UHF SQUID-magnetometer at 77 K

ICEC 14 Proceedings

UHF

SQUID-MAGNETOMETER

AT 77 K

Valery P. Timofeev, Sergej S. Khvostov, Georgy M. Tsoi, and Vladimir I.Shnyrkov B.I. Verkin Institute for Low Temperature Physics & Engineering, 47 Lenin Ave.,Kharkov 310164, Academy of Sciences of Ukraine, Ukraine

Using an interferometer made of the bulk YBaCuO ceramics a SQUID-based magnetometer has been designed. Magnetic flux sensitivity of the n { t r o g e ~ o o l e d UHF SQUID pumped at 390 MHz is found to be 5-10-U~o/HZ ±/~ in the quasiwhite noise region. The energy sensitivity of the SQUID is about 2.6"10 -29 J/Hz which is by an order of magnitude higher than that of available HTS commercial SQUIDs. The receiving area of the quantization loop is 0.8mm in l~iamet~2 resulting in the field sensitivity as high as 6-10T/Hz / . On the basis of the device constructed, it is expected to design SQUID-magnetometers for nondestructive control, geophysical and biomagnetic researches.

INTRODUCTION It is well known from the theory that the RF SQUID sensitivity can be improved by increasing its pumping frequency co. However, this condition should be true only for the case when some requirements are satisfied . So, the pumping frequency should be much less than the characteristic interferometer frequency, i.e.

03 <<

R / L

(i)

which is defined by the ratio between the normal resistance of Josephson contact R and geometrical inductance of quantization loop L. Second, the pumping frequency should also be considerably less than the characteristic frequency of the HTS Josephson contact, i.e.

cO <
0

(2)

, magnetic flux quantum.

The basic problem in a development of RF SQUIDs made from HTS materials (including the bulk ceramics) is to manufacture the reliable Josephson contacts with a satisfactory value of R needed for the conditions (I) and (2) to be satisfied.

SQUID

DESIGN

The quantum interferometer has been made of HTS bulk ceramics YBa2Cu307_ x rather high

critical current

having a

density (Jc >102A/cm2at 77 K). The sintered bar used

has a form of cylinder ( 4.0_+0.2 mm in diameter and 5.0±0.5ram in length). Subsequent machining allows the quantization loop to be obtained as an a l i g ~ d cylindrical hole ( 0.9_+0.1 mm in diameter with a "weak" link of 0.i ~ O . i X O . l mm V) .The critical current of the junction was reduced to the value required by means of high voltage electric discharge pulses. Tank circuit of SQUID is build on a long one-wire conductor line above the conducting shield. One end of the line is earthed and the other is terminated in capacitance. Since the peak of UHF magnetic field component is near the earthed end, Cryogenics 1992 Vol 32 ICEC Supplement

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ICEC 14 Proceedings

just at this point a link coupling with an interferometer circuit has been formed. The preamplifier cooled is a compact shielded module consisting of GaAsFET amplifying stage. The amplifier provides a stable operation with a gain of about 30 db over the band of 390-400 MHz. In addition, the amplifying channel circuit of HTS SQUID comprises a broadband amplifier of the gain factor of more than 60 db, which is placed at the cryostat top. The effective noise temperature of amplifying channel at 300 K was found to be 16 K. After being detected, the signal is supplied to the direct current amplifier (with the bandwidth of about 40 KHz) of the amplitude-frequency analyser. To prevent an ingress of moisture into the interferometer which is condensed whilst thermocycling, the HTS SQUID together with the preamplifying stage has been placed into a hermetic stainless steel probe.ln our experiments, the fibre glass cryostat has been used. To eliminate the external electromagnetic noise the cryostat has been contained within a three-layer n-metal shield with an attenuation factor of over 65 db.

EXPERIMENTAL RESULTS -iO

From the experimental and calculated parameters of HTS SQUID (L =2-I0 H, ~=8+I, UD/2 ~ 390 MHz) one can deduce that at 77 K the value of normal resistance should be much than 0.22 Ohms and that of the characteristic contact frequency much than 3 GHz. The spectral noise density measured in the i0 KHz bandwidth permits us to estimate the m~gneti9 .~flux sensitivity of HTS SQUID in the quasiwhite region as at the pumping frequency of 390 MHz (Fig.l, see, for _g ~/z __ 5-10- ~o/Hz-/Z comparison the spectral noise density of the same interferometer at the pumping frequency of 20 MHz). The energy sensitivity ~E= S ~ / 2L should be as high as 2.6 •lO-29j/Hz that exceeds the sensitivity of available commercial SQUIDs by an order of magnitude. It should be noted that even though the three-layer shields have been used the external low-frequency electromagnetic fields can penetrate within a cryostat thus distorting the low-frequency spectrum. Therefore, the i/f noise has not been studied in this work. However, to estimate the contribution of environmental noise into spectral characteristics of SQUID under study the interferometer was tested being placed into superconducting shield. The standard RF SQUID pumped at a frequency of 20 MHz and the bulk Y-Ba-C-O ceramics shield have been used. The shield was made in the form of cylinder 16mm in I.D. and 65mm long, the wall thickness was 3mm. Fig.2 shows the noise spectra of HTS SQUID placed into (I) and removed (2) from the shield. As is seen from the Figure, the HTS shield considerably decreases the noise level in the I/f region. Hence, the noticeable rise observed earlier in spectral characteristic [2] is caused by the environmental noise. One of the important characteristic of the SQUID-magnetometer is the magnetic field sensitivity. It seems to us that in this respect the SQUID designed by the authors can be favorably compared with the high-Tc DC SQUIDs. Indeed, the highperformance DC SQUIDs prepared by the thin.~i.lm technology [3] manifest the energy sensitivity as high as 3" i0- J/Hz but because of the poor character~tics of the HTS input antennas only the small quantization loop area (50x50~m or even less)contributes into the reception process Therefore, the magnetmc field sens.~vitYl, Of the instruments having the best characteristics approaches 3.8"10- T/Hz /z only. In the UHF HTS SQUID design discussed, the receiving area of the quantization_,.loop 0.8 mm in diameter thus resulting in the a ~ is, ~/~_ sensitivity being as high as 6-10 ~T/mz . From the above discussion increase in the pumping frequency is believed to be an efficient way (at least at the current status of technologies used in a fabrication of HTS materials) to improve the HTS bulk ceramics RF SQUID performance.

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ICEC 14 Proceedings

REFERENCES

1 2 3

Silver,A.H. and Zimmerman,J.E., Quantum states and transitions in weakly connected superconducting rings ~ ( 1 9 6 7 ) 157 317-341 Shnyrkov,V.I., Timofeev,V.P., Khvostov,S.S. and Tsoi,G.M., UHF high-TcSQUID ~g~i~1~ay~Le~ (1991) 5 1281-1286 Oh,B., Koch,R.H., Gallagher,W.J., Robertazzi,R.P. and Eidelloth,W., Multilevel YBaCuO flux transformers with high T c SQUIDs: A prototype high T c SQUID magnetometers working at 77K

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