Anisotropic transport critical current density and its field dependence in a Bi2Sr2CaCu2Ox single crystal

Anisotropic transport critical current density and its field dependence in a Bi2SrzCaCuzOx single crystal Y. Y a m a d a * S. Nomura t, K. Ando and O. Horigami Toshiba Research and Development Center, 4-1, Ukishima-cho, Kawasaki-ku, Kawasaki 210, Japan The authors observed an anisotropy in the transport critical current density, Jct, at 77 K and 500 G, for a Bi2Sr2CaCu2Ox single crystal. The transport critical current density, Jct, along the c-axis for a field parallel to the a-axis, with the Lorentz force parallel to the b-axis, Was-ab0ut three times larger than that for the field parallel to the b-axls, wi-th the-Lorent~ force parallel to the a-axis. This shows the possibility that a bismuth atom modulation structure, existing only along the b-axis, may act as an effective pinning site. Jct with the Lorentz force perpendicular to the B i - O insulating layers was about 10 times larger than that with the force along the layers, indicating that the B i - O layers also serve as pinning centres. A correlation was observed between the decrease in Jct and the field dependence rate, with Jct oc B-n (where n = 0.8-1.3). This is attributed to the pronounced flux creep effect in this material.

Keywords: oxide superconductors; Bi- Sr- Ca- Cu- 0 compounds; anisotropy; critical currents; flux pinning

Intrinsic pinning, i.e. pinning by the crystal structure itself, has been proposed by Tinkham I and Tachiki and Takahashi 2 for the high Tc oxide superconductors. This originates from the very short coherence length, ~(0), typically 0.2 nm, along the c-axis in Bi2Sr2CaCu2Ox3 and the anisotropic crystal structure. These authors have suggested that the insulating layer characteristics of the oxide high T~ superconductors or other crystallographic peculiarities act as effective pinning sites. This also results in anisotropic electrical properties. Anisotropic critical current densities for the crystallographic axes have already been reported. For describing the anisotropy in critical current density and thus flux pinning, we define here the quantity Jc(i), J the critical current density along the i-axis with the magnetic field parallel to the j-axis. In single crystal Bi2Sr2CaCu20 x Biggs et al. 4 reported J cc~a,b)values about 20 times larger than J a,b c(c>,as derived from magnetization measurements. Jb~a) and Ja(b) were also found to be much higher than J~(,,b). They concluded that the insulating B i - O layers serve as pinning centres because J~(,) and J~co)are determined by the Lorentz force perpendicular to the insulating layers. For single crystal YBa2C3075, a similar anisotropy was recently *Present address: Karlsruhe Nuclear Research Centre, Institute of Technical Physics, Postfach 3640, D-7500 Karlsruhe 1, FRG ~Present address: Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

reported, where Jai~) was 30 times smaller than J~(a) and J ca(b), from magnetization measurements. Looking at transport current density, large anisotropy was found 6 in single crystal Bi2Sr2CaCu20/, i.e. Jc(a,b)/Jc(c) = 1000, at zero magnetic field and near To. However, no magnetic field dependence has been reported for the transport critical current density, JctThis paper reports the anisotropic transport critical current density, or the anisotropic pinning force density, where the Lorentz force is parallel to the ab plane for single crystal Bi2Sr2CaCu2Ox, indicating the possibility of a bismuth atom modulation structure serving as a pinning centre. Moreover, the authors also investigated the anisotropy of Jct values within the ab plane and along the c-axis, and the field dependence for each Jct.

Experimental details Single crystals, of composition Bi2.3Srl.9Ca0.sCu2.0Os+d, determined by energy dispersive X-ray spectroscopy (EDS), were grown by the self-flux method reported previously 7, using a solution of a B i : S r : C a : Cu(2 : 2 : 1 : 2) oxide mixture. The crystals grew as rectangular thin platelets; the thin plane is the ab plane, which was confirmed by the X-ray diffraction pattern. Crystallographic a- and b-axes were determined by the Laue pattern with C u - K s radiation. Electrical contacts were made by ultrasonic bonding with I n - S n solder. Copper wire 100/~m in diameter was attached to the crystal with a specific contact resistivity

OOll - 2 2 7 5 / 9 0 / 0 7 0 6 4 3 - 0 4 © 1990 Butterworth-Heinemann Ltd

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Transport critical current density in Bi2Sr2CaCu20x crystal: Y. Yamada et al. of less than 1 m~ cm. The T~ onset and zero resistivity temperatures were 84 and 81 K 8, respectively, as determined by the d.c. four probe method. Typical resistivities at 100 K were 80 #f~ cm along the a-axis, 150/x~ cm along the b-axis and 5 mfl cm along the c-axis. A discussion of the anisotropic resistivities has been presented elsewhere 8. For the anisotropy measurements of the pinning force densities along the a- and b-axes, the critical current density along the c-axis was measured with a dc current and a 2 s rise in a 500 G field within the ab plane. A 1 #V cm -~ criterion was used. For the anisotropy measurements of J~ values along the c-axis and within the ab plane, and their field dependence, the authors used a pulsed current with a 5 ms rise and 1 ms fall in a magnetic field of up to 14 T. A 1 mV cm -~ criterion for J~ was used in this high speed measurement.

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,

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I

I

I

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200

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Figure 1 Measured c u r r e n t - v o l t a g e curves at 77 K and 500 G for magnetic field within the ab plane. Critical current density, Jc (shown by arrows), is defined by 1 /~V cm -1. Voltage peaks are due to flux jumps

Results and discussion The angular dependence of the critical current density, Jc(O), has been measured along the c-axis. Figure 1 presents the curent-voltage curves for the measurement configuration shown in the inset: current is along the caxis, magnetic field within the ab plane and the angle, O, defined for the b-axis. The applied magnetic field within the ab plane was 500 G and was sufficiently larger than the lower critical field, Be1, of 9 G reported earlier 9 for this magnetic field direction. The critical current density at O = 0 ° is 15 A cm -2. The critical current density first increased slowly with increasing angle O and then decreased. The peak occurred at 75 o. The critical current density at 90 °, along the a-axis, was 42 A cm -2. The anisotropic ratio was = 3. Furthermore, the critical current density was assumed to vary periodically with the 180 ° period. This angular dependence was observed for two crystals. Some voltage peaks can be also seen, probably due to flux jumps; these are usually also observed in conventional superconductors, such as N b - T i and Nb3Sn. However, the critical current, defined by the I #V cm-1 criterion, varied considerably with angle 0. From the microstructural point of view, it is well known that the bismuth atom modulation structure exists only along the b-axis 1°'11. The periodicity is 2.7 nm, i.e. 4.8 times the unit cell length. The Be2 measurement using the d.c. transport method showed that Be2 for the field parallel to the ab plane was 14 T at 77 K. Therefore, the coherence length, ~(77 K), along the c-axis was = 4 nm. Figure 2 shows the conditions between the vortex and the crystallographic axes. The sine wave curve indicates the bismuth atom density fluctuation. The periodicities for both the bismuth atom density and the vortex core diameter are similar. The C u - O layers are strongly superconductive, while the B i - O layers and the regions bet-

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Figure 2 Schematic representation of conditions between vortex and the a- or b-axes in Figure I. The vertical axis is the density fluctuation of bismuth atoms caused by bismuth atom modulation. No modulation exists along the a-axis

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Transport critical current density in Bi2Sr2CaCu20x crystal: Y. Yamada et al. i

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triangles in Figure 3) and 1 - 10 T (open circles in Figure 3). In Figure 3 the dependence J¢ oc B-1 is shown by the solid line. This B-1 dependence is known to be typical of a Josephson current. However, this is not the case for a single crystal, at least for current directions within the ab plane, because there are no insulating layers limiting the current path in this direction. For the Y - B a - C u - O •ms, the magnetic field dependence of Jet was reported to be J¢ oc B -~/2 in the field range 0.5 - 1.5 T 13. Mannhart et al. 13assumed that this dependence was due to the flux creep effect. Using the flux creep model, J~ is expressed by the following equation 14

J~(B, t)= [NpU(B, 0)/1.07($0B)1/2] (1 - - a t - bt 2)

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Figure 3 Reduced magnetic field dependence of normalized transport critical current density for various current and magnetic field directions (at 77 K). - - . , Jc oc B -1 dependence. The plane in the inset is the ab plane. (Key to symbols shown on figure)

ween the C u - O layers are weakly superconductive. Thus, the region poor in bismuth atoms, where the CU-O layers are closest to each other, and the region dense in bismuth atoms, have the strongest and weakest superconducting order parameter, respectively. Therefore, the superconducting order parameter is modulated along the b-axis. The region dense in bismuth atoms may attract a vortex. This implies that the bismuth modulation structure serves as an effective pinning centre. The authors also investigated the anisotropy in Jc values along the c-axis and within the ab plane, and their magnetic field dependence. Figure 3 shows the magnetic field dependence of / c t reduced by the upper critical field, B~2, and Jet at zero magnetic field, respectively, for various directions of magnetic field and current. The critical current density at zero magnetic field was 4000 A cm-2 within the ab plane and 250 A cm-2 along the c-axis 12. The anisotropic ratio was 16. When the magnetic field was applied, J~t for the field perpendicular to the ab plane was drastically lowered. This was due to the low Be2 value of 0.7 T, compared to that of 14 T when the field was parallel to the ab plane. J~t along the c-axis is also much smaller in a magnetic field than that within the ab plane. Vortices move along the B i - O layers in the case where the current travels along the c-axis and the field is parallel to the ab plane (open triangles in Figure 3). On the other hand, vortices move across the B i - O layers when the current and the field are applied within the ab plane (open circles in Figure 3). Thus, it seems that B i - O insulating layers act as pinning centres. Furthermore, the magnetic field dependence is given by J~ o: B-", with n = 0 . 8 - 1.3, within the field ranges 0.01-0.1 T (closed circles in Figure 3), 0.5 - 10 T (open

where: Np = density of the pinning centres; U(B, O) = pinning potential energy; $0 = flux quantum; B = magnetic field; and t = reduced temperature, T/Tc. Following Tinkham 15, a is the coefficient given by a = [kTJU(B, 0)]In [nd~/Emin] and b is the coefficient given by the temperature dependence in the form U(B, t)= U(B, 0 ) ( 1 - bt2). Here: k = the Boltzmann constant; d = average hopping distance for the flux quanta; fl = attempt frequency for escape; and Emi n = electric field criterion that defines Jc. Thus, the B -1/2 dependence is indicative of the flux creep, as predicted by the above equation. However, Mannhart 13 ignored the field dependence for U. Theoretically and experimentally, a strong field dependence on U is proposed, varying as U oc B -1 (Reference 16) or B -~ (Reference 17), respectively. Furthermore, very small U values, nearly 10 meV, have recently been reported for single crystal Bi2Sr2CaCu208+d4'18. This also indicates accelerated flux creep phenomena in the bismuth compound, compared to the Y - B a - C u - O crystal. Both these facts, the field dependence of U and the small U value, result in the above field dependence of JcocB-(°8-13) for this bismuth based single crystal, which is stronger than J~ cc B -1/2 for Y - B a - C u - O crystals. This pronounced creep effect might also affect the I - V curves in Figure 1. Further studies are needed in which the modulation period is varied and the critical current densities are compared.

Conclusions The present results suggest the possibility of intrinsic pinning by the bismuth atom modulation structure as a consequence of the anisotropic current density along the c-axis for the vortex movements along the a- and b-axes. In addition, the observed anisotropy between the critical current densities along the c-axis and within the ab plane indicates that the B i - O insulating layers also play a role as pinning sites. The field dependence for the J~t values is expressed by Jc oc B-" (with n = 0.8-1.3). This is attributed to a pronounced flux creep effect in bismuth based single crystals.

Acknowledgements The authors are grateful to Mr Yamashita, Dr Yoshino and Dr Murase for useful discussions.

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Transport critical current density in Bi2Sr2CaCu20 x crystal: Y. Yamada et al.

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