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946 IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 2, MARCH 1989

METAL-OXIDE CERAMIC RF SQUID OPERATING AT 77 K

V.M.Zakosarenko, E.V.Ilyichyov, VA.Tulin Institute of Problems of Microelectronics Technology and Superpure hkterhls , USSR Academy of Sciences, 142432 Chernogolovka, Moscow District, USSR

Abstract

W e have fabricated several versions of Zimmer- man-type R F SQUID s from bulk ceramic YBa2Cu;jOx samples and studied their characteristics. The SQUIE s operated reliably at liquid nitrogen tem- perature and proved to be stable in time. In some features their properties were similar to those of R F SQUID s based on conventional superconduc- tors, but there were also some prinsiple distinc- tions. The latter may be-due to the fact that because of the macroscopic size of the weak link, the magnetic fLuc vortex enters reversibly with respect to R F current the body of the weak link without crossing it. One of the SQUID s operated as a conventional nonhysteretic SQUID, but the small value of the critical.current of the weak link led to a lower operating temperature ( T 4 60K)

Introduction

The discovery of high temperature supercon-

ductivity of metal oxide compounds' has genera- ted interest in the use of these materials for fabrication Of conventional superconducting de- vices operating a t liquid nitrogen temperature. The most simple device i s the superconducting quantum interferometer (SQUID). The first investi- S t i o n s of high temperature SQUID s have revealed their distinctions as opposed to conventional RF (for instance, niobium) SQUID s. The distinctions

be caused by the fact that size of the weak link d i s actually macroscopic ( d +A, 5 , where

and a r e the superconducting penetration depth and coherence length, respectively). This refers both to bulk ceramic devices and film samples.2-6

W e have made a comprehensive study of different parameters of SQUID 5 made of bulk ce- r a m i c yBa cu o samples and fabricated a n R F SQUID fro,$ th3mxniobium films which displays the properties identical to those of ceramic SQUIDS. The investigations performed gave a n insight in- to the principle of operation of these devices. In addition, a number of RF electrical characteris- tics of YBaZCu30x superconducting ceramics have been obtained.

Sample and e a s u r e m e n t Procedure

The body of the SQUID was fabricated from a pillet of superconducting metal oxide YBa2Cu 0 ceramics. First a rectangular parallelepiped me2-x suring 3 x 4 x 5 mm was cut from the pillet. Then two holes 1.3 mm in diameter were made in it with a diamond drill (as required by the geometry

sensitive to the variation of external magnetic field gradients.

Another version of interferometer was an identical ceramic bulk with one hole and a saw cut to connect the hole with outer space. The junction was of the s a m e s i z e - a s in the first variant and determined by the possibility to form manually a minimum solid bridge.

Since the interferometers operated reliably up to liquid nitrogen temperature (77K), i.e. they we- re sensitive to variations of external magnetic field, we have specified two problems, namely, to obtain information on the current-carrying proper- ties of ceramics and to understand the principles of operation of a ceramic RF SQUID with a macro- scopic bridge. To this end, a setup was assemblw to take off various characteristics of the device comprising a ceramic ring with a thin (as compa- red to the size of the ring) junction. The block- diagram of the setup is shown in Fig.1. The current from the drive R F generator, I,, was supplied to the tank circuit, the coil of which was put into the SQUID hole. The resonant frequency of the tank circuit and, hence, that of the genera- tor was about 30 MHz. The signal from the gene- rator of slowly varying voltage (0.1-0.01 Hz) with an audiofrequency modulating component (630 Hz) could either control the amplitude of the RF generator (30 MHz) or be fed directly to the tank coil to supply a bias magnetic flux to the SQUID. The signal was also put to the x-axis of a two-coordinate recorder. The RF voltage across the tank circuit, U,., , entered a resonant R F amplifier by a wide band preamplifier and after de- tection c a m e to the y-axis of the recorder either directly or upon lock-in amplification.

1

SWEEP AUDIO ' GEN. c

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Results of Measurements and Discussion. First SQUIDs

After the first S Q U I D s had displayed reliable operation? we undertook more comprehensive stu- dies of their properties. It was particularly in- teresting to compare their characteristics with those of conventional SQUIDs described in de-

tail in l i t e r a t ~ r e . ~ ” Fig.2 shows teristic of one of such SQUIDs. It i s a smooth curve ttith a pronounced nonlinearity. The R F V-I characteristics of conventional SQUIDs a r e well known step-like curves. Yet, the characte- rptic i s realized for typical values of the product k Q % 1, where k i s the tank coil-SQUID circuit coupling coefficient and Q i s the quality factor of the tank circuit.

the V-I charac-

h

T-4.2K

61 10 2 0 3 0 40 C C 0 1 I I 1 I,

DRIVE CURRENT, I-(-) DRIVE CURRENT, I,,,(nA) Fig. 2 Fig. 3

Figure 2. R F V-I characteristics for one of the first SQUIDs. The sca les a r e given for 0 dB curves.

Figure 3. dU,/dI, as a function of R F drive current I,. The scale of I i s given for ,O dE curves. The form of the 0 dB curves i s due to the nonlinearity of the modulator. The vertical sections indicate 10”/0 variations of the tank circuit impedance.

Fig.3 shows a derivative of the curves of Fig.2 with respect to the amplitude of R F current energizing the circuit,. The curves exhibit several singularities corresponding to the quasi-steps in Fig.2. It could be assumed that for the devices in question the inequality i s not realized. The Q-factor i s ea sy to rxeasure and in our c a s e it was r 3 0 , i.e. k & l . However, the shift of the tank circuit resonant frequency occurring when the ceramics became superconducting pointed to a good coupling between the coil and the super- conductor. This suggests that the operation of the SQUID does not correspond to complete penetra- tion of the magnetic flux through the junction (“weak link”) into the SQUID hole and back.

Fig.4 shows the derivative of the SQUID signal characteristic for a few closely spaced va- lues of R F drive power. The characteristic con- sisted of a number of fairly sharp widely-spaced peaks. The signal characteristic allowed detecting, by crude estimates, variation of the magnetic flux in the SQUIE hole within IO-? @o using a feed- back circuit. Such sensitivity, the simplicity of working with liquid nitrogen and the comparatively simple design showed that the SQUID described above can compete with different fluxmeters.

I

FA BIAS DIRECT CURRENT, r= Figlire 4. dU,/dI, a s a function of direct current I=for three values of I, in the vicinity of the first singularity on the curves of Fig.3.

Critical Current of a Ceramic Rino with a Necking

A macroscopic ring with a necking can be considered as a device for R F measuring of the junction by the noncontact method. In the ring the R F magnetic flux induces circulating currents that a r e concentrated in the necking. A reduction of the cross-section of the necking brought about a sharp bend with l e s s sloping in the V-I charac- teristic (Fig.5), which corresponds to a decrease of the Q-factor of the tank-circuit connected with the ring. The foregoing distinctions can be seen on the derivative of the R F V-I characteristic a t R F current amplitudes l e s s than the critical ampli- tude of the bend (Fig.5b). The singularities a r e sensitive to the variations in bias current or ex- ternal magnetic field. N o such sensitivity i s ob- served if the RF current amplitude exceeds the critical one. -

1‘=4.2 K

DRIVE I‘\F CUP-IhI;, I , ( r A ) I I 1 I -

O L . 4 6 L O

Figure 5. RF V-I characteristic ( a ) and the der- ivative of its section marked by a heavy line (b).

In the next experiment the amplitude of R F drive current was slightly modulated with respect to the value marked by a n arrow on the RF V-I characteristic shown on the insert of Fig.6. V a - riations in direct current through the tank coil re- sulted in restoration of the part of the R F v-I characteristic corresponding to the section (a ,b)

948

shown on the insert. Some of the peculiarities reveal the correspondeKce between the peaks on We section of the RF V-I characteristic and the curves shown in Fig.6.

w

-40 -20 0 20 40 BIAS DIRECT CURRENT, I, (?A)

Figure 6. dU,/dI, as a function of direct current I, a t RF drive current close to its critical value. The horizontal arrows indicate the direction of I, sweep. The curves a r e vertically shifted for clar- ity. The insert shorn-s a section of the derivative of the R F V-I characteristic with a marked value of drive current I-. The interval(a,b) i s the section of the R F V-I characteristic restored at I, sweep.

These experiments can be described on the basis of the model of hard type I1 superconduc-

tors." Magnetic vortices (vortices irr ceramics a r e a subject for a separate discussion) enter reversibly into the neck region of the sample and remain there till the RF current amplitude reaches its critical value. Each act of vortex en- tering i s represented on the RF V-I and other characteristics under study. The vortex enters the junction surface involving losses in the tank cir- cuit which ensures operation of the SQUIDS des- cribed. The critical RF current amplitude corres- ponds to the occu-rance of the critical state of the junction when the vortices a r e not retarded but move to c ross the junction. Fig.6 shows the data of the experiment in which the RF current ampli- tude was se t up somewhat below its critical value. A s bias current i s applied, it i s added to RF curr- ent and when the total current reaches its critical value, it causes the magnetic flux to pass over the junction corresponds to the jumps following recor- ding of the regular structure (Fig.6). The jumps a re random in character but they occur invariably. Upon reaching its critical value, the total current in the junction remains constant within variations of the current corresponanding to the jumps shown in Fig.6 (10-2W of the peak width). With a chan- ge of the sweeping direction of bias current the value of total current becomes immediately less than its critical value and, hence, the "regular" pattern of singularities reoccurs. At a certain in- stant the bias current drops to zero and the sys- tem i s back to its initial state marked by the arrow on the insert of Fig.6. Then the bias curr- ent is reversed and all the processes a re repea- ted for the opposite direction of bias current and the opposite RF current phase. The change of RF current amplitude in the tank coil estimated from the difference of the drive current I,,, between its initial and critical values (i.e.between the arrow and a sharp drop of the signal on the in- sert of Fig.6) corresponds to half of a n interval

of bias currents at which the "regular" pattern i s observed. It follows from the foregoing that the position of the singularities defining the operation of a n RF SQUID is dictated by the currents flow- ing through the junction rather than the magnetic flux in the SQUID hole.

W e should also dwell on the "regular structu- re". It does not appear as regular on any single RF V-I characteristic. If the family of the deriva- tives of RF V-I characteristics i s written with different values of bias current as a parameter the former looks fairly regular (Fig.7). Such patterns a r e calied "quantum interference" and can be explained on the assumption that absorp- tion occurs in the circuit at some peak current values and the object (junction) in question dis- plays nonlinear properties in the high frequency range.

4.6 4.8 5.0 5.2 5.4 5.6

DRIVE R F CURRENT, I,,, P A ) Figure 7. A series of the derivatives of R F V-I characteristics a t different values of direct current I=. On recording the value of I, (marked on the curves) changed with a step of 1 p A and the curves were shifted upwards for clarity.

SQUID Operation Variants

The operating characteristics of the SQUID can be improved by reducing the size of the junction. In this case the SQUID passes over several stages distinguished by different opera- ting modes. A s a n illustration of two modes, con- sider a SQUID at different temperatures. Fig.8 shows the RF V-I characteristics of one of the SQUIDS (No.13). Curves a,b,c, are strongly en-

la rged initial sections of the RF V-I characteris- tics of Fig.8 (1) for various operating temperatures. The RF V-I characteristics shown in Fig.8 a r s typical of RF SQUIDS with a , p a l l value of k Q (for the sample in question k Q ~ 0 . 3 ) . At low temperatures curves a and b correspond to the hysteretic mode of the RF SQUID. Curve c cor- responds to the nonhysteretic mode, but, as in the previous case, the magnetic flux enters the SQUID junction without crossing it.

At 4.2 K the parameters of SQUID No.13 cal-

culated in accordance with the theoryg are as fol-

lows: pk:2rLsIc/*o 4.5 where the SQUID inductance

LsY 2 *lo-'' H and the critical current of the

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Josephson contact I r 6 0 , ~ A ; k2Q ~ 0 . 2 5 and

y =dU,,,/ d q , signal characteristic under the assumption that its period or. external magnetic flux I$ e equals

All the SQUIDs described operate reliably a t liquid nitrogen temperature ( 77 E; ).

2.1c1011 sec-' derived from the

*o.

DRIVE R F CURRENT, I, (PA) Figure 8. The RF V-I characteristic of SQUID No.13 a t T=35 K (1)1 the initial section of the RF V-I characteristic a t different temperatures ( 2 ) and a family of signal characteristics corresponding to curve a and recorded a t different values of I, and, hence,of U,.,, respectively (3). Curves 1, b and c a r e shifted upwards for clarity. For curve 1 the values of U, and I, marked on the a x e s should be increased tenfold.

5 10 15 20 DRIVE RF CURRENT. I, (nA)

Figure 9. The RF V-I characteristics of SQUID No.14 in the nonhysteretic mode a t different mag- netic flux values and signal characteristics recor- ded a t different values of RF drive current. The curves were recorded a t the frequency of the R F generator exceeding the tank circuit resonance frequency by 1%. It i s easily seen that a t small values of drive current the signal characteristic i s periodic with QJ 2 l1

Fig.9 illustrates the characteristics of R F SQUID No.14 with a junction brought up to the s ize com- parable to that of a vortex i r ceramics. The SQUID operated in the nonhysteretic mode as a conven-

tional niobium device with a point contact. The mag- netic flux quantum penetrated through the junction into the SQUID hole. The SQUID operated up to 60K and did not work a t higher temperatures which may be due to the small value of critical current and thermal smearing of the SQUID characteristic.

A thin film niobium SQUID was fabricated to simulate the operation of a cerawic SQUID. A bium film 200 nm thick was deposited on the surface of a small silicon txr (except for the face surfaces). Or, one of the side surfaces a microbridge 5 m wide with pronounced half-shadows was formed by the mask method, i.e. the width of the bridge excee- ded the s ize of the vortex. The characteristics of the SQUID were identical to those described above (but for No.14). The operation of the device was a l so determined by reversible entering of the vortex into the bridge (without crossing it).

nio-

Conclusion

W e have shown that, unlike the operation of con- ventional RF SQUIDs with the s ize of the weak link comparable to that of the magnetic flux vortex, ce- ramic RF S Q U I D s operate on the principle of rever- sible entering of the vortex into the junction mate- rial. The body of the SQUID with the junction a c t s as a flux transformer, whilst the junction itself ser- ves as a bulk When working with the SQUID we were certainly aware that other scientists were engaged in similar studies. For now, we do not have a complete list of papers on the subject, therefore, we must apologize for not having referred to some of the papers on RF SQUIDs based on high temperature superconductor ceramics. Unfor- tunately, some of the results obtained a r e a l so bey- ond the scope of this paper.

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