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Magnetic identification of 3d to 2d vortex transitions in Bi2Sr2CaCu2Ox single crystals
Physica C 235-240 (1994) 2713-2714
PHYSICA
North-Holland
Magnetic identification of 3d to 2d vortex transitions in Bi2Sr2CaCu20 x single crystals G. Yang*, C.E. Gough® and J.S. Abeli*, Superconductivity Research Group, *School of Metallurgy and Materials,®School of Physics and Space Research, The University of Birmingham, Birmingham, B 15 2TI', UK. VSM measurements on Bi2Sr2CaCu20 x single crystals as a function of oxygen and vacuum annealing reveal a strongly processing dependent "arrow-head" magnetic anomaly, which we associate with the decomposition of a 3d-flux line to a 2d pancake vortex, as recently observed in ~tSR and neutron diffraction measurements. The value of the decomposition field is correlated with the irreversibility line, both of which are strongly dependent on oxygen content, consistent with associated changes in anisotropy factor ~,. Oxygen or vacuum annealing may effectively change superconductivity in the B i t players therefore the anisotropy of the crystal.
1. INTRODUCTION For highly anisotropic layered superconductors, such as Bi2Sr2CaCu20 x (BSCCO), vortex lines can be regarded as 2d pancake vortices confined to the C u t 2 layers but weakly interacting between layers by Josephson and magnetic coupling [11. At low fields, such coupling rosy.Its in essentially 3d flux lines. At high fields, the in-plane repulsion between pancakes exceeds their interplane attraction. Uncorrelated pinning in different C u t 2 layers can then break up flux lines in the field direction, leading to the so called flux line decomposition as observed in measurements of ~tSR and small angle neutron diffraction [2,31 in BSCCO crystals, where the signature of a flux lattice disappears for applied field above ,-650 Oe. This decomposition has been predicted to occur at a field B2D=~O/(Sy)2, which does not vary significantly with temperature, but depends strongly on the spacing between the C u t 2 layers, s, and the anisotropy factor 7 [4]. Differences between the pinning of 3d flux lines and the decomposed 2d pancake vortices lead 1o dramatic change in magnetic behaviour, which can be used to identify the decomposition transition. In tn~ proccssmg this paper, we present lebUltb . . . . "- o-"t "'-^ dependent arrow-head anomaly of BSCCO crystals, which we associate with this decomposition transition.
parallel to the c-axis of the crystals at a field sweep rate 1 mT/s using an Oxford VSM 3001. ,00 S
~K
| -511 -IO0 -~ooo
2~,~0
100o
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.1000
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2000
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2000
1000
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~
~
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o .,oo
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2. EXPERIMENTS
single crystals before (filled squares) and after (empty squares) annealing treatme~.ts. (a) Oxygen annealed crystal A at 35 K, (b) vacuum annealed crystal B at 30 K and (c) oxygen annealed oxTgen crystal C at 30 K.
BSCCO single crystals were grown using a large temperature gradient technique [5]. VSM measurements were performed in a magnetic field
Figure l(a) shows hestercsis loops of a BSCCO crystal A at 35 K before and after oxygen annealing. An "arrow-head" anomaly, with a sudden increase
g21-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)01922-3
G. Yang et al./Physica C 235-240 (1994) 2713-2714
2714
in magnetisation above a well-defined field, can be clearly observed in both cases. The onset of the anomaly moves to significantly higher field (from 550 Oe to 950 Oe) after oxygen annealing. Detailed measurements [6] showed that the arrow-head anomaly only appears above -20 K and gradually disappears at about 40 K. However, the field at which the maximum occurs, Hp, is almost independent of temperature. Processing dependent arrow-head anomaly is further demonstrated in Fig. l(b), the anomaly disappeared completely in a vacuum annealed crystal B, and in Fig. l(c), it appeared only in an oxygen annealed crystal C. Figure 2 plots the irreversibility lines deduced for the crystal A both in the as-grown and oxvgen annealed states. Identifying this with the flux lattice melting field [7] and assuming no change in penetration depth, we estimate Ya=108 for the asgrown crystal and 7o=70 after oxygen annealing. The derived value 0,o/Ya)2~--0.4 is close to the value Hpa / Hp°~.5., There is internal consistency with the theoretical predictions for both flux lattice melting and the 3d to 2d decomposition field.
pinning centres will be ineffective. However, existed other important defects, such as planar dislocation networks with low density but large pinning energy, will remain effective at higher temperatures. At low temperaturc ~, the high density of effective pinning centres means that there is very little difference in pinning force per unit length of flux line above and below B2D. However, at high temperatures, when only the lower density, deeper lying pinning centres from extended defects are effective, the pinning force per unit length is increased after decomposition because the now independent 2d-pancakes can move in the CuO2 planes to maximise their pinning energy. This would interpret the appearance of the arrowhead anomaly at the higher temperatures. Angle-Resolved-Photoemission measurements has suggested that oxygen annealing may change BiO layers from the metallic to superconducting [8]. This may account for the reduction in the anisotropy of BSCCO crystal after oxygen annealing and enhancement after vacuum annealing, and therefore the processing dependent arrow-head anomaly.
REFERENCES
1200
--
o--- --¢.-
o
800
is
w
. . . .
~r--
.--. . . . . . . . . . .
400
o
3o
oo
1
9o
Fig. 2. Irreversibility lines for the crystal A before (filled squares) and meter o×Tgen annealing (empty squares). The solid lines are predicted by melting theory [7]. The dashed lines indicate the mean decomposition field deduced from the plotted measurements (circles) of the peak positions for the as-grown and oxygen annealing states. 3: DISCUSSION In BSCCO crystals, the effective pinning centres at low temperature (<20 K) are believed to be mainly oxygen vacancies in the CuO2 layers. The features of such point defects are high density but low pinning energy. At high temperature such
1. J.R. Clem, Phys. Rev. B 43 (1991) 7837. 2. S.L. Lee, P. Zimmermann, H. Keller, M. Warden, I.M. Savic, R. Schauwecker, D. Zech, R. Cubbik E.M. Forgan, P.H. Kes, T.W. Li, A.A. Menovsky, Z. Tarnawski, Phys. Rev. Lett. 71 (1993) 3862. 3. R. Cubbit, E.M. Forgan, G. Yang, S.L. Lee, D.McK. Paul, H.A. Mook, M. Yethiraj, P.H. Kes, T.W. Li, A.A. Menovsky, Z. Tarnawski, and K. Mortensen, Nature 365 (1993) 407. 4. V.M. Vinokur, P.H. Kes and A.E. Koshelev, Physica 168C (1990) 29. 5. G. Yang, S.D. Sutton, P. Shang, C.E. Gough, and J.S. Abell, IEEE Trans. Appl. Superconduc. 3 (1993) 1663. 6. G. Yang, J.S. Abell and C.E. Gough, IEEE Trans. Appl. Superconduc. 3 (1993) 1671; G. Yang, P. Shang, S.D. Sutton, !.P. Jones, J.S. AbeU and C.E. Gough, Phys. Rev. B 48 (1993) 4054. 7. A. Houghton, R.A. Pelcovi~s, and S. Sudb~), Phys. Rev. B 40 (1989) 6763 .... 8. B.O Wells, Z.X. Shen, D.$. Dessau, W.E. Spicer, C.G. Olson, D.B. Mitzi, A. Kapitulnik, R.S. List and A. Arko~ Phys. Rev. Lelt. 65 (1990) 3056.
PHYSICA
North-Holland
Magnetic identification of 3d to 2d vortex transitions in Bi2Sr2CaCu20 x single crystals G. Yang*, C.E. Gough® and J.S. Abeli*, Superconductivity Research Group, *School of Metallurgy and Materials,®School of Physics and Space Research, The University of Birmingham, Birmingham, B 15 2TI', UK. VSM measurements on Bi2Sr2CaCu20 x single crystals as a function of oxygen and vacuum annealing reveal a strongly processing dependent "arrow-head" magnetic anomaly, which we associate with the decomposition of a 3d-flux line to a 2d pancake vortex, as recently observed in ~tSR and neutron diffraction measurements. The value of the decomposition field is correlated with the irreversibility line, both of which are strongly dependent on oxygen content, consistent with associated changes in anisotropy factor ~,. Oxygen or vacuum annealing may effectively change superconductivity in the B i t players therefore the anisotropy of the crystal.
1. INTRODUCTION For highly anisotropic layered superconductors, such as Bi2Sr2CaCu20 x (BSCCO), vortex lines can be regarded as 2d pancake vortices confined to the C u t 2 layers but weakly interacting between layers by Josephson and magnetic coupling [11. At low fields, such coupling rosy.Its in essentially 3d flux lines. At high fields, the in-plane repulsion between pancakes exceeds their interplane attraction. Uncorrelated pinning in different C u t 2 layers can then break up flux lines in the field direction, leading to the so called flux line decomposition as observed in measurements of ~tSR and small angle neutron diffraction [2,31 in BSCCO crystals, where the signature of a flux lattice disappears for applied field above ,-650 Oe. This decomposition has been predicted to occur at a field B2D=~O/(Sy)2, which does not vary significantly with temperature, but depends strongly on the spacing between the C u t 2 layers, s, and the anisotropy factor 7 [4]. Differences between the pinning of 3d flux lines and the decomposed 2d pancake vortices lead 1o dramatic change in magnetic behaviour, which can be used to identify the decomposition transition. In tn~ proccssmg this paper, we present lebUltb . . . . "- o-"t "'-^ dependent arrow-head anomaly of BSCCO crystals, which we associate with this decomposition transition.
parallel to the c-axis of the crystals at a field sweep rate 1 mT/s using an Oxford VSM 3001. ,00 S
~K
| -511 -IO0 -~ooo
2~,~0
100o
.2000
.1000
0
Io00
2000
lO00
1000
2000
1000
•15o -250
-3~,0 -3000
,0o Ic)
~
~
30K
1013
~
o .,oo
-200
1000
F|~.
1.
-1000
.I O00
OU[.)l~lllllildt.,j,~t,,, u
0 H(O¢) ii2,..3t~.,l~u•,~t,.,~
i000
1000
3000
,vv[.',...~
2. EXPERIMENTS
single crystals before (filled squares) and after (empty squares) annealing treatme~.ts. (a) Oxygen annealed crystal A at 35 K, (b) vacuum annealed crystal B at 30 K and (c) oxygen annealed oxTgen crystal C at 30 K.
BSCCO single crystals were grown using a large temperature gradient technique [5]. VSM measurements were performed in a magnetic field
Figure l(a) shows hestercsis loops of a BSCCO crystal A at 35 K before and after oxygen annealing. An "arrow-head" anomaly, with a sudden increase
g21-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)01922-3
G. Yang et al./Physica C 235-240 (1994) 2713-2714
2714
in magnetisation above a well-defined field, can be clearly observed in both cases. The onset of the anomaly moves to significantly higher field (from 550 Oe to 950 Oe) after oxygen annealing. Detailed measurements [6] showed that the arrow-head anomaly only appears above -20 K and gradually disappears at about 40 K. However, the field at which the maximum occurs, Hp, is almost independent of temperature. Processing dependent arrow-head anomaly is further demonstrated in Fig. l(b), the anomaly disappeared completely in a vacuum annealed crystal B, and in Fig. l(c), it appeared only in an oxygen annealed crystal C. Figure 2 plots the irreversibility lines deduced for the crystal A both in the as-grown and oxvgen annealed states. Identifying this with the flux lattice melting field [7] and assuming no change in penetration depth, we estimate Ya=108 for the asgrown crystal and 7o=70 after oxygen annealing. The derived value 0,o/Ya)2~--0.4 is close to the value Hpa / Hp°~.5., There is internal consistency with the theoretical predictions for both flux lattice melting and the 3d to 2d decomposition field.
pinning centres will be ineffective. However, existed other important defects, such as planar dislocation networks with low density but large pinning energy, will remain effective at higher temperatures. At low temperaturc ~, the high density of effective pinning centres means that there is very little difference in pinning force per unit length of flux line above and below B2D. However, at high temperatures, when only the lower density, deeper lying pinning centres from extended defects are effective, the pinning force per unit length is increased after decomposition because the now independent 2d-pancakes can move in the CuO2 planes to maximise their pinning energy. This would interpret the appearance of the arrowhead anomaly at the higher temperatures. Angle-Resolved-Photoemission measurements has suggested that oxygen annealing may change BiO layers from the metallic to superconducting [8]. This may account for the reduction in the anisotropy of BSCCO crystal after oxygen annealing and enhancement after vacuum annealing, and therefore the processing dependent arrow-head anomaly.
REFERENCES
1200
--
o--- --¢.-
o
800
is
w
. . . .
~r--
.--. . . . . . . . . . .
400
o
3o
oo
1
9o
Fig. 2. Irreversibility lines for the crystal A before (filled squares) and meter o×Tgen annealing (empty squares). The solid lines are predicted by melting theory [7]. The dashed lines indicate the mean decomposition field deduced from the plotted measurements (circles) of the peak positions for the as-grown and oxygen annealing states. 3: DISCUSSION In BSCCO crystals, the effective pinning centres at low temperature (<20 K) are believed to be mainly oxygen vacancies in the CuO2 layers. The features of such point defects are high density but low pinning energy. At high temperature such
1. J.R. Clem, Phys. Rev. B 43 (1991) 7837. 2. S.L. Lee, P. Zimmermann, H. Keller, M. Warden, I.M. Savic, R. Schauwecker, D. Zech, R. Cubbik E.M. Forgan, P.H. Kes, T.W. Li, A.A. Menovsky, Z. Tarnawski, Phys. Rev. Lett. 71 (1993) 3862. 3. R. Cubbit, E.M. Forgan, G. Yang, S.L. Lee, D.McK. Paul, H.A. Mook, M. Yethiraj, P.H. Kes, T.W. Li, A.A. Menovsky, Z. Tarnawski, and K. Mortensen, Nature 365 (1993) 407. 4. V.M. Vinokur, P.H. Kes and A.E. Koshelev, Physica 168C (1990) 29. 5. G. Yang, S.D. Sutton, P. Shang, C.E. Gough, and J.S. Abell, IEEE Trans. Appl. Superconduc. 3 (1993) 1663. 6. G. Yang, J.S. Abell and C.E. Gough, IEEE Trans. Appl. Superconduc. 3 (1993) 1671; G. Yang, P. Shang, S.D. Sutton, !.P. Jones, J.S. AbeU and C.E. Gough, Phys. Rev. B 48 (1993) 4054. 7. A. Houghton, R.A. Pelcovi~s, and S. Sudb~), Phys. Rev. B 40 (1989) 6763 .... 8. B.O Wells, Z.X. Shen, D.$. Dessau, W.E. Spicer, C.G. Olson, D.B. Mitzi, A. Kapitulnik, R.S. List and A. Arko~ Phys. Rev. Lelt. 65 (1990) 3056.