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Enhanced flux pinning in Bi-2212 single crystals by planar defects introduced via Ti-substitution
N
ELSEVIER
PHYSICA@ PhysicaC274 (1997) 197-203
Enhanced flux pinning in Bi-2212 single crystals by planar defects introduced via Ti-substitution T.W. Li a R.J. Drost a,* P.H. Kes a C. Tra~holt b, H.W. Zandbergen b N.T. Hien c, A.A. Menovsky c, J.J.M. Franse c a Kamerlingh Onnes Laboratory, Leiden University, Nieuwsteeg 18, P.O. Box 9506, 2300 RA Leiden, The Netherlands" b Laboratory of Materials Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands c Van der Waals-Zeeman Laboratory, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands
Received 31 August 1996; revised manuscript received 11 December t996
Abstract Ti-doped Bi-2212 single crystals were ~own by the travelling solvent floating zone technique. Concentrations of about 1 at% and 2 at% (with respect to Cu) were shown to be incorporated in the structure which leads to lowering of Tc to about 73 K from 85 K. High resolution microscopy revealed high densities of planar defects parallel to the a,c-planes. The flux pinning properties at elevated temperatures are significantly improved with respect to undoped single crystals. Keywords: Critical current density; Flux pinning; Substitution effects; Defect structure; Single crystal growth
1. Introduction One of the most promising high-T~ superconducting compounds from the point of view of applications is Bi2Sr2CaiCu2Os+ x (Bi-2212). It has been shown that the Bi-based cuprates have high critical current densities (J0) at 4.2 K, even in magnetic fields above 15 T [1]. Grain oriented B i - 2 2 1 2 / A g composite tapes were found to have Jo values above 105 A / c m 2 in magnetic fields up to 25 T [2]. However, the high (effective mass) anisotropy and the short coherence length considerably enhance the role of thermal fluctuations at elevated temperatures,
* Corresponding author. Fax: + 31 71 527 5404; e-mail: [email protected].
which leads to thermally activated flux motion. This magnetic relaxation process generates significant resistance even at T << Tc in a modest magnetic field. As a result, energy dissipation is present at finite current densities which seriously limits the number o f applications. Therefore, improvement of flux pinning in Bi-2212 is of great technological interest. With this aim, several methods have been attempted to introduce effective pinning centers. To our knowledge, this has not yet b e e n achieved so far in Bi-2212 through any type o f substitution. The only method to successfully introduce pinning centers is by irradiation of heavy ions [3-6], neutrons [7,8] or protons [9] of certain energy ranges to create microscopic defects in the sample. For instance, it has been reported [10] that the introduction o f columnar defects in Bi-2212 single crystals, by means
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T.W. Li et al./Physica C 274 (1997) 197-203
of heavy-ion irradiation, can overcome the detrimental effect of the extremely large anisotropy. At fields up to the dose-equivalent field B,~, the pancake vortices which are trapped at the columnar defects, behave as non-segmented flux lines. Since heavy ions can have only a small damage range in solid matter (less than 100 ~m), this technique thus imposes an upper limit to the thickness of current leads or tapes. Furthermore, the enormous costs of the irradiation makes this method infeasible for application. In this paper, we show that doping of Ti in Bi-2212 single crystals can create a high density of planar defects extended along the a,c-plane. These defects form effective pinning centers when the field is applied along the c-direction. We present various magnetization experiments and compare the results with the pinning in pure and heavy-ion irradiated Bi-2212 single crystals.
2. Experimental Single crystals of Bi-2212 doped with Ti were prepared using the travelling solvent floating zone (TSFZ) technique. A " w e t " preparation route from nitrates was used in order to achieve homogeneous starting materials. Powders of Bi203, SrCO 3, CaCO 3, CuO, and Ti (all 3N or purer) were weighted in the ratios of Bi:Sr:Ca:Cu:Ti = 2.05:1.95:1.0:(2.0 - x ) : x ( x = 0 . 0 4 , and x = 0 . 1 0 ) for the feed rods and Bi:Sr:Ca:Cu=2.6:l.9:l.0:2.6 for the solvent. These mixtures were dissolved in HNO 3. After a drying procedure, the mixtures were calcined in air at 810°C. The calcined powders were pressed into boules of 7 mm in diameter and 60 mm in length under hydrostatic pressure of 3.5 kbar. These rods were then sintered in a vertical hanging furnace at 840°C for 48 hrs in air. Prior to the crystal growth, the feed rods were first densified by passing through a melting zone at a velocity of 60 m m / h r . A Bi-2212 crystalline boule from a previous growth run was used as a seed. The growth was carried out with a rotation speed of 25 rpm and a growth speed of 0.26 m m / h r . The ambient atmosphere was air. We add here that the growth is less smooth than that of pure Bi-2212. In addition to X-ray diffraction, the as-grown Ti-substituted single crystals were checked for corn-
position, homogeneity and single crystallinity by Electron Probe Micro Analysis (EPMA). Powder X-ray diffraction data were collected with a Philips PW-1710 setup. The EPMA was carried out using a JEOL JXA8621. The microstructure was studied by high resolution electron microscopy (HREM), using a Philips CM30ST equipped with a field emission gun and a Link EDX elemental analysis system. AC susceptibility, magnetization and magnetic relaxation measurements, in the temperature range 2 120 K, were carried out using a commercial SQUID magnetometer (Quantum Design MPMS-5S). In all measurements, the DC and AC fields were applied parallel to the c-axis of the crystal and the scan length was set at 3 - 4 cm in order to suppress effects related to a small amount of magnetic field inhomogeneity. The procedure for the relaxation measurements was to cool the sample in zero field. The field was then ramped to 3 T in order to ensure a full penetration of the sample by a shielding current of uniform direction. Subsequently, the field was reduced to the respective measurement values and the decay of the magnetic moment with time was recorded.
3. Results and discussion
3.1. Crystal characterization Single crystalline boules of typically 5.5 mm in diameter and 45 mm in length were obtained. The as-grown single crystals exhibit a less shiny appearance than pure Bi-2212 single crystals. Furthermore, the cleaving of the crystals appears to be more difficult than the cleaving of pure Bi-2212 crystals. EPMA data were typically taken at five points on several single crystals. The averaged composition was normalized to a Ca-stoichiometry of unity. The uniformity was studied by comparing the compositions of a few single crystals cleaved from the top part of the as-grown boule. Within the accuracy of the EPMA apparatus, the measurement gave the same stoichiometry. The results are listed in Table 1, from which it is seen that the actual Ti to Cu ratios are 0.86 at% and 1.95 at% for x = 0.04 and 0.1, respectively. We will further denote these crystals as 1 at% and 2 at% Ti-doped Bi-2212. The data in
T.W. Li et al./Physica C 274 (1997) 197-203
199
Table 1 EPMA characterization of the 1 at% and 2 at% Ti-doped Bi-2212 single crystals Nominal feed composition
Bi2.05Srl.95CaCul.96Ti0.040 x (2 at% Ti)
Bi2.05SrL95CaCul.90Ti0.1oO x (5 at% Ti)
Bi Sr Ca Cu Ti
2.01 _+ 0.04 1.92 + 0.02 1 1.97 _ 0.03 0.017 + 0.008 ( = 0.86 at% Ti)
1.99 +__0.03 1.93 + 0.04 1 1.95 _~ 0.02 0.038 + 0.011 ( = 1.95 at% Ti)
Table 1 reveal that the actual Ti-content is much lower than the nominal composition for both the 1 at% and the 2 at% samples. This is consistent with the observation of some SrTiO 3 grains between Bi2212 platelets in the crystal boule, indicating that the actual doping level is limited to low values. X-ray powder diffraction was performed on some ground single crystals. The unit-cell parameters were calculated from the reflections in the X-ray diffraction spectra. Within experimental resolution, we can index the diffraction pattern with a tetragonal symmetry I 4 / m m m , analogous to the crystal symmetry of pure Bi-2212 [11]. The lattice parameters are not significantly different as those of pure, overdoped Bi-2212 crystals [12], namely a = 0.540(1) nm and c = 3.0838(3) nm. 3.2. Transition temperature
The transition temperature for our 1 and 2 at% Ti doped crystals are determined from AC-susceptibility measurements in an AC-field of 30 ~T and a frequency of 22 Hz. As shown in Fig. 1, Ti-doping results in a shift of Tc towards a lower temperature, typically from 85 to 73 K (midpoint). Similar downward shifts of T~ were observed in crystals doped with other 3d elements (for example: Fe, Co, Zn [11], V, Cr [13], Fe, Co [14] or Fe, Ni, Zn [15]), but in those cases Tc decreased continuously with increasing doping level. In our Ti-doped Bi-2212, there is no further decrease when the doping level is changed from 1 at% to 2 at%. 3.3. HREM studies
Fig. 2 reveals the presence of defects which consist of pairs of antiphase boundaries (APB's, indi-
cated by the black arrows in Fig. 2b) parallel to the a,c-plane. The defect can only be clearly seen when one views along the a-axis. At the antiphase boundaries the shift of the lattice is about 0.4 nm along the c-axis, being two (001) planes. This shift results in one CuO2-plane which continues through the antiphase boundary whereas the other CuO2-plane terminates to continue as a Bit-plane. Because the shift of the second antiphase boundary is opposite to the first, the structure is restored after the second antiphase boundary, resulting in atom positions equal to that of the matrix and having one CuO2-plane which is continuous throughout the whole defect. The distance between antiphase boundaries is equal to the vector of the modulation in the Bi-2212 structure which is about 4.5b [16]. This distance is comparable to the superconducting coherence length at low temperature. Therefore, it is expected that these defects may act as effective pinning centers when the field is applied parallel to the c-direction. 0.2 =~
0.0
[{{'[{([[[[i [ [ (111([[[~~11[[(1[[(1 [
-0.2 Ti-doped Bi-2212
[
{
[
~
d, ~}
-0.4
~t
~"
-0.6
A 2at. %
-~ -0.8 ~ . . q ~ . . . . . ~ -1.0
lat.%
3'0 ' 4'0 ' 5b ' 6'0 ' 7'0 ' 80 ' 9b ' 100 Temperature [K] Fig. 1. Zero-field AC-susceptibility measurements (30 IxT, 22 Hz) on as-grown 1 at% (open symbols) and 2 at% (closed symbols) Ti-doped Bi-2212 single crystals. The AC-field is parallel to the c-axis.
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T.W. Li et a l . / P h y s i c a C 274 (1997) 197-203
a)
C
T
~b
b)
C
3.4. Magnetic measurements in the irreversible regime In order to study the effect of the planar defect structure on the flux pinning, we have carried out magnetization and AC-susceptibility measurements. We compare the results with those of as-grown and heavy-ion irradiated Bi-2212 single crystals. It is well documented that high energy heavy-ion irradiation creates amorphous tracks of about 7 nm in diameter and that these defects are optimal pinning centers for flux densities below the dose equivalent field. Our 2 at% sample contained too many macroscopic defects (mainly misorientations) to allow a reliable Bean analysis and therefore the results of this sample will not be considered in the calculation of critical current densities. In Fig. 3 we compare the critical current densities at 0.5 T obtained from SQUID magnetization measurements at similar sweep rates on a 1 at% Ti-doped sample with data of an as-grown Bi-2212 crystal [12] and of a heavy-ion irradiated Bi-2212 crystal (dose equivalent field is 0.5 T) measured by Prost et al. [17]. In all cases, the current densities were calculated from the widths of the magnetization loops at various temperatures using the Bean model for a cylindrical geometry. The magnetization curves of the Ti-doped sample are characterized by a smooth
~b Fig. 2. Low magnification (a) and high magnification (b) HREM images of 1 at% Ti-doped Bi-2212 along the a-direction, revealing the defect structure induced by the presence of Ti. The antiphase boundaries (APB's) are indicated by arrows. We refer to the accompanying paper [16] for more details.
Irradiated Bi-2212 v v v v ~ 10~
V []
.~o
We stress here that the defect structure solely arises as a consequence of the Ti substitution, because elemental analysis (EDX), using a small spotsize down to 2 nm, showed the sole presence of Ti in the vicinity of the anti-phase boundaries. To our knowledge, this is the first time that such planar defects have been introduced in Bi-2212 crystals by chemical doping. A more detailed analysis of this peculiar defect structure will be presented in a separate paper [16].
~toHa= 0.5T
v ~xz
< ~
•
V
A
V
lat.% Ti-Dope
m
105
A
PureBi-2212 D
5
'
1'0
'
1'5
'
Temperature [K] Fig. 3. Comparison of current densities obtained from the widths of magnetization loops for: 1 at% doped Bi-2212 ( • ) , as-grown Bi-2212 (D), and Bi-2212 with columnar defects (B a, = 0.5 T) created by Pb-irradiation ( v ) [17].
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T.W. Li et al./Physica C 274 (1997) 197-203
penetration (at all temperatures) and the absence of a second peak in the magnetization [18]. It follows from Fig, 3 that the current carrying capability of the Ti-doped crystal is comparable to that of pure Bi2212 at low temperature. At higher temperatures however, the current density is significantly higher, e.g. a factor of roughly 4 at 15 K, but not as much as for the irradiated crystal. In order to compare the properties of the 2 at% Ti-doped crystals, it suits to determine the relative creep rate S ( T ) = - 1 / M o ( d M / d l n t). Here, M 0 denotes the initial magnetization at t = t o (in our case 150 s after the field was stopped). This quantity has been measured for fields between 0.05 and 2 T in a temperature range between 2 and 50 K. A typical result is shown in Fig. 4 for a field of 0.2 T. It is seen that the creep rate of the Ti-doped sample lies considerably below the rate of our pure Bi-2212 crystal and just above the result of Prost et al. [17] on a crystal containing columnar defects. Finally a comparison is made, for pure and Tidoped crystals, of the irreversibility lines (IL) determined by the peak in the out-of-phase component of the AC susceptibility in an AC field of 30 tzT and frequency 22 Hz superimposed on DC fields ranging between 500 mT and 5 T. The data in Fig. 5 show that the IL lines of the 1 at% and 2 at% Ti-doped crystals almost coincide. It is also seen that the doping increases the irreversibility fields at the same reduced temperature by a factor of 2-4. We note that these results should still be corrected for different
0.15 g0Hex = 0.2T zx "~ ,.-i
0.10
2at.% Ti-Doped A
0.05 Z~. A
•
A
~vV
v
v
•
•
•
V
v After Irradiation
0.00
5
10
ZX
•
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1
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ZX
~"
/4
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0.1
O oe O•
zx
• o []
=t. Pure Bi-2212
0.01
~a
e• A
0.001
0.2
0.4
0.6
0.8
1.0
T/Tc Fig. 5. Comparison of the irreversibility lines of 1 at% ( O ) and 2 at% ([]) Ti-doped Bi-2212 with pure Bi-2212 (zx) single crystals. The irreversibility lines (IL) are defined by the peak positions of the dissipative component X" of the AC susceptibility.
effective sample sizes, since our 2 at% Ti-doped sample shows some cracks due to the high density of defects. However, such a correction will always lead to a higher location of the IL. To give an idea how serious this effect is, we can estimate from a TAFF analysis [19] and taking into account the general observation that the IL is approximately determined by the condition U/kBT..~ 20-25 (U is the pinning energy and k B the Boltzmann constant), that a reduction in sample size by a factor of 10 leads to a small upward shift of the IL over 20%. Together with the current densities of Fig. 3 and the relative creep rates Shown in Fig. 4, this shift of the irreversibility line evidences the increase of bulk pinning in Ti-doped Bi-2212, as compared to undoped Bi-2212.
3.5. Magnetic measurements in the reversible regime
Pure Bi-2212 zx
"7,
10
15
20
Temperature [K] Fig. 4. Comparison of the temperature dependence of normalized relaxation rate S(T) between: 2 at% Ti-doped Bi-2212 ( O ) , pure(A) and irradiated ( v ) Bi-2122 single crystals (B,/, = 0.2 T) [17] at field 0.2 T parallel to the c-axis.
It has been recently shown [20] that the presence of columnar defects is reflected in the behaviour of the magnetization curves above the depinning temperature. Instead of a monotonous in(B) dependence, the magnetization in irradiated Bi-2212 crystals displays a minimum at the dose equivalent field. The reason for such a minimum follows from an expression for the free energy proposed by Van der Beek et al. [20a] who added two terms: one expressing the entropy gain related to the fluctuations of the unpinned vortices, the other one representing the pin-
202
T.W. Li et al./Physica C 274 (1997) 197-203
Oo ,~N2at~o/oTi-doped Bi-2212~.....
-8.0x102 ,-----,
x
3
-1.2x10
< -1.6xl03 £
Io
}¢ 65K
-2.0xl( 10 -2
10 -1
10 ° [T]
101
Fig. 6. The reversible magnetization as function of magnetic field at temperatures of 63 K (O), 65 K (El), 67 K ( v ) , and 70 K ( O ) for 2 at% Ti-doped Bi-2212 crystal. The dashed lines represent the average of the decreasing and increasing (see arrows) branches. A minimum occurs in all curves at approximately 0.3 T. The ins& shows the construction for the estimation of Up. More details on this construction are given in the text.
ning energy for vortices trapped at a columnar defect. Similar behaviour may be expected in samples with extended defects along the field direction (correlated disorder), such as the planar defects in our Ti-doped samples. The magnetizations, for temperatures close to Tc of the 2 at% Ti-doped crystal are plotted in Fig. 6. It is seen that the irreversibility sets in below about 20 mT. The reversible magnetization in this field regime was estimated by taking the average (indicated by the dotted line) of the increasing and decreasing branches, as denoted by the arrows. The overall behaviour of the reversible magnetization is very similar to that observed in ref. [20a], i.e. a minimum is observed at, in our case, a field of 0.3 T. Assuming that we may analyze our data as if the planar defects can be treated as columnar defects, we can roughly estimate the typical pinning energy per pancake vortex Up from the shape of the magnetization curves and the formalism given in [20a]: sgb0 A M = Up, where s is the interlayer distance and ~b0 denotes the flux quantum. The construction for our estimate of A M (by substraction of extrapolated values of M of the low and high field regime) is shown in the inset of the figure. The
value for Up (65 K) obtained from this construction is 70 -t- 35 K. The occurrence of the minimum in the reversible magnetization demonstrates the extra bulk pinning due to the presence of the planar defects created by Ti-doping. It is as yet not clear what the prevailing pinning mechanism is, because not much is known about the microscopic properties of the anti-phase boundaries. Since one of the CuO2-planes is continuous throughout the defect one may speculate that the anti-phase boundary is conducting, perhaps even superconducting. Nevertheless, some scattering of the charge carriers at the planar defect is to be expected which would give rise to gK or (8l) pinning [21]. Here, K is the Ginzburg-Landan parameter and l the elastic scattering length. However, a local depression of T~ at the defects is also possible which would give rise to STc pinning. Whatever the precise mechanism is, the planar defects improve the current carrying capability of Bi-2212. The price we have to pay is a slightly lower T~. Therefore our finding only seems to have possible consequences for low temperature applications.
3.6. Directions towards application With possible applications in mind, we have checked the stability of the planar defect structure on various heat treatments. We therefore annealed several Ti-doped crystals in N 2 and in air at temperatures between 500°C and 850°C (for 12 hours or longer). HREM images reveal that the defect structure is stable up to at least 850°C in air. Measurements of the transition temperature and the IL of various annealed Ti-doped Bi-2212 crystals, show that these quantities do not undergo any significant shift. This behaviour is completely different from that of pure Bi-2212 where the out-diffusion of oxygen changes parameters such as To, critical current, the pinning energy, and the position of the first order transition line in the H,T-diagram [22,23].
4. Conclusions
Ti has been partially incorporated in Bi-22i2 single crystals prepared by the TSFZ technique in
T.W. Li et al./Physica C 274 (1997) 197-203
concentrations of about 1 at% and 2 at%. HREM images of these crystals reveal a peculiar defect structure consisting of clusters of antiphase boundaries in the a,c-plane. The T~ of the Ti-doped crystals is about 10 K lower than Tc of undoped Bi-2212. Magnetization measurements reveal the effect of the planar defects on both the irreversible and the reversible regime, indicating the enhanced bulk pinning in the Ti-doped crystals, as compared to pure Bi-2212. Since chemical doping would provide an easy and cheap route to improve the pinning properties of this material, a continuous research program on ceramic material is carried out.
Acknowledgements The authors wish to thank Prof. P. Bongers for valuable discussions, and Ton Gortenmulder for EPMA measurements. This work is part of research program of the Amsterdam-Leiden materials research cooperation ( F O M - A L M O S ) which is financially supported by the Dutch Foundation for Fundamental Research on Matter (FOM).
References [1] L.R. Motowidlo, G.B. Gaslinski and G.M. Ozeryanksy, 1995 Int. Workshop on Superconductivity (Co-sponsored by Japan ISTEC and U.S. MRS), Maui, 18-21 June 1995. [2] H. Kumakura, K. Togano, H. Maeda, J. Kase and T. Morimoto, Appl. Phys. Lett. 58 (1991) 2830. [3] L. Civale, A.D. Marwick, T.K. Worthington, M.A. Kirk, J.R. Thompson, L. Krusin Elbaum, Y. Sun, J.R. Clem and F. Holtzberg, Phys. Rev. Lett. 67 (1991) 648. [4] M. Konczykowski, F. Rullier-Albenque, E.R. Yacoby, A. Shaulov, Y. Yeshurun and P. Lejay, Phys. Rev. B 44 (1991) 7167. [5] H. Kumakura, H. Kitaguchi, K. Togano, H. Maeda, J. Shi-
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moyama, A. Okayasu and Y. Kazumata, Jpn. J. Appl. Phys. 31 (1992) L1408. [6] V. Hardy, J. Provost, D. Groult, M. Hervieu, B. Raveau, S. Durcok, E. Pollert, J.C. Frison, J.P. Chaminade and M. Pouchard, Physica C 191 (1992) 85. [7] H.W. Weber, Supercond. Sci. Technol. 5 (1992) 19; H.W. Weber and G.W. Crabtree, Studies of High Temperature Superconductors, Vol. 9, ed. A.V. Narlikar (Nova Science, New York, 1991). [8] F.M. Sauerzopf, H.P. Wiesinger and H.W. Weber, Phys. Rev. B 51 (1995) 6002. [9] M.P. Maley, H.F. Safar, L.N. Bulaevskii, J.O. Willis, J.H. Cho and Y.J. Coulter, 10th Anniversary HTS Workshop on Physics, Materials and Applications, t2-16 March 1996, Houston. [10] C.J. van der Beek, M. Konczykowski, V.M. Vinokur, T.W. Li, P.H. Kes and G.W. Crabtree, Phys. Rev. Lett. 74 (1995) 1214. [11] J.M. Tarascon, Y. Lepage, P. Bardoux, B.G. Bagley, L.H. Greene, W.R. McKinnon, G.W. Hull, M. Giroud and D.M. Wang, Phys. Rev. B 37 (1988) 9382. [12] T.W. Li, P.H. Kes, N.T. Hien, J.J.M. Franse and A.A. Menovsky, J. Cryst. Growth 135 (1994) 481. [13] B. Lonnberg, T. Lundstrom and P. Norling, Physica C 191 (1992) 147. [14] P. Sumana Prabhu, M.S. Ramachandra Rao and G.V. Subba Rao, Physica C 211 (1993) 279. [15] B: vom Hedt, W. Lisseck, K. Westerholt and H. Bach, Phys. Rev. B 49 (1994) 9898. [16] C. Traeholt, H.W. Zandbergen, T.W. Li, R.J. Drost, A.A. Menovsky, J.J.M. Franse and P.H. Kes, to be published. [17] D. Prost, L. Fruchter, I.A. Campbell, N. Motohira and M. Konczykowski, Phys. Rev. B 47 (1993) 3457. [18] V.N. Kopylov, A.E. Koshelev, I.F. Schegolev and T.G. Togonidze, Physica C 170 (1990) 291. [19] P.H. Kes, J. Aarts, J. van den Berg, C.J. van der Beek and J.A. Mydosh, Supercond. Sci. Techn. 1 (1989) 242. [20] (a) C.J. van der Beek, M Koncykowsky, T.W. Li, P.H. Kes and W. Benoit, Phys. Rev. B 54(2) (1996) R792; (b) A. Wahl, V. Hardy, J. Provost, Ch. Simon and A. Buzdin, Physica C 250 (1995) 163. [21] P.H. Kes and J. van der Berg, Studies of High Temperature Superconductors, Vol. 4/5, ed. A.V. Narlikar (Nova Science, New York, i989). [22] B. Kaykovieh, E. Zeldov, D. Majer, T.W. Li, P.H. Kes and M. Konczykowski, Phys. Rev. Lett. 76 (1996) 2555. [23] T.W. Li, P.H. Kes, W.T. Fu, A.A. Menovsky and J.J.M. Franse, Physica C 224 (1994) 110.
ELSEVIER
PHYSICA@ PhysicaC274 (1997) 197-203
Enhanced flux pinning in Bi-2212 single crystals by planar defects introduced via Ti-substitution T.W. Li a R.J. Drost a,* P.H. Kes a C. Tra~holt b, H.W. Zandbergen b N.T. Hien c, A.A. Menovsky c, J.J.M. Franse c a Kamerlingh Onnes Laboratory, Leiden University, Nieuwsteeg 18, P.O. Box 9506, 2300 RA Leiden, The Netherlands" b Laboratory of Materials Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands c Van der Waals-Zeeman Laboratory, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands
Received 31 August 1996; revised manuscript received 11 December t996
Abstract Ti-doped Bi-2212 single crystals were ~own by the travelling solvent floating zone technique. Concentrations of about 1 at% and 2 at% (with respect to Cu) were shown to be incorporated in the structure which leads to lowering of Tc to about 73 K from 85 K. High resolution microscopy revealed high densities of planar defects parallel to the a,c-planes. The flux pinning properties at elevated temperatures are significantly improved with respect to undoped single crystals. Keywords: Critical current density; Flux pinning; Substitution effects; Defect structure; Single crystal growth
1. Introduction One of the most promising high-T~ superconducting compounds from the point of view of applications is Bi2Sr2CaiCu2Os+ x (Bi-2212). It has been shown that the Bi-based cuprates have high critical current densities (J0) at 4.2 K, even in magnetic fields above 15 T [1]. Grain oriented B i - 2 2 1 2 / A g composite tapes were found to have Jo values above 105 A / c m 2 in magnetic fields up to 25 T [2]. However, the high (effective mass) anisotropy and the short coherence length considerably enhance the role of thermal fluctuations at elevated temperatures,
* Corresponding author. Fax: + 31 71 527 5404; e-mail: [email protected].
which leads to thermally activated flux motion. This magnetic relaxation process generates significant resistance even at T << Tc in a modest magnetic field. As a result, energy dissipation is present at finite current densities which seriously limits the number o f applications. Therefore, improvement of flux pinning in Bi-2212 is of great technological interest. With this aim, several methods have been attempted to introduce effective pinning centers. To our knowledge, this has not yet b e e n achieved so far in Bi-2212 through any type o f substitution. The only method to successfully introduce pinning centers is by irradiation of heavy ions [3-6], neutrons [7,8] or protons [9] of certain energy ranges to create microscopic defects in the sample. For instance, it has been reported [10] that the introduction o f columnar defects in Bi-2212 single crystals, by means
0921-4534/97/$17.00 Copyright (C) 1997 Elsevier Science B.V. All fights reserved. PII S0921-4534(96)00699-5
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T.W. Li et al./Physica C 274 (1997) 197-203
of heavy-ion irradiation, can overcome the detrimental effect of the extremely large anisotropy. At fields up to the dose-equivalent field B,~, the pancake vortices which are trapped at the columnar defects, behave as non-segmented flux lines. Since heavy ions can have only a small damage range in solid matter (less than 100 ~m), this technique thus imposes an upper limit to the thickness of current leads or tapes. Furthermore, the enormous costs of the irradiation makes this method infeasible for application. In this paper, we show that doping of Ti in Bi-2212 single crystals can create a high density of planar defects extended along the a,c-plane. These defects form effective pinning centers when the field is applied along the c-direction. We present various magnetization experiments and compare the results with the pinning in pure and heavy-ion irradiated Bi-2212 single crystals.
2. Experimental Single crystals of Bi-2212 doped with Ti were prepared using the travelling solvent floating zone (TSFZ) technique. A " w e t " preparation route from nitrates was used in order to achieve homogeneous starting materials. Powders of Bi203, SrCO 3, CaCO 3, CuO, and Ti (all 3N or purer) were weighted in the ratios of Bi:Sr:Ca:Cu:Ti = 2.05:1.95:1.0:(2.0 - x ) : x ( x = 0 . 0 4 , and x = 0 . 1 0 ) for the feed rods and Bi:Sr:Ca:Cu=2.6:l.9:l.0:2.6 for the solvent. These mixtures were dissolved in HNO 3. After a drying procedure, the mixtures were calcined in air at 810°C. The calcined powders were pressed into boules of 7 mm in diameter and 60 mm in length under hydrostatic pressure of 3.5 kbar. These rods were then sintered in a vertical hanging furnace at 840°C for 48 hrs in air. Prior to the crystal growth, the feed rods were first densified by passing through a melting zone at a velocity of 60 m m / h r . A Bi-2212 crystalline boule from a previous growth run was used as a seed. The growth was carried out with a rotation speed of 25 rpm and a growth speed of 0.26 m m / h r . The ambient atmosphere was air. We add here that the growth is less smooth than that of pure Bi-2212. In addition to X-ray diffraction, the as-grown Ti-substituted single crystals were checked for corn-
position, homogeneity and single crystallinity by Electron Probe Micro Analysis (EPMA). Powder X-ray diffraction data were collected with a Philips PW-1710 setup. The EPMA was carried out using a JEOL JXA8621. The microstructure was studied by high resolution electron microscopy (HREM), using a Philips CM30ST equipped with a field emission gun and a Link EDX elemental analysis system. AC susceptibility, magnetization and magnetic relaxation measurements, in the temperature range 2 120 K, were carried out using a commercial SQUID magnetometer (Quantum Design MPMS-5S). In all measurements, the DC and AC fields were applied parallel to the c-axis of the crystal and the scan length was set at 3 - 4 cm in order to suppress effects related to a small amount of magnetic field inhomogeneity. The procedure for the relaxation measurements was to cool the sample in zero field. The field was then ramped to 3 T in order to ensure a full penetration of the sample by a shielding current of uniform direction. Subsequently, the field was reduced to the respective measurement values and the decay of the magnetic moment with time was recorded.
3. Results and discussion
3.1. Crystal characterization Single crystalline boules of typically 5.5 mm in diameter and 45 mm in length were obtained. The as-grown single crystals exhibit a less shiny appearance than pure Bi-2212 single crystals. Furthermore, the cleaving of the crystals appears to be more difficult than the cleaving of pure Bi-2212 crystals. EPMA data were typically taken at five points on several single crystals. The averaged composition was normalized to a Ca-stoichiometry of unity. The uniformity was studied by comparing the compositions of a few single crystals cleaved from the top part of the as-grown boule. Within the accuracy of the EPMA apparatus, the measurement gave the same stoichiometry. The results are listed in Table 1, from which it is seen that the actual Ti to Cu ratios are 0.86 at% and 1.95 at% for x = 0.04 and 0.1, respectively. We will further denote these crystals as 1 at% and 2 at% Ti-doped Bi-2212. The data in
T.W. Li et al./Physica C 274 (1997) 197-203
199
Table 1 EPMA characterization of the 1 at% and 2 at% Ti-doped Bi-2212 single crystals Nominal feed composition
Bi2.05Srl.95CaCul.96Ti0.040 x (2 at% Ti)
Bi2.05SrL95CaCul.90Ti0.1oO x (5 at% Ti)
Bi Sr Ca Cu Ti
2.01 _+ 0.04 1.92 + 0.02 1 1.97 _ 0.03 0.017 + 0.008 ( = 0.86 at% Ti)
1.99 +__0.03 1.93 + 0.04 1 1.95 _~ 0.02 0.038 + 0.011 ( = 1.95 at% Ti)
Table 1 reveal that the actual Ti-content is much lower than the nominal composition for both the 1 at% and the 2 at% samples. This is consistent with the observation of some SrTiO 3 grains between Bi2212 platelets in the crystal boule, indicating that the actual doping level is limited to low values. X-ray powder diffraction was performed on some ground single crystals. The unit-cell parameters were calculated from the reflections in the X-ray diffraction spectra. Within experimental resolution, we can index the diffraction pattern with a tetragonal symmetry I 4 / m m m , analogous to the crystal symmetry of pure Bi-2212 [11]. The lattice parameters are not significantly different as those of pure, overdoped Bi-2212 crystals [12], namely a = 0.540(1) nm and c = 3.0838(3) nm. 3.2. Transition temperature
The transition temperature for our 1 and 2 at% Ti doped crystals are determined from AC-susceptibility measurements in an AC-field of 30 ~T and a frequency of 22 Hz. As shown in Fig. 1, Ti-doping results in a shift of Tc towards a lower temperature, typically from 85 to 73 K (midpoint). Similar downward shifts of T~ were observed in crystals doped with other 3d elements (for example: Fe, Co, Zn [11], V, Cr [13], Fe, Co [14] or Fe, Ni, Zn [15]), but in those cases Tc decreased continuously with increasing doping level. In our Ti-doped Bi-2212, there is no further decrease when the doping level is changed from 1 at% to 2 at%. 3.3. HREM studies
Fig. 2 reveals the presence of defects which consist of pairs of antiphase boundaries (APB's, indi-
cated by the black arrows in Fig. 2b) parallel to the a,c-plane. The defect can only be clearly seen when one views along the a-axis. At the antiphase boundaries the shift of the lattice is about 0.4 nm along the c-axis, being two (001) planes. This shift results in one CuO2-plane which continues through the antiphase boundary whereas the other CuO2-plane terminates to continue as a Bit-plane. Because the shift of the second antiphase boundary is opposite to the first, the structure is restored after the second antiphase boundary, resulting in atom positions equal to that of the matrix and having one CuO2-plane which is continuous throughout the whole defect. The distance between antiphase boundaries is equal to the vector of the modulation in the Bi-2212 structure which is about 4.5b [16]. This distance is comparable to the superconducting coherence length at low temperature. Therefore, it is expected that these defects may act as effective pinning centers when the field is applied parallel to the c-direction. 0.2 =~
0.0
[{{'[{([[[[i [ [ (111([[[~~11[[(1[[(1 [
-0.2 Ti-doped Bi-2212
[
{
[
~
d, ~}
-0.4
~t
~"
-0.6
A 2at. %
-~ -0.8 ~ . . q ~ . . . . . ~ -1.0
lat.%
3'0 ' 4'0 ' 5b ' 6'0 ' 7'0 ' 80 ' 9b ' 100 Temperature [K] Fig. 1. Zero-field AC-susceptibility measurements (30 IxT, 22 Hz) on as-grown 1 at% (open symbols) and 2 at% (closed symbols) Ti-doped Bi-2212 single crystals. The AC-field is parallel to the c-axis.
200
T.W. Li et a l . / P h y s i c a C 274 (1997) 197-203
a)
C
T
~b
b)
C
3.4. Magnetic measurements in the irreversible regime In order to study the effect of the planar defect structure on the flux pinning, we have carried out magnetization and AC-susceptibility measurements. We compare the results with those of as-grown and heavy-ion irradiated Bi-2212 single crystals. It is well documented that high energy heavy-ion irradiation creates amorphous tracks of about 7 nm in diameter and that these defects are optimal pinning centers for flux densities below the dose equivalent field. Our 2 at% sample contained too many macroscopic defects (mainly misorientations) to allow a reliable Bean analysis and therefore the results of this sample will not be considered in the calculation of critical current densities. In Fig. 3 we compare the critical current densities at 0.5 T obtained from SQUID magnetization measurements at similar sweep rates on a 1 at% Ti-doped sample with data of an as-grown Bi-2212 crystal [12] and of a heavy-ion irradiated Bi-2212 crystal (dose equivalent field is 0.5 T) measured by Prost et al. [17]. In all cases, the current densities were calculated from the widths of the magnetization loops at various temperatures using the Bean model for a cylindrical geometry. The magnetization curves of the Ti-doped sample are characterized by a smooth
~b Fig. 2. Low magnification (a) and high magnification (b) HREM images of 1 at% Ti-doped Bi-2212 along the a-direction, revealing the defect structure induced by the presence of Ti. The antiphase boundaries (APB's) are indicated by arrows. We refer to the accompanying paper [16] for more details.
Irradiated Bi-2212 v v v v ~ 10~
V []
.~o
We stress here that the defect structure solely arises as a consequence of the Ti substitution, because elemental analysis (EDX), using a small spotsize down to 2 nm, showed the sole presence of Ti in the vicinity of the anti-phase boundaries. To our knowledge, this is the first time that such planar defects have been introduced in Bi-2212 crystals by chemical doping. A more detailed analysis of this peculiar defect structure will be presented in a separate paper [16].
~toHa= 0.5T
v ~xz
< ~
•
V
A
V
lat.% Ti-Dope
m
105
A
PureBi-2212 D
5
'
1'0
'
1'5
'
Temperature [K] Fig. 3. Comparison of current densities obtained from the widths of magnetization loops for: 1 at% doped Bi-2212 ( • ) , as-grown Bi-2212 (D), and Bi-2212 with columnar defects (B a, = 0.5 T) created by Pb-irradiation ( v ) [17].
201
T.W. Li et al./Physica C 274 (1997) 197-203
penetration (at all temperatures) and the absence of a second peak in the magnetization [18]. It follows from Fig, 3 that the current carrying capability of the Ti-doped crystal is comparable to that of pure Bi2212 at low temperature. At higher temperatures however, the current density is significantly higher, e.g. a factor of roughly 4 at 15 K, but not as much as for the irradiated crystal. In order to compare the properties of the 2 at% Ti-doped crystals, it suits to determine the relative creep rate S ( T ) = - 1 / M o ( d M / d l n t). Here, M 0 denotes the initial magnetization at t = t o (in our case 150 s after the field was stopped). This quantity has been measured for fields between 0.05 and 2 T in a temperature range between 2 and 50 K. A typical result is shown in Fig. 4 for a field of 0.2 T. It is seen that the creep rate of the Ti-doped sample lies considerably below the rate of our pure Bi-2212 crystal and just above the result of Prost et al. [17] on a crystal containing columnar defects. Finally a comparison is made, for pure and Tidoped crystals, of the irreversibility lines (IL) determined by the peak in the out-of-phase component of the AC susceptibility in an AC field of 30 tzT and frequency 22 Hz superimposed on DC fields ranging between 500 mT and 5 T. The data in Fig. 5 show that the IL lines of the 1 at% and 2 at% Ti-doped crystals almost coincide. It is also seen that the doping increases the irreversibility fields at the same reduced temperature by a factor of 2-4. We note that these results should still be corrected for different
0.15 g0Hex = 0.2T zx "~ ,.-i
0.10
2at.% Ti-Doped A
0.05 Z~. A
•
A
~vV
v
v
•
•
•
V
v After Irradiation
0.00
5
10
ZX
•
A
1
~
2at.% Ti-doped •
ZX
~"
/4
lat.% Ti-doped
0.1
O oe O•
zx
• o []
=t. Pure Bi-2212
0.01
~a
e• A
0.001
0.2
0.4
0.6
0.8
1.0
T/Tc Fig. 5. Comparison of the irreversibility lines of 1 at% ( O ) and 2 at% ([]) Ti-doped Bi-2212 with pure Bi-2212 (zx) single crystals. The irreversibility lines (IL) are defined by the peak positions of the dissipative component X" of the AC susceptibility.
effective sample sizes, since our 2 at% Ti-doped sample shows some cracks due to the high density of defects. However, such a correction will always lead to a higher location of the IL. To give an idea how serious this effect is, we can estimate from a TAFF analysis [19] and taking into account the general observation that the IL is approximately determined by the condition U/kBT..~ 20-25 (U is the pinning energy and k B the Boltzmann constant), that a reduction in sample size by a factor of 10 leads to a small upward shift of the IL over 20%. Together with the current densities of Fig. 3 and the relative creep rates Shown in Fig. 4, this shift of the irreversibility line evidences the increase of bulk pinning in Ti-doped Bi-2212, as compared to undoped Bi-2212.
3.5. Magnetic measurements in the reversible regime
Pure Bi-2212 zx
"7,
10
15
20
Temperature [K] Fig. 4. Comparison of the temperature dependence of normalized relaxation rate S(T) between: 2 at% Ti-doped Bi-2212 ( O ) , pure(A) and irradiated ( v ) Bi-2122 single crystals (B,/, = 0.2 T) [17] at field 0.2 T parallel to the c-axis.
It has been recently shown [20] that the presence of columnar defects is reflected in the behaviour of the magnetization curves above the depinning temperature. Instead of a monotonous in(B) dependence, the magnetization in irradiated Bi-2212 crystals displays a minimum at the dose equivalent field. The reason for such a minimum follows from an expression for the free energy proposed by Van der Beek et al. [20a] who added two terms: one expressing the entropy gain related to the fluctuations of the unpinned vortices, the other one representing the pin-
202
T.W. Li et al./Physica C 274 (1997) 197-203
Oo ,~N2at~o/oTi-doped Bi-2212~.....
-8.0x102 ,-----,
x
3
-1.2x10
< -1.6xl03 £
Io
}¢ 65K
-2.0xl( 10 -2
10 -1
10 ° [T]
101
Fig. 6. The reversible magnetization as function of magnetic field at temperatures of 63 K (O), 65 K (El), 67 K ( v ) , and 70 K ( O ) for 2 at% Ti-doped Bi-2212 crystal. The dashed lines represent the average of the decreasing and increasing (see arrows) branches. A minimum occurs in all curves at approximately 0.3 T. The ins& shows the construction for the estimation of Up. More details on this construction are given in the text.
ning energy for vortices trapped at a columnar defect. Similar behaviour may be expected in samples with extended defects along the field direction (correlated disorder), such as the planar defects in our Ti-doped samples. The magnetizations, for temperatures close to Tc of the 2 at% Ti-doped crystal are plotted in Fig. 6. It is seen that the irreversibility sets in below about 20 mT. The reversible magnetization in this field regime was estimated by taking the average (indicated by the dotted line) of the increasing and decreasing branches, as denoted by the arrows. The overall behaviour of the reversible magnetization is very similar to that observed in ref. [20a], i.e. a minimum is observed at, in our case, a field of 0.3 T. Assuming that we may analyze our data as if the planar defects can be treated as columnar defects, we can roughly estimate the typical pinning energy per pancake vortex Up from the shape of the magnetization curves and the formalism given in [20a]: sgb0 A M = Up, where s is the interlayer distance and ~b0 denotes the flux quantum. The construction for our estimate of A M (by substraction of extrapolated values of M of the low and high field regime) is shown in the inset of the figure. The
value for Up (65 K) obtained from this construction is 70 -t- 35 K. The occurrence of the minimum in the reversible magnetization demonstrates the extra bulk pinning due to the presence of the planar defects created by Ti-doping. It is as yet not clear what the prevailing pinning mechanism is, because not much is known about the microscopic properties of the anti-phase boundaries. Since one of the CuO2-planes is continuous throughout the defect one may speculate that the anti-phase boundary is conducting, perhaps even superconducting. Nevertheless, some scattering of the charge carriers at the planar defect is to be expected which would give rise to gK or (8l) pinning [21]. Here, K is the Ginzburg-Landan parameter and l the elastic scattering length. However, a local depression of T~ at the defects is also possible which would give rise to STc pinning. Whatever the precise mechanism is, the planar defects improve the current carrying capability of Bi-2212. The price we have to pay is a slightly lower T~. Therefore our finding only seems to have possible consequences for low temperature applications.
3.6. Directions towards application With possible applications in mind, we have checked the stability of the planar defect structure on various heat treatments. We therefore annealed several Ti-doped crystals in N 2 and in air at temperatures between 500°C and 850°C (for 12 hours or longer). HREM images reveal that the defect structure is stable up to at least 850°C in air. Measurements of the transition temperature and the IL of various annealed Ti-doped Bi-2212 crystals, show that these quantities do not undergo any significant shift. This behaviour is completely different from that of pure Bi-2212 where the out-diffusion of oxygen changes parameters such as To, critical current, the pinning energy, and the position of the first order transition line in the H,T-diagram [22,23].
4. Conclusions
Ti has been partially incorporated in Bi-22i2 single crystals prepared by the TSFZ technique in
T.W. Li et al./Physica C 274 (1997) 197-203
concentrations of about 1 at% and 2 at%. HREM images of these crystals reveal a peculiar defect structure consisting of clusters of antiphase boundaries in the a,c-plane. The T~ of the Ti-doped crystals is about 10 K lower than Tc of undoped Bi-2212. Magnetization measurements reveal the effect of the planar defects on both the irreversible and the reversible regime, indicating the enhanced bulk pinning in the Ti-doped crystals, as compared to pure Bi-2212. Since chemical doping would provide an easy and cheap route to improve the pinning properties of this material, a continuous research program on ceramic material is carried out.
Acknowledgements The authors wish to thank Prof. P. Bongers for valuable discussions, and Ton Gortenmulder for EPMA measurements. This work is part of research program of the Amsterdam-Leiden materials research cooperation ( F O M - A L M O S ) which is financially supported by the Dutch Foundation for Fundamental Research on Matter (FOM).
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