- Email: [email protected]
Magnetic microscope based on YBCO bicrystal thin film do SQUID operating at 77 K
ICEC 15 Proceedings
Magnetic Microscope Based on YBCO Bicrystal Thin Film dc SQUID Operating at 77 K
S.A. Gudoshnikov*, I.I. Vengrus**, K.E. Andreev**, and O.V. Snigirev** * Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences, 142092, Troitsk, Moscow Region, Russia **Department of Physics Moscow State University, 119899 GSP Moscow, Russia We have developed two versions of magnetic flux microscope which use an YBCO thin film bicrystal dc SQUID to image the two-dimensional distribution of magnetic field. With the size of the inner hole of the SQUID close to 30 ~tm a spatial resolution of about 200 I-tm and a field of view 3×4 mm 2 have been obtained. The equivalent magnetic field noise in the SQUID ranges from 100 pT/Hz -1/2 at 1 Hz to 10 pT/Hz -1/2 at 100 Hz. We have employed the microscope to image 50 I.tA 110 Hz ac currents flowing in the thin parallel wires.
INTRODUCTION Recently first scanning magnetic microscopes based on low-Tc SQUID operating at T=4.2 K have been fabricated and successfully tested with different objects [1-3]. The main element of all developed magnetic imaging systems is the cryogenic two-coordinates positioning mechanism. This mechanism provides scanning along two perpendicular directions over a typical sample area of about 5×5 mm 2 with controlled steps of about 10 I-tm and with fixed separation between the SQUID and sample surface. The main problem o f such systems is that the all scanning parts of microscope, including two-coordinates positioning mechanism, should be able to work at cryogenic temperatures. The application of the thin film YBCO dc SQUID [3] cooled by liquid nitrogen as a sensor of magnetic microscope is very attractive idea. The operation at T=77 K allows us simplify the design and the operating conditions. In spite of simplicity of the idea to use the dc SQUID in the magnetic imaging system, the practical realization of this device is very difficult. One of the most important problems which should be solved is to eliminate the influence of external noise on the SQUID. The other two problems, which can be mentioned here, are the stable and reproducible performance o f the X-Y scanning mechanism and Z-coordinate positioner and the selection of the construction materials with the low level of the magnetic impurities. In this paper we present the first version of the developed device, discussion of the obtained results, and description of the new version, which based on the experience obtained during the of measurements.
D E S C R I P T I O N OF THE FIRST S C A N N I N G SYSTEM A cryogenic part of the first version of scanning magnetic microscope is shown in Fig. 1. In our microscope, the SQUID was mounted on a kapron-based holder and is kept at fixed position during the measurements. The SQUID holder was shifted down before the measurements and sample was placed and glued to the sample holder through the windows in the brass container. Then the separation between the sample and SQUID is adjusted using a lever and a fine screw: The specific features of manufacturing SQUID contact pads and position of the soldered wires at the chip did not allow us to make this separation less than 200 I.tm. The sample holder was made of fabricbased laminate and attached to the X-Y translation table. The brass container was fixed on the stainless steel thin wall tube with outer diameter 40 ram. This tube Cryogenics 1994 Vo134 ICEC Supplement 883
ICEC 15 Proceedings
connects the room temperature parts and cryogenic parts of the device. Inside the brass container driving tubes with the plastic rods at the ends were connected to the worm gear arrangement and brass screwwasher assemblies. As the result the 1 turn of the motor rotation was transfered in the 10 iam of the table linear movement. Typical value of the sample scanning rate was about 10 gm/sec.
DC I N T E R F E R O M E T E R S D E S I G N AND S Q U I D S O P E R A T I O N We used a dc SQUID made of YBCO thin film as a magnetic field sensor [4]. Thin film was laser deposited on a SrTiO3 36 ° bicrystal substrate and was patterned using photolithography and laser ablation to form a quasi-square washer with 30×30 lam2 inner hole and outer dimension of about 1 mm. The effective magnetic field pick-up area of the sensor was 0.01 mm 2 ,and the interferometer inductance was close to 60 pH. At 77 K its critical current is about 80 laA, the normal resistance is close to 0.6 f2, the maximum slope of the voltage-to-flux transfer function is about 10 !aV/~0. During operation, the SQUID was kept in a flux locked loop under control of the computer-driven electronics. Two cylindrical mu-metal shields around the dewar were used in order to reduce the ambient magnetic field. The main sources of the radiointerference noise deteriorating the SQUID normal operation were a two-wire line between the computer and SQUID electronics and a common ground wire in the interferometer bias current and voltage leads. An optical decoupling pair and additional ground wire for the voltage amplifier was inserted in the electronics. After that we ran the experiment in the usual laboratory environment without radio-frequency shielded room. During imaging, we operated with feedback transform factor from 0.4 V/~0 to 2.2 V/~0. Using the small test coil at the sample holder we found that the effective pickup area of the SQUID has provided 200 nT per one flux quantum q~0 at the device input. While the equivalent magnetic flux noise in the SQUID ranges from 5×10 -4 ~0/Hz 1/2 at 1 Hz to 5×10 -5 qb0/Hz 1/2 at 100 Hz the dynamic range of the device was as high as t 10 dB. A New Version dc SQUIDs Design Several SQUIDs with new different interferometer designs were abricated and successfully tested. There were devices with 30x30 ~tm2 inner hole and 46x46 ~tm2 outer washer, 50x50 gm 2 inner hole and 70x70 ~.tm2 outer washer, 50x50 j.tm2 inner hole - 200x200 I.tm2 and 500x500 ~.tm2 outer. Josephson junctions were formed as two microbridges on bicrystal line with typical width of each equal to 5 ILtm. Each type of SQUIDs was twice fabricated at the same substrate. Thus we could measure two parallel tracks (for example Bz(x, Yi) and Bz(x , yi+Ay), Ay - distance between sensors) along one direction (x) simultaneously. On the other hand we could subtract signals from sensors and realize gradiometer configuration. Modulating coil of each SQUID was patterned in superconducting film near the interferometer. Such configuration allow us to localize modulating field in small area and minimize influence of feedback signal on testing sample. In order to minimize influence from the big superconducting parts of film on the sample the gold contact pads (with superconducting underlayer) were made with the area as small as possible at the distance 3.5 mm from the interferometer. With the same purpose superconducting thin film between contact pads and dc interferometer were made as narrow as possible (from I00 ~tm near the contact pads to 10 I-tm near the body ofinterferometer).
PRELIMINARY MEASUREMENTS At the beginning we tested the system operation without the sample. We found that during one X-scan Z component of the measured magnetic field (Bz) varied from - 10 nT to + 10 nT with coordinate period close to 400 ~tm, which well corresponded to one turn of the brass screw in the cryogenic worm mechanism. We added the locking amplifier PAR 5209 between the SQUID output and AD converter in order to avoid significant background signal produced by magnetic impurities in the translation mechanism.
884 Cryogenics 1994 Vo134 ICEC Supplement
ICEC 15 Proceedings
Built-in generator was used as a current source to measure the currents flowing in the thin wires or printed board circuits. The separation between the SQUID and sample table was found from the measurements of magnetic field produced by the single wire. The result was 600 j.tm instead of 200 t-tm fixed at room temperature. After that we made images of a variety of the samples. For example, Fig. 2(a) shows the resulting magnetic image of Z component of the magnetic field (Bz) of the sample consisting of several wires with an ac 50 I-tA currents flowing in the same direction. One can see that the observed pattern is in a good agreement with the simulated field configuration shown in Fig. 2(b).
DISCUSSION AND CONCLUSIONS In conclusion, we have designed and tested the first version of the magnetic flux microscope at 77 K based on YBCO thin film dc SQUID as a magnetic field sensor. During our experiments the de SQUID demonstrated stable and reproducible operation in the each run. With the aim to estimate the field sensitivity of the microscope, we measured the equivalent flux noise spectrum at the SQUID output. The equivalent flux noise for the given effective pickup area of our SQUID close to 0.0t mm 2 results in field sensitivity ranged from 100 pT/Hz 1/2 at 1 Hz to 10 pT/Hz 1/2 at 100 Hz. On the other hand in the first version of the magnetic microscope we were limited in the space resolution by the unpredicted deformations of the X-Y table and SQUID holder at 77 K. In addition, significant background signal from X-Y table translations made us carry out measurements using ac modulation of magnetic field. This problems were solved in the new version of apparatus. Cryogenic part of the new version of microscope is shown in Fig 3. SQUID sensor is placed at the top of kapron holder. One can move this holder inside 14-mm hole in the base stage (Z coordinate) pushing the rod along X direction via the wedge 4:1. Bottom part of the holder is pressed against by CuBe spring that makes available reverse movement. This system allows us set sensor position at the distance 0.05+2 mm from the sample with minimum step of about 10 ~tm. Matching transformer between SQUID and electronics, and magnetic field coils are placed at the base stage. X-Y translation system (see Fig. 3) consists of base part that can move along X direction and small table with the sample holder placed inside base part. Table with the sample holder is moved along Y coordinate by pushing along X direction rod through the wedge 2:1. At the opposite side it is held against by Cu-Be spring. X-Y translation system is pressed face-to-face with the plane of the base stage, so the sample holder slides along base stage plane at the front of the sensor. Sample is glued in the recess at the sample holder surface. All moving rods (X,Y and Z) are driven by push-tubes and held against by Cu-Be springs. Pushtubes and cables are placed inside connecting room temperature and cryogenic parts 40-ram tube. Room temperature part consists of three stepper motors, SQUID electronics, and stepper motors drive electronics. During X-Y-Z translations the stepper motors turn special drive screws moving down push-tubes. Measurements are carried out in automatic computer driven mode. We are going to obtain new experimental results at the nearest future. This work is supported by Russian Fund of Fundamental Research (Grant No. 93-02-17495). REFERENCES Mathai, A., Song, D., Gim, Y. and Wellstood, F.C., High resolution magnetic microscopy using a dc SQUID" IEEE Transactions on Applied Superconductivity (1993) 3 2609-2612. Vu, L.N:, Wistrom, M.S.and Van Harlingen, L, Imaging of magnetic vortices in superconducting network and clusters by scanning SQUID microscopy AppL Phys. Lett. (1993) 63 1693-1695. Black, R.C., Mathai, A., Wellstood, F.C., Dantscer, E., Miklich, A.H:, Nemeth, D.T., Kingston, J.J. and Clarke, J., Magnetic microscopy using a liquid nitrogen cooled YBaCuO superconducting quantum interference device Appl. Phys. Lett. (1993) 62 2128-2130. Vengrus, I.I., Kupriyanov, M.Yu., Maresov, A.G., Pirogov, V.G., Snigirev, O.V., Krasnosvobodtsev, S.I., Thin film HTSC SQUID-magnetometer on bicrystal substrate SrTiO3 Russian Phys.: Sverkhprovodimost': fizika, khimiya, tekhnika (1993) 6 1730-1748 1993. Cryogenics 1994 Vo134 ICEC Supplement 885
ICEC 15 Proceedings
Rotating tubes
f
less
netal
(
]
Brass
container Worm driv-~"e of X eoord. Worm drive-"-of Y eoord, DC SQUID
)rd.
lg plate
)rd. lg plate
Test coill SQUID
holde_rj~
,te
f
ling former
Figure 1 Cross-section of the cryogenic part of the scanning magnetic microscope (first version).
150
~..
6b
c 100 ~ 50 -6
~ ,
d
-5o
o~
&
~; -100
-150 0
1
2
5
4
5
6
X (ram) b)
a)
Figure 2 Magnetic image of the sample consisting of several wires with currents flowing in the same direction: a) observed pattern; b) expected field configuration. l~4D mm
1~40 e , ~
J
Bsse
stage
r
~Springs Base part of the
mo~.ng rod
X-Y translation "--.4yaem Y-coord.
SQUID
/
holder
SQb'IDj
__Sample pSample holder X-Y translation /
\table
/
Figure 3 New version of cryogenic part of magnetic microscope 886
Cryogenics 1994 Vol 34 ICEC Supplement
/
Magnetic Microscope Based on YBCO Bicrystal Thin Film dc SQUID Operating at 77 K
S.A. Gudoshnikov*, I.I. Vengrus**, K.E. Andreev**, and O.V. Snigirev** * Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences, 142092, Troitsk, Moscow Region, Russia **Department of Physics Moscow State University, 119899 GSP Moscow, Russia We have developed two versions of magnetic flux microscope which use an YBCO thin film bicrystal dc SQUID to image the two-dimensional distribution of magnetic field. With the size of the inner hole of the SQUID close to 30 ~tm a spatial resolution of about 200 I-tm and a field of view 3×4 mm 2 have been obtained. The equivalent magnetic field noise in the SQUID ranges from 100 pT/Hz -1/2 at 1 Hz to 10 pT/Hz -1/2 at 100 Hz. We have employed the microscope to image 50 I.tA 110 Hz ac currents flowing in the thin parallel wires.
INTRODUCTION Recently first scanning magnetic microscopes based on low-Tc SQUID operating at T=4.2 K have been fabricated and successfully tested with different objects [1-3]. The main element of all developed magnetic imaging systems is the cryogenic two-coordinates positioning mechanism. This mechanism provides scanning along two perpendicular directions over a typical sample area of about 5×5 mm 2 with controlled steps of about 10 I-tm and with fixed separation between the SQUID and sample surface. The main problem o f such systems is that the all scanning parts of microscope, including two-coordinates positioning mechanism, should be able to work at cryogenic temperatures. The application of the thin film YBCO dc SQUID [3] cooled by liquid nitrogen as a sensor of magnetic microscope is very attractive idea. The operation at T=77 K allows us simplify the design and the operating conditions. In spite of simplicity of the idea to use the dc SQUID in the magnetic imaging system, the practical realization of this device is very difficult. One of the most important problems which should be solved is to eliminate the influence of external noise on the SQUID. The other two problems, which can be mentioned here, are the stable and reproducible performance o f the X-Y scanning mechanism and Z-coordinate positioner and the selection of the construction materials with the low level of the magnetic impurities. In this paper we present the first version of the developed device, discussion of the obtained results, and description of the new version, which based on the experience obtained during the of measurements.
D E S C R I P T I O N OF THE FIRST S C A N N I N G SYSTEM A cryogenic part of the first version of scanning magnetic microscope is shown in Fig. 1. In our microscope, the SQUID was mounted on a kapron-based holder and is kept at fixed position during the measurements. The SQUID holder was shifted down before the measurements and sample was placed and glued to the sample holder through the windows in the brass container. Then the separation between the sample and SQUID is adjusted using a lever and a fine screw: The specific features of manufacturing SQUID contact pads and position of the soldered wires at the chip did not allow us to make this separation less than 200 I.tm. The sample holder was made of fabricbased laminate and attached to the X-Y translation table. The brass container was fixed on the stainless steel thin wall tube with outer diameter 40 ram. This tube Cryogenics 1994 Vo134 ICEC Supplement 883
ICEC 15 Proceedings
connects the room temperature parts and cryogenic parts of the device. Inside the brass container driving tubes with the plastic rods at the ends were connected to the worm gear arrangement and brass screwwasher assemblies. As the result the 1 turn of the motor rotation was transfered in the 10 iam of the table linear movement. Typical value of the sample scanning rate was about 10 gm/sec.
DC I N T E R F E R O M E T E R S D E S I G N AND S Q U I D S O P E R A T I O N We used a dc SQUID made of YBCO thin film as a magnetic field sensor [4]. Thin film was laser deposited on a SrTiO3 36 ° bicrystal substrate and was patterned using photolithography and laser ablation to form a quasi-square washer with 30×30 lam2 inner hole and outer dimension of about 1 mm. The effective magnetic field pick-up area of the sensor was 0.01 mm 2 ,and the interferometer inductance was close to 60 pH. At 77 K its critical current is about 80 laA, the normal resistance is close to 0.6 f2, the maximum slope of the voltage-to-flux transfer function is about 10 !aV/~0. During operation, the SQUID was kept in a flux locked loop under control of the computer-driven electronics. Two cylindrical mu-metal shields around the dewar were used in order to reduce the ambient magnetic field. The main sources of the radiointerference noise deteriorating the SQUID normal operation were a two-wire line between the computer and SQUID electronics and a common ground wire in the interferometer bias current and voltage leads. An optical decoupling pair and additional ground wire for the voltage amplifier was inserted in the electronics. After that we ran the experiment in the usual laboratory environment without radio-frequency shielded room. During imaging, we operated with feedback transform factor from 0.4 V/~0 to 2.2 V/~0. Using the small test coil at the sample holder we found that the effective pickup area of the SQUID has provided 200 nT per one flux quantum q~0 at the device input. While the equivalent magnetic flux noise in the SQUID ranges from 5×10 -4 ~0/Hz 1/2 at 1 Hz to 5×10 -5 qb0/Hz 1/2 at 100 Hz the dynamic range of the device was as high as t 10 dB. A New Version dc SQUIDs Design Several SQUIDs with new different interferometer designs were abricated and successfully tested. There were devices with 30x30 ~tm2 inner hole and 46x46 ~tm2 outer washer, 50x50 gm 2 inner hole and 70x70 ~.tm2 outer washer, 50x50 j.tm2 inner hole - 200x200 I.tm2 and 500x500 ~.tm2 outer. Josephson junctions were formed as two microbridges on bicrystal line with typical width of each equal to 5 ILtm. Each type of SQUIDs was twice fabricated at the same substrate. Thus we could measure two parallel tracks (for example Bz(x, Yi) and Bz(x , yi+Ay), Ay - distance between sensors) along one direction (x) simultaneously. On the other hand we could subtract signals from sensors and realize gradiometer configuration. Modulating coil of each SQUID was patterned in superconducting film near the interferometer. Such configuration allow us to localize modulating field in small area and minimize influence of feedback signal on testing sample. In order to minimize influence from the big superconducting parts of film on the sample the gold contact pads (with superconducting underlayer) were made with the area as small as possible at the distance 3.5 mm from the interferometer. With the same purpose superconducting thin film between contact pads and dc interferometer were made as narrow as possible (from I00 ~tm near the contact pads to 10 I-tm near the body ofinterferometer).
PRELIMINARY MEASUREMENTS At the beginning we tested the system operation without the sample. We found that during one X-scan Z component of the measured magnetic field (Bz) varied from - 10 nT to + 10 nT with coordinate period close to 400 ~tm, which well corresponded to one turn of the brass screw in the cryogenic worm mechanism. We added the locking amplifier PAR 5209 between the SQUID output and AD converter in order to avoid significant background signal produced by magnetic impurities in the translation mechanism.
884 Cryogenics 1994 Vo134 ICEC Supplement
ICEC 15 Proceedings
Built-in generator was used as a current source to measure the currents flowing in the thin wires or printed board circuits. The separation between the SQUID and sample table was found from the measurements of magnetic field produced by the single wire. The result was 600 j.tm instead of 200 t-tm fixed at room temperature. After that we made images of a variety of the samples. For example, Fig. 2(a) shows the resulting magnetic image of Z component of the magnetic field (Bz) of the sample consisting of several wires with an ac 50 I-tA currents flowing in the same direction. One can see that the observed pattern is in a good agreement with the simulated field configuration shown in Fig. 2(b).
DISCUSSION AND CONCLUSIONS In conclusion, we have designed and tested the first version of the magnetic flux microscope at 77 K based on YBCO thin film dc SQUID as a magnetic field sensor. During our experiments the de SQUID demonstrated stable and reproducible operation in the each run. With the aim to estimate the field sensitivity of the microscope, we measured the equivalent flux noise spectrum at the SQUID output. The equivalent flux noise for the given effective pickup area of our SQUID close to 0.0t mm 2 results in field sensitivity ranged from 100 pT/Hz 1/2 at 1 Hz to 10 pT/Hz 1/2 at 100 Hz. On the other hand in the first version of the magnetic microscope we were limited in the space resolution by the unpredicted deformations of the X-Y table and SQUID holder at 77 K. In addition, significant background signal from X-Y table translations made us carry out measurements using ac modulation of magnetic field. This problems were solved in the new version of apparatus. Cryogenic part of the new version of microscope is shown in Fig 3. SQUID sensor is placed at the top of kapron holder. One can move this holder inside 14-mm hole in the base stage (Z coordinate) pushing the rod along X direction via the wedge 4:1. Bottom part of the holder is pressed against by CuBe spring that makes available reverse movement. This system allows us set sensor position at the distance 0.05+2 mm from the sample with minimum step of about 10 ~tm. Matching transformer between SQUID and electronics, and magnetic field coils are placed at the base stage. X-Y translation system (see Fig. 3) consists of base part that can move along X direction and small table with the sample holder placed inside base part. Table with the sample holder is moved along Y coordinate by pushing along X direction rod through the wedge 2:1. At the opposite side it is held against by Cu-Be spring. X-Y translation system is pressed face-to-face with the plane of the base stage, so the sample holder slides along base stage plane at the front of the sensor. Sample is glued in the recess at the sample holder surface. All moving rods (X,Y and Z) are driven by push-tubes and held against by Cu-Be springs. Pushtubes and cables are placed inside connecting room temperature and cryogenic parts 40-ram tube. Room temperature part consists of three stepper motors, SQUID electronics, and stepper motors drive electronics. During X-Y-Z translations the stepper motors turn special drive screws moving down push-tubes. Measurements are carried out in automatic computer driven mode. We are going to obtain new experimental results at the nearest future. This work is supported by Russian Fund of Fundamental Research (Grant No. 93-02-17495). REFERENCES Mathai, A., Song, D., Gim, Y. and Wellstood, F.C., High resolution magnetic microscopy using a dc SQUID" IEEE Transactions on Applied Superconductivity (1993) 3 2609-2612. Vu, L.N:, Wistrom, M.S.and Van Harlingen, L, Imaging of magnetic vortices in superconducting network and clusters by scanning SQUID microscopy AppL Phys. Lett. (1993) 63 1693-1695. Black, R.C., Mathai, A., Wellstood, F.C., Dantscer, E., Miklich, A.H:, Nemeth, D.T., Kingston, J.J. and Clarke, J., Magnetic microscopy using a liquid nitrogen cooled YBaCuO superconducting quantum interference device Appl. Phys. Lett. (1993) 62 2128-2130. Vengrus, I.I., Kupriyanov, M.Yu., Maresov, A.G., Pirogov, V.G., Snigirev, O.V., Krasnosvobodtsev, S.I., Thin film HTSC SQUID-magnetometer on bicrystal substrate SrTiO3 Russian Phys.: Sverkhprovodimost': fizika, khimiya, tekhnika (1993) 6 1730-1748 1993. Cryogenics 1994 Vo134 ICEC Supplement 885
ICEC 15 Proceedings
Rotating tubes
f
less
netal
(
]
Brass
container Worm driv-~"e of X eoord. Worm drive-"-of Y eoord, DC SQUID
)rd.
lg plate
)rd. lg plate
Test coill SQUID
holde_rj~
,te
f
ling former
Figure 1 Cross-section of the cryogenic part of the scanning magnetic microscope (first version).
150
~..
6b
c 100 ~ 50 -6
~ ,
d
-5o
o~
&
~; -100
-150 0
1
2
5
4
5
6
X (ram) b)
a)
Figure 2 Magnetic image of the sample consisting of several wires with currents flowing in the same direction: a) observed pattern; b) expected field configuration. l~4D mm
1~40 e , ~
J
Bsse
stage
r
~Springs Base part of the
mo~.ng rod
X-Y translation "--.4yaem Y-coord.
SQUID
/
holder
SQb'IDj
__Sample pSample holder X-Y translation /
\table
/
Figure 3 New version of cryogenic part of magnetic microscope 886
Cryogenics 1994 Vol 34 ICEC Supplement
/