- Email: [email protected]
Study of pinning mechanisms in YBCO thin films by means of magnetic force microscopy
Physica C 460–462 (2007) 732–733 www.elsevier.com/locate/physc
Study of pinning mechanisms in YBCO thin films by means of magnetic force microscopy T. Shapoval *, V. Neu, U. Wolff, R. Hu¨hne, J. Ha¨nisch, E. Backen, B. Holzapfel, L. Schultz IFW Dresden, Institute for Metallic Materials, P.O. Box 270116, D-01171 Dresden, Germany Available online 28 March 2007
Abstract Low-temperature magnetic force microscopy is used for the direct visualization of vortex distribution in high-temperature superconducting films. With the scanning probe microscope (Omicron Cryogenic SFM) magnetic fields up to 7 T (vertical) and 3 T (transversal) can be applied to the sample during in situ imaging at temperatures ranging from 7 K to 300 K in ultra high vacuum. The samples investigated were 300 nm thick YBa2Cu3O7 d films grown epitaxially on SrTiO3 (0 0 1) substrates by off-axis pulsed laser deposition with a mean roughness less than 10 nm. Prior to imaging, the samples were cooled down in the microscope to 7.7 K in a magnetic field of 3 mT. Flux lines have been successfully imaged by measuring their stray field in the scan height of 40 nm above the sample surface. The number of vortices observed corresponds to the theoretically expected one. Whereas a constant current leads to unacceptable heating of the sample, the use of current pulses allows transport measurements without thermal movement of vortices. 2007 Elsevier B.V. All rights reserved. Keywords: YBCO films; Vortex; Imaging; Magnetic force microscopy; PLD
1. Introduction Low-temperature magnetic force microscopy (MFM) has been demonstrated to be a powerful tool in visualization of the vortex structure in high-temperature superconducting films [1]. In comparison to scanning tunneling spectroscopy (STS), MFM is a rather fast vortex imaging technique, which is not too sensitive to the surface quality and tolerates nonconductive inclusions on the sample surface. A remarkable influence of artificial defects on the vortex pinning and, hence, on an increased in-field critical current density of YBa2Cu3O7 d(YBCO) films has been recently observed in Jc measurements [2–4]. The microscopic imaging of flux lines can help to understand the vortex pinning mechanism at natural and artificial defects. Direct observation of vortex movement during applied cur-
*
Corresponding author. Tel.: +49 351 4659 741; fax: +49 351 4659 541. E-mail address: [email protected] (T. Shapoval).
0921-4534/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.156
rent flow gives information about the magnitude of the pinning force at different defects. 2. Experimental For the visualization of the vortex structure the lowtemperature scanning probe microscope (Omicron Cryogenic SFM) is used in MFM mode. Magnetic fields of 7 T (vertical) and 3 T (transversal) can be applied to the sample during in situ imaging at temperatures ranging from 7 K to 300 K in ultra high vacuum. In addition, it is possible to pass a current through the field-cooled sample in the microscope. About 300 nm thick YBCO films are grown epitaxially on SrTiO3 (0 0 1) substrates by off-axis pulsed laser deposition (PLD) [5]. Off-axis PLD is chosen in order to avoid the formation of precipitates and droplets on the surface of the film. For optimized deposition temperature and oxygen pressure a mean roughness of less than 10 nm is achieved, which is a prerequisite for MFM imaging. The cantilevers are commercial MFM tips (Nanoworld
T. Shapoval et al. / Physica C 460–462 (2007) 732–733
733
MFMR) with a force constant of about 3 N/m and a resonance frequency of 60–70 kHz. Flux lines are imaged by measuring a frequency shift Df at a scan height of 40 nm above the sample surface, which is caused by the second z-derivative of the stray field of the vortex and which amounts to about 30 Hz. 3. Experimental results and discussion Fig. 1 shows MFM images of an YBCO film cooled down to 7.7 K in a magnetic field of 3 mT. The temperature was selected as low as possible to minimize the noise level and increase the contrast of individual vortices [1]. The field was chosen small enough to avoid an overlap of magnetic contrast arising from neighboring vortices. Twenty-four individual vortices are clearly visible in the 4 · 4 lm2 scan (Fig. 1a) as bright objects on a dark background. The number precisely agrees with the total flux frozen in during cool-down. The distribution of vortices reveals no short-range order. This demonstrates that in thin films vortices are pinned on natural defects, such as dislocations, grain boundaries, etc. Our first experiment of applying current to the film shows that the contact resistance between the copper wires and the YBCO surface prepared only by conductive silver glue is too high (about 100 X) and leads to the dramatic increase in sample temperature and, hence, to the movement of vortices due not to the current flow but rather to thermal excitation (Fig. 1b). To avoid this disturbing effect, we switched to current pulses of 1 s length. Although the current pulse leads to a small increase of temperature (about 1 K), within about 10 min the temperature stabilizes sufficiently for the next MFM scan. We have performed a series of measurements applying current pulses of 2, 5, 10, 20, 50, 100, 200 and 300 mA, all well below the critical current Ic of the sample investigated. The data show that the distribution of vortices does not change after applying currents much lower than Ic. This investigation also proved the stability of our scanning equipment and the small value of piezo drift with time (about 500 nm a week). As a next step, we have improved the contact resistance of the contacts from 100 to 2 X by covering the contact surface with a thin gold layer and heating it up to the temperature of 300 C in a 400 mbar oxygen atmosphere for 30 min. With the new contacts we are now going to observe the movement of vortices by currents higher than Ic in pure YBCO films and further to visualize the vortices and their movement by current flow in YBCO off-axis PLD films deposited on nano-
Fig. 1. 4 · 4 lm2 MFM image of an off-axis PLD YBCO film cooled down to the temperature of 7.7 K in a magnetic field of 3 mT without applying a current (a) and during a constant current flow (b).
structured templates. It will allow us to visualize the correlation between artificial defects and vortex pinning centers. 4. Summary In this paper we have presented our first successful experiments on the way towards direct visualization of vortex pinning in YBa2Cu3O7 d off-axis pulsed laser deposited films by low-temperature magnetic force microscopy. We have shown the ability of the Omicron Cryogenic SFM microscope to perform in situ transport measurements, which should help us to understand the vortex pinning at natural and artificial defects. Acknowledgements The work and results reported in this publication were obtained with research funding from the European Community under the Sixth Framework Programme Contract Number 516858: HIPERCHEM. References [1] A. Moser, H.J. Hug, B. Stiefel, H.J. Guntherodt, Journal of Magnetism and Magnetic Materials 190 (1998) 114. [2] J. Ha¨nisch, C. Cai, R. Hu¨hne, L. Schultz, B. Holzapfel, Appl. Phys. Lett. 86 (2005) 122508. [3] J.L. MacManus-Driscoll, S.R. Foltyn, Q.X. Jia, H. Wang, A. Serquis, L. Civale, B. Maiorov, M.E. Hawley, M.P. Maley, D.E. Peterson, Nature Mater. 3 (2004) 439. [4] J. Ha¨nisch, C. Cai, V. Stehr, R. Hu¨hne, J. Lyubina, K. Nenkov, G. Fuchs, L. Schultz, B. Holzapfel, Supercond. Sci. Technol. 19 (2006) 534. [5] B. Holzapfel, B. Roas, L. Schultz, P. Bauer, G. Saemannischenko, Appl. Phys. Lett. 61 (1992) 3178.
Study of pinning mechanisms in YBCO thin films by means of magnetic force microscopy T. Shapoval *, V. Neu, U. Wolff, R. Hu¨hne, J. Ha¨nisch, E. Backen, B. Holzapfel, L. Schultz IFW Dresden, Institute for Metallic Materials, P.O. Box 270116, D-01171 Dresden, Germany Available online 28 March 2007
Abstract Low-temperature magnetic force microscopy is used for the direct visualization of vortex distribution in high-temperature superconducting films. With the scanning probe microscope (Omicron Cryogenic SFM) magnetic fields up to 7 T (vertical) and 3 T (transversal) can be applied to the sample during in situ imaging at temperatures ranging from 7 K to 300 K in ultra high vacuum. The samples investigated were 300 nm thick YBa2Cu3O7 d films grown epitaxially on SrTiO3 (0 0 1) substrates by off-axis pulsed laser deposition with a mean roughness less than 10 nm. Prior to imaging, the samples were cooled down in the microscope to 7.7 K in a magnetic field of 3 mT. Flux lines have been successfully imaged by measuring their stray field in the scan height of 40 nm above the sample surface. The number of vortices observed corresponds to the theoretically expected one. Whereas a constant current leads to unacceptable heating of the sample, the use of current pulses allows transport measurements without thermal movement of vortices. 2007 Elsevier B.V. All rights reserved. Keywords: YBCO films; Vortex; Imaging; Magnetic force microscopy; PLD
1. Introduction Low-temperature magnetic force microscopy (MFM) has been demonstrated to be a powerful tool in visualization of the vortex structure in high-temperature superconducting films [1]. In comparison to scanning tunneling spectroscopy (STS), MFM is a rather fast vortex imaging technique, which is not too sensitive to the surface quality and tolerates nonconductive inclusions on the sample surface. A remarkable influence of artificial defects on the vortex pinning and, hence, on an increased in-field critical current density of YBa2Cu3O7 d(YBCO) films has been recently observed in Jc measurements [2–4]. The microscopic imaging of flux lines can help to understand the vortex pinning mechanism at natural and artificial defects. Direct observation of vortex movement during applied cur-
*
Corresponding author. Tel.: +49 351 4659 741; fax: +49 351 4659 541. E-mail address: [email protected] (T. Shapoval).
0921-4534/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.156
rent flow gives information about the magnitude of the pinning force at different defects. 2. Experimental For the visualization of the vortex structure the lowtemperature scanning probe microscope (Omicron Cryogenic SFM) is used in MFM mode. Magnetic fields of 7 T (vertical) and 3 T (transversal) can be applied to the sample during in situ imaging at temperatures ranging from 7 K to 300 K in ultra high vacuum. In addition, it is possible to pass a current through the field-cooled sample in the microscope. About 300 nm thick YBCO films are grown epitaxially on SrTiO3 (0 0 1) substrates by off-axis pulsed laser deposition (PLD) [5]. Off-axis PLD is chosen in order to avoid the formation of precipitates and droplets on the surface of the film. For optimized deposition temperature and oxygen pressure a mean roughness of less than 10 nm is achieved, which is a prerequisite for MFM imaging. The cantilevers are commercial MFM tips (Nanoworld
T. Shapoval et al. / Physica C 460–462 (2007) 732–733
733
MFMR) with a force constant of about 3 N/m and a resonance frequency of 60–70 kHz. Flux lines are imaged by measuring a frequency shift Df at a scan height of 40 nm above the sample surface, which is caused by the second z-derivative of the stray field of the vortex and which amounts to about 30 Hz. 3. Experimental results and discussion Fig. 1 shows MFM images of an YBCO film cooled down to 7.7 K in a magnetic field of 3 mT. The temperature was selected as low as possible to minimize the noise level and increase the contrast of individual vortices [1]. The field was chosen small enough to avoid an overlap of magnetic contrast arising from neighboring vortices. Twenty-four individual vortices are clearly visible in the 4 · 4 lm2 scan (Fig. 1a) as bright objects on a dark background. The number precisely agrees with the total flux frozen in during cool-down. The distribution of vortices reveals no short-range order. This demonstrates that in thin films vortices are pinned on natural defects, such as dislocations, grain boundaries, etc. Our first experiment of applying current to the film shows that the contact resistance between the copper wires and the YBCO surface prepared only by conductive silver glue is too high (about 100 X) and leads to the dramatic increase in sample temperature and, hence, to the movement of vortices due not to the current flow but rather to thermal excitation (Fig. 1b). To avoid this disturbing effect, we switched to current pulses of 1 s length. Although the current pulse leads to a small increase of temperature (about 1 K), within about 10 min the temperature stabilizes sufficiently for the next MFM scan. We have performed a series of measurements applying current pulses of 2, 5, 10, 20, 50, 100, 200 and 300 mA, all well below the critical current Ic of the sample investigated. The data show that the distribution of vortices does not change after applying currents much lower than Ic. This investigation also proved the stability of our scanning equipment and the small value of piezo drift with time (about 500 nm a week). As a next step, we have improved the contact resistance of the contacts from 100 to 2 X by covering the contact surface with a thin gold layer and heating it up to the temperature of 300 C in a 400 mbar oxygen atmosphere for 30 min. With the new contacts we are now going to observe the movement of vortices by currents higher than Ic in pure YBCO films and further to visualize the vortices and their movement by current flow in YBCO off-axis PLD films deposited on nano-
Fig. 1. 4 · 4 lm2 MFM image of an off-axis PLD YBCO film cooled down to the temperature of 7.7 K in a magnetic field of 3 mT without applying a current (a) and during a constant current flow (b).
structured templates. It will allow us to visualize the correlation between artificial defects and vortex pinning centers. 4. Summary In this paper we have presented our first successful experiments on the way towards direct visualization of vortex pinning in YBa2Cu3O7 d off-axis pulsed laser deposited films by low-temperature magnetic force microscopy. We have shown the ability of the Omicron Cryogenic SFM microscope to perform in situ transport measurements, which should help us to understand the vortex pinning at natural and artificial defects. Acknowledgements The work and results reported in this publication were obtained with research funding from the European Community under the Sixth Framework Programme Contract Number 516858: HIPERCHEM. References [1] A. Moser, H.J. Hug, B. Stiefel, H.J. Guntherodt, Journal of Magnetism and Magnetic Materials 190 (1998) 114. [2] J. Ha¨nisch, C. Cai, R. Hu¨hne, L. Schultz, B. Holzapfel, Appl. Phys. Lett. 86 (2005) 122508. [3] J.L. MacManus-Driscoll, S.R. Foltyn, Q.X. Jia, H. Wang, A. Serquis, L. Civale, B. Maiorov, M.E. Hawley, M.P. Maley, D.E. Peterson, Nature Mater. 3 (2004) 439. [4] J. Ha¨nisch, C. Cai, V. Stehr, R. Hu¨hne, J. Lyubina, K. Nenkov, G. Fuchs, L. Schultz, B. Holzapfel, Supercond. Sci. Technol. 19 (2006) 534. [5] B. Holzapfel, B. Roas, L. Schultz, P. Bauer, G. Saemannischenko, Appl. Phys. Lett. 61 (1992) 3178.