Superconductive quantum interference magnetometer with high sensitivity achieved byan induced resonanceA. Vettoliere and C. Granata Citation: Review of Scientific Instruments 85, 085006 (2014); doi: 10.1063/1.4893655 View online: http://dx.doi.org/10.1063/1.4893655 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Experimental study of a hybrid single flux quantum digital superconducting quantum interference devicemagnetometer J. Appl. Phys. 104, 024509 (2008); 10.1063/1.2958327 Miniaturized superconducting quantum interference magnetometers for high sensitivity applications Appl. Phys. Lett. 91, 122509 (2007); 10.1063/1.2785129 High-temperature superconducting quantum interference device with cooled L C resonant circuit for measuringalternating magnetic fields with improved signal-to-noise ratio Rev. Sci. Instrum. 78, 054701 (2007); 10.1063/1.2735561 Superconducting-quantum-interference-device array magnetometers with directly coupled pickup loop and serialflux dams J. Appl. Phys. 100, 064510 (2006); 10.1063/1.2353396 Operation of high-sensitivity radio frequency superconducting quantum interference device magnetometers withsuperconducting coplanar resonators at 77 K Appl. Phys. Lett. 71, 704 (1997); 10.1063/1.119835

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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 085006 (2014)

Superconductive quantum interference magnetometer with high sensitivityachieved by an induced resonance

A. Vettoliere and C. Granataa)

Istituto di Cibernetica “E. Caianiello” del Consiglio Nazionale delle Ricerche, I-80078 Pozzuoli, Napoli, Italy

(Received 1 July 2014; accepted 10 August 2014; published online 29 August 2014)

A fully integrated low noise superconducting quantum interference device (SQUID) in a magnetome-ter configuration is presented. An intrinsic high voltage responsivity as high as 500 μV/�0 has beenobtained by introducing a resonance in the voltage – magnetic flux characteristic. This resonance isinduced by an integrated superconducting coil surrounding the pick-up coil and connected to oneend of the SQUID output. The SQUID magnetometer exhibits a spectral density of magnetic fieldnoise as low as 3 fT/Hz1/2. In order to verify the suitability of the magnetometer, measurements ofbandwidth and slew rate have been performed and compared with those of the same device withoutthe resonance and with additional positive feedback. Due to their good characteristics such devicescan be employed in a large number of applications including biomagnetism. © 2014 AIP PublishingLLC. [http://dx.doi.org/10.1063/1.4893655]

Sensors based on Superconducting Quantum InterferenceDevice (SQUID) exhibit an ultra high sensitivity limited onlyby quantum effects and are capable of measuring any physi-cal quantity that can be converted in a magnetic flux threadingthe superconducting loop such as a magnetic field, a voltageor electric current, a displacement, a magnetic dipole.1, 2 Dueto their unique characteristics SQUID devices are employedin a wide range of applications3 such as biomagnetism, non-destructive evaluation or testing, geophysics, magnetic mi-croscopy, quantum computation, and interesting basic physicsapplications like the detection of axion dark-matter,4 the dy-namical Casimir effect,5 Majorana fermions,6 and the Joseph-son heat interferometer.7 In the last years, there has also beena growing interest into nanomagnetism applications.8–10 Alsoworth mentioning is the application of SQUID as readoutin Transition Edge Sensors (TES) for astrophysics investiga-tions. Recently two fundamental experiments regarding kine-matic Sunyaev-Zel’dovich effect and the detection of the in-flationary gravitational waves have been performed.11, 12

In its simplest implementation a SQUID consists of a su-perconducting ring interrupted by two Josephson junctions.This basic core is usually integrated in a more complex cir-cuit whose design depends on the application for which thedevice is intended. Two of the most widely used configura-tions are the magnetometer and gradiometer design usuallyemployed in large multichannel system for biomagnetism.1, 2

These configurations include a superconducting pick-up loopthat is either connected in series with an input coil magnet-ically coupled to the SQUID loop13 or in a parallel multi-loop design.14 Although SQUID magnetometers have beenused for several decades, there is a constant research effortto optimize their performances and useful improvements havebeen made also in recent years.15, 16 One of the most impor-tant issues is the optimization of the readout scheme.17 It isa well-known fact that the intrinsic voltage noise and the in-

a)Author to whom correspondence should be addressed. Electronic mail:c.granata@cib.na.cnr.it

ternal impedance of a SQUID device are much smaller thanthose of a low noise semiconductor based amplifier. In or-der to preserve the ultra low noise of a SQUID sensor, sev-eral read-out schemes have been developed. These schemesallow to increase the magnetic flux to voltage transfer fac-tor or the voltage responsivity (V� = ∂V/∂�), so that themagnetic flux contribution of the room temperature electron-ics S�

1/2 = SV,el1/2/V� can be reduced (S�

1/2 and SV1/2 are

the spectral density of magnetic flux and voltage noise, re-spectively). A common readout scheme consists of modulatedelectronics based on a quartz oscillator and a cold step-upamplifier (transformer or LC circuit)18 or, alternatively, a di-rect coupled electronics based on Additional Positive Feed-back (APF)17 or adaptive noise cancellation circuits.19 Otherreadout concepts that allow to increase the voltage respon-sivity and to neglect the noise of the room temperature ampli-fier have also been developed. Examples are double relaxationoscillation SQUID (DROS),20 serial array of SQUIDs,21 anddirectly two-stage configuration.22 It is worth noting that inthe case of magnetoencephalograhic or magnetocardiographicsystems which include several hundred of SQUID magne-tometers and/or gradiometers,23, 24 it is necessary to employa simple readout electronic based on a direct coupled schemein order to reduce the complexity of the room temperatureelectronics.17, 25

In this letter, we present a fully integrated SQUID mag-netometer displaying a high intrinsic voltage responsivityobtained by inducing a resonance in the magnetic flux vs.voltage characteristic (V-�). The basic principle is to use anappropriate resonance to increase the slope of the V-� char-acteristic which results in an increase of the voltage respon-sivity. If the SQUID sensor is biased on the steeper side of theV-� a simple direct coupled scheme can be employed, whileat the same time keeping the overall magnetic flux noise atan acceptable level as in the case of an APF circuit. It is im-portant to point out that we are not claiming that the ultimatesensitivity of a SQUID is obtained by introducing the reso-nance, but that the resonance allows to get a magnetic flux

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085006-2 A. Vettoliere and C. Granata Rev. Sci. Instrum. 85, 085006 (2014)

noise of few μ�0/Hz1/2 (�0 = 2.07 × 10−15 Wb is the quan-tum flux) which corresponds to a magnetic field noise of fewfT/Hz1/2 in the case of a SQUID magnetometer. This SQUIDmagnetometer turns the presence of resonances into a virtuemaking it suitable for all the high sensitivity applications.

In a fully integrated SQUID magnetometer, there are sev-eral types of resonances that usually arise from the interactionbetween the Josephson ac effect and circuit made up by sev-eral elements of the SQUID device.26 Well-known examplesare the LC resonance generated by the SQUID inductance (L)and the junction capacitance (C) or the resonance created bythe microwave transmission line consisting of the input coiland the SQUID washer acting, respectively, as stripline andground plane (input coil stripline resonance) or vice versa(washer resonance).26 Typically these resonances are dampedby resistive elements like shunt and/or damping resistors orare moved far from the Josepshon frequency operation of theSQUID given by the operation voltage divided by the quan-tum flux (f = V/�0, 1 μV corresponds to 484.34 MHz). Inthe SQUID magnetometer introduced in this work, a reso-nant superconducting coil surrounding the pick-up coil in-duces the resonance. The coil inductance and the distributedcapacitance between the coplanar lines of both pick-up andresonant coils form a LC circuit with a resonant frequency offew tens of GHz, corresponding to the operation frequency ofthe SQUID magnetometer. The resonance only occurs if theresonance coil is connected to the SQUID.

The full design of the SQUID magnetometer here pre-sented includes a superconducting flux transformer consist-ing of a single square pick-up coil having an effective areaof 64 mm2 and an inductance of 27 nH connected in se-ries with a spiral input coil magnetically coupled to theSQUID in a washer configuration. The inductance of the in-put coil is 33 nH. An improvement in device performanceshas been achieved by increasing the flux capture area of themagnetometer.15 For such a purpose the mutual inductancebetween the input coil and the SQUID has been increasedby a large inductance SQUID (L = 250 pH). In such a waya magnetic flux to magnetic field transfer factor (magneticfield sensitivity) B� = 0.7 nT/�0 corresponding to an effec-tive flux capture area of 3 mm2 has been obtained. However,being the SQUID critical current IS = 30 μA, a non-optimalnoise condition βL = ISL/�0 = 3.6 occurred. The consequentsensitivity degradation due to a non-optimal βL value hasbeen avoided inserting a damping resistor across the washer.27

The resonant coil is also integrated on the device. It consistsof a square coil of length side of 8.5 mm and linewidth of40 μm, surrounding the pick-up coil. The spacing betweenthe resonant and pick-up coil is 100 μm. Such a distance playsa fundamental role for the determination of the resonant fre-quency value. This is due to the fact that the distributed capac-itance increases by decreasing the spacing between the twocoils.

In Figure 1, a picture of the SQUID magnetometer isshown. In the top portion of the figure, the whole device show-ing the resonant coil, the pick-up coil, the washer, and thecontact pads is reported. The bottom portion depicts someof the device details: on the left the SQUID washer and theunderlying input coil, on the right the Josephson junctions

FIG. 1. Picture of a dc SQUID magnetometer (top figure) and details of thedevice (bottom figure) showing the Josephson junctions, the washer, and theshunt resistors. The resonant coil surrounds the pick-up coil is also displayedin the top picture.

(4 × 4 μm2), the gold-palladium shunt, and damping re-sistors. The device fabrication process, capable to routinelyproduce high quality window-type junctions having an arearanging from (3 × 3) μm2 to (100 × 100) μm2, is basedon conventional Nb/Al-AlOx/Nb trilayer and it has been welldescribed elsewhere.25

The SQUID sensors have been characterized in liquidhelium at T = 4.2 K in a two coaxial magnetic shield con-sisting of a lead superconducting cylinder (inner), and a cry-operm one (outer) having a high magnetic permeability. Allthe electrical connections to room temperature were radio fre-quency filtered.28 The spectral density of magnetic flux noisehas been measured using a very low noise readout electron-ics. The electronic circuit is based on flux-locked-loop (FLL)with a direct coupled scheme. In such a configuration theSQUID is directly coupled to a low noise preamplifier. TheFLL linearizes the SQUID output increasing the linear dy-namic range. The whole amplification stage is integrated ona miniaturized circuit and carefully shielded by a copper box.In order to verify the effectiveness of the SQUID design, theperformances have been compared with those of the same de-vice characterized without the resonant coil and with a con-ventional APF circuit.

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085006-3 A. Vettoliere and C. Granata Rev. Sci. Instrum. 85, 085006 (2014)

FIG. 2. Voltage vs. external magnetic flux characteristics measured atT = 4.2 K for the SQUID magnetometer in three different configurations:resonance coil connected (top panel), with additional positive feedback cir-cuit (middle panel), and without the resonant coil connection (bottom panel).

In Figure 2, the voltage-magnetic flux characteristics ofthe SQUID magnetometer under investigation are reported forthree different configurations. The bias current maximizingthe V-� was IB = 40 μA and corresponds to about the SQUIDcritical current. In the top figure, the V-� with the resonantcoil is shown. As expected, it is possible to observe an evidentresonance on one side of the V-�. In the lower branch of theresonance side, the slope of the characteristic and the linearrange are high enough to ensure both a satisfactory noise leveland a good stability in FLL. The middle figure shows a V-�measured with a APF circuit consisting in an external coppercoil having an area of about 1 cm2 printed on the chip carrierin series with a SMD (Surface Metal Device) resistor. The re-sistance value was specifically chosen to obtain an APF gain(GA = V�,i LA/RA, LA, and RA are the APF coil inductanceand the APF resistance values, respectively) of about 0.9, en-suring an amplification of the intrinsic responsivity V�/V�,i= 1/(1 − GA) ≈ 10. It is evident from the figure what theeffect of the APF is. It renders asymmetric the V-� charac-teristic resulting in an increase of the slope of one side ofthe characteristic. The bottom figure reports the V-� withoutany resonant coil connection. Apart from a slight structureon the right side of the characteristic, there are no resonances,demonstrating that the resonant coil undoubtedly produces theresonances. The voltage swings �V are about 60 μV for allconfigurations. In the case of APF, the voltage swing is a bit

FIG. 3. Voltage responsivity (V�

) as a function of the external magneticfield measured at T = 4.2 K. These data have been obtained by taking thederivative of the V-� with the resonance coil connected.

smaller than the other two configurations; this is due to theeffect of APF resistor that loads the SQUID reducing the us-able �V. In Figure 3, the voltage responsivity as a function ofthe external magnetic flux is reported in the case of the mag-netometer with the resonance. It is possible to observe twopeaks in correspondence of the left side of the characteristicswhere two regions with high slope are present. The zero valueplateau corresponds to the almost constant area of the charac-teristics between the two steeper branches (V = 25 μV). Thenegative peak of the V� corresponds to the right side of theV-� characteristic having a negative slope, but it is smallerand broader than the other two peaks. The maximum V�, cor-responding to about 0.1 �0, is as high as 500 μV/�0. Such avalue is about 4 times higher than the value of a device with-out the resonance (120 μV/�0). Figure 4 reports the spectraldensity of the magnetic field as a function of the frequencyfor the three different configurations. It has been measured inFLL, by using a room temperature readout electronics hav-ing a voltage noise of SV

1/2 = 0.7 nV/Hz1/2. The magneticflux contribution of the electronics is S�,el

1/2 = SV,el1/2/V�.

In the case of the device with active resonance, biased on thesteepest side of the V-� characteristic, the flux contribution ofthe readout electronics is 1.4 μ�0/Hz1/2. Figure 4 highlightsthat the magnetic filed noise of the resonance and APF con-figurations (blue and red lines) are about the same SB

1/2 = 3fT/Hz1/2, while the noise of the SQUID magnetometer with-out resonance (black line) SB

1/2 = 10 fT/Hz1/2 is much higherthan the value of the previous configurations. It is worth not-ing that, in the case of the APF and resonance configurations,the noise contribution of the room amplifier is negligible. Infact, being B� = 0.7 nT/�0, the magnetic flux noise is S�

1/2

= SB1/2/B� = 4.2 μ�0/Hz1/2; the overall noise is S�,T

1/2

= (S�,S+S�,el)1/2, so the intrinsic SQUID noise is S�,S =

4.0 μ�0/Hz1/2. The peaks at 50 Hz and their harmonics aredue to electric power network disturbances.

In order to verify the suitability of the superconductivemagnetometer under investigation, measurements of slew rateand bandwidth have been performed and compared with thoseof the same device with APF. Generally these characteristics

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085006-4 A. Vettoliere and C. Granata Rev. Sci. Instrum. 85, 085006 (2014)

FIG. 4. Spectral density of the magnetic field measured at T = 4.2 K in fluxlocked loop mode for the three configuration reported in Figure 2. The blackline (upper trace) refers to the device without resonance and or APF, whilethe other two lower traces correspond to the V-� with resonance (blue line)and APF (red line).

depend on both readout electronics and SQUID intrinsic pa-rameters such as the voltage responsivity and the linear mag-netic flux range that is the magnetic flux span on the V-�characteristics, corresponding to a linear output SQUID volt-age. In particular the SQUID dynamic (slew rate and band-width) will improve if the previous SQUID parameters in-crease. The bandwidth measurements have been performed byapplying a small sinusoidal magnetic field signal and measur-ing the SQUID output in FLL as a function of the frequency.The resulting value (frequency at −3 dB attenuation) was of40 KHz for both configurations. Such a value is essentiallydue to the low frequency read-out electronic employed. InFigure 5, the maximum magnetic field Bmax in FLL as a func-tion of the frequency and the slew rate (ωBmax) are reported.The Bmax was obtained by measuring the sinusoidal magneticfield corresponding to the loss of the magnetic flux lock. Forlow frequency, it corresponds to the saturation magnetic fieldas observed in Figure 5(a). For these measurements readoutelectronics with a voltage saturation of 4.5 V and a reasonableelectronic gain of 0.22 V/�0 have been used. Since the fieldsensitivity is 0.7 nT/�0, a low frequency saturation field of14 nT has been obtained. From Figure 5(b) it is possible to es-timate a maximum slew rate of 1.4 × 104 nT s, correspondingto 2 × 104 �0 s which can be considered a suitable value formost of SQUID applications. Note that, such a value can beeasily increased by a factor 3, by using electronics with a satu-ration level of 12 V and an electronic gain of 0.1 V/�0 whichis still an acceptable value. It is worth pointing out that thesame measurements have been performed with the APF con-figuration, showing, within the measurement uncertainty, thesame results. Moreover, in order to verify the working opera-tion stability, the output of the device in FLL was monitoredfor more than 3 h with and without a test signal. During thistime the device has shown no loss of optimal working pointnor significant output drifts were observed.

In conclusion, a SQUID magnetometer with a high intrin-sic voltage responsivity due to an induced resonance has been

FIG. 5. Maximum magnetic field (Bmax) and slew rate (ωBmax) as a functionof frequency measured at T = 4.2 K in flux locked loop. Bmax was obtainedby measuring the sinusoidal magnetic field corresponding to the loss of themagnetic flux lock (FLL).

presented. The main advantage of the proposed device lies inthe fact that to avoid the performance degradation, it needsno external circuits that sometimes can lead working oper-ation instability as the APF approaches the unit value. Thenoise and dynamic characteristic measurements have shownthat there are no appreciable differences with respect to thesame device characterized by using the conventional APF cir-cuit. The good performance in terms of magnetic field noise,dynamical characteristics, and operation stability makes suchdevice suitable for all high sensitivity magnetometry applica-tions, including biomagnetism, geophysics, and non destruc-tive testing.

The authors are grateful to Luigi Longobardi for the forhelpful discussions.

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