Substitution of 3d metals for Cu in (Bi, Pb)2Sr2Ca2Cu3Oy

Physica C 202 (1992) 371-378 North-Holland

Substitution of 3d metals for Cu in (Bi, Pb ) &Ca2Cu30y R.K. Nkum alb, A. Punnett ’ and W.R. Datars B ’ Department ofPhysics B Astronomy, M&aster University, Hamilton, Ont. L8S 4h41, Canada b Department of Physics, University of Science and Technology, Kumasi, Ghana Received 23 July 1992

The effect of the substitution of Mn, Fe, and Zn for Cu on the superconducting properties of (Bi, Pb),SrrCa&us_&i,O, (0 0.1 are not superconducting while the corresponding Zndoped samplesare superconducting with the amount of the 22 12 phase increasing with increasing Zn concentration. The effect of Mn doping on the superconductivity is less than that of Zn and Fe. There is evidence of weak links between superconducting grains. Pair-breaking effects due to the effect of doping seem to give a satisfactory explanation of the reduction in T,. The resultsfor the low dopant concentrations lead to the conclusion that local disorder rather than magnetism is the important factor for the suppression of superconductivity in the samples.

1. Introduction Substitutional studies wherein one or more chemical constituents of a parent material are replaced either partially or completely by other elements is one of interesting areas of research of high-Tc superconductors. The importance of substitutional studies stems from their role as a probe of the chemical and structural environment which determines whether or not the system exhibits superconductivity. In the cuprate-based LnBazCusO, system (Ln = Y or some of the lanthanides) work has been done with the effect of substitution on the physical properties [ 1,2]. The substitution of 3d metals for Cu is expected to provide important information on the pairing mechanism of high-temperature superconductivity. Since the pairing of carriers in the Cu-O2 planes is responsible for high-temperature superconductivity in all copper-oxide ceramics, it is of interest to alter the electronic structure of, as well as introduce magnetic impurities into, the Cu-O2 planes. The 3d metals are often chosen because their valences and ionic radii suggest that they should substitute for Cu in these materials. There is evidence from neutron diffraction experiments that the 3d metals substitute for Cu 13941.

For the La- and Y-based compounds, solid solutions for various 3d metals on the Ctr site exist over a broad range of composition. In the 2212 Bi-based compounds, the substitution of Cu by the 3d metals has been found to lower the transition temperature [ 5-7 1. The largest effect is produced by iron and cobalt which lower the transition temperature to 38 K and 40 K, respectively. Recently, the substitution of and chromium for copper in vanadium BizSrzCaCuzOs has been reported [ 8 1. The suppression of T,by the substitution of 3d transition metals for Cu has been attributed to the pair-breaking mechanism. It is well known that a chief mechanism responsible for superconductivity is the formation of bound electrons in a singlet state. The exchange interaction between the electrons and the spinning impurity atoms leads to nonconservation of the electron spin, which affects the formation of Cooper pairs. The spin of the impurity inhibits the appearance of the superconductivity and hence, causes a decreases in T,. In conventional superconductors, it is well known that paramagnetic impurities are much more effeo tive T=-suppressors than nonmagnetic ones. Timereversal symmetry-breaking perturbations such as paramagnetic scattering have a strong pair-breaking

0921-4534/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

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R.K. Nkum et al. /Substitution of 3d metals

effect for the spin-singlet, s-wave Cooper pairs [ 91. This does not seem to be the case in the high-Tc cuprates. For example, the whole family of the YBa#&O, type compounds with magnetic rare earth ions has essentially the same T,s [lo]. Another significant observation is that there seem to be no significant differences in the strengths of T, suppression between magnetic and nonmagnetic impurities. The present study deals with the substitution of Cu by the 3d metals Mn, Fe and Zn in the (Bi, Pb),Sr,Ca$Lt,O,, system. X-ray diffraction, DC magnetization and electrical resistivity results are presented. Zn is used in order to compare the effect of a nonmagnetic impurity with the magnetic impurities. In the low concentration region, the effects of Fe and Zn on the superconductivity are found to be the same. Pair-breaking effects explain the suppression of superconductivity.

2. Experimental method with a composition nominal Samples Bi1.6Pb0.4Sr2Ca2Cu3 _J&O,, (M = Mn, Fe, Zn) (0 5x1; 0.5 ) were prepared using the conventional solid state reaction method. Chemically pure pow-

ders of B&OS, PbO, SrC03, CaCOS, CuO, MnOz, Fez03 and ZnO were used as starting materials. The powders were mixed in the appropriate amounts in acetone in an agate mortar and calcined in air at 820’ C for 5 h. The resulting materials were ground, pressed into pellets and sintered at 850°C for 100 h with several intermediate grindings and pressings. The pellets were cooled at O.Z”C/min to 7OO”C, 0.1’ C to 200’ C and then cooled to room temperature with the furnace turned off. All samples were heat treated in alumina boats. The electrical resistivity of the samples was measured in the temperature range 50-300 K by the standard four-probe technique. The 50 K temperature was obtained by pumping on liquid nitrogen. A constant current of 1 mA was used for all the samples. The temperature was monitored with carbon glass and platinum resistance thermometers. Phase identification of the samples was carried out by recording X-ray diffraction patterns in the 26 range of 10 to 70” at room temperature with a Nicolet powder diffractometer using CuK, radiation. DC susceptibility measurements were made with a superinterference device conducting quantum magnetometer in an applied magnetic field of 20 G.

Table 1 Lattice parameters of the 2223 phases of the 3d metal doped Bi,.6Pbo.,Sr2Ca2Cu~_~~~ (M = Fe, Zn, Mn) samples x

2 8 (deg) Fig. 1. X-ray diffraction patterns of some of the Bi,.6Pb0.4Sr2Ca2(=U3--xMnxOysamples: (a) x=0.2, (b) x=0.3, (c) x=0.4 and (d) x=0.5 (L) 2212 phase, (H) 2223 phase.

Mndoped 0 0.01 0.05 0.20 0.30 0.40 0.50 Zndoped 0.01 0.05 0.20 0.40 Fe-doped 0.01 0.05 0.07 0.10

a (A)

c(A)

5.41 5.40 5.39 5.41 5.40 5.41 5.40

37.12 37.00 36.81 37.04 36.98 36.94 36.80

5.40 5.42 5.42 5.38

37.03 37.09 37.16 37.09

5.40 5.39 5.39 5.38

37.00 37.14 37.08 36.89

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RX Nkum et al. /Substitution of 3d metals

(b)

1

20 Temperature

40

I

I,

I

60

80

Temperature

(K)

I

I

100

I

I

120

1 0

(K)

t

IL

20

10

40

11

60

18

1

80

Temperature

100

’ 120

0 1

(K)

Fig. 2. Temperature dependence of the DC magnetic susceptibility of some of the samples, (a) x=0, (b) Mndoped x=0.05, (~1 Fedoped x=0.05.

3. Results and discussion

The X-ray diffraction (XRD) patterns of some of the samples are shown in fig. 1. Most of the peaks belong to the 2223 phase and a few peaks of low intensities belong to the 22 12 phase for the samples with low dopant concentrations. The number and intensities of the 22 12 reflections increase with increasing dopant concentration. Thus, the doping of the 3d metals causes a degeneration of the 2223 phase

and enhances the 22 12 phase. All the reflections are indexed with the pseudotetragonal structures of the 2223 and the 2212 phases. No impurity reflections belonging to any of the oxides such as FezOp, ZnO and MnOa are observed indicating that the dopants are incorporated into the crystalline structure. The lattice parameters of the 2223 phase of the samples are given in table 1. The temperature dependence of the DC susceptibility (in arbitrary units) in fig. 2 show the two su-

RX Nkum et al. /Substitution of 3d metals

374

perconducting transitions of the 22 12 and 2223 phases. The transition temperature of the 2223 phase of the samples, determined by the temperature at which the diamagnetic signal becomes zero, is 105.3 K for the undoped sample, 103.8 K for the x=0.05 Mn sample and 95.5 K for the x=0.05 Fe sample. Thus, for the same amount of dopant concentration, Fe suppresses the transition temperature more than Mn. There is a small amount of 2212 phase in the undoped sample. This is in agreement with the XRD results. Figures 3-5 show the temperature dependence of the electrical resistivity of the samples. The resistivity curve of the undoped sample has a 2223 phase with an onset temperature (T,““) of 110.1 K. The temperature of the onset of the superconducting transition is determined as demonstrated in fig. 6 (a). The temperature of zero resistivity is 99.7 K. Mndoped samples with 0 SxS 0.3 have one-step resistive transitions. The onset of the superconducting transition, however, decreases slightly with increasing x, as shown in table 2, and remains constant for XT 0.3. The temperature of zero resistivity decreases with increasing Mn concentration. Samples with x20.4 have a two-step resistive transition, suggestive of the presence of two phases, the 2223 and 22 12 phases. The xS 0.1 Zn-doped samples have single re-

10.0

,

8.0 Bi,,gPbo.4Sr2C02CuJ_xZniOg (a)

Temperature

(K)

50.0

I

0.0

? 0

50

100

150

Temperature

200

250

300

(K)

6.0

Fig. 4. Temperature dependence of the electrical resistivity of the Zndoped samples, (a) x50.4, (b) x-0.5. The x=0.5 sample shows a semiconducting behavior in the normal state.

2.0

0.0

Temperature

(K)

Fig. 3. Temperature dependence of the electrical resistivity of the Mndoped samples.

sistive transitions whereas the x2 0.2 samples have transitions of the two phases. The resistivity of the sample with x=0.5 in fig. 4(b) has a semiconducting behaviour in the normal state but has a single 22 12 superconducting transition with zero resistivity. The results of the electrical resistivity measurements of the Fe-doped samples are given in fig. 5. Samples with OSx10.07 are metallic in the normal state and have a one-step resistive transition that is

R.K. Nkum et al. /Substitution of 3d metals

4.0

2

r

3.0

2 x .e .e

2.0

E t cf

1 .o

0.0

0

50

100

150

Temperature

Fig. 5. Temperature

200

250

3JoO

(K)

dependenceof the electricalresistivityof the

Fedoped samples. similar to that of the samples with low concentrations of Mn and Zn. Another similarity is that the transition temperature of the Fe-doped samples decreases with increasing Fe concentration. This behavior has been observed by Sun et al. [ 111 who reported the suppression of T, when Fe was used as an additive to the 110 K Bi-Pb-Sr-Ca-Cu-0 system. Unlike the Mn- and Zn-doped samples, however, the x= 0.1 sample has a superconducting transition at 94 K but the resistivity does not go to zero even at 55 K. This indicates that not all the sample is superconducting. Thus Fe suppresses superconductivity more than both Mn and Zn. Although all the samples with low dopant concentrations contain some amounts of the 22 12 phase, as seen from the XRD and the magnetic susceptibility results, they have a one-step resistive transition. This is because the volume fraction of the 2223 phase is sufficient to.form a continuous network of grains with most) weak links interconnecting the islands of only this phase. Upon lowering the temperature, the current by-passes the islands of the 22 12 phase and goes through paths traversing the grains of the 2223 phase coupled together via some weak links. Evidence of weak links between the superconducting grains in these samples is deduced from the dp/dT versus T curves. Representative plots are given in fig. 6. There

375

is a single peak for the undoped sample indicating a single superconducting transition. However, there is a hump on the low temperature side of the dp/dT curve of the doped samples. The other doped samples have a similar behaviour. The shoulders in the dp/dT versus T curves are a signature of weak links between grains. Two types of superconducting grains, one formed by the 2223 phase and the second by the 2212 phase, apparently exist in the doped samples. A sufficiently thin layer of the 2212 phase can play a role as the weak link. It can be seen from table 2 that the dopants cause decreases in both T:” and TC’O. The T:” of the Mndoped samples decreases gradually and becomes constant for x20.3. The T{‘O, however, decreases almost monotonically with x. The T:* of the Zn- and Fe-doped samples decrease with x. The suppression of Tcao in the region of low dopant concentrations give interesting results. The T<” versus x for the low dopant concentration of Zn and Fe in fig. 7 indicates that the transition temperature decreases proportionally to the concentration. This is in agreement with the pair-breaking mechanism. According to the Abrikosov-Gor’kov theory [ 12 ] for a BCS superconductor, T, can be expressed as ln(TJT,)=W[(l/2)

where Tco is the value of T, in the absence of magnetic impurities, w is the digamma function, and r is the pair-breaking parameter proportional to the magnetic impurity concentration. This formula indicates that the initial reduction in T, is linear in p Tco/ TC- 1 - (fl/4k,T,). At low impurity concentrations the transition temperature decreases proportionally to the impurity concentration. It is necessary to point out here that pair-breaking effects in the superconducting oxides are not restricted to magnetic scattering. In addition to magnetic impurity, pair-breaking effects can be due to scattering by nonmagnetic disorder or changes in the spin fluctuations. Ample evidence for the phasebreaking scattering from nonmagnetic impurities and defects comes in systems with atomic substitution as an upturn in the zbehavior of the resistivity at low temperatures, as seen in the x=0.5 Zn-doped sample. This may be attributed to electron localization

R.K. Nkum et al. /Substitutionof 3d metals

376 0.16 ,

iY

0.12

0.10

I-

0.08

-

0.12

0.08

@)

0.06 -

0.04

0.00

90

100

110

120

130

140

1

Temperature (K)

Temperature (K)

0.25

(4 c 'Y E 0 s k \ 3

0.20 -

0.15 -

0.10 -

1::

0

Temperature (K)

Fig. 6. Temperature dependence of the temperate doped samples.

derivative ofthe resistivity of (a) undoped, (b) x=0.01 Zn-doped, (c) x=0.05 Zn-

driven by a random impurity potential [ 13 J. The localization regime becomes evident for La-Sr-Cu-0 and YBCO compounds having quite small amounts of dopants and can seemingly be independent of whether impurities are magnetic or nonmagnetic [ 13-151. As a result, T, in these systems is quickly reduced with increasing impurity concentration. On the other hand, the B&based compounds are more stable against localization effects, in the sense that the resistivity upturn behavior is observed for sys-

terns with much higher dopant concentrations. The T, reduction in La-Sr-Cu-0 and YBCO through the nonmagnetic impurity Zn has been explained by a model in which the impurity also behaves as a magnetic scatterer [ 16 1. A number of experiments indicate that the nonmagnetic ions induce magnetic moments in the host high-T, materials [ 17191. It is, however, not clear at the present stage if the same explanation would apply to the Bi-based systems. Maeda et al. [ 201 have claimed that Zn-

R.K. Nkum et al. /Substitution of 3d metals Table 2 Transition temperatures of some samples from the msistivity measurements. F is the onset temperature and TiSo is the temperature at which the msistivity goes to zero 3d composition, x Mn-doped 0 o..os 0.20 0.30 0.40 0.50 Zndoped 0.01 0.05 0.07 0.10 0.20 0.40 0.50 Fe-doped 0.01 0.05 0.07 0.10

TP (K)

rc-0

110.1 109.7 108.4 106.8 106.8 106.8

99.7 95.9 95.0 88.4 65.9 63.0

101.1 107.7 107.1 105.2 106.0 108.1,81.8 14.3

95.9 86.5 79.0 78.1 78.0 63.8 55.0

109.7 101.9 98.8 94.2

96.8 85.6 79.9

100

90

80 B 0 2 70

I

0.02

I

0.04

Nominal concentration

#

0.06

0

of dopant

Fig. 7. The dopant concentration dependence of the transition temperature of the Zn and Fe samples (xr 0.07), ( 0 ) Zn, (A ) Fe. The Linear tit agrees with the Abrikosov-Gor’kov pairbreaking theory.

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doped Bi-based systems in the 80 K phase, which has a T, reduction similar to that for the case of Ni doping, shows no evidence of localized magnetic moments. It is interesting to note that in the small dopant concentrations, Fe and Zn similar effects on the T,. In this region, both Fe and Zn depress the T, at approximately the same rate. Within the conventional low-temperature superconductors the presence of magnetic impurities depress the T, much stronger than nonmagnetic impurities [ 12 1. However, for the La- and Y-based superconductors the magnetism of the dopant has no effect on the T,. In the superconsystem ducting Bii.zPbo.&o.sSr&ur -,Mxo, (M =Ni, Zn) , both Zn and Ni were found to depress T, at approximately the same rate [ 211. These and our results for the low dopant concentration samples lead to the conclusion that local disorder rather than magnetism is the important factor for the suppresin the cuprate sion of superconductivity superconductors. It is known that disorder affects superconductivity in several theoretical models based on both BCS theory [ 221 and a theory [ 231 developed for high-Tc superconductivity. These models predict an increase in the normal-state resistivity due to impurity scattering introduced by disorder, accompanied by the depression of superconductivity. As seen in figs. 35, the normal-state resistivity of all the samples increases with increasing dopant concentration, In summary, X-ray diffraction, DC magnetic susceptibility and electrical resistivity measurements have been used to investigate the effect of the substitution of 3d elements for Cu in the (Bi, Pb ) ,SrzCa2Cus0,, system. Both XRD and magnetic susceptibility measurements give evidence of the presence of the 22 12 and 2223 phases in the systems with the amount of the 2212 phase increasing with increasing dopant concentration. Shoulders are obServed in the dp/dT versus T curves of the samples which have single resistive transitions. This is a signature of weak links between grains in the samples. In the low concentration region, Zn and Fe affect the T, by the same amount. However, samples with higher Fe concentrations (~2 0.1) are no longer superconducting while all the Mn- and Zn-doped samples studied in this work are superconducting. Fe suppresses superconductivity in this system more

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R.K. Nkum et al. /Substitution of 3d metals

than the other two elements. Mn has the least effect on the superconductivity. Pair-breaking effects due to the doping seem to give a satisfactory explanation for the reduction in T,.

Acknowledgements We are grateful to the McMaster University Institute’ of ,Materials Research for the X-ray diffraction measurements. The research was supported by the Natural Sciences and Engineering Research Council of Canada. Technical assistance was provided by T. Olech.

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