber, N. Dieckman, M. Schilling, Appl. Phys. Lett. 70 (1997) 2189. [9] X.-J. Wu, N. Inoue, T. Sugano, K. Tanabe, Physica C, in press. [10] Y. Wu, N. Inoue, J.G. Wen, T. Sugano, S. Adachi, K. Tanabe, Physica 357±360 (2001), these Proceedings.