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Sensitive dc-SQUIDs and magnetometers using step-edge junctions
me
Eiii
PHYSICA 8
me
&SEWER
Sensitive
Physica C 282-287
dc-SQUIDS and magnetometers
J.C.Nie,L.Chen,T.Yang,M.Q.Huang,P.J.Wu,
(I 997) 2477-2478
using step-edge junctions G.R.Liu,andL.Li
National Laboratory for Superconductivity, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing, 100080, China.
Superconducting quantum interference devices (SQUIDS) using YHa&r307 step-edge grain boundary junctions on LaA103 substrates were fabricated. The I-V and V-4 characteristics of these step-junction dcSQUIDS were studied. At 77K, a square washer configuration with inductance L,=150 pH yields flux noise of about ~.OSX~O~~&/~HZ, corresponding to a magnetic fie1.d and energy resolutions were 270 ff/dHz and 3.68x lo-*’ J/Hz respectively above 100 Hz.
35665A Dynamic Signal Analyzer for the device operating in a feedback loop.
1. INTRODUCTION Currently most YDCO based devices rely on step edge grain boundary or SNS junctions and planar bicrystal or biepitaxial junctions. Recent progress on magnetometer and gradiometer using &SQUIDS with double step-edge junctions as low noise weak links[l] is quite encouraging. In this paper we describe the experimental results for step junction d&SQUIDS on LaA103 (LAO) substrates.
”
2. EXPERIMENTAL
I
-1 I,/((,
i
-80
Cr metal masks have been used to prepare the step edges with Ar+ ion milling. Details of the fabrication process is published elsewhere[2]. The resulting step edge has a high angle of 70”. All the step heights are 200 run. Epitaxial YBCO films were deposited by dc-magnetron sputtering on LAO substrates with pre-etched steps. The films were patterned by low energy (300eV) Ar-ion milling. To fabricate the SQUID, two parallel bridges of 2-3 urn in width, 10 pm in length and a square loop were patterned over the step. The values of inductance of the square loop with 40, 70, 100 urn inner holes are estimated to be about 60, 100, 150 pH respectively. The outer dimensions of the washers are about 4, 5, 10 mm respectively. The inset of Fig.1 shows a layout of a fabricated step junction dc-SQUID. SQUIDS are characterized in a u-metal shielded temperature controlled holder. Noise measurements are performed by using a HP 0921-4534/97/$17.00 0 Elsevier Science B.V PI1 SO921-4534(97)01325-7
c
-4”
All rights reserved.
-120
-240
-1KO
,
,
-80
0
, 80
, 160
,I 240
Voltage
Fig.1 I-V characteristics of a step-junction dcSQUID with different applied magnetic flux to the loop at 77 K. The inset is the layout of a fabricated step junction dc-SQUID. 3. RESULTS AND DISCUSSION
The typical critical current 10=21, of the step-edge d&SQUIDS is about 40 kA. Thus, bt,=2LIJ$0 varies from 1.16 to 2.9. These values are near to unity indicating that our SQUIDS will have large modulation currents. The I-V curves of a dcSQUID with 60 pH hole inductance and 4x4 mm2 outer washer dimension with different applied magnetic flux to the loop at 77 K are shown in Fig. 1. These Z-V curves are modulated by & the
2478
J.C. Nie er al. /Pltysicn
applied magnetic flux to the SQUID-hole, oscillating between a curve with &=n& and a curve with &=(n+l)&. The I-I/ characteristics of the dc-SQUIID also exhibit the existence of a “rounding” effect due to the thermal fluctuation. This can be well explained by the theory of thermally activated phase slip[3]. Modulation voltages of about 10 PV per flux quantum #,, were obtained at 77K for L,
C 282-287
(1997)
2477-2478
10x10 mm’ outer washer dimension. The effective area is A+O.39dD and the factor of flux focusing effect is about 39. With increasing inductance L, the transfer function avla4 decreases, accompanied by a corresponding increase in flux noise. We measured a flux noise of 1.1 x 10v4 &+/Hz at 1 Hz corresponding to a magnetic field sensitivity of 710 ff/dHz for a 150 pH dc-SQUID. The energy resolution of this device is 1.8~10‘~~J/Hz at 1 Hz. At 100 Hz, the flux noise is about 5.08x 10m5@/Hz, the best magnetic field and energy resolutions are 270 fT/dHz and 3.68x 1O-2gJ/Hz respectively.
Fig.3 Flux noise and field noise spectrum of a 150 pH dc-SQUID at 77 K meaasure. 4. SUMMARY The Z-V and V-4 characteristics of the stepjunction dc-SQUIDS were studied. At 77K, a modulation voltage exceeding 16 PV was attained with L,=60 pH. A white noise level of 5.08~10“ $&/Hz at 100 H z and a low frequency noise around 1. lx 10-44ddHz at 1Hz in a 150pH device was measured at 77K. At 100 Hz, the magnetic field and energy resolutions were 270 fT’/dHz and 3.68~10~~’J/Hz respectively. REFERENCE
E
I
-2
I
I........
-I
D
Magnetic
Field
I........
I
I.....
2
.J
(mG)
Fig.2 V-4 curves of a step-junction d&QUID different bias currents at 77 K.
at
Figure 3 shows the spectral flux noise at 77 K for a SQUID with 150 pH hole in ductance and
1. Y.Tavrin, Y.Zhang, M.Miick, A.I.Braginski, and C.Heiden, Appl.Phys.Lett. 62,1824( 1993). 2. J.C.Nie, L.Chen, T.Yang, M.Q.Huang, P.J.Wu, G.R.Liu, and L.Lin, Physica C 258, 183(1996). 3. R.Gross, P.Chaudhari, D.Dimos, A.Gupta, and G.Koren, Phys.Rev.Lett. 64, 228(1990).
Eiii
PHYSICA 8
me
&SEWER
Sensitive
Physica C 282-287
dc-SQUIDS and magnetometers
J.C.Nie,L.Chen,T.Yang,M.Q.Huang,P.J.Wu,
(I 997) 2477-2478
using step-edge junctions G.R.Liu,andL.Li
National Laboratory for Superconductivity, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing, 100080, China.
Superconducting quantum interference devices (SQUIDS) using YHa&r307 step-edge grain boundary junctions on LaA103 substrates were fabricated. The I-V and V-4 characteristics of these step-junction dcSQUIDS were studied. At 77K, a square washer configuration with inductance L,=150 pH yields flux noise of about ~.OSX~O~~&/~HZ, corresponding to a magnetic fie1.d and energy resolutions were 270 ff/dHz and 3.68x lo-*’ J/Hz respectively above 100 Hz.
35665A Dynamic Signal Analyzer for the device operating in a feedback loop.
1. INTRODUCTION Currently most YDCO based devices rely on step edge grain boundary or SNS junctions and planar bicrystal or biepitaxial junctions. Recent progress on magnetometer and gradiometer using &SQUIDS with double step-edge junctions as low noise weak links[l] is quite encouraging. In this paper we describe the experimental results for step junction d&SQUIDS on LaA103 (LAO) substrates.
”
2. EXPERIMENTAL
I
-1 I,/((,
i
-80
Cr metal masks have been used to prepare the step edges with Ar+ ion milling. Details of the fabrication process is published elsewhere[2]. The resulting step edge has a high angle of 70”. All the step heights are 200 run. Epitaxial YBCO films were deposited by dc-magnetron sputtering on LAO substrates with pre-etched steps. The films were patterned by low energy (300eV) Ar-ion milling. To fabricate the SQUID, two parallel bridges of 2-3 urn in width, 10 pm in length and a square loop were patterned over the step. The values of inductance of the square loop with 40, 70, 100 urn inner holes are estimated to be about 60, 100, 150 pH respectively. The outer dimensions of the washers are about 4, 5, 10 mm respectively. The inset of Fig.1 shows a layout of a fabricated step junction dc-SQUID. SQUIDS are characterized in a u-metal shielded temperature controlled holder. Noise measurements are performed by using a HP 0921-4534/97/$17.00 0 Elsevier Science B.V PI1 SO921-4534(97)01325-7
c
-4”
All rights reserved.
-120
-240
-1KO
,
,
-80
0
, 80
, 160
,I 240
Voltage
Fig.1 I-V characteristics of a step-junction dcSQUID with different applied magnetic flux to the loop at 77 K. The inset is the layout of a fabricated step junction dc-SQUID. 3. RESULTS AND DISCUSSION
The typical critical current 10=21, of the step-edge d&SQUIDS is about 40 kA. Thus, bt,=2LIJ$0 varies from 1.16 to 2.9. These values are near to unity indicating that our SQUIDS will have large modulation currents. The I-V curves of a dcSQUID with 60 pH hole inductance and 4x4 mm2 outer washer dimension with different applied magnetic flux to the loop at 77 K are shown in Fig. 1. These Z-V curves are modulated by & the
2478
J.C. Nie er al. /Pltysicn
applied magnetic flux to the SQUID-hole, oscillating between a curve with &=n& and a curve with &=(n+l)&. The I-I/ characteristics of the dc-SQUIID also exhibit the existence of a “rounding” effect due to the thermal fluctuation. This can be well explained by the theory of thermally activated phase slip[3]. Modulation voltages of about 10 PV per flux quantum #,, were obtained at 77K for L,
C 282-287
(1997)
2477-2478
10x10 mm’ outer washer dimension. The effective area is A+O.39dD and the factor of flux focusing effect is about 39. With increasing inductance L, the transfer function avla4 decreases, accompanied by a corresponding increase in flux noise. We measured a flux noise of 1.1 x 10v4 &+/Hz at 1 Hz corresponding to a magnetic field sensitivity of 710 ff/dHz for a 150 pH dc-SQUID. The energy resolution of this device is 1.8~10‘~~J/Hz at 1 Hz. At 100 Hz, the flux noise is about 5.08x 10m5@/Hz, the best magnetic field and energy resolutions are 270 fT/dHz and 3.68x 1O-2gJ/Hz respectively.
Fig.3 Flux noise and field noise spectrum of a 150 pH dc-SQUID at 77 K meaasure. 4. SUMMARY The Z-V and V-4 characteristics of the stepjunction dc-SQUIDS were studied. At 77K, a modulation voltage exceeding 16 PV was attained with L,=60 pH. A white noise level of 5.08~10“ $&/Hz at 100 H z and a low frequency noise around 1. lx 10-44ddHz at 1Hz in a 150pH device was measured at 77K. At 100 Hz, the magnetic field and energy resolutions were 270 fT’/dHz and 3.68~10~~’J/Hz respectively. REFERENCE
E
I
-2
I
I........
-I
D
Magnetic
Field
I........
I
I.....
2
.J
(mG)
Fig.2 V-4 curves of a step-junction d&QUID different bias currents at 77 K.
at
Figure 3 shows the spectral flux noise at 77 K for a SQUID with 150 pH hole in ductance and
1. Y.Tavrin, Y.Zhang, M.Miick, A.I.Braginski, and C.Heiden, Appl.Phys.Lett. 62,1824( 1993). 2. J.C.Nie, L.Chen, T.Yang, M.Q.Huang, P.J.Wu, G.R.Liu, and L.Lin, Physica C 258, 183(1996). 3. R.Gross, P.Chaudhari, D.Dimos, A.Gupta, and G.Koren, Phys.Rev.Lett. 64, 228(1990).