2549 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 7, NO. 2, JUNE 1997

Magnetic Hysteresis in YBa,Cu,O,-, Magnetometer Sense Loops

John W. Purpura and Roy F. Wiegert NSWC Coastal Systems Station, Panama City, FL,

Abstract -We have measured the magnetic hysteresis of a YBa2Cu30,, thin-film magnetometer test sample at 77 K for applied magnetic field ramp amplitudes up to 1 mT. The test sample was designed with two distinct magnetometer input sense loops connected in series with each other and also with an output washer located outside of the magnetic field applica- tion region. In addition to permitting fields to be applied to the input loops in the same direction, as in a conventional mag- netometer arrangement, this sample design allowed opposing fields to be applied to the loops, providing a measure of hys- teresis behavior expected for a gradiometer loop. The frac- tional hysteresis in the loops for field excursions of 100 pT, as encounteredl for rotation in the earth's field, was 2.1 X1WS for the gradiometer mode measurement, and 3.5 X lo"' for a mag- netometer measurement. We found that hysteresis measured with the sample cooled down in a 50 pT field was identical to that measured with the sample cooled in "zero field".

1. INTRODUCTION

The effect of magnetic hysteresis on the performance of superconducting quantum interference devices (SQUIDS) is a concern for applications which demand the utmost in sen- sitivity and accuracy. Magnetic hysteresis occurs when a time-varying magnetic field imposed on the superconducting material of the sensor causes an irreversible change in the number andfor position of the flux vortices trapped in the material. Tlie result is that the signal measured by the sen- sor differs from the actual signal applied because the sensor no longer responds in a linear, reversible manner.

There is much interest in using SQUID magnetometers and gradiometers made from high transition temperature (T',) superconducting films for applications which require low- noise, unshielded operation in the earth's magnetic field at fi-equencies,>t down to 0.1 Hz. Previous studies have indi- cated that the earth's field is sufficiently large to cause hys- teresis in high-T', SQUIDS and sense loops [l], [2]. Sun, Gallagher, aad Koch [l] have developed and verified a mod- el for flux entry around the edges of thin-film superconduct- ing structures which emphasizes the importance of high quality edges with high critical current density to reduce hysteresis. 'hey have demonstrated that a single weak

Manuscript received August 27, 1996. This work was suppoad by the U.S. Ofice of Naval Research. The appearance of trade names in this document does not constitute endorse- ment by the Department of Defense; the Navy; or the Coastal Systems Sta- tion, Dahlgren Division, Naval Surface Warfare Center.

pinniig site along the edge of a washer can significantly in- crease the overall hysteresis of the device. They also noted that hysteresis increases as the ratio of Iinewidth to film thickness increases. Our earlier measurements agree with these last two observations 121.

Though improvements continue to be made, present high-Tc film processing is not capable of routinely providing low Ilf noise, patterned trilayer devices with interconnects; such as flux transformen and gradiometer loops. As a re- sult, high-resolution SQUID magnetometers must typically be fabricated with wide-linewidth, directly coupled sense loops to improve matching to the low inductance of the SQUID, at the expense of increasing hysteresis. The three SQUID gradiometer [3] was conceived as a way to avoid high-T', device fabrication limitations and magnetic hystere- sis by eliminating the need for large area gradiometer loops and nulling the field on the SQUID sense loops, though it is not practical for all applications.

In this paper, we present the results of hysteresis mea- surements on a single-layer yBa2Cu307-, (YEKO) test sam- ple designed to allow us to measure hysteresis originating strictly in the sense loops, for both magnetometer and gra- diometer configurations. Our sample design eliminates the contributions to hysteresis from possible weak features which exist in optimized devices, such as Josephson junc- tions, crossovers, and vias.

11. EXPERIMENT

Single-layer thin-film magnetometer samples were fabri- cated for us by Conductus, Inc. [4] from a high-quality epi- taxial YBCO film grown by laser ablation on a SrTiO, substrate. The sample geometry is identical to that of the GdBa2Cu,07., samples we studied previously [2]. To re- view, the sample consisted of a pair of series-connected square input magnetometer loops, attached to each other and to an output magnetometer washer, by closely spaced, par- allel striplines (see Fig. la). The input loop centers were spaced 10 mm apart and the outer edges of each loop mea- sured 2 mm x 2 mm as shown in Fig. IC. The input loops of the sample studied in this investigation both had linewidths of 400 p. The outer dimensions of the output washer were 240 pm x 240 pm, and the inner hole measured 40 pm x 40 pm. The connecting striplines all had 20-pm linewidths with their inner edges separated by 20 pm (see Fig. Id).

1051-8223/97$10.00 0 1997 IEEE

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Magnetometer Sample {a) 1-10”--1

I J

I-- 36 mm 4

Ground-Plane Sample

Output Washer \-3

Fig 1 (a) Geometry of thin film YBCO magnetometer sample. In- put loops are on left side of the substrate and output washer is on the right end @) YBCO ground-plane sample. Two 4 mm x 4 mm windows allow field to be applied to sense loops of sample (c) Detail of leftmost input loop Linewidth was 400 pm. (d) Detail o f sample output washer.

The sample design alIows each sense loop to have a localized magnetic field applied to it with its own magnet coil. Thus, field can be applied in the same direction to the input loops (the common mode configuration) to measure hysteresis for a magnetometer loop, or the field can be ap- plied to the loops in opposite directions (the differential mode configuration), minimizing the sample’s circulating current, to examine hysteresis expected for a gradiometer confguration.

The output magnetic flux of the magnetometer samples was measured with a YBCO de SQUID which was flip-chip coupled to the sample output washer. The SQUID was a bi- crystal p i n boundary junction device obtained from Con- ductus. It had a square washer, with 500-pm sides and a 25 pm x 25 pm hole. A single-turn, thin-Elm gold coil was de- posited over the SQUID washer to provide flux modulation and feedback. The bare SQUID had a field sensitivity of about 5Qd pT and a white noise of 10 pQ&i”*. With the test sample in place, the sensitivity was reduced to O.SS@J pT, because of the significant inductance of the sample’s parallel striplines. In ow setup, with a magnetometer sam- ple coupled to the SQUID, and using a static bias current, we had a noise level of 75 p@&z-”* at 1 Hz, with the 1/f knee being at 100 Hz.

The experimental arrangement is illustrated in Fig. 2. The SQUID was attached face-up to a sapphire sample hold- er (88 mm long x 10 mm wide x 3 mm thick) with rubber cement. The SQUID was wire-bonded to a printed-circuit board mounted next to it. In an effort to increase the signal from the sample by reducing the inductance of the Iong striplimes, a YBCO ground-plane sample (see Fig. Ib),

which we obtained froin IBM [ 5 ] , was mounted face-up 011 the sample holder adjacent to the SQUID. The magnetome- ter sample was placed face-down over the groumi-plme sample and the SQUID, with the hole of its output washer aligned directry over that of the SQUID washer. The see- through sample holder made it easy to line up the washers by sighting through their holes. A 1.5-pm thick sheet of mylar film was sandwiched between the SQUID and the sample washer to electrically insulate them from one anoth- er. The ground-plane sample was fabricated with a SrTiO, insulating layer, which in hull, was coated with two layers of teflon.

The plane of the sample holder was mounted horizontal- ly such that the sense axes of the sample and readout SQUID were along the vertical direction. The SQUID was centered inside a matchbox-shaped, bulk-YBCO magnetic shield, with the sample input loops extending outside of the shield as shown in Fig. 2. The shield greatly reduced direct pickup between the field application coils and the SQUID. The two field application coils were mounted side-by-side with their axes parallel to one another, with each centered on an input loop. One coil was made to have a slightly larger coil cun- stant than the other so that a shunt resistor connected across it could be adjusted to null the response of the sample for the differential mode experiment. A polarity-reversing switch connected to the shunted coil was used to convert from the differential mode to the c~mrnon mode configura- tion. The unshunted magnet coil was calibrated by compar- ing the sample response to it with the sample response to a known, uniform field provided by a solenoid wound around the cryostat.

The experiments were conducted inside a 2-layer p metal shield room in an evacuated cryostat inserted into research dewar containing liquid nitrogen. The dewar wa placed inside a p-metal cylinder having a bottom endcar This arrangement reduced the magnitude of the backgroun magnetic field to less than 0.2 pT.

’

Magnet Coils

Magnetometer Sample

SQUID Substrate /

Sample Holder YBCO Shield

Fig. 2. Experimental arrangement to measure the hysteresis of mag- netometer loops. Small magnet coils apply fields, rn either the same or opposite directions, to the sample‘s input loops. A high-T, shield’ prevents the applied field Erom directly reachtng the readout SQUID

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To perform the hysteresis experiments, the readout SQUID was operated with static bias current in a flux- locked loop using a 240 kHz flux modulation signal. The sample, SQUID, and YBCO shield were cooled down in the "zero field" environment, except for the measurements in which the cryostat solenoid was used to provide a 50 pT field cooldown. The shunt resistor of the local field applica- tion coil was fine-tuned for sample flux nulling in trial runs using 1 - 2 pT fields. Heaters were used to thermally cycle the superconducting components above their respective T;s prior to eaclh new data collection run. The sweeping field ramp for the experiments was a linear ramp waveform which increased from zero to a maximum value of B,, then ramped down to -B,, and finally returned to zero.

111. RESULTS

Hysteresis measurements in the differential mode con- figuration were performed for B, up to 1 mT. The raw data for the case of B, = 300 pT are shown in Fig. 3. The plot actually shows two complete hysteresis loops carried out consecutively, showing the high degree of reproducibility in the measureiinent. The sweep waveform had a period of about two minutes, which was typical for most of our mea- surements. 1The trace starts out in the center of the plot with the sample's initial ideal linear behavior indicated by the lack of response of the sample to applied field. As the ap- plied field continues its rise, at a certain point the mag- netometer output begins to increase, signaling the onset of hysteretic behavior. We began detecting hysteresis for a B, value of 30 pT in our differential mode measurements. The output of the test sample continues its increase as the applied field reaches the maximum value a,. When the ap- plied field is ramped downward to zero. the flux measured by the SQUID has a non-zero remanance value, which we will denote as Likewise, once the field reaches - B, and then ranips upward to zero, the flux recorded by the SQUID has a value Q U p . The hysteresis, as measured in terms of the flux at the SQUID, is simply the difference

The SQUID-referenced hysteresis is plotted as a function of the peak-to-peak field excursion Bp in Fig. 4. For our linear-ramp sweep waveform, Bpp = 2 B,. The hysteresis varies as a smoothly increasing function of Bpp. It appears to increase approximately linearly for Bpp 2 600 pT.

Experiments were also conducted in the differential mode configuration for the case in which the sample, SQUID, and shield were heated above T, and allowed to cool in a uniform 50 pT field. This constant background field was mai~ntahed during the field sweep on the mag- netometer sample loops. The amount of hysteresis

@DOWN- %P.

, .. ........................... 1 YBa,Cu,O,, Sense

E 400 urn linewidth

f

Loopi I

Fig. 3. Flux measured at the readout SQUID forthe 4oo-)un test sample as a function of applied magnetic field for the differential mode configuration

measured in these experiments was identical to that mea- sured in the "zero field" cooled measurements. This ob- servation held true over the entire range of B, covered in this investigation.

Hysteresis measured in the common mode configuration, which is the experiment in which the loop applied fields point in the same direction, is shown in Fig. 4 as filled circles. In this case, the hysteresis was significantly larger than that observed in the Werential mode configuration. The onset of hysteresis could fm be detected for a value of B, = 10 pT. For a value of B, = 100 pT, the common

mode hysteresis was a factor of 19 times greater than the Merentid mode hysteresis. The difference cannot be com- pletely accounted for by the additional field generated at the

0 0

0

0

- 8 0 0

0

J

0

1

Differential Mode ZZzJ 4

0 500 loo0 1500 2000 2500 Peak-To-Peak Applied Magnetic Field (pT)

Fig. 4. Hysteresis measured as a function of the peak-to-peak applied field for the differential mode experiment and the common mode experiment.

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edge of the loop stripline by the transport current which is induced in the common mode configuration. We estimate that the transport current increases the field at the stripline edge by only an additional 40 pT for an applied field of 100 pT; whereas it would have to provide an increase of over 100 pT according to our differential mode measurements to cause the difference observed. The remaining difference can be explained by the cancellation of a portion of the hystere- sis of one loop by the other loop.

Because of the large inductance of the connecting strip- lines of our sample, and the imperfect coupling from our flip-chip alignment, we measured that it takes a total of 4586 Qo applied to the sample input loops to couple one a0 into our SQUID. We can use this coupling factor, k, to de- termine the hysteresis with respect to the loops, to allow a c ~ m p ~ s o n with the results of other samples. Following Koch et al. [6] , we can define the fractional hysteresis error, I z , as

where 24 is the total area of both sample input loops. A plot o f k versus Bpp is given in Fig. 5 for both the differential mode and the common mode configmations. In the com- mon mode configuration, we have an h-value of 3.5 x for Epp= 100 pT, which is somewhat higher than the values of a~pro~imately 8 x 10'' which were reported for measure- ments on YBCO SQUID washer structures for the same val- tie of Bpp 171. The larger size and different geometry of our samples may explain the difference.

We obtain an h-value of 2.1 x in the differential mode conQpation for Bpp = 100 pT. This value is compa- rable with recently reported measurements [SI made on a high-T, gradiometer consisting of a YBCO multilayered gtadiometric SQUID with a flip-chipped flux transformer. The value of h increases exponentially for Bpp C: 400 pT, and then begins leveling-off. For larger linewidth samples of the same thickness, we have observed that the onset of hysteresis occurs at lower values of applied field.

Iv. SUMMARY

We have measured hysteresis in YBCO sense loops in experiments performed at 77 K. A sample design with dual magnetometer input loops allowed us to measure hysteresis pertinent in a gradiometer configuration as well as a mag- netometer configuration, allowing a direct comparison. In a gradiometer codiguration, we observed cancellation of the

0 0

e 0

*. O

0

0

0 0

0 0

Fig. 5 . Fractional hysteresis error, h, as a fimction ofBpp. for the differ- ential and common mode configuration experiments.

hysteresis from one loop by the other loop. In the case of our 400-pm linewidth sample, the fractional hysteresis in the loops for Bpp= 100 pT, such as encountered for a rotation in h e earth's field, was 2.1 x for the gradiometer mode, and 3.5 x for a magnetometer. The hysteresis did not increase when the sample was cooled and measured in a 50 pT field.

ACKNOWLEDGMENT

The authors wish to than!s Ted R. Clem for many useful discussions.

REFERENCES

J Z Sun, W J Gallagher, and R.H. Koch, "hiitial-vortex-entryrelated magnetic hysteiesis m thm-film SQUID magnetometers," Phys Rev E 50 (18), 13644 (1994) John W. Purpura, Ted R Clem and Roy F Wiegert, "Nonhear r sponse in thin fihn inagnetonieter sense loops at 77 K," IEEE Tr Appl. Sup. 5,3123 (1995) R H Koch, J R Rozen, J Z Sun, and W.J Gallagher, "Three SOU1 gradiometei," App1 Phys kit 63 (3), 403 (1993). Conductus, Jnc ,969 W. Maude Avenue, Sunnyvale, CA 94086 IBM Research Division, P.O. Box 218, Yorktown Heights, NY 10 RH. Koch, M B Ketchen, W.J. Gallagher, R L Sandstrom, Klemsasser, U.R Gambrel, T.H. Field, and H. Matz, "Magnehc h: teresis in integrated low T' SQUID gradiometers," Appl Phys Lett (IG), 1786 (1991) J Z Sun, W J Gallagher, and R H Koch, "Magnets hysteresis ui tl. film dc SQUID magnetometers." Appl. Phys. T ~ t t 61 (26), 31 (1992) M.N Keene, N J Exon, R G Humphreys, and N G Chew, "The mf

ence of ambient magnetx envvonments on high-T, superconduct1 quantum inte&erence device gradiometers," J Appl Phys 79 (1 8783 (1996)