Physica C 162-164 (1989) 725-726 North-Holland

UPPER CRITICAL MAGNETIC FIELD OF Yl_2xCaxThxBa2Cu307_~

Magnus ANDERSSON and 0sten RAPP

Solid State Physics, The Royal Institute of Technology, S-100 44 Stockholm, Sweden

The upper critical magnetic field of Yl_2xCaxThxBazCu307-~ is measured up to 12 T for x<=0.1. The addition of (Ca,Th) decreases T c and broadens the resistive transitions in magnetic field. A slow decrease with x in the magnitude of the slope of the linear part of Hc2(T ) is observed.

i. INTRODUCr ION

Soon after the discovery of superconductivity

in YBa2Cu307_ ~ it was found that most of the

lanthanides could replace yttrium in these com-

pounds without appreciably c~mnging the super-

conducting transition ter~erature I . In contrast,

the replacement of yttrium by an actinide element

seems to be difficult. Our attempts to substitute

thorium for yttrium have failed, while we have

succeeded in dissolving thorium in the YBa2Cu307_ ~

phase when an equivalent amount of calcium is

present 2 . The introduction of calcium in the

structure ensures the preservation of local charge

neutrality.

The solubility limit of (Ca,Th) in YBa2Cu307_~

was estimated from X-ray analysis and supercond-

uctivity measurements to be around x=0.1 in the

series Yl_2xCaxThxBa2Cu307_ 6. For x>0.1 a multi-

phase material was obtained. The superconducting

T was found to decrease in the orthorhombic phase c

with a rate of 1.3 K per atomic percent of (Ca,Th)

within the solubility region.

In this paper we report on resistive measure-

ments of the upper critical field Hc2 in magnetic

fields up to 12 T for Yl_2xCaxThxBa2Cu307_ ~ with

x<=0. i.

2. EXPERIMENTAL

2. i. Sangple preparation

Samples were prepared by the standard solid

state method according to the following route.

i. Heating 900°C, 16 h in air

2. Heating 950°C, 16 h in air

3. Sintering 900°C, 1 h and equilibrating 24 h

at 500°C in flowing oxygen

The samples were reground between the different

steps in the preparation. The resulting tablets

were checked for inpurities by X-ray analysis and

the oxygen content was determined by thermogravi-

metric measurements. Further details and results

were given previously 2.

2.2. Resistivity measurements

Electrical resistivity was measured with a

standard four-probe method using low current

densities (<0.2 A/cm2). The measurements were

performed in a flowing gas cryostat equipped

with a 12 T superconducting magnet.

3. RESULTS

Our results for the resistive transitions of

YBazCu3OT_ 6 ill magnetic field, shown in Fig i,

are similar to several other reports 3 . The resi-

stive transition broadens strongly with magnetic

field B from about 2 to 14 K when B increases

from 0 to 12 T. In particular, a "knee" develops

in the resistance curves at small resistances

already for low magnetic fields, suggesting weak

pinning forces in part of the sample.

These characteristics are further pronounced

by the addition of (Ca,Th) as seen in the lower

part of Fig 1 for the x=0.1 sar~le. The transi-

tion width at 12 T, measured as the temperature

0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

726 M. Andersson and O. Rapp / Upper critical magnetic field of Y~_exCaxThxBaeCu3Oz_a

g t~

I - v o,.

v

c~.

c~.

0.4

0.3

0.2

0.1

0.0

0.6

0.4

• v

• v • v z~

• • v V &

v ~ • v a,

v • • ~ &

i i I

• • v n

• • v ~ ,

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0,0 v--- 'v I ~ - ~ . v , . ~ , i 50 60 70 80 9O

Temperature (K)

FIGURE 1 Normalized transition curves for YBa2Cu307-6 (upper figure) and Yo.sCa0.1Tho.iBa2Cu307-~ (lower figure) at zero field (up triangle), 1 T (down triangle), 5 r (up filled triangle) and 12 T (down filled triangle).

interval between i0 and 90 % of the normal state

resistance, is now 20 K, with an extended tail

down to 50 K. The feature of the knee is also

strengthened and it develops at a larger fraction

of the normal state resistance.

In Fig 2, Hc2(T) (at S0 % of normal resistance)

is shown for three samples in the series of

Yl_2xCaxThxBa2Cu307_8. The upward curvature at

low B is a co,anon feature in ceramic superconduc-

tors and has been ascribed to critical fluctua-

tions 4 or to flux creep 5. The results in Fig 1

suggest that the addition of (Ca,Th) increase

disorder and weakens pinning forces. Therefore,

if the non-linear He2 curves in Fig 2 were due to

flux creep, one would expect this behaviour to

become more pronounced with increasing (Ca,Th)

concentration. The fact that the curved regions

15

lO I - -

"1-

• A

0 70 95

i ! l i

B C v A

v A

v ~

• v A

• v A

• v A

, "-.,_ ~ , "r'. 75 80 85 90

Temperature (K)

FIGURE 2 The upper critical field, taken as the 50 % level of the resistive transition, versus temperature for samples Y0 8Ca0 ITh0 iBa2Cu307-6 (A), Y0.9- Ca0.0sTh0.0sBa~Cu307_ ~ (B)and YBa2Cu307-~ (C).

are similar in all samples would therefore seem

to give some support for fluctuation models.

In the linear region, -dHc2/dT decreases with

increasing (Ca,Th) concentration with 2.2, 2.1

and 1.8 T/K for x=0, 0.05 and 0.i respectively.

One sample with x=0.05 was sintered at 900 ° C

for 5 hours instead of one hour as for the samp-

les above. This had at most a small influence of

the critical field curve.

ACKNOWLEDGEMENTS

We are grateful to Mats Nygren and Zsolt Heg-

edNs, Stockholm University, for help and advice

in sample preparation. This work has been suppor-

ted by the Swedish National Research Foundation

and by the Board for Technical Development.

REFERENCES

i. P.H. NOr et al, Phys. Rev. Lett. 58 (1987) 1891.

2. M. Andersson et al, submitted to Physica C

3. E.g.I. Apfelstedt et al, Jap. J. Appl. Phys. 26 (1987) 1181.

4. C.J. Lobb, Phys. Rev. B36 (1987) 3930. A. Kapitulnik et al, Phys. Rev. B37 (1988) 537

5. Y. Yeshurun and A.P. Malozemoff, Phys. Rev. Lett. 60 (1988) 2202. D. Dew-~ghes, Cryogenics 28 (1988) 674.