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Addition of ferromagnetic CoFe2O4 to YBCO thin films for enhanced flux pinning
Physica C 470 (2010) S223–S224
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Addition of ferromagnetic CoFe2O4 to YBCO thin films for enhanced flux pinning Stuart C. Wimbush *, Rong Yu 1, Rantej Bali 2, John H. Durrell, Judith L. MacManus-Driscoll Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK
a r t i c l e
i n f o
Article history: Accepted 26 October 2009 Available online 29 October 2009 Keywords: Flux pinning Critical current Magnetic pinning Cuprate superconductors
a b s t r a c t An attempt has been made to prepare a YBa2 Cu3 O7d (YBCO) thin film doped with ferromagnetic CoFe2O4. Transmission electron microscopy of the resultant samples shows, however, that Y(Fe, Co)O3 forms as a nanoparticulate dispersion throughout the film in preference to CoFe2O4, leaving the YBCO yttrium deficient. As a consequence, the superconducting properties of the sample are poor, with a self-field critical current density of just 0.25 MA cm2. Magnetic measurements indicate however that the Y(Fe, Co)O3 content, together with any other residual phases, is also ferromagnetic, and some interesting features are present in the in-field critical current behaviour, including a reduced dependence on applied field and a strong c-axis peak in the angular dependence. The work points the way towards future attempts utilising YFeO3 as an effective ferromagnetic pinning additive for YBCO. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The discovery of a ferromagnetic pinning additive, chemically compatible with YBa2 Cu3 O7d (YBCO), offering the potential of enhanced flux pinning and consequently improved critical current density J c [1] in this technologically vital material ‘beyond BZO’ [2] is a long sought-after goal [3]. To date, attempts have been made with Fe, Co and Ni, as well as with simple metal oxides such as Fe2O3 [4], but all have resulted in a detrimental effect on the superconducting properties through cross-contamination into the YBCO matrix and a subsequent suppression of the superconducting transition temperature T c . We have switched the focus to complex magnetic oxide materials offering a greater likelihood of compatibility, and here report on our attempt to dope YBCO with CoFe2O4. 2. Experimental Deposition targets were prepared from stoichiometric mixtures of powder starting materials of YBa2Cu3Ox (SCI Engineered Materials, 99.99%), Co3O4 (Aldrich, 99.8%) and FeO (Aldrich, 99.9%) by pressing and sintering. A target comprising 1 wt.% CoFe2O4 was used in the preparation of thin film samples of nominal thickness 100 nm by off-axis [5] pulsed laser deposition (KrF 248 nm, 10 Hz, 2 J cm2) onto SrTiO3 (1 0 0) single crystal substrates held at an elevated temperature in an oxygen atmosphere and annealed * Corresponding author. E-mail address: [email protected] (S.C. Wimbush). 1 Present address: Beijing National Center for Electron Microscopy, Tsinghua University, Beijing 100084, China. 2 Present address: Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany. 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.10.117
in oxygen post-deposition. A control sample of pure YBCO without any additions was also prepared by the same process. Pairs of samples were deposited simultaneously, with one of the pair being photolithographically patterned and ion beam etched into a bridge structure for four-point electrical transport measurements and the other being characterised structurally by X-ray diffraction and magnetically in a vibrating sample magnetometer before being prepared and imaged in a transmission electron microscope (TEM).
3. Results Due to the low dopant level, no evidence of secondary phases could be seen in the X-ray analysis of the samples (not shown), which yielded only peaks related to epitaxially grown YBCO. High resolution TEM imaging of the doped sample (Fig. 1), however, reveals a distribution of nanoparticulate precipitates embedded throughout the YBCO matrix. Analysis of these precipitates showed them not to be CoFe2O4, but rather Y(Fe, Co)O3. The incorporation of the yttrium species into the precipitates results in a deficiency for the formation of YBCO, with a consequent detrimental effect on the superconducting properties. As a result of this observation, further work focussed on intentionally introducing just the YFeO3 material, ensuring that sufficient yttrium was present in the target to allow undisturbed YBCO formation [6,7]. The magnetic behaviour of the doped sample was also investigated (Fig. 2). For fields applied out of the plane of the sample ðHjjcÞ, a standard superconducting hysteresis loop was obtained, with typically large diamagnetic response. Due to the low J c of the sample, the signal is small enough that the ferromagnetic moment can be observed at higher fields, where the superconductivity
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Fig. 1. HRTEM image of a Co–Fe–O doped YBCO thin film showing a nanoparticle Y(Fe, Co)O3 precipitate within the YBCO matrix.
Fig. 3. Superconducting properties of a Co–Fe–O doped YBCO thin film (broken line) compared to a pure YBCO control sample (solid line) prepared under the same conditions. Upper inset shows the resistive transitions of the two samples. Main panel shows the field dependence of J c at 77 K, Hkc. Lower inset is the angular dependence of J c at 77 K, 1 T. The curves are normalised to the ab peak to highlight the strong Jc enhancement for Hkc in the doped film; absolute Jc values are given on the left (doped) and right (pure) axes.
50 K (not shown), with normalised J c values in the doped sample around 30% higher in the range 1–6 T. Additionally, a pronounced c-axis peak exists in the angular J c dependence, exceeding even the in plane ab peak value, although no evidence of columnar pins was seen in the TEM. 4. Conclusion
Fig. 2. Magnetic properties of a Co–Fe–O doped YBCO thin film at 77 K, showing a superconducting hysteresis loop for fields applied out of plane ðHkcÞ and (inset) a ferromagnetic hysteresis loop (with remnant of the superconducting loop at low fields) for fields applied in plane ðHkabÞ.
is suppressed. For fields applied in plane ðHjjabÞ, the superconducting signal is barely observed (aside from a small remnant at low fields) due to the thin film geometry of the sample, and the ferromagnetic response is clearly seen, indicating that the dopant material remains magnetic. The superconducting properties of the samples were then evaluated (Fig. 3). The doped sample exhibited a slightly depressed resistive T c of 87 K (transition width DT c ¼ 1:7 K), compared to 92 K ðDT c ¼ 1:2 KÞ for the pure control sample. The self field J c , however, was drastically reduced, having a value of just 0.25 MA cm2 at 77 K compared to a more typical 1.2 MA cm2 in the case of the control sample. This is attributed, as mentioned above, to an yttrium deficiency in the YBCO film following Y(Fe, Co)O3 formation. The in-field J c behaviour of the doped sample is of some interest. When compared to the self-field value, the J c of the doped sample is observed to drop off less rapidly under an applied field than that of the pure sample, with a crossover occurring at about 2 T at 77 K. At lower temperatures, this effect extends further, up to 7 T at
The attempt to introduce CoFe2O4 pinning centres into YBCO was only partly successful, but has pointed the way towards other compatible ferromagnetic pinning additives. Nonetheless, a magnetically-doped superconducting YBCO sample has been prepared, and shown to have a number of interesting features, including a sustained J c under an applied field and a strong c-axis peak in the angular J c dependence. Acknowledgment SCW is supported by The Leverhulme Trust with supplemental funding from The Isaac Newton Trust. References [1] M.G. Blamire, R.B. Dinner, S.C. Wimbush, J.L. MacManus-Driscoll, Supercond. Sci. Technol. 22 (2009) 025017. [2] 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, Nat. Mater. 3 (2004) 439. [3] US Department of Energy, Office of Science Report, ‘‘Basic Research Needs For Superconductivity”, 2006, p. 88. [4] J. Wang, C.F. Tsai, Z. Bi, D.G. Naugle, H. Wang, IEEE Trans. Appl. Supercond. 19 (2009) 3503. [5] B. Holzapfel, B. Roas, L. Schultz, P. Bauer, G. Saemann-Ischenko, Appl. Phys. Lett. 61 (1992) 3178. [6] S.C. Wimbush, J.H. Durrell, R. Bali, R. Yu, H. Wang, S.A. Harrington, J.L. MacManus-Driscoll, IEEE Trans. Appl. Supercond. 19 (2009) 3148. [7] S.C. Wimbush, J.H. Durrell, C.F. Tsai, H. Wang, Q.X. Jia, M.G. Blamire, J.L. MacManus-Driscoll, Supercond. Sci. Technol., submitted for publication.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Addition of ferromagnetic CoFe2O4 to YBCO thin films for enhanced flux pinning Stuart C. Wimbush *, Rong Yu 1, Rantej Bali 2, John H. Durrell, Judith L. MacManus-Driscoll Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK
a r t i c l e
i n f o
Article history: Accepted 26 October 2009 Available online 29 October 2009 Keywords: Flux pinning Critical current Magnetic pinning Cuprate superconductors
a b s t r a c t An attempt has been made to prepare a YBa2 Cu3 O7d (YBCO) thin film doped with ferromagnetic CoFe2O4. Transmission electron microscopy of the resultant samples shows, however, that Y(Fe, Co)O3 forms as a nanoparticulate dispersion throughout the film in preference to CoFe2O4, leaving the YBCO yttrium deficient. As a consequence, the superconducting properties of the sample are poor, with a self-field critical current density of just 0.25 MA cm2. Magnetic measurements indicate however that the Y(Fe, Co)O3 content, together with any other residual phases, is also ferromagnetic, and some interesting features are present in the in-field critical current behaviour, including a reduced dependence on applied field and a strong c-axis peak in the angular dependence. The work points the way towards future attempts utilising YFeO3 as an effective ferromagnetic pinning additive for YBCO. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The discovery of a ferromagnetic pinning additive, chemically compatible with YBa2 Cu3 O7d (YBCO), offering the potential of enhanced flux pinning and consequently improved critical current density J c [1] in this technologically vital material ‘beyond BZO’ [2] is a long sought-after goal [3]. To date, attempts have been made with Fe, Co and Ni, as well as with simple metal oxides such as Fe2O3 [4], but all have resulted in a detrimental effect on the superconducting properties through cross-contamination into the YBCO matrix and a subsequent suppression of the superconducting transition temperature T c . We have switched the focus to complex magnetic oxide materials offering a greater likelihood of compatibility, and here report on our attempt to dope YBCO with CoFe2O4. 2. Experimental Deposition targets were prepared from stoichiometric mixtures of powder starting materials of YBa2Cu3Ox (SCI Engineered Materials, 99.99%), Co3O4 (Aldrich, 99.8%) and FeO (Aldrich, 99.9%) by pressing and sintering. A target comprising 1 wt.% CoFe2O4 was used in the preparation of thin film samples of nominal thickness 100 nm by off-axis [5] pulsed laser deposition (KrF 248 nm, 10 Hz, 2 J cm2) onto SrTiO3 (1 0 0) single crystal substrates held at an elevated temperature in an oxygen atmosphere and annealed * Corresponding author. E-mail address: [email protected] (S.C. Wimbush). 1 Present address: Beijing National Center for Electron Microscopy, Tsinghua University, Beijing 100084, China. 2 Present address: Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany. 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.10.117
in oxygen post-deposition. A control sample of pure YBCO without any additions was also prepared by the same process. Pairs of samples were deposited simultaneously, with one of the pair being photolithographically patterned and ion beam etched into a bridge structure for four-point electrical transport measurements and the other being characterised structurally by X-ray diffraction and magnetically in a vibrating sample magnetometer before being prepared and imaged in a transmission electron microscope (TEM).
3. Results Due to the low dopant level, no evidence of secondary phases could be seen in the X-ray analysis of the samples (not shown), which yielded only peaks related to epitaxially grown YBCO. High resolution TEM imaging of the doped sample (Fig. 1), however, reveals a distribution of nanoparticulate precipitates embedded throughout the YBCO matrix. Analysis of these precipitates showed them not to be CoFe2O4, but rather Y(Fe, Co)O3. The incorporation of the yttrium species into the precipitates results in a deficiency for the formation of YBCO, with a consequent detrimental effect on the superconducting properties. As a result of this observation, further work focussed on intentionally introducing just the YFeO3 material, ensuring that sufficient yttrium was present in the target to allow undisturbed YBCO formation [6,7]. The magnetic behaviour of the doped sample was also investigated (Fig. 2). For fields applied out of the plane of the sample ðHjjcÞ, a standard superconducting hysteresis loop was obtained, with typically large diamagnetic response. Due to the low J c of the sample, the signal is small enough that the ferromagnetic moment can be observed at higher fields, where the superconductivity
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Fig. 1. HRTEM image of a Co–Fe–O doped YBCO thin film showing a nanoparticle Y(Fe, Co)O3 precipitate within the YBCO matrix.
Fig. 3. Superconducting properties of a Co–Fe–O doped YBCO thin film (broken line) compared to a pure YBCO control sample (solid line) prepared under the same conditions. Upper inset shows the resistive transitions of the two samples. Main panel shows the field dependence of J c at 77 K, Hkc. Lower inset is the angular dependence of J c at 77 K, 1 T. The curves are normalised to the ab peak to highlight the strong Jc enhancement for Hkc in the doped film; absolute Jc values are given on the left (doped) and right (pure) axes.
50 K (not shown), with normalised J c values in the doped sample around 30% higher in the range 1–6 T. Additionally, a pronounced c-axis peak exists in the angular J c dependence, exceeding even the in plane ab peak value, although no evidence of columnar pins was seen in the TEM. 4. Conclusion
Fig. 2. Magnetic properties of a Co–Fe–O doped YBCO thin film at 77 K, showing a superconducting hysteresis loop for fields applied out of plane ðHkcÞ and (inset) a ferromagnetic hysteresis loop (with remnant of the superconducting loop at low fields) for fields applied in plane ðHkabÞ.
is suppressed. For fields applied in plane ðHjjabÞ, the superconducting signal is barely observed (aside from a small remnant at low fields) due to the thin film geometry of the sample, and the ferromagnetic response is clearly seen, indicating that the dopant material remains magnetic. The superconducting properties of the samples were then evaluated (Fig. 3). The doped sample exhibited a slightly depressed resistive T c of 87 K (transition width DT c ¼ 1:7 K), compared to 92 K ðDT c ¼ 1:2 KÞ for the pure control sample. The self field J c , however, was drastically reduced, having a value of just 0.25 MA cm2 at 77 K compared to a more typical 1.2 MA cm2 in the case of the control sample. This is attributed, as mentioned above, to an yttrium deficiency in the YBCO film following Y(Fe, Co)O3 formation. The in-field J c behaviour of the doped sample is of some interest. When compared to the self-field value, the J c of the doped sample is observed to drop off less rapidly under an applied field than that of the pure sample, with a crossover occurring at about 2 T at 77 K. At lower temperatures, this effect extends further, up to 7 T at
The attempt to introduce CoFe2O4 pinning centres into YBCO was only partly successful, but has pointed the way towards other compatible ferromagnetic pinning additives. Nonetheless, a magnetically-doped superconducting YBCO sample has been prepared, and shown to have a number of interesting features, including a sustained J c under an applied field and a strong c-axis peak in the angular J c dependence. Acknowledgment SCW is supported by The Leverhulme Trust with supplemental funding from The Isaac Newton Trust. References [1] M.G. Blamire, R.B. Dinner, S.C. Wimbush, J.L. MacManus-Driscoll, Supercond. Sci. Technol. 22 (2009) 025017. [2] 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, Nat. Mater. 3 (2004) 439. [3] US Department of Energy, Office of Science Report, ‘‘Basic Research Needs For Superconductivity”, 2006, p. 88. [4] J. Wang, C.F. Tsai, Z. Bi, D.G. Naugle, H. Wang, IEEE Trans. Appl. Supercond. 19 (2009) 3503. [5] B. Holzapfel, B. Roas, L. Schultz, P. Bauer, G. Saemann-Ischenko, Appl. Phys. Lett. 61 (1992) 3178. [6] S.C. Wimbush, J.H. Durrell, R. Bali, R. Yu, H. Wang, S.A. Harrington, J.L. MacManus-Driscoll, IEEE Trans. Appl. Supercond. 19 (2009) 3148. [7] S.C. Wimbush, J.H. Durrell, C.F. Tsai, H. Wang, Q.X. Jia, M.G. Blamire, J.L. MacManus-Driscoll, Supercond. Sci. Technol., submitted for publication.