~ Solid State C~munications, Vol. 76, No. 7, Pp. 929-931, 1990. Printed in Great Britain.

0038-1098/9053.00+.00 Pergamon Press plc

pmm, VICOa OF ~ l ~ ~ B~Lq~aC~0 RF ~ AT 77 K

Neeraj Khare, A.K. Gupta, Sangeeta Chaudhry and V.S. Tcmar

National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi - 110012, India

(Received 23 August 1990 by C. N. R. Rao)

Rf SQUID effect has been observed in BiSrCaCuO thick film at 77 K. The films were prepared using screen printing technique on Mg0 (I00) substrate with the starting cumposition of 1112. The superconducting film has Tc(R=0) -~i00 K. A hole shunted with a microbridge is carved manually for observing rf SQUID effect. Flux noise spectrum of the SQUID has been studied at 77 K. The flux noise in this device is relatively less than that observed in thick film YBaCu0 rf SQUID.

The BiSrCaCu0 high temperature superconductors have several advantages over the YBaCuO system such as compositional stability, inertness to moisture, ease of oxidation in air and lower reactivity with the substrate material during film growth. This material with high Tc phase (ii0 K phase) [1-2] is quite attractive for microelectronics applications. Recently there has been considerable progress in the study of SQUID behaviour in high Tc superconductors [3-26]. This includes both dc SQUID [6-14] as well as rf SQUID [15-26] effects. However most of the studies reported are on YBaCuO material either in the form of bulk [6-8, 15-23] or in films [9-12, 24-26]. Contrary to so many reports on SQUID effects in YBaCA/0 there are only a few on BiSrCaCuO [13,14,16]. PegrtuG et al [16] reported the rf SQUID behaviour in 1112 BiSrCaCu0 bulk at 4.2 K. Gergis et al [13] and Gupta et al [14] reported the dc SQUID effect in BiSrCaCuO at temperatures much lower than 77 K. Gergis et al [13] found SQUID behaviour upto 30 K whereas Gupta et al [ 14 ] observed the complete disappearance of SQUID modulation at 51 K. SQUID behaviour observed in high Tc superconductors are due to naturally present grain boundary %~ak links. Howe, er, behaviour of the grain boundary weak link is sometimes quite unpredictable. For observing dc SQUID effect in films a geometry consisting of two weak links in a superconducting ring is required whereas for rf SQUID effect only one weak link in a superconducting ring is needed. Realization of two identical grain boundary weak links for dc SQUID in high Tc superconductor is rather difficult. However, it is always easy to get one weak link with a superconducting ring for rf SQUID. Morever the observation of rf SQUID effect does not require any direct contact , so the problem of contact resistance [28] doesn't come into picture. Thus the study of rf SQUID effect in high Tc superconducting films is highly desirable. Recently there have been some work on the study of rf SQUID effect in YBaCuO films [24-26]. However, no work is yet reported on rf SQUID effect in BiSrCaCuO film. In the present paper we report for the first time the

929

observation of rf SQUID effect in BiSrCaCuO thick superconducting film at 77 K. Flux noise spectrum of the BiSrCaCu0 rf SQUID is also reported.

Thick films of BiSrCaCu0 were prepared by screen p~'inting technique with the starting composition of 1112. The details of the preparation of the films is described elsewhere [29]. The films were prepared on (i00) Mg0 substrates and given two step heat treatment. The films were first annealed at 880°C for 45 min followed by slow cooling (2°C/rain) to 864°C. In the second step the films were maintaind at 864°C for 80 hours and then cooled slowly to ambient temperature. Tc (offset) of the film has been found to be i00 K. X-ray diffraction reveals the presence of high Tc 2223 phase as well as low Tc 2212 phase. All peaks were indexed with (00 [ ) planes. Comparison of peak height of (002) plane of high Tc phase to low Tc phase shows that the film consists of predominently high Tc phase. Scanning electron micrograph shows that the film has needle like grains of average dimensions of ,~50 microns.

The geometry of a hole interrupted with a microbridge was carved manually on the film. Radius of the central hole was 1 mm whereas~the dimension of the microbridge was 0.5K0.2 ram-. A i0 turn spiral shaped coil wound from 38 SWG copper wire was used as an inductance of the tank circuit. Resonance frequency and Q of the tank circuit were 20.4 MHz and 33 respectively. The patterned film was mounted face down over the coil. A 20 turn copper coil of radius 4ram was used for applying the magnetic field. This coil was kept beneath the film such that centres of the coil and the hole of SQUID structure coincide. A modified commercial rf SQUID electronics was used to observe the SQUID behaviour. The inbuilt rf oscillator in the commercial unit was disabled and an external rf oscillator (HP 3325B) was connected. External af oscillator (Wavetek Model 166) was connected to the field coil for applying ac magnetic field. The reflected rf signal was amplified using low noise preamplifier and was detected using a

930 BEHAVIOUR OF THICK FILM BiSrCaCuO RF SQUID AT 77 K

diode detector. The overall gain of amplifier was 63 dB. The SQUID was shielded with three layers of ~ metal.

Figure 1 shows the V-B behaviour of the BiSrCaCuO thick film rf SQUID. The observed triangular pattern is typical characteristic for a rf SQUID. The amplitude of the triangular pattern is found to be susceptible to the rf voltage ,V T, applied across the tank circuit. At Vm=48 mV good triangular V-B characteristic is oSserved whereas at lower or higher values it becomes noisy and irregular. The mutual inductance between the SQUID loop and field coil can be estimated using the formula M= ~./I~ where I~ is the current in the field coil e required~o change flux coupled to the SQUID by one quanta. For the p{~ent case I~_a= 24 ~A which gives M ~ 8.33xi0 -- H. The inductance of the SQUID loop can be estimated using the 10 2 .... I formula L^^ = ~ W R/2. Based on the radius of the SQUID~oop ~0.5 mm) the estimated value of the in_d~tance of the SQUID loop is LSQ~ 9.86xi0 -~ H.

Flux noise density of the rf SQUID was studied using lock in amplifier (~ 530) and dynamic signal analyser (HP 35660) [30]. Current in the coil for producing the magnetic field was reduced to a value less than I~/4. The detector output was connected to the lock in amplifer whose reference signal was taken from the af oscillator. The output of the lock in amplifier was connected to the input of dynamic signal analyser for recording the voltage noise spectrum, S (f). Flux noise density S (f) is

V . calculated uslng the formula

Js~(¥) = [sv(f)/(~Vla~)2] % where (~/~) being the transfer function

of the SQUID.

Figure 2 shows the flux noise density~ of the BiSrCaCuO thick film rf SQUID -for a

Vol. 76, NO. 7

Fig.l Voltage-flux characteristic of a BiSrCaCuO thick film rf SQUID at 77 K.

L<

' ' ' ' ' ' " i ' ' ' I t " ' i I

lo -4,, . . . . . . . . . . . . . . . . . . .

05 1 I0 100 200

FREQUENCY (H~)---

Fig.2 Spectral density of flux noise of the SQUID at 77 K.

frequency range of 500 mHz to 200 Hz. 0"S-~ remains~nearly constant for f > 5 Hz and is-- 3.5xi0 -e ~o/~'~ at 70 Hz. Below 5 Hz, the value of. ~ S~ increases due to predominance of i/f noise.

Table 1 : Spectral density of flux noise, JS~of various thick/thin film rf SQUIDS.

Film ~ Operating Material Temperature

at i0 Hz in white noise (K) ( ~/ ~'~) region (~-~/~)

Reference

YBaCuO 2. OxlO -3 2. OxlO -4 4.2 thin film (i00 Hz)

YBaCuO 4. ix 10 -3 1. ixlO -3 60-65 thin film (i000 Hz)

YBaCuO 1.6xlO -2 1.7xlO -3 77 thick film (70 Hz)

BiSrCaCuO 5. OxlO -4 3.5xlO -4 77 thick film (70 Hz)

24

25

26

Present w~rk

Vol. 76, NO. 7

Table 1 shows a comparison of flux noise density of various rf SQUIDS made of thick or thin films of high Tc superconductors. The flux noise density of the present SQUID at 77 K is atleast an order of magnitude better than the SQUID based on YBaCuO SQUID.

In conclusion a rf SQUID using thick film of BiSrCaCu0 superconductor has been fabricated and operated at 77 K. The flux noise density of the

BEHAVIOUR OF THICK FILM BiSrCaCuO RF SQUID AT 77 K 931

SQUID is better than that those fabricated earlier using YBaCuO superconductor.

Acknowledgement

The authors would like to thank Professor S.K. Joshi, Director, National Physical Laboratory and Dr. A.V. Narliksr for their keen interest and encouragement in the present work.

REFE~ENCES

[i] H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Jpn. J. Appl. Phys. 27, L209 (1988).

[2]J.M. Tarascon, W.R. McKinnon, P. Barboux, D.M. Hwang, B.G. Bagley, L. H. Greene, G.W. Hull, Y. LePage, N Stoffel and M. Giroud, Phys. Rev. B 38, 8885 (1988).

[3] A.K. Gupta, S.K. Agarwal, B. Jayaram, A. Gupta and A.V. Narlikar, Pramana - J. Phys. 28, L705 (1987).

[4] C.E. Gough, M.S. Colclough, E.M. Forgan, R.G. Jordan, M. Keene, C.M. Muirhead, A.I.M. Rae, N. Thomas, J.S. Abell and S. Sutton, Nature 326, 855 (1987).

[5] M.S. Colclough, C.E. Gough, M. Keene, C.M. Muirhead, N. T~omas, J.S. Abell and S. Suttan, Nature 328, 47 (1987).

[6] A. Th. A. M. de Waele, R.T.M. Smokers, R.W. vander Heijden, K. Kadowski, Y.K. Huang, M. van Sprang and A.A. Menovsky, Phys. Rev., B 35, 8858 (1987).

[7] N.D. Kataria, V.S. Tomar, V.N. Ojha, M. Kumar, G.S.N. Reddy and A.K. Gupta, Mod. Phys. Lett. B 3, 519 (1989).

[8] A Sigishita, M. Yanagisawa and I. Iguchi, Jpn. J. Appl. Phys. 26, L1472 (1987).

[9] R.H. Koch, C.P. Umback, G.J. Clark, P. Chauc~hari and R.B. Leibewitz, Appl. Phys. Lett. 51, 200 (1987).

[lO]I. Takeuchi, J. Tsai, H. Tsuge and N. Matsukura, Jpn. J. Appl. Phys. 27. 2265 (1988).

[II]Y. Katoh, M. Asahi, H. Asano and O. Michikani, Jpn. J. Appl. Phys. 27, LIII0 (1988).

[12]G.S.N. Reddy, A.K. Gupta, V.N. Ojha, D.K. Walia, N.D. Kataria, Neeraj Khare and V.S. Tomar, Mod. Phys. Lett. B3, 1311 (1989).

[13]I.S. Gergis, J.A. Titus, P.H. Kobrin and A.B. Harker, Appl. Phys. Lett 53, 2226 (1988).

[14]A.K. Gupta, Neeraj Khare, D.K. Walia, V.S. To[mar, V.N. Ojha, H.C. Pandey, S.K. Bhatnagar and Y.K. Jain, Pramana - J. Phys. 35, L193 (1990).

[15]C.M. Pegrum, G.B. Donaldson, A.H. Cart and A. Hendry, Appl.Phys.Lett 51, 1364 (1987).

[16]C.M. Pegrum, J.R. Buckley and M. Odehnal, IEEE Trans. Magn. 25, 872 (1989).

[17]R. Tichy, M. Odehnal, V. Petricek, S. Safrata, E. Pollert, J. Kamarad, J. Heytmanek, O. Smrckova and D. Sykorova, J. Low Temp. Phys. 70, 187 (1988).

[18]Neeraj Khare, A.K. Gupta, S.K. Arora, V.S. Tomar and V.N. Ojha, Pramana J. Phys. (to be published).

[19]J.E. Zimmerman, J.A. Beall, M.W. Cromar, and R.H. Ono, Appl. Phys. Lett. 51, 617 (1987).

[20]S. Harrop, C.M. Muirhead, M.S. Colclough and C.E. Gough, Physica C, 153-155, 1411 (1988).

[21]V.F. Bobrakov, V.N. Polushkin and B.V. Vasiliev, Mod. Phys. Lett. B3, 415 (1989).

[22]A.G. Likhachev, V.N. Polushkin, S.V. Uchaikin, and B.V. Vasiliev, Supercond. Sci. Technol. 3, 148 (1990).

[23]S.S. Tinchev, and J.H. Hinken, Supercond. Sci. Technol. 3, i00 (1990).

[24]K. Betts, M.B. Burbank, A. Cragg, A. A. Fife, P.R. Kubik, S. Lee, J. McCubbin, D. McKenzie, M. Tillotson, B. Taylor, H. Tran, J. Vrba, A.C.D. Chaklader, G. Roemer, B. Heinrich, J. Chrzanowski and J.C. Irwin, IEEE Trans. Magn. 25, 965 (1989).

[25]K.P. Daly, A.H. Silver, R.W. Simon, C.E. Platt, A.E. Lee, M.S. Wire and J.E. Zimmerman, IEEE Trans. Magn. 25, 1305 (1989).

[26]A.K. Gupta, Neeraj Khare, Sangeeta Chaudhry, S.K. Arora and V.S. Tormar, Pramana-J. Phys. (to be published).

[27]John Clarke, Proc. IEEE 77, 1208 (1989)

[28]NeerajKhare, S.K. Arora, G.S.N. Reddy, V.S. Tomar, V.N. Ojha, N.D. Kataria and A.K. Gupta, Pramana - J. Phys. 33, L333 (1989).

[29]Sangeeta Chaudhry, Neeraj Khare, A.K. Gupta, K.C. Nagpal, V.N. Ojha, G.S.N. Reddy and V.S. Tomar, Bull. Mater. Sc. (1990).

[30]M.N. Keene, S.P. Harrop, T.J. Jackson, C.M. Muirhead and C.E. Gough, Supercond. Sci. Technol. 3,263 (1990).