Integrated high T c superconducting magnetometer with multiturn input coil and grainboundary junctionsY. Q. Shen, Z. J. Sun, R. Kromann, T. Holst, P. Vase, and T. Freloft Citation: Applied Physics Letters 67, 2081 (1995); doi: 10.1063/1.115085 View online: http://dx.doi.org/10.1063/1.115085 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/67/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Induction magnetometer using a high- T c superconductor coil J. Appl. Phys. 107, 09E721 (2010); 10.1063/1.3360770 Effect of capacitive feedback on the characteristics of direct current superconducting quantum interferencedevice coupled to a multiturn input coil J. Appl. Phys. 82, 457 (1997); 10.1063/1.365838 Modeling the dc superconducting quantum interference device coupled to the multiturn input coil. III J. Appl. Phys. 72, 1000 (1992); 10.1063/1.351824 Modeling the direct current superconducting quantum interference device coupled to the multiturn input coil. II J. Appl. Phys. 71, 2338 (1992); 10.1063/1.351353 Modeling the dc superconducting quantum interference device coupled to the multiturn input coil J. Appl. Phys. 69, 7295 (1991); 10.1063/1.347576

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Integrated high Tc superconducting magnetometer with multiturn input coiland grain boundary junctions

Y. Q. Shen, Z. J. Sun, R. Kromann, T. Holst, P. Vase, and T. FreloftNKT Research Center A/S, Sognevej 11, DK-2605 Bro”ndby, Denmark

~Received 21 March 1995; accepted for publication 28 July 1995!

We have fabricated and tested integrated magnetometers based on the superconducting quantuminterference device~SQUID!. The magnetometer consists of two patterned films of YBa2Cu3O7,separated by an insulating layer of SrTiO3. A multiturn input coil was integrated on top of theSQUID, where the misorientation angle in a SrTiO3 bicrystal substrate was used to form the grainboundary junctions. The noise spectrum was characterized at 77 K and showed that above 10 Hz themagnetometer sensitivity is limited by a white noise level of around 431025 F0 /Hz

1/2. In the 4 mm3 4 mm detection area of the input coil, this translates into a magnetic field sensitivity of 320 fT/Hz1/2 at 100 Hz. Compared to the theoretical value of an optimized SQUID the white noise level ofthe magnetometer is two times higher. Below 10 Hz the noise is dominated by 1/f noise mainly dueto the critical current fluctuations. ©1995 American Institute of Physics.

By far the most widely exploited superconducting deviceis the superconducting quantum interference device~SQUID!. One of the applications of SQUIDs is the measure-ment of extremely weak magnetic fields such as biomagneticsignals. A particular problem to be solved for this applicationis the efficient coupling of these low-inductance devices toexternal magnetic fields. A possible solution is to use a su-perconducting flux transformer consisting of a large areapickup loop connected to a small area multiturn input coil.Fabrication of the multiturn coil, and especially the integra-tion of the multiturn coil with the SQUID, requiresmultilayer deposition and patterning. For highTc supercon-ductors the requirement of epitaxial growth of the successivelayers makes the fabrication process even more complicated.Previously, highTc superconducting magnetometers, operat-ing at 77 K have been demonstrated by a flip-chip arrange-ment of a dc SQUID chip and a flux transformer chip.1–5

However, only few groups have reported single chipmultilayer highTc magnetometers operating at 77 K.6–9

In this letter we describe the fabrication and character-ization of a single chip YBa2Cu3O7 ~YBCO! magnetometer.The substrates used were 10 mm3 10 mm ~100! SrTiO3

bicrystal with a misorientation angle at the boundary of 36°.The magnetometer was formed in a YBCO/SrTiO3/YBCOtrilayer structure. The SQUID was patterned in the bottomYBCO layer with the junctions at the grain boundary. Rightabove the SQUID washer, the input coil was patterned in thetop YBCO layer. The inner turn of the input coil was con-nected to the SQUID washer through a window in theSrTiO3 insulation layer, meaning that the washer also servesas the crossunder for the input coil~see Fig. 1!. A 4 mm3 4mm pickup loop was patterned in the top YBCO layer. Oneend of the pickup loop was connected to the SQUID washer,via a 60mm wide strip line prepared in the bottom YBCOlayer. Above this strip line, a 50mm wide strip line patternedin the top YBCO layer, connects the other end of the pickuploop to the outer turn of the input coil. All three layers havea thickness of around 150 nm. In Table I the design param-eters for the magnetometer are listed.

The YBCO and SrTiO3 layers were deposited by pulsedlaser ablation with a setup described elsewhere.10 Both ma-terials were deposited at 0.2 mbar oxygen partial pressureand at a substrate temperature of 800 °C. Single layer YBCOfilms obtained in the above-mentioned way haveTc above 89K and critical current density,j c , above 53106 A/cm2 at 77K. After the deposition of the top YBCO layer, a layer of Auwas laser ablatedin situ to form a contact layer. Each layerwas patterned by electron beam lithography and Ar ion mill-ing. The Ar ion beam was provided by a 5 cmAnatechsource, using a beam acceleration voltage of 450 V. Thebeam current density was 0.5 mA/cm2. During the ion mill-ing of the bottom YBCO layer and the SrTiO3 insulationlayer, a rotating sample holder was used, in order to obtainstep edges with low angle ramps and round corners. Prior toeach deposition of a new layer the sample surface wascleaned by oxygen plasma etching and Ar ion milling. Intotal, there are four patterning processes involved, one for

FIG. 1. Scanning electron micrography image of the magnetometer. A de-tailed image of the SQUID washer with the input coil on top of it, is placedin the inset.

2081Appl. Phys. Lett. 67 (14), 2 October 1995 0003-6951/95/67(14)/2081/3/$6.00 © 1995 American Institute of Physics

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each ceramic layer and one for the Au contact pads.The deposition and the lithography procedures for fabri-

cation of the YBCO/SrTiO3/YBCO trilayer structure havebeen optimized by characterizing a simple crossover teststructure. Table II shows the results obtained by these cross-over studies. Four-point resistance measurements carried outdirectly on the trilayer input coils show that the typical criti-cal current at 77 K was 3 mA.

The characterization of the magnetometer was carriedout at 77 K by immersing the sample into a liquid-nitrogenbath. Shielding against external interference fields was pro-vided by a ceramic cylinder coated with YBCO in the liquid-nitrogen bath and am-metal foil which was wrapped uparound the dewar. A magnetic test field was applied to themagnetometer by a calibrated copper coil placed on the topof the chip. Critical current, voltage response to magneticfield, and noise characteristics have been investigated. Themodulation period of the magnetometer is 8 nT/F0 , corre-sponding to an effective detection area of 0.26 mm2, or to1.6% of the area of the pickup loop. Compared to the esti-mated effective area of the magnetometer of 0.55 mm2, weobtain a coupling factor of 0.47 between the flux transformerand the SQUID washer. Figure 2 shows the voltage modula-tion as a function of magnetic field for the magnetometer,and the measured data are listed in Table III.

The noise characterization was carried out by using aflux-locked loop configuration with a 100 kHz modulationfrequency. After the loop has been locked, the output voltageof the integrator was recorded by a spectrum analyzer. Thetransfer coefficient of the modulation system was also mea-sured. The flux noise spectrum is then calculated from thevoltage spectrum by using the transfer coefficient. The fluxnoise characteristics of the magnetometer are shown in Fig.3. By using the field modulation period of 8 nT/F0, the fieldsensitivity is indicated on the right vertical scale. As shown,the field sensitivity is 2.4 pT/Hz1/2 at 1 Hz and 320

fT/Hz1/2 at 100 Hz. Below 10 Hz the noise is 1/f-like flickernoise. The 1/f noise is normally contributed by flux noise andcritical current fluctuations.11 We have also studied the low-frequency noise characteristics by applying bias current re-versal technique,11–13which can reduce the 1/f noise due tocritical current fluctuations. We have found that the whitenoise region can be extended down to 1 Hz when bias cur-rent reversal is applied. This means that the major part of the1/f noise of 331024 F0 /Hz

1/2 at 1 Hz is due to criticalcurrent fluctuations. The critical current fluctuation noise,which is about two times higher than what we observed inour single layer bicrystal SQUID,13 may be caused by anoveretch of the junction during the ion milling of theSrTiO3 and the top YBCO layer. After the critical currentfluctuation noise has been removed, the noise level of631025 F0 /Hz

1/2 at 1 Hz is dominated by the white noise.There may still be 1/f noise left due to 1/f flux noise, whichmay come from the SQUID13 and the input coil.14,15 Any-way, the 1/f noise at 1 Hz is not higher than631025 F0 /Hz

1/2. Above 10 Hz the noise characteristics arelimited by the white noise at 431025 F0 /Hz

1/2. This is twotimes higher than the calculated value by using the theoreti-cal formula for an optimized SQUID~i.e., with reduced in-ductanceb51 and noise parameterG50.05!.11 However, thismagnetometer is still not fully optimized, since the corre-sponding parameters are as follows:b'1.3 andG50.12. Thedecrease of the flux noise spectrum above 300 Hz representthe high-frequency rolloff of the flux-lock loop. The esti-mated rolloff frequency in case of the modulation coil usedfor recording this spectrum, is 400 Hz.

In summary, we have developed a multilayer depositionand patterning technique providing high quality YBCO/

TABLE I. Design parameters of the magnetometer.

Pickup loop outer dimension 4 mm3 4 mmPickup loop effective area 11.7 mm2

Estimated inductance of pickup loop 7.8 nHNumber of turns in input coil 8

12

Estimated inductance of input coil 3.3 nHSQUID washer dimension 300mm 3 300mmSQUID hole dimension 30mm 3 30 mmJunction width 2mmEstimated hole inductance 47 pHEstimated mutual inductance 400 pHEstimated effective area 0.55 mm2

TABLE II. Typical performance of YBCO/SrTiO3/YBCO trilayer cross-overs.

Tc of top YBCO 86–88 Kj c of top YBCO at 77 K 0.5–13106 A/cm2

Tc of bottom YBCO 88–90 Kj c of bottom of YBCO at 77 K 1–53106 A/cm2

Tc of window contact 86–88 KI c ~77 K! of a 5mm 3 10 mm window contact 5 mALeak current density per volt through SrTiO3 800 A/cm2 at 300 K

TABLE III. Performance of the magnetometer at 77 K.

I c per junction 20mARn per junction 10VPeak-to-peak voltage of SQUID modulation 10mVWhite noise 431025 F0 /Hz

1/2

Effective area 0.25 mm2

Field sensitivity at 100 Hz 320 fT/Hz1/2

FIG. 2. Voltage modulation as a function of applied magnetic field measuredon the integrated SQUID magnetometer at 77 K.

2082 Appl. Phys. Lett., Vol. 67, No. 14, 2 October 1995 Shen et al.

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SrTiO3/YBCO structures. Based on this technique, we havefabricated a highTc magnetometer on a SrTiO3 bicrystaloperating at 77 K. The response to the field modulation andthe noise characteristics of the magnetometer have been in-vestigated. With a 4 mm3 4 mm pickup loop, the magne-tometer has a field sensitivity of 2.4 pT/Hz1/2 at 1 Hz and 320fT/Hz1/2 at 100 Hz. The low-frequency noise is mainly due tothe critical current fluctuations, which may be reduced byavoiding the degradation of the bottom YBCO layer at thegrain boundary, during the etching process. This noise canalso be reduced by applying bias current reversal technique.The white noise may also be reduced by optimizing the de-sign of the magnetometer.

The authors would like to thank Dr. J. Flokstra at Uni-versity of Twente, who has supplied the magnetometer de-

sign, and the CEC for funding of BRITE/EURAM projectBE-4020.

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FIG. 3. Noise spectrum for the SQUID magnetometer measured at 77 K,using flux-locked loop configuration with static bias current.

2083Appl. Phys. Lett., Vol. 67, No. 14, 2 October 1995 Shen et al.

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