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Strong influence of the oxygen content on the electrical and magnetic properties of Bi2Sr1.6La0.4CuOy
PHYSICA
Physica C 194 (1992) 337-342 North-Holland
Strong influence of the oxygen content on the electrical and magnetic properties of Bi2Srl.6Lao.aCuOy C. Allgeier a.b and J.J. N e u m e i e r a,b a Sektion Physik. Universitgit Miinchen, Schellingstr. 4, 14/-8000 Miinchen 40, German), b Walther Meiflner Institut, I4/-8046 Garching, Germany
J.S. Schilling Department of Physics, Washington University, Campus Box 1105, One Brookings Drive, St. Louis, MO 63130-4899, USA
Received 21 January 1992 Revised manuscript received 2 March 1992
The electrical resistivity and the static magnetic susceptibility of Bi2Sri.6Lao.4CuOywere studied as a function of the hole concentration by varying the oxygen content y. An increase of the superconducting transition temperature from 23.2 K to 30.6 K is observed when the oxygen content is reduced by only 0.5% from y=6.32 to y=6.29. Reduction of the oxygen content also results in a sizeable decrease in both the electrical conductivity and the magnetic susceptibility in the normal state. These results emphasize the sensitivity of the superconducting and normal state properties of Bi2Sr2_xLaxCuOyto the oxygen concentration.
1. Introduction An intriguing aspect o f high temperature superconductivity is that all high temperature superconductors can be m a p p e d onto a similar phase diagram [1]. This phase diagram begins with an antiferromagnetic insulator, whereby the addition of holes (or electrons) through chemical substitution leads to a metal which becomes superconducting below T~. Further doping leads to an enhancement o f the metallic state, but to the ultimate destruction o f superconductivity. The value o f T~ is strongly influenced by the hole content, although disorder effects can also affect Tc as well as vary the normal state properties [2,3 ]. Within the family o f high temperature superconductors, different regions o f the phase diagram are mapped out [ 4 - 8 ] . For example, in the YBa2Cu30~, system a decrease o f the hole concentration via substitution o f La 3+ for Ba 2+ leads first to an increase o f T~ followed by a decrease [ 9 ]. Further reduction o f the hole concentration, through the removal o f oxygen, results in a continuous decrease in Tc and a gradual reduction o f metallicity [5], which finally culminates in an antiferromagnetic in-
sulating state. On the high hole side of the phase diagram, increased hole doping via substitution of Ca 2÷ for y3+ results in a continuous reduction of Tc [ I 0 ]. Unfortunately, YBazCu3Oy and all other high-To systems [ 4 - 8 ] , except for La2_xMxCuO4 (where M = Ba or Sr) [ 11 ], are unable to span the entire phase diagram because o f restrictions due to the chemistry of these complicated compounds. It would be clearly desirable to have a second simple model system, like Laz_xM,.CuO4, where the entire phase diagram can be observed through the variation of one chemical constituent. One possible candidate for such a model system is Bi2Sr2_xLaxCuO.~,. This c o m p o u n d has, like La2_xM~CuO4, only one CuO2 layer per formula unit, thus limiting complications due to interactions between multiple CuO2 layers or between CuO2 layers and CuO chains. This system reportedly [12] exhibits an antiferromagnetically ordered state for x = l . 0 with an insulator-metal transition and a m a x i m u m in Tc as x is reduced (i.e. the hole content is increased). Some uncertainty exists regarding the m a x i m u m value of T~ and the hole content at which
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.
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c. Allgeier et al. / Electrical and magnetic properties Qf Bi 2Srl.nLao.~CuQ.
it occurs [ 12-14]. This uncertainty can be isolated to two possibilities: ( I ) difficulties in sample preparation, and (2) possible variations in the oxygen concentration which was not studied in the experiments in refs. [ 12-14 ]. In a recent publication, Sales et al. [ 15 ] investigated the influence of the oxygen content on To. In this report we concentrate on the preparation of high quality samples with a fixed lanthanum content of x=0.4; we discuss the results of experiments which reveal the influence of the oxygen content on To, the electrical resistivity, and the magnetic susceptibility.
2. Experimental A polycrystalline sample of nominal composition Bi2Srl.6Lao.aCuOy was prepared from Bi203, SrO, La203 and CuO with minimum purity levels of 99.98%. The starting materials were weighed in the appropriate amounts, mixed with an agate mortar and pestle, and reacted in A1203 crucibles. The mixture was calcinated at 850°C for one day. After the first reaction, the sample was reground and reacted at 860°C for two days with one intermediate grinding, then reground and reacted for a further day at 865 ° C. Finally, the specimen was reground, pressed into a pellet, and reacted for 40 h at 870°C, 12 h at 890°C, and then slow cooled to 30°C over a period of 6 h. A piece of the resulting sample was again fired at 890°C for one day and rapidly quenched by placing the specimen quickly on a metal plate. Structural studies carried out with standard X-ray powder diffractometry confirmed that the specimens are single phase. The diffraction patterns were measured repeatedly over a period of 6 months and found to be unchanged, thereby indicating that the specimens were stable. Using an orthorhombic unit cell [ 16] with 18 of the observed peaks, we obtained, for the slow-cooled sample, values of a = 5 . 3 8 4 ( 4 ) A, b = 5 . 4 0 7 ( 4 ) A and c = 2 4 . 4 9 1 ( 1 7 ) A. The oxygen content of the slow-cooled sample was determined by iodometric titration [ 17 ] to be y = 6.32 _+0.02 under the assumption that the valencies in the presence of excess I - in acidic solution for Bi (La), Sr, Cu and O are +3, +2, +1 and - 2 , respectively. A controlled reduction in the oxygen content of this sam-
pie was carried out in a Faraday magnetometer described elsewhere [ 18 ] where heating the sample in high vacuum drives off oxygen, leading to a measurable weight loss. A mass spectrometer was used to assure that the observed weight loss could be attributed to the removal of oxygen. The change in oxygen content was measured to an accuracy of better than Ay= + 0.005. This method has the advantage that the static magnetic susceptibility in the normal and superconducting states can be measured in-situ for different oxygen contents. After the measurements were completed, the sample was removed from the magnetometer and a powder X-ray pattern was taken at room temperature; no alteration of the crystal structure or additional peaks from an impurity phase were observed. The changes in the lattice parameters are given below. Electrical resistivity measurements were carried out with a standard four-point DC method with typical current densities of ~ 200 m A / c m 2. For the measurement of the Meissner effect, we determined the volume of the cylindrical sample with a microscope and corrected for demagnetization effects using the tables of Crabtree [ 19 ].
3. Results and discussion With the Faraday system we were able to reduce the oxygen content of Bi2Sr~.6Lao.4CuOv from y = 6.32 to 6.26. Further reduction was not possible as indicated by the absence of further degasing of the specimen at our highest attainable temperature of 630°C. The reduction of the oxygen content by Ay=0.06 led to small increases in the c-axis by +0.29% to c = 2 4 . 5 6 3 ( 1 8 ) A, in the a-axis by +0.29% to a = 5 . 4 0 0 ( 4 ) , and in the b-axis by +0.25% to b = 5 . 4 2 0 ( 4 ) A. The relative change in the c-axis per change in oxygen content, d l n c / dy=4.9%, is comparable to the values found for the double-layer system Bi2Sr2CaCu20,,, where d l n c / d y = 4 % [7]. In fig. 1 we show measurements of the Meissner effect for different oxygen contents. The reduction from y = 6 . 3 2 to 6.29 leads to a marked increase in the temperature of the onset of flux expulsion from 23.2 K to 30.6 K. This is the highest transition temperature thus far reported for the single-layer Bi system. This increase in Tc is accompanied by an in-
C. Allgeier et al. / Electrical and magnetic properties of BieSr~.6Lao.4CuOy .
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Fig. I. Static magnetic susceptibility (Meissner effect) of Bi2Sr~.6Lao.4CuOr as a function of the oxygen content y vs. temperature measured in a field of 2 mT. The susceptibility is given in cgs units. Note that 4nX=-I corresponds to ideal diamagnetism. crease in the superconducting volume fraction from 14% to 18% o f ideal diamagnetism. Further reduction o f the oxygen content to y = 6.26 results in both a rapid decrease of the transition temperature as well as a reduction of the Meissner fraction to a value below 5%. In fact, for this oxygen concentration the Meissner effect does not saturate even at 4 K. As mentioned above, powder X-ray diffraction gave no sign that the removal o f oxygen resulted in the degradation of this sample. These changes of the transition temperature with oxygen content are in qualitative agreement with the results o f Sales et al. [ 15 ] who determined Tc using an AC susceptibility technique, although our m a x i m u m transition temperature is considerably higher. In fig. 2 the influence o f the oxygen content on the electrical resistivity is presented. The slow-cooled sample with y = 6.32 shows a resistive transition to the superconducting state with Tmid=24.6 K. The width of the resistive transition is relatively broad, A T c ( 1 0 % - 9 0 % ) = 6 . 8 K, although somewhat narrower than that o f the Meissner transition in a field of 2 m T (see fig. 1 ). Although we attempted various heat treatments, we were not able to obtain resistive transition widths less than 5 K. Other studies [ 1215 ] yield similar results, thus implying that the large transition width may be an intrinsic problem associated with this system and possibly caused by inhomogeneities in the oxygen content a n d / o r distor-
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Fig. 2. Logarithm of the resistivity of Bi2SrL6Lao.4CuO~,as a function of the temperature for three oxygen contents. tions of the crystal lattice. Partial substitution of Bi with Pb seems to remove the incommensurable superstructure o f the crystal lattice and improve the transition width remarkably [ 20 ]. Rapid quenching o f the specimen, as described in the previous section, results in an increase o f the transition temperature to T ~ d = 2 9 . 0 K as displayed in fig. 2. Unfortunately, we could not measure the oxygen content of the quenched specimen due to an insufficient quantity o f specimen although we note that a similarly prepared specimen possessed an oxygen content o f y = 6 . 3 0 + 0 . 0 2 [21]. In addition, comparison with the Meissner effect data o f fig. 1, and the fact that an increase in preparation temperature resulted in a decrease in oxygen content, followed by a subsequent increase o f Tc, clearly allows the conclusion that the quenching procedure results in a lower oxygen content. We also note that a sample which was heated for 18 h at 500°C followed by slow cooling in argon yielded a similar value of T mid = 2 9 . 2 K. The resistivity o f the normal state at 295 K increases from 1.8 mf~ cm for the slow-cooled sample to 2.8 m f ~ c m for the quenched sample; thus the removal of oxygen leads to a reduction of the metallicity o f the sample, as expected when the carrier concentration is reduced. Further reduction of the oxygen content to y = 6.26 leads to a reversal from metallic to non-metallic behavior and an increase o f p ( 2 9 6 K) by almost two orders o f magnitude. As a final observation, we note that the value of p.(295 K ) / p ( 50 K) is somewhat higher for the slow-cooled
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C. Allgeier et al. /Electrical and magnetic properties of BieSrt.6Lao.4CuOy
sample (2.8) than for the rapidly quenched sample (2.3). We believe that these changes in the properties of Bi2SrL6Lao.4CuOy arise from the influence of the oxygen concentration on the hole content. The sample with high oxygen content ( y = 6 . 3 2 ) is situated on the right (high-hole) side of the T : m a x i m u m . Thus T~ does not possess its optimum value in the slowcooled specimen. Reducing the oxygen content is synonymous with reducing the hole concentration. Thus in the phase diagram one moves in the direction of the insulating state, the sample becoming less metallic, and reaching the hole concentration where the transition temperature T¢ takes on its maximum value. Further reduction to y = 6 . 2 6 leads to semiconducting behavior with no full transition to superconductivity observable in the electrical resistivity, as seen in fig. 2. The small Meissner signal observed in fig. 1 and the drop in resistivity seen in fig. 2 are probably due to a somewhat inhomogeneous oxygen distribution resulting in a small number of tiny islands of superconducting material. The temperature dependence of the normal-state magnetic susceptibility at different oxygen concentrations, shown in fig. 3, agrees well with the behavior of several other high temperature superconductors, such as the Bi2Sr2CaCu2Os +y system [ 7,11,22 ], which exhibit a correlation between the slope of the magnetic susceptibility d z / d T and the magnitude of the transition temperature T¢. Here as there, for the sample with the highest oxygen content (also the 1.5 .:~
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highest hole content), the slope d z / d T is negative, resembling a weak Curie-Weiss behavior. As one approaches the hole content where Tc has its optimal value (near y = 6.29 for Bi2Srl.6La0.4CuOy), the slope of the susceptibility changes its sign, exhibiting no indication of Curie-Weiss behavior. Further reduction of the hole concentration to y = 6.26 results in a specimen which displays a positive value of the slope d z / d T for temperatures above 100 K, with a Curie tail at low temperatures. We cannot exclude the possibility that this Curie tail is caused by a small amount of impurity phase arising from the oxygen reduction, since the amount necessary for such a small increase, corresponding to 0.4% of spin S = ½ magnetic moments, is far below the limit of X-ray detection. We note that the susceptibilities of both La2_xSrxCuO4 [23] and YBaECU306+y [5] display Curie tails in the semiconducting region of the phase diagram. The decrease in the value of the susceptibility as the oxygen content is reduced is another similarity of the data in fig. 3 to those of other high temperature superconductors. We believe this to be caused by the change in the Pauli susceptibility of the charge carriers, whose concentration decreases as the oxygen content is lowered. This would be consistent with the results of the resistivity measurements. For the BiaSrl.6Lao.4CuO6.29 sample we deduce the magnitude of the Pauli susceptibility using the relation. •=Zcore "~-×van Vleck -]-)~spin ,
( 1)
where Z¢ore=- 1.65× 10 -4 cm3/mole can be estimated from tabulated values [ 24 ]. The Landau diamagnetism can be neglected here because it is at least one order of magnitude lower than the Pauli paramagnetism [25 ]. Since a unit cell of Bi2Srl.6Lao.4CuOy contains only one CuO2 layer, we use for the Van Vleck paramagnetism the value Zva, v~eck= + 0 . 4 3 × l0 -4 cm3/mole estimated for YBa2Cu307 per CuO2 layer [26 ]. This value should be a good first approximation because of the similarity in the environment of the CuO2 layers in all high-T¢ superconductors [27]. With these assumptions, we obtain Xspi,= + 1.Sl × 10 -4 cm3/mole using the room temperature value of Z; similarly we obtain Zspi,= + 1.21X 10 -4 cm3/mole, if we use the value of g at 50 K. The question remains as to the origin of the temperature dependence. If the spin
C. Allgeier et al. / Electrical and magnetic properties of Bi~r l.~Lao.4CuOy susceptibility can be d e c o m p o s e d into two independent parts, the Pauli susceptibility ZPauli, which we assume t e m p e r a t u r e i n d e p e n d e n t , a n d a t e m p e r a ture d e p e n d e n t part from residual t w o - d i m e n s i o n a l a n t i f e r r o m a g n e t i c correlations Xzo [28,29], then Zspin=ZPauli"[-Z2v(T)- The value o f the d e d u c e d spin susceptibility can also be regarded as the Pauli susceptibility Zpau~i=Zspi, if the spin susceptibility consists o f only one c o m p o n e n t [30,31 ]. Although positive values o f the slope d z / d T can be u n d e r s t o o d within the f r a m e w o r k o f the antiferromagneticF e r m i - l i q u i d theory [32,33], this theory is not yet able to explain negative slopes in the susceptibility o f samples with high oxygen (high h o l e ) concentrations. A recent e v a l u a t i o n o f the m a g n e t i c susceptibility by O d a et al. in the La2_xMxCuO4 systems [ 11] notes that the C u r i e - W e i s s tail f o u n d on the high hole side o f the peak in Tc versus hole concentration indicates the existence o f localized magnetic m o m e n t s which can be a t t r i b u t e d to the c o p p e r ions. They speculate that these magnetic m o m e n t s m a y be responsible for the r e d u c t i o n o f Tc with increasing hole c o n c e n t r a t i o n due to magnetic p a i r breaking o f the s u p e r c o n d u c t i n g electron pairs. The a p p e a r a n c e o f a C u r i e - W e i s s tail on the high hole side o f the Tc peak is d i s p l a y e d in this work, a n d is also seen in Bi2SrzCaCuzOa+y a n d TlzBa2CuO6+y [7], suggesting that m a g n e t i c pair breaking m a y also be occurring in these systems. F u r t h e r e x p e r i m e n t s on specimens with various La substitution levels will be n e e d e d to investigate this possibility in the singlelayer b i s m u t h system. To s u m m a r i z e , we find in o u r e x p e r i m e n t s that the oxygen content plays an i m p o r t a n t role not only for the superconducting, but also for the n o r m a l - s t a t e p r o p e r t i e s o f the Bi2Sr2_xLaxCuOy system. T h e position a n d height o f the m a x i m u m Tc value is a sensitive function o f both the l a n t h a n u m a n d oxygen concentrations. The present m e a s u r e m e n t s o f the electrical resistivity and magnetic susceptibility yield evidence for the direct influence o f the oxygen content on the hole c o n c e n t r a t i o n in this system. I f Bi2Sr2_xLaxCuOy is to b e c o m e a n o t h e r m o d e l syst e m like La2_xMxCuO4, it will be necessary to accurately control the oxygen c o n c e n t r a t i o n as a function o f l a n t h a n u m content.
341
Acknowledgements We t h a n k A n d r e a s Schlrgl for i n f o r m a t i v e discussions. This work was s u p p o r t e d in part by the Bund e s m i n i s t e r i u m fiir Forschung a n d Technology und e r grant n u m b e r 13N5490-7.
References [1 ] J.B. Torrance, A. Bezinge, A.I. Nazzal and S.S.P. Parkin, Physica C 162-164 (1989) 291. [2] H. Claus, S. Yang, A.P. Paulikas, J.W. Downey and B.W. Veal, Physica C 171 (1990) 205, and references therein. [3 ] J.J. Neumeier, C. Allgeier, J.S. Schilling and A.K. Klehe, in preparation. [4 ] J.M. Tranquada, A.H. Mouden, A.I. Goldmann, P. Zolliker, D.E. Cox, G. Shirane, S.K. Sinha, D. Vaknin, D.C. Johnston, M.S. Alvarez, A.J. Jacobson, J.T. Lewandowski and J.M. Newsam, Phys. Rev. B 38 (1988) 2477. [ 5 ] Y. Nakazawa and M. lshikawa, Physica C 158 ( 1989 ) 381. [ 6 ] Y. Shimakawa, Y. Kubo, T. Manakom and H. Igarashi, Phys. Rev. B 40 (1989) 11400. [ 7 ] C. Allgeier and J.S. Schilling, Physica C 168 (1990) 499. [8] W.A. Groen and D.M. de Leeuw, Physica C 159 (1989) 417. [ 9 ] J.J. Neumeier, in preparation. [10]J.J. Neumeier, T. Bjornholm, M.B. Maple and Ivan K. Schuller, Phys. Rev. Lett. 63 (1990) 2516, and references therein. [ I 1] M. Oda, T. Nakano, Y. Kamada and M. Ido, Physica C 183 ( 1991 ) 234, and references therein. [ 12 ] A. Maeda, M. Hase, I. Tsukada, K. Noda, S. Takebayashi and K. Uchinikura, Phys. Rev. B 41 (1990) 6418. [ 13 ] W. Bauhofer, H. Mattausch, R.K. Kremer, P. Murugaraj and A. Simon, Phys. Rev. B 39 (1989) 7244. [ 14] W.A. Groen, D.M. de Leeuw and G.M. Stollman, Solid State Commun. 72 (1989) 697. [ 15 ] B.C. Sales and B.C. Chakoumakus, Phys. Rev. B 43 ( 1991 ) 12994. [16]C.C. Torardi, M.A. Subramanian, J.C. Calabrese, J. Gopalakrishnan, E.M. McCarron, K.H. Morrissey, T.R. Askew, R,B. Flippen, U. Chowdhry and A.W. Sleight, Phys. Rev. B 38 (1988) 225. [ 17 ] A.I. Nazzal, V.Y. Lee, E.M. Engler, R.D. Jacowilz, Y. Tokura and J.B. Torrance, Physica C 153-155 ( 1988 ) 1367. [18]C. Allgeier, R. Sieburger, H. GoBner, J. Diederichs, H. Neumaier, W. Reith, P. MiJller and J.S. Schilling, Physiea C 162 (1989) 741. [ 19] G.W. Crabtree, Phys. Rev. B 16 (1977) 1117. [20] V. Manivannan, N.Y. Vasanthacharya and J. Gopalakrishnan, Mater. Res. Bull. 26 (1991) 349. [ 21 ] A. Schlrgl, C. Allgeier, J.J. Neumeier, J. Diederichs and J.S. Schilling, in preparation.
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[22] D.C. Johnston, S.K. Sinha, A.J. Jacobsen and J.M. Newsam, Physica C 153-155 (1988) 572. [23] H. Takagi, T. ldo, S. lshibashi, M. Uota, S. Uchida and Y. Tokura, Phys. Rev. B 40 (1989) 2254. [24] K.-H. Hellwege and A.M. Hellwege, Landolt-B6rnstein New Series II/16, (Springer, Heidelberg, 1986) p. 402. [25] C. Allgeier, Doctoral Thesis, Sektion Physik, Universit/it Miinchen ( 1991 ); C. Allgeier and J.S. Schilling, in preparation. [26] M. Takigawa, P.C. Hammel, R.H. Heffner, Z. Fisk, J.L. Smith and R.B. Schwarz, Phys. Rev. B 39 (1989) 300; M. Takigawa, P.C. Hammel, R.H. Heffner, Z. Fisk, K.C. Ott and J.D. Thompson, Phys. Rev. Lett. 63 (1989) 1865.
[27] W.C. Lee and D.C. Johnston, Phys. Rev. B 41 (1990) 1904. [28] V.J. Emery, Phys. Rev. Lett. 58 (1987) 2794. [29] D.C. Johnston, Phys. Rev. Lett. 62 (1989) 957. [30 ] F.C. Zhang and T.M. Rice, Phys. Rev. B 37 ( 1988 ) 9423. [31 ] R.E. Walstedt and W.W. Warren, Science 248 (1990) 1082. [ 32 ] A.J. Millis, H. Monien and D. Pines, Phys. Rev. B 42 (1990) 167. [33] H. Monien, D. Pines and M. Takigawa, Phys. Rev. B 43 (1991) 258; H. Monien, P. Monthoux and D. Pines, Phys. Rev. B 43 ( 1991 ) 275.
Physica C 194 (1992) 337-342 North-Holland
Strong influence of the oxygen content on the electrical and magnetic properties of Bi2Srl.6Lao.aCuOy C. Allgeier a.b and J.J. N e u m e i e r a,b a Sektion Physik. Universitgit Miinchen, Schellingstr. 4, 14/-8000 Miinchen 40, German), b Walther Meiflner Institut, I4/-8046 Garching, Germany
J.S. Schilling Department of Physics, Washington University, Campus Box 1105, One Brookings Drive, St. Louis, MO 63130-4899, USA
Received 21 January 1992 Revised manuscript received 2 March 1992
The electrical resistivity and the static magnetic susceptibility of Bi2Sri.6Lao.4CuOywere studied as a function of the hole concentration by varying the oxygen content y. An increase of the superconducting transition temperature from 23.2 K to 30.6 K is observed when the oxygen content is reduced by only 0.5% from y=6.32 to y=6.29. Reduction of the oxygen content also results in a sizeable decrease in both the electrical conductivity and the magnetic susceptibility in the normal state. These results emphasize the sensitivity of the superconducting and normal state properties of Bi2Sr2_xLaxCuOyto the oxygen concentration.
1. Introduction An intriguing aspect o f high temperature superconductivity is that all high temperature superconductors can be m a p p e d onto a similar phase diagram [1]. This phase diagram begins with an antiferromagnetic insulator, whereby the addition of holes (or electrons) through chemical substitution leads to a metal which becomes superconducting below T~. Further doping leads to an enhancement o f the metallic state, but to the ultimate destruction o f superconductivity. The value o f T~ is strongly influenced by the hole content, although disorder effects can also affect Tc as well as vary the normal state properties [2,3 ]. Within the family o f high temperature superconductors, different regions o f the phase diagram are mapped out [ 4 - 8 ] . For example, in the YBa2Cu30~, system a decrease o f the hole concentration via substitution o f La 3+ for Ba 2+ leads first to an increase o f T~ followed by a decrease [ 9 ]. Further reduction o f the hole concentration, through the removal o f oxygen, results in a continuous decrease in Tc and a gradual reduction o f metallicity [5], which finally culminates in an antiferromagnetic in-
sulating state. On the high hole side of the phase diagram, increased hole doping via substitution of Ca 2÷ for y3+ results in a continuous reduction of Tc [ I 0 ]. Unfortunately, YBazCu3Oy and all other high-To systems [ 4 - 8 ] , except for La2_xMxCuO4 (where M = Ba or Sr) [ 11 ], are unable to span the entire phase diagram because o f restrictions due to the chemistry of these complicated compounds. It would be clearly desirable to have a second simple model system, like Laz_xM,.CuO4, where the entire phase diagram can be observed through the variation of one chemical constituent. One possible candidate for such a model system is Bi2Sr2_xLaxCuO.~,. This c o m p o u n d has, like La2_xM~CuO4, only one CuO2 layer per formula unit, thus limiting complications due to interactions between multiple CuO2 layers or between CuO2 layers and CuO chains. This system reportedly [12] exhibits an antiferromagnetically ordered state for x = l . 0 with an insulator-metal transition and a m a x i m u m in Tc as x is reduced (i.e. the hole content is increased). Some uncertainty exists regarding the m a x i m u m value of T~ and the hole content at which
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.
338
c. Allgeier et al. / Electrical and magnetic properties Qf Bi 2Srl.nLao.~CuQ.
it occurs [ 12-14]. This uncertainty can be isolated to two possibilities: ( I ) difficulties in sample preparation, and (2) possible variations in the oxygen concentration which was not studied in the experiments in refs. [ 12-14 ]. In a recent publication, Sales et al. [ 15 ] investigated the influence of the oxygen content on To. In this report we concentrate on the preparation of high quality samples with a fixed lanthanum content of x=0.4; we discuss the results of experiments which reveal the influence of the oxygen content on To, the electrical resistivity, and the magnetic susceptibility.
2. Experimental A polycrystalline sample of nominal composition Bi2Srl.6Lao.aCuOy was prepared from Bi203, SrO, La203 and CuO with minimum purity levels of 99.98%. The starting materials were weighed in the appropriate amounts, mixed with an agate mortar and pestle, and reacted in A1203 crucibles. The mixture was calcinated at 850°C for one day. After the first reaction, the sample was reground and reacted at 860°C for two days with one intermediate grinding, then reground and reacted for a further day at 865 ° C. Finally, the specimen was reground, pressed into a pellet, and reacted for 40 h at 870°C, 12 h at 890°C, and then slow cooled to 30°C over a period of 6 h. A piece of the resulting sample was again fired at 890°C for one day and rapidly quenched by placing the specimen quickly on a metal plate. Structural studies carried out with standard X-ray powder diffractometry confirmed that the specimens are single phase. The diffraction patterns were measured repeatedly over a period of 6 months and found to be unchanged, thereby indicating that the specimens were stable. Using an orthorhombic unit cell [ 16] with 18 of the observed peaks, we obtained, for the slow-cooled sample, values of a = 5 . 3 8 4 ( 4 ) A, b = 5 . 4 0 7 ( 4 ) A and c = 2 4 . 4 9 1 ( 1 7 ) A. The oxygen content of the slow-cooled sample was determined by iodometric titration [ 17 ] to be y = 6.32 _+0.02 under the assumption that the valencies in the presence of excess I - in acidic solution for Bi (La), Sr, Cu and O are +3, +2, +1 and - 2 , respectively. A controlled reduction in the oxygen content of this sam-
pie was carried out in a Faraday magnetometer described elsewhere [ 18 ] where heating the sample in high vacuum drives off oxygen, leading to a measurable weight loss. A mass spectrometer was used to assure that the observed weight loss could be attributed to the removal of oxygen. The change in oxygen content was measured to an accuracy of better than Ay= + 0.005. This method has the advantage that the static magnetic susceptibility in the normal and superconducting states can be measured in-situ for different oxygen contents. After the measurements were completed, the sample was removed from the magnetometer and a powder X-ray pattern was taken at room temperature; no alteration of the crystal structure or additional peaks from an impurity phase were observed. The changes in the lattice parameters are given below. Electrical resistivity measurements were carried out with a standard four-point DC method with typical current densities of ~ 200 m A / c m 2. For the measurement of the Meissner effect, we determined the volume of the cylindrical sample with a microscope and corrected for demagnetization effects using the tables of Crabtree [ 19 ].
3. Results and discussion With the Faraday system we were able to reduce the oxygen content of Bi2Sr~.6Lao.4CuOv from y = 6.32 to 6.26. Further reduction was not possible as indicated by the absence of further degasing of the specimen at our highest attainable temperature of 630°C. The reduction of the oxygen content by Ay=0.06 led to small increases in the c-axis by +0.29% to c = 2 4 . 5 6 3 ( 1 8 ) A, in the a-axis by +0.29% to a = 5 . 4 0 0 ( 4 ) , and in the b-axis by +0.25% to b = 5 . 4 2 0 ( 4 ) A. The relative change in the c-axis per change in oxygen content, d l n c / dy=4.9%, is comparable to the values found for the double-layer system Bi2Sr2CaCu20,,, where d l n c / d y = 4 % [7]. In fig. 1 we show measurements of the Meissner effect for different oxygen contents. The reduction from y = 6 . 3 2 to 6.29 leads to a marked increase in the temperature of the onset of flux expulsion from 23.2 K to 30.6 K. This is the highest transition temperature thus far reported for the single-layer Bi system. This increase in Tc is accompanied by an in-
C. Allgeier et al. / Electrical and magnetic properties of BieSr~.6Lao.4CuOy .
.
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Bi2Srt.6LaoACuOv 0.0
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. . . . . . . . . . . . . . . . . 10 20 30 40 Temperature (K)
Fig. I. Static magnetic susceptibility (Meissner effect) of Bi2Sr~.6Lao.4CuOr as a function of the oxygen content y vs. temperature measured in a field of 2 mT. The susceptibility is given in cgs units. Note that 4nX=-I corresponds to ideal diamagnetism. crease in the superconducting volume fraction from 14% to 18% o f ideal diamagnetism. Further reduction o f the oxygen content to y = 6.26 results in both a rapid decrease of the transition temperature as well as a reduction of the Meissner fraction to a value below 5%. In fact, for this oxygen concentration the Meissner effect does not saturate even at 4 K. As mentioned above, powder X-ray diffraction gave no sign that the removal o f oxygen resulted in the degradation of this sample. These changes of the transition temperature with oxygen content are in qualitative agreement with the results o f Sales et al. [ 15 ] who determined Tc using an AC susceptibility technique, although our m a x i m u m transition temperature is considerably higher. In fig. 2 the influence o f the oxygen content on the electrical resistivity is presented. The slow-cooled sample with y = 6.32 shows a resistive transition to the superconducting state with Tmid=24.6 K. The width of the resistive transition is relatively broad, A T c ( 1 0 % - 9 0 % ) = 6 . 8 K, although somewhat narrower than that o f the Meissner transition in a field of 2 m T (see fig. 1 ). Although we attempted various heat treatments, we were not able to obtain resistive transition widths less than 5 K. Other studies [ 1215 ] yield similar results, thus implying that the large transition width may be an intrinsic problem associated with this system and possibly caused by inhomogeneities in the oxygen content a n d / o r distor-
0.1
§Q
100 150 200 Temperature (K)
........
250
300
Fig. 2. Logarithm of the resistivity of Bi2SrL6Lao.4CuO~,as a function of the temperature for three oxygen contents. tions of the crystal lattice. Partial substitution of Bi with Pb seems to remove the incommensurable superstructure o f the crystal lattice and improve the transition width remarkably [ 20 ]. Rapid quenching o f the specimen, as described in the previous section, results in an increase o f the transition temperature to T ~ d = 2 9 . 0 K as displayed in fig. 2. Unfortunately, we could not measure the oxygen content of the quenched specimen due to an insufficient quantity o f specimen although we note that a similarly prepared specimen possessed an oxygen content o f y = 6 . 3 0 + 0 . 0 2 [21]. In addition, comparison with the Meissner effect data o f fig. 1, and the fact that an increase in preparation temperature resulted in a decrease in oxygen content, followed by a subsequent increase o f Tc, clearly allows the conclusion that the quenching procedure results in a lower oxygen content. We also note that a sample which was heated for 18 h at 500°C followed by slow cooling in argon yielded a similar value of T mid = 2 9 . 2 K. The resistivity o f the normal state at 295 K increases from 1.8 mf~ cm for the slow-cooled sample to 2.8 m f ~ c m for the quenched sample; thus the removal of oxygen leads to a reduction of the metallicity o f the sample, as expected when the carrier concentration is reduced. Further reduction of the oxygen content to y = 6.26 leads to a reversal from metallic to non-metallic behavior and an increase o f p ( 2 9 6 K) by almost two orders o f magnitude. As a final observation, we note that the value of p.(295 K ) / p ( 50 K) is somewhat higher for the slow-cooled
340
C. Allgeier et al. /Electrical and magnetic properties of BieSrt.6Lao.4CuOy
sample (2.8) than for the rapidly quenched sample (2.3). We believe that these changes in the properties of Bi2SrL6Lao.4CuOy arise from the influence of the oxygen concentration on the hole content. The sample with high oxygen content ( y = 6 . 3 2 ) is situated on the right (high-hole) side of the T : m a x i m u m . Thus T~ does not possess its optimum value in the slowcooled specimen. Reducing the oxygen content is synonymous with reducing the hole concentration. Thus in the phase diagram one moves in the direction of the insulating state, the sample becoming less metallic, and reaching the hole concentration where the transition temperature T¢ takes on its maximum value. Further reduction to y = 6 . 2 6 leads to semiconducting behavior with no full transition to superconductivity observable in the electrical resistivity, as seen in fig. 2. The small Meissner signal observed in fig. 1 and the drop in resistivity seen in fig. 2 are probably due to a somewhat inhomogeneous oxygen distribution resulting in a small number of tiny islands of superconducting material. The temperature dependence of the normal-state magnetic susceptibility at different oxygen concentrations, shown in fig. 3, agrees well with the behavior of several other high temperature superconductors, such as the Bi2Sr2CaCu2Os +y system [ 7,11,22 ], which exhibit a correlation between the slope of the magnetic susceptibility d z / d T and the magnitude of the transition temperature T¢. Here as there, for the sample with the highest oxygen content (also the 1.5 .:~
Bi2Srl.6Lao.4CuOy
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0
100
200 Temperature (K)
300
Fig. 3. The magnetic susceptibility of the normal state of Bi2Srt.rLao.4CuOy, given in cgs units vs. temperature measured in a magnetic field of 5.7 T.
highest hole content), the slope d z / d T is negative, resembling a weak Curie-Weiss behavior. As one approaches the hole content where Tc has its optimal value (near y = 6.29 for Bi2Srl.6La0.4CuOy), the slope of the susceptibility changes its sign, exhibiting no indication of Curie-Weiss behavior. Further reduction of the hole concentration to y = 6.26 results in a specimen which displays a positive value of the slope d z / d T for temperatures above 100 K, with a Curie tail at low temperatures. We cannot exclude the possibility that this Curie tail is caused by a small amount of impurity phase arising from the oxygen reduction, since the amount necessary for such a small increase, corresponding to 0.4% of spin S = ½ magnetic moments, is far below the limit of X-ray detection. We note that the susceptibilities of both La2_xSrxCuO4 [23] and YBaECU306+y [5] display Curie tails in the semiconducting region of the phase diagram. The decrease in the value of the susceptibility as the oxygen content is reduced is another similarity of the data in fig. 3 to those of other high temperature superconductors. We believe this to be caused by the change in the Pauli susceptibility of the charge carriers, whose concentration decreases as the oxygen content is lowered. This would be consistent with the results of the resistivity measurements. For the BiaSrl.6Lao.4CuO6.29 sample we deduce the magnitude of the Pauli susceptibility using the relation. •=Zcore "~-×van Vleck -]-)~spin ,
( 1)
where Z¢ore=- 1.65× 10 -4 cm3/mole can be estimated from tabulated values [ 24 ]. The Landau diamagnetism can be neglected here because it is at least one order of magnitude lower than the Pauli paramagnetism [25 ]. Since a unit cell of Bi2Srl.6Lao.4CuOy contains only one CuO2 layer, we use for the Van Vleck paramagnetism the value Zva, v~eck= + 0 . 4 3 × l0 -4 cm3/mole estimated for YBa2Cu307 per CuO2 layer [26 ]. This value should be a good first approximation because of the similarity in the environment of the CuO2 layers in all high-T¢ superconductors [27]. With these assumptions, we obtain Xspi,= + 1.Sl × 10 -4 cm3/mole using the room temperature value of Z; similarly we obtain Zspi,= + 1.21X 10 -4 cm3/mole, if we use the value of g at 50 K. The question remains as to the origin of the temperature dependence. If the spin
C. Allgeier et al. / Electrical and magnetic properties of Bi~r l.~Lao.4CuOy susceptibility can be d e c o m p o s e d into two independent parts, the Pauli susceptibility ZPauli, which we assume t e m p e r a t u r e i n d e p e n d e n t , a n d a t e m p e r a ture d e p e n d e n t part from residual t w o - d i m e n s i o n a l a n t i f e r r o m a g n e t i c correlations Xzo [28,29], then Zspin=ZPauli"[-Z2v(T)- The value o f the d e d u c e d spin susceptibility can also be regarded as the Pauli susceptibility Zpau~i=Zspi, if the spin susceptibility consists o f only one c o m p o n e n t [30,31 ]. Although positive values o f the slope d z / d T can be u n d e r s t o o d within the f r a m e w o r k o f the antiferromagneticF e r m i - l i q u i d theory [32,33], this theory is not yet able to explain negative slopes in the susceptibility o f samples with high oxygen (high h o l e ) concentrations. A recent e v a l u a t i o n o f the m a g n e t i c susceptibility by O d a et al. in the La2_xMxCuO4 systems [ 11] notes that the C u r i e - W e i s s tail f o u n d on the high hole side o f the peak in Tc versus hole concentration indicates the existence o f localized magnetic m o m e n t s which can be a t t r i b u t e d to the c o p p e r ions. They speculate that these magnetic m o m e n t s m a y be responsible for the r e d u c t i o n o f Tc with increasing hole c o n c e n t r a t i o n due to magnetic p a i r breaking o f the s u p e r c o n d u c t i n g electron pairs. The a p p e a r a n c e o f a C u r i e - W e i s s tail on the high hole side o f the Tc peak is d i s p l a y e d in this work, a n d is also seen in Bi2SrzCaCuzOa+y a n d TlzBa2CuO6+y [7], suggesting that m a g n e t i c pair breaking m a y also be occurring in these systems. F u r t h e r e x p e r i m e n t s on specimens with various La substitution levels will be n e e d e d to investigate this possibility in the singlelayer b i s m u t h system. To s u m m a r i z e , we find in o u r e x p e r i m e n t s that the oxygen content plays an i m p o r t a n t role not only for the superconducting, but also for the n o r m a l - s t a t e p r o p e r t i e s o f the Bi2Sr2_xLaxCuOy system. T h e position a n d height o f the m a x i m u m Tc value is a sensitive function o f both the l a n t h a n u m a n d oxygen concentrations. The present m e a s u r e m e n t s o f the electrical resistivity and magnetic susceptibility yield evidence for the direct influence o f the oxygen content on the hole c o n c e n t r a t i o n in this system. I f Bi2Sr2_xLaxCuOy is to b e c o m e a n o t h e r m o d e l syst e m like La2_xMxCuO4, it will be necessary to accurately control the oxygen c o n c e n t r a t i o n as a function o f l a n t h a n u m content.
341
Acknowledgements We t h a n k A n d r e a s Schlrgl for i n f o r m a t i v e discussions. This work was s u p p o r t e d in part by the Bund e s m i n i s t e r i u m fiir Forschung a n d Technology und e r grant n u m b e r 13N5490-7.
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