Physica 148B (1987) 453-455 North-Holland, A m s t e r d a m
HIGH-T~ SUPERCONDUCTIVITY MAGNETISM
Tsuyoshi T A M E G A I , Isamu O G U R O , Kei-ichi K O G A , Akiko W A T A N A B E and Yasuhiro IYE The Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan Received 10 A u g u s t 1987
( R E ) B a 2 C u 3 0 x c o m p o u n d s were studied by structural, transport and magnetic m e a s u r e m e n t s . ( R E ) B a 2 C u 3 0 x compounds with orthorhombic oxygen deficient perovskite structure are h i g h - T c superconductors, while those with tetragonal phase are semiconductors. Clear correlation is found between the superconducting transition temperature and the lattice parameters. ( R E ) B a 2 C u 3 0 x c o m p o u n d s with smaller lattice parameters have lower T c except for La. U p p e r critical fields He2 were m e a s u r e d resistively up to 90 kOe. T h e initial slope [dH¢2/dTIrc have e n o r m o u s values of about 20 -- 29 k O e / K . Magnetic susceptibility m e a s u r e m e n t s revealed that the R E ions are in their trivalent states for both the orthorhombic and the tetragonal phases. Antiferromagnetic orderings of R E ions were found in G d B a 2 C u 3 0 x and D y B a 2 C u 3 0 x at about 2 K and 0.9 K, respectively. Magnetoresistance m e a s u r e m e n t s on tetragonal G d B a 2 C u 3 0 x showed cusp-like anomalies below about 2 K.
Since the discovery of high-T c superconductivity in the L a - B a - C u - O system , the race for much higher T c was activated. The first superconductor with T~ above liquid nitrogen temperature was found in the Y - B a - C u - O system [2, 3]. Chemical modification is a popular way in materials science. Substitution of Sr for Ba was very successful in raising Tc in the L a B a - C u - O system . In the Y - B a - C u - O system, substitution of R E (rare earth) for Y is interesting to study not only the variation of the superconductivity but also the magnetic properties of the R E ions and their relation with the superconductivity. ( R E ) B a 2 C u 3 0 x compounds were prepared by a solid-state reaction of CuO, BaCO 3 and R E oxides. Details of the synthesis of orthorhombic (RE)Ba2Cu3Ox have been reported already . We were able to synthesize almost single phased ( R E ) B a 2 C u 3 0 x compounds of orthorhombic oxygen deficient perovskite ( O D P ) structure similar to that of YBa2Cu30 x except for Ce, Pr, and Tb. Ce and Tb compounds form an unknown structure similar to each other. The Pr compound shows a mixture of tetragonal O D P structure and another structure similar to that of Ce 0378-4363/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Y a m a d a Science Foundation
and Tb. To make the tetragonal samples, pellets of the orthorhombic samples were retired at 930°C for 5 h in air and then quenched in liquid nitrogen. Electrical measurements were performed by the ordinary four-probe method. Upper critical field and magnetoresistance were measured up to 90 kOe. Magnetic susceptibility was measured under a magnetic field of 11.8 kOe by a pendulum magnetometer. Magnetization was measured by a moving-sample magnetometer up to 80 kOe. The lattice parameters of orthorhombic ( R E ) B a z C u 3 0 x are shown in fig. 1. In case of LaBa2Cu3Ox, fittings by both tetragonal and orthorhombic cells are shown. The lattice parameters decrease monotonically with increasing atomic number of R E element, as expected from the trend of the ion radius of RE. Changes in lattice parameters are not isotropic; contraction in the b-axis is larger than those of the aand the c-axes. (RE)Ba2Cu3Ox with orthorhombic O D P structure shows superconductivity with T c in the 90 K range except for La which has a slightly lower To. Table I summarizes T c (onset and midpoint), the transition width from 90% to
T. Tamegai et al. / High-T superconductivity
Table I Superconducting characteristics for (RE)Ba2Cu30 ~ compounds. RE
Pr Nd Sm Eu Gd
. 94.6 94.6 94.4 93.9
. 93.6 94.2 93.9 93.6
Dy Ho Er Tm Yb Lu
92.3 92.3 92.3 91.0 89.8 88.7
690 430 1500 350
20 21 20 29
680 840 730 360 600 640
27 26 20 21 24 24
. 2.3 1.0 2.6 0.6 .
2.0 2.5 1.5 1.9 2.3 3.0
" i ....
91.8 91.7 91.4 90.2 88.7 86.9
'~ '1' +
*'~-~1 . 4
o o o o o o o
in (RE)Ba2Cu30 x
a positive pressure coefficient of T~ . The difference might be due to anisotropic change in the lattice parameters in case of R E substitution. Upper critical fields of ( R E ) B a 2 C u 3 0 ~ were measured resistively up to 90 kOe. Some ( R E ) BazCu30 x samples show resistive tails even under a magnetic field of l k O e . The resistive tail is not specific to certain ( R E ) B a z C u 3 0 x compounds, but depends on the sintering condition. Enough sintering in 02 atmosphere can reduce the resistive tail to some extent. Fig. 2 shows the temperature dependence of Hc2 of ( R E ) Ba2Cu30 x defined by the midpoint of the resistive transition. The H c z - T curves have upward curvature and the slopes are nearly the same for all ( R E ) B a 2 C u 3 0 x. We can estimate H c 2 a t T = 0 by the formula H c 2 ( 0 ) = 0 . 6 9 T~]dHcz/dTJrc. We defined JdHcz/dT]T c by the slope for higher fields, which are also summarized in Table I. The estimated values of He2(0 ) range from 1250 kOe to 1850kOe for the ( R E ) B a z C u 3 0 x system, which are comparable to that of Y B a z C u 3 0 x . These values are also comparable to the PauliClogston limit of about 1700 kOe for these materials. The temperature dependence of the magnetic susceptibility can be well described by the C u r i e Weiss law for most of the samples. Effective moments of the R E ions correspond to those in trivalent state. Magnetization measurements
÷ + + f JLu
Er 14o DvNdEu SmGd
L La Ce Pr Nd PmSmEu Gd Tb Dy Ho Er TmYb Lu
Fig. 1. Lattice parameters for orthorhombic (RE) Ba2fu30 x and the ion radii of the RE elements. In case of LaBa2Cu3Ox, triangles show lattice parameters fitted assuming a tetragonal unit cell.
10% of normal resistance, and the resistivity at 100K. It is seen that T c is lower for ( R E ) Ba2Cu30 x with smaller RE. Combined with the structural data, it is clear that ( R E ) B a 2 C u 3 0 x with smaller lattice parameters have lower T c. This seems to be inconsistent with the recent report on the pressure effect on T c which shows
Temperot u~(lg -
Fig. 2. Temperature dependence of the upper critical fields defined by the midpoint of the resistive transition for (RE) Ba2Cu30 x compounds. Solid lines are guides for the eye.
T. Tamegai et al. / High-T~ superconductivity in (RE)Ba2Cu30 ~
showed antiferromagnetic orderings of the RE ions in both phases of GdBa2Cu30 x and DyBa2Cu3Ox; at about 2 K and 0.9K, respectively. Details of the magnetic measurements are reported in a separate paper . Fig. 3 shows the temperature dependence of the resistivity of tetragonal GdBa2Cu30 x. Though the temperature dependence of the resistivity of the tetragonal phase is semiconductorlike, simple thermally activated behavior is not obeyed. In the tetragonal GdBa2Cu30 x, the magnetic ordering also manifests itself as a shoulder in the R - T curve as shown in the inset of fig. 3. Fig. 4 shows the magnetoresistance data on tetragonal GdBa2Cu30 x below 4.2 K. Cusplike anomalies are seen below about 2 K, where Gd ions order antiferromagnetically. The inset shows the temperature dependence of the magnetic field where the cusp occurs. The cusp occurs at around the the field where the M - H curve turns toward saturation . The cusps are considered to occur when the magnetic field crosses the phase boundary between the spinflop and the paramagnetic phases. Scattering of conduction electrons by magnetic ions will decrease when the magnetic moments are aligned in one direction. The fact that this cusp shows up sharply in GdBa2Cu30~ compared with other compounds may be connected with the small
GdBa2CU30x( tet ragonal) v
0£2K 0.95K 1.BOK 200 K 249K
100 5O Magnetic Field (kOe)
Fig. 4. Magnetoresistance for tetragonal GdBa2Cu30 x up to 90kOe at some temperatures below 4.2K. Cusp-like anomalies can be seen below about 2 K, where the Gd ion orders antiferromagnetically. The inset shows the temperature dependence of the magnetic field where the cusp occurs. The broken line is a guide for the eye.
anisotropy of Gd ions, where the smearing out by averaging over randomly oriented grains may be small. The authors would like to thank W. Utsumi, H. Takahashi, N. Mori and T. Yagi for stimulating discussions and comments. They also thank M. Katahashi and A. Yamaguchi for their technical assistance. References
ld GdBa2Cu30x (tetragona[)
2'0 730 ~ 50 60 IO001T (IIK)
'70 I~3 90
Fig. 3. Temperature dependence of the resistivity for tetragonal GdBaECU3Ox . The inset shows the lower temperature part. A shoulder is seen corresponding to the antiferromagnetic ordering of Gd ions.
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