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Observation of square array of nickel dots by using high-Tc SQUID microscope
Physica C 392–396 (2003) 1406–1410 www.elsevier.com/locate/physc
Observation of square array of nickel dots by using high-Tc SQUID microscope Kuniaki Sata, Mitsuyuki Tsuji, ShinÕichiro Nakata, Takekazu Ishida
*
Department of Physics and Electronics, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan Received 13 November 2002; accepted 21 February 2003
Abstract We have developed a SQUID microscope using a high-Tc SQUID sensor. The SQUID sensor is cooled by liquid nitrogen down to 77 K. The distance between the sensor and a sapphire window is less than 100 lm. The distance between a sample and the window along the Z-direction can be adjusted to less than 200 lm manually by monitoring with a CCD camera. A motion controller and a computer interface drive the stage. A SQUID sensor operates at the level near the critical current. A digital multimeter can read a SQUID output voltage coming from a local magnetic field. A sample is scanned in the two dimensions while the SQUID sensor is fixed to the bottom of the liquid nitrogen Dewar. The performance of the SQUID microscope is examined by a regular array of Ni dots. The sample is made by evaporating nickel on to a cover glass with a mask of metallic mesh (17 lm), and then magnetized in a magnetic field of 0.8 T. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.60.Ge; 74.25.Ha; 85.25.Dq Keywords: SQUID; Microscope; X –Y stage; High-Tc
1. Introduction A SQUID microscope has been developed rapidly in recent years. The SQUID microscope using a conventional superconductor consumes a lot of liquid helium [1]. The recent progress of high-Tc SQUID extends to the application in the field of magnetic field measurements. Since the SQUID resolution reaches 10 12 T, the apparatus becomes
*
Corresponding author. Fax: +81-72-254-9908. E-mail address: [email protected] (T. Ishida).
extremely sensitive compared to other means. Black et al. [3] pioneered a high-Tc SQUID microscope having a spatial resolution of 60 lm. Several groups have utilized the SQUID microscopes with higher spatial resolution to investigate superconductors [4–6]. This apparatus may find a wide variety of the applications in the material sciences. We eventually intend to improve a spatial resolution of the SQUID microscope to observe a Bitter pattern of vortices. In this paper, we report the construction of a high-Tc SQUID microscope, and describe the results of the test experiments by using a regular array of the tiny nickel dots.
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01062-1
K. Sata et al. / Physica C 392–396 (2003) 1406–1410
2. SQUID sensor As shown in Fig. 1, a SQUID sensor is made by the Sumitomo Hightechs Inc. (Osaka, Japan) is V
Bias current
Josephson junction
Thin film superconductor SQUID hole
Fig. 1. The schematic diagram of the SQUID sensor. A thinfilm superconductor is Eu–Ba–Cu–O. The sensitive area of the SQUID sensor is indicated in the figure as SQUID hole.
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fabricated on the STO substrate, of which the thickness is 1.0 mm. The details of the sensor have been reported elsewhere [2]. The SQUID sensor consists of a couple of high-Tc Josephson junctions. The signal of the SQUID sensor can be modulated with a period of flux U0 in the SQUID hole. This makes it possible to carry out the sensitive measurements of the magnetic field. The SQUID sensor has a sensitivity of 1 pT and the effective sensor area is 0.08 mm2 .
3. SQUID microscope In Fig. 2, we show the schematic diagram of our SQUID microscope constructed by us. A sample can be scanned in the two dimensions while the SQUID sensor is fixed to the nitrogen Dewar. An X –Y stage (Suruga K201-30LMS) is able to scan over 30 30 mm area and its spatial resolution is
Fig. 2. Schematic diagram of our SQUID microscope. (1) A high-Tc SQUID sensor, (2) a liquid nitrogen Dewar, (3) a doublepermalloy shield of the SQUID microscope, (4) a light source guide, (5) a sapphire window (100 lm in thickness), (6) an XY stage with stepper motors, (7) a Z stage in manual operation, (8) a driver, (9) a stage control board, (10) a CCD camera with a scope, (11) a power source for the scope, (12) a CRT monitor to examine a distance between sapphire window and sample, (13) an electronic SQUID controller, (14) a control computer, (15) a LabVIEW-6i software developed by us, (16) a motion interface and a control box for the XY stage, (17) a vacuum pump for the liquid nitrogen Dewar, (18) a digital multimeter to measure the SQUID output, (19) a PCI-GPIB board and (20) an active band pass filter for the SQUID output.
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K. Sata et al. / Physica C 392–396 (2003) 1406–1410
as good as 0.05 lm. The SQUID sensor can be cooled down to 77 K for 1 h by using the evacuated Dewar (5 10 4 Pa). A distance between the SQUID sensor and the sapphire window can be minimized as short as 100 lm. The sample space is subjected to light when one monitors a distance between the Dewar window and the sample. A motion interface (National Instruments UMI-7764) and a control box for the XY stage were assembled by us. The sample temperature is limited to room temperature. The whole device is controlled by LabVIEW-6i is the trademark of the National Instruments Inc. Fig. 3. The photograph of the Ni dots. The Ni dots are regularly arranged in a square lattice of lattice constant 17 lm.
4. Experimental We prepared a Ni rod and a square array of Ni dots. The diameter and length of the Ni rod is 0.1/ and 4 mm. The Ni dots were prepared by evaporating nickel on to a cover glass with a mask of metallic mesh (17 lm) (Fig. 3). We magnetized both sample in the field of 0.8 T.
We scanned the sample along the X - and Y -axes. We measured the Ni rod in the step of 200 lm over 3000 3000 lm region. For the Ni dots, the scan step was typically chosen as 3 lm. First, we fixed the position of the Y -coordinate, and started the scan along the X -direction to measure a SQUID signal. After completion, we added by 3 lm to the
Fig. 4. The two-dimensional graph of the SQUID signal on the Ni rod. The magnetic field is stronger in the thicker colors, but they have the opposite signs. The SQUID signal V is relative in scale.
K. Sata et al. / Physica C 392–396 (2003) 1406–1410
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Fig. 5. The two-dimensional graph of the SQUID signal on the Ni dots. The magnetic field is stronger in the thicker colors. The SQUID signal V is relative in scale.
X -coordinate, and repeat a scan along the X direction. A band pass filter of the bandwidth from 10 to 20 Hz was necessary to reduce the electronic noise. The whole measurements were controlled from a computer by the LabVIEW-6i software.
5. Results and discussion Fig. 4 represents the two-dimensional graph of the SQUID signal on the Ni rod. The distance between the sample and the window was chosen as 1 mm. When the distance becomes smaller than 1 mm, the SQUID sensor was overloaded due to the enhanced signal. It turns out that the Ni rod was magnetized, and generated the dipole field around the rod. The field profile is spread than the actual sample size because of the distance between the sample and the sensor. In Fig. 5, we show the two-dimensional field profile from the Ni dot sample where the distance
between the sample and the window is 0.3 mm. The square is the size of the Ni dot, and the squares are overlaid on the two-dimensional data as a measure of period. We can see the shape of the Ni dot from the magnetic field profile. It seems that the periodicity is apparent along the X -axis. However, the periodicity is not perfect along the Y -axis. We explain this by the fact that one is the SQUID hole is rectangular (see Fig. 1). The scan along the X -axis is more sensitive to a change in the magnetic field. The spatial resolution in the literature is less than 200 lm [2]. It is necessary to improve a spatial resolution of our system further.
6. Conclusions We have constructed the SQUID microscope using the high-Tc SQUID sensor. The field profile reproduces the shape of the Ni rod and dot sample well.
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K. Sata et al. / Physica C 392–396 (2003) 1406–1410
Acknowledgements This work was partially supported by a special grant for the practical applications in Osaka Prefecture University and by a grant to Prefecture Colleges and Universities from the Ministry of Education, Science, and Culture of Japan.
References [1] K. Chinone, S. Nakayama, T. Morooka, A. Odawara, M. Ikeda, IEEE Trans. Appl. Supercond. 7 (1997) 3271.
[2] K. Minamimura, T. Nagaishi, H. Itozaki, in: 8th Int. Super. Electron. Conf., Extended Abstract, Osaka, 2001, pp. 201– 202. [3] R.C. Black, A. Mathai, F.C. Wellstood, E. Dantsker, A.H. Miklich, D.T. Nemeth, J.J. Kingston, J. Clarke, Appl. Phys. Lett. 62 (1993) 2128. [4] T. Ishida, M. Yoshida, K. Okuda, JAERI-Review 2001-008, 2001, pp. 92–99. [5] I. Iguchi, A. Sugimoto, T. Yamaguchi, JAERI-Review 2001008, 2001, pp. 51–53. [6] A. Nagata, K. Chinone, JAERI-Review 2001-008, 2001, pp. 41–49.
Observation of square array of nickel dots by using high-Tc SQUID microscope Kuniaki Sata, Mitsuyuki Tsuji, ShinÕichiro Nakata, Takekazu Ishida
*
Department of Physics and Electronics, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan Received 13 November 2002; accepted 21 February 2003
Abstract We have developed a SQUID microscope using a high-Tc SQUID sensor. The SQUID sensor is cooled by liquid nitrogen down to 77 K. The distance between the sensor and a sapphire window is less than 100 lm. The distance between a sample and the window along the Z-direction can be adjusted to less than 200 lm manually by monitoring with a CCD camera. A motion controller and a computer interface drive the stage. A SQUID sensor operates at the level near the critical current. A digital multimeter can read a SQUID output voltage coming from a local magnetic field. A sample is scanned in the two dimensions while the SQUID sensor is fixed to the bottom of the liquid nitrogen Dewar. The performance of the SQUID microscope is examined by a regular array of Ni dots. The sample is made by evaporating nickel on to a cover glass with a mask of metallic mesh (17 lm), and then magnetized in a magnetic field of 0.8 T. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.60.Ge; 74.25.Ha; 85.25.Dq Keywords: SQUID; Microscope; X –Y stage; High-Tc
1. Introduction A SQUID microscope has been developed rapidly in recent years. The SQUID microscope using a conventional superconductor consumes a lot of liquid helium [1]. The recent progress of high-Tc SQUID extends to the application in the field of magnetic field measurements. Since the SQUID resolution reaches 10 12 T, the apparatus becomes
*
Corresponding author. Fax: +81-72-254-9908. E-mail address: [email protected] (T. Ishida).
extremely sensitive compared to other means. Black et al. [3] pioneered a high-Tc SQUID microscope having a spatial resolution of 60 lm. Several groups have utilized the SQUID microscopes with higher spatial resolution to investigate superconductors [4–6]. This apparatus may find a wide variety of the applications in the material sciences. We eventually intend to improve a spatial resolution of the SQUID microscope to observe a Bitter pattern of vortices. In this paper, we report the construction of a high-Tc SQUID microscope, and describe the results of the test experiments by using a regular array of the tiny nickel dots.
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01062-1
K. Sata et al. / Physica C 392–396 (2003) 1406–1410
2. SQUID sensor As shown in Fig. 1, a SQUID sensor is made by the Sumitomo Hightechs Inc. (Osaka, Japan) is V
Bias current
Josephson junction
Thin film superconductor SQUID hole
Fig. 1. The schematic diagram of the SQUID sensor. A thinfilm superconductor is Eu–Ba–Cu–O. The sensitive area of the SQUID sensor is indicated in the figure as SQUID hole.
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fabricated on the STO substrate, of which the thickness is 1.0 mm. The details of the sensor have been reported elsewhere [2]. The SQUID sensor consists of a couple of high-Tc Josephson junctions. The signal of the SQUID sensor can be modulated with a period of flux U0 in the SQUID hole. This makes it possible to carry out the sensitive measurements of the magnetic field. The SQUID sensor has a sensitivity of 1 pT and the effective sensor area is 0.08 mm2 .
3. SQUID microscope In Fig. 2, we show the schematic diagram of our SQUID microscope constructed by us. A sample can be scanned in the two dimensions while the SQUID sensor is fixed to the nitrogen Dewar. An X –Y stage (Suruga K201-30LMS) is able to scan over 30 30 mm area and its spatial resolution is
Fig. 2. Schematic diagram of our SQUID microscope. (1) A high-Tc SQUID sensor, (2) a liquid nitrogen Dewar, (3) a doublepermalloy shield of the SQUID microscope, (4) a light source guide, (5) a sapphire window (100 lm in thickness), (6) an XY stage with stepper motors, (7) a Z stage in manual operation, (8) a driver, (9) a stage control board, (10) a CCD camera with a scope, (11) a power source for the scope, (12) a CRT monitor to examine a distance between sapphire window and sample, (13) an electronic SQUID controller, (14) a control computer, (15) a LabVIEW-6i software developed by us, (16) a motion interface and a control box for the XY stage, (17) a vacuum pump for the liquid nitrogen Dewar, (18) a digital multimeter to measure the SQUID output, (19) a PCI-GPIB board and (20) an active band pass filter for the SQUID output.
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K. Sata et al. / Physica C 392–396 (2003) 1406–1410
as good as 0.05 lm. The SQUID sensor can be cooled down to 77 K for 1 h by using the evacuated Dewar (5 10 4 Pa). A distance between the SQUID sensor and the sapphire window can be minimized as short as 100 lm. The sample space is subjected to light when one monitors a distance between the Dewar window and the sample. A motion interface (National Instruments UMI-7764) and a control box for the XY stage were assembled by us. The sample temperature is limited to room temperature. The whole device is controlled by LabVIEW-6i is the trademark of the National Instruments Inc. Fig. 3. The photograph of the Ni dots. The Ni dots are regularly arranged in a square lattice of lattice constant 17 lm.
4. Experimental We prepared a Ni rod and a square array of Ni dots. The diameter and length of the Ni rod is 0.1/ and 4 mm. The Ni dots were prepared by evaporating nickel on to a cover glass with a mask of metallic mesh (17 lm) (Fig. 3). We magnetized both sample in the field of 0.8 T.
We scanned the sample along the X - and Y -axes. We measured the Ni rod in the step of 200 lm over 3000 3000 lm region. For the Ni dots, the scan step was typically chosen as 3 lm. First, we fixed the position of the Y -coordinate, and started the scan along the X -direction to measure a SQUID signal. After completion, we added by 3 lm to the
Fig. 4. The two-dimensional graph of the SQUID signal on the Ni rod. The magnetic field is stronger in the thicker colors, but they have the opposite signs. The SQUID signal V is relative in scale.
K. Sata et al. / Physica C 392–396 (2003) 1406–1410
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Fig. 5. The two-dimensional graph of the SQUID signal on the Ni dots. The magnetic field is stronger in the thicker colors. The SQUID signal V is relative in scale.
X -coordinate, and repeat a scan along the X direction. A band pass filter of the bandwidth from 10 to 20 Hz was necessary to reduce the electronic noise. The whole measurements were controlled from a computer by the LabVIEW-6i software.
5. Results and discussion Fig. 4 represents the two-dimensional graph of the SQUID signal on the Ni rod. The distance between the sample and the window was chosen as 1 mm. When the distance becomes smaller than 1 mm, the SQUID sensor was overloaded due to the enhanced signal. It turns out that the Ni rod was magnetized, and generated the dipole field around the rod. The field profile is spread than the actual sample size because of the distance between the sample and the sensor. In Fig. 5, we show the two-dimensional field profile from the Ni dot sample where the distance
between the sample and the window is 0.3 mm. The square is the size of the Ni dot, and the squares are overlaid on the two-dimensional data as a measure of period. We can see the shape of the Ni dot from the magnetic field profile. It seems that the periodicity is apparent along the X -axis. However, the periodicity is not perfect along the Y -axis. We explain this by the fact that one is the SQUID hole is rectangular (see Fig. 1). The scan along the X -axis is more sensitive to a change in the magnetic field. The spatial resolution in the literature is less than 200 lm [2]. It is necessary to improve a spatial resolution of our system further.
6. Conclusions We have constructed the SQUID microscope using the high-Tc SQUID sensor. The field profile reproduces the shape of the Ni rod and dot sample well.
1410
K. Sata et al. / Physica C 392–396 (2003) 1406–1410
Acknowledgements This work was partially supported by a special grant for the practical applications in Osaka Prefecture University and by a grant to Prefecture Colleges and Universities from the Ministry of Education, Science, and Culture of Japan.
References [1] K. Chinone, S. Nakayama, T. Morooka, A. Odawara, M. Ikeda, IEEE Trans. Appl. Supercond. 7 (1997) 3271.
[2] K. Minamimura, T. Nagaishi, H. Itozaki, in: 8th Int. Super. Electron. Conf., Extended Abstract, Osaka, 2001, pp. 201– 202. [3] R.C. Black, A. Mathai, F.C. Wellstood, E. Dantsker, A.H. Miklich, D.T. Nemeth, J.J. Kingston, J. Clarke, Appl. Phys. Lett. 62 (1993) 2128. [4] T. Ishida, M. Yoshida, K. Okuda, JAERI-Review 2001-008, 2001, pp. 92–99. [5] I. Iguchi, A. Sugimoto, T. Yamaguchi, JAERI-Review 2001008, 2001, pp. 51–53. [6] A. Nagata, K. Chinone, JAERI-Review 2001-008, 2001, pp. 41–49.