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Magnetism and superconductivity in Ba2GdCu3O7−y
Solid State Communications, Vol. 65, No. 11, pp. 1395-1398, 1988. Printed in Great Britain.
0038-1098/88 $3.00 + .00 Pergamon Press plc
MAGNETISM AND SUPERCONDUCTIVITY IN Ba2GdCu3OT_y* Zhao Guo-meng, Wang Rui-lan and Dong Zi-wen Institute of Physics, Academia Sinica, Beijing, China
(Received 24 October 1987 by IF. Y. Kuan) The structure, resistivity, Meissner effect and magnetization measurements were performed on Ba~GdCu3OT_y. It was found that oxygen contents not only influence drastically on structure and superconductivity but also on magnetic behaviors at low temperatures while magnetic transition temperature remains unchanged. Moreover, paramagnetic susceptibility obeys different Curie-Weiss laws with 0 positive or negative value over different temperature ranges.
THE RECENT discovery of superconductivity above 90K [1, 2] in the compound [3] YBa2CU3OT_y raises many questions about the origin and properties of superconducting electrons. Among the initial studies characterizing these compounds are many involving substitutions of elements on the Y site [4, 5]. Surprisingly, a depression of T~ is rarely observed for full substitutions on the Y sites even if the substituted atom is a rare earth with large magnetic moment and crystal field. The specific heat measurement [6] seemed to indicate that there appears AF ordering at 2.2 K in Ba2GdCu3 O 7 _ y , which awakes other interesting questions of whether superconductivity and AF ordering in this compound could coexist and what is the cause of the AF ordering. In this Communication, we report other experimental results for Ba2GdCu307_y , which strongly suggest that the magnetic transition in Ba2GdCu307_y is not simply correspondent to AF ordering and that the dominant magnetic interaction in these materials is not R K K Y type interaction but dipole-dipole interaction. The samples were prepared by mixing powders of Gd203, CuO, BaCO 3 in appropriate ratios. The powders were ground, reacted in air at 930°C for 16 h and cooled to room temperature within 6h. Then, the powders were reground, pressed into ~b13 mm pellets, sintered at 970°C for 14h and cooled to room temperature within 9 h. Sample 1 was produced by annealing the as-grown sample in vacuum (4 x 10-2 torr.) at 800°C for 10h. Sample 2 was prepared by annealing the as-grown one in air at 700°C for 2 h and cooling it to room temperature within 3 h. Sample 3 is
* The project is supported by National Natural Science Foundation of China.
the as-grown one. Sample 4 was obtained by annealing the as-grown one at 700°C for 15 h and cooling to room temperature within 6 h in flowing oxygen. Sample 5 was produced by annealing the as-grown with nominal composition Ba0.65Gd0.35CuOy, in an almost sealed tube at 800°C for 15 h and cooling to room temperature within 6 h in flowing oxygen. X-ray diffraction was carried using monochromated CuKct radiation. The standard four probe technique was used to measure the resistivity. The magnetization and Meissner effect measurements were made on an extracting magnetometer in which the magnetic field can be continuously varied from 0-8 x 104Oe, the inhomogeneity of the field being less than 10 -4 in the diameter of 60mm. The temperature was computer controlled within the range of 1.5-300 K to be an accuracy of _ 0.01 K. The experimental error of the magnetization was better than 5 x 10-4emu. The temperature dependence of resistivity for sample 5 is shown in Fig. l(a). The sample exhibits a good metal behavior with Q300K~ 750#f~cm and Q300K/QL00k~ 2.5, and a good superconductivity with Tc onset about 98 K, T~ zero-resistance about 94.5 K and AT~ < 1.5 K. The Meissner flux expulsion for the sample (Fig. 1 cross line) reaches about 43% with no paramagnetism correction. Fig. l(b) also shows the susceptibility result for the sample of cooling it in 10 KOe (solid circle). At low temperatures, no kink in susceptibility could be observed down to 1.5 K. It seems to be that no magnetic transition takes place in this sample down to 1.5 K. However, our specific heat measurement [7] does show magnetic transition at 2.23 K in this sample. Figure 2 shows the magnetization curve at 1.5 K for this sample. The curve saturates at about 3.5 tesla indicating that magnetic ordering
1395
1396
MAGNETISM AND SUPERCONDUCTIVITY IN Ba2GdCu3OT_y
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Vol. 65, No. 11
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Fig 1 Fig. 1. (a) Temperature dependence of resistivity for sample 5 annealed in flowing oxygen. (b) Meissner effect (cross line) and susceptibility cooled in 10 KOe (solid circle). has taken place at this temperature. The saturation magnetization is about 40 emu g- ~with diamagnetism correction (4emu g-~), which is about 80% of ideal value (,-~ 54 emu g ~). However, the effective magnetic moment of Gd ion in the sample is about 8.0 #B at high temperature (see Table 1), which is very close to the ideal value. Figure 3 shows the temperature dependences of susceptibility for samples 3 and 4. The irreversibility is clearly seen for both the samples but their behaviors are very different. For sample 3 the cooling curve (solid circle) is higher than the warming one (open circle) and the difference between them becomes very large for T < 2 K, which can not be simply attributed to shielding effect. Moreover, the cooling curve appears a kink at about 2.2 K but the warming one exhibits an inclined plane between 2.3 to 4.2K. For sample 4, however, the cooling curve (solid triangle)
Fig. 2. The magnetization curve at 1.5 K for sample 5. be lower than warming one at low temperature and there exists kinks at about 2.1 K and 2.4 K respectively. In Fig. 4, we show the susceptibility at tem-peratures between 1.5 to 15K for five samples of cooling them in 10KOe. According to sample preparation conditions, we believe that the oxygen contents should increase with the sample number. For sample 1 showing no superconductivity and tetragonal structure, the oxygen content should be less than 6.5. For the others having orthorhombic structure, the oxygen content should be larger than 6.7. Therefore, one could see that the kinks are lowered and smeared out as oxygen contents are increased. Table 1 lists electrical and magnetic properties for all five samples. The electrical and magnetic properties for these samples are greatly different. However, our specific heat measurements [7] on samples 2 and 5 showed no variation of magnetic transition temperature (2.23 K).
Table 1. Electrical and magnetic properties for different samples No. Q3ooK(QCm) Tcm(K) AT,(K) M.F. (%) O(T < 200K) P~(T < 200K) O(T > 200K) Pe~(T > 200K) T,-(K)
1
2
3
4
5
5.4 6.0K 8.0 6.0K 8.0 2.2
3 × 10 3 96 5.0 35 3.5K 8.0 -18K 7.6 2.3
1.8 X 10 -3 93 3.0 50 4.0K 8.0 -16K 7.6 2.2
9.0 × 10 -4 95.5 1.0 62 2.0K 8.0 -13K 7,7 2.1
7.5 × 10 -4 96.5 1.5 43 3.0K 8.0 -20K 7.6 -
M.F. - - Meissner fraction. * in units of #8. Tr - the temperatures corresponding to the kinks in Fig. 4.
Vol. 65, N o . 11
MAGNETISM
AND
I N Ba~GdCu307_y
SUPERCONDUCTIVITY
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T(K) Fig. 3. The temperature dependences of susceptibility for samples 3 and 4. The solid circle presents the susceptibility cooled in 10KOe and open circle presents one warmed in 10KOe for sample 3. The solid triangle and open triangle present the susceptibility cooled and warmed in 10 KOe for sample 4, respectively. The products z T via T for three samples are shown in Fig. 5. For sample 3 (solid triangle) and sample 5 (solid circle), the products tend to increase with temperature between 100 to 200 K and decrease with temperature above 200 K. The peaks at about 210 and 240 K in the curves can not be attributed to experiment fluctuation since all four superconducting samples have almost the same characters. The increase
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Fig. 4. The susceptibility for five samples of cooling them in 10 KOe. Solid circle for sample 1, open circle for sample 2, solid triangle for sample 3, open triangle for sample 4, solid square for sample 5.
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T0 ) Fig. 5. Temperature dependence of the products z T for three samples. Solid circle for sample 5, open circle for sample 1 and triangle for sample 3. of z T with T means paramagnetism followed by Z = C/(T + O) with positive 0 while the decrease of z T with T means paramagnetism followed by g = C/(T + O) with negative 0. As seen from table l, the effective moment of Gd ion for four superconducting samples are all equal to about 8.0#8 in the temperature range between 100 to 200 K. Above 200 K, we neglect those fine structures and obtain fitting parameters of Pefr ~ 7.6/~8/Gd and 0 ,-~ - 1 3 to -20K. The novel behaviors appearing in superconducting samples do not exist in nonsuperconducting samples as seen from Fig. 5 (open circle for sample 1). Above results indicate that oxygen vacancies of these materials play a crucial role in the structure symmetry, electrical properties and magnetic behaviors at low temperatures but they have little effect on magnetic transition temperature [7]. This suggests that the dominant magnetic interaction in these materials cannot be the R K K Y interaction since the R K K Y interaction and superconductivity are both sensitive to the electronic states at Fermi level. Unaffected transition temperature by oxygen vacancies suggest that dipole~tipole interaction may be a main interaction which is responsible for the magnetic transition in these materials. The novel behaviors like irreversibility
1398
MAGNETISM AND SUPERCONDUCTIVITY IN Ba2GdCu307_y
of susceptibility at low temperatures, sign change of 0 with temperature, decrease of effective moment at T > 200 K, inconsistency of transition temperature between magnetic and specific heat measurement, etc, cannot be understood if one thinks that the magnetic transition observed in specific heat is simply correspondent to antiferromagnetic ordering.
2.
3.
4. Acknowledgements - - The authors would like to thank Zhao Yu-ying for X-ray analysis and Dr. Xu Ru-fong for useful discussion.
REFERENCES M.K. Mu, J.R. Ashbun, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang & C.W. Chu, Phys. Rev. Lett. 58, 908 (1987).
5. 6. 7.
Vol. 65, No. 11
P.H. Hor, L. Gao, R.L. Meng, Z.J. Huang, Y.Q. Wang, K. Forster, J. Vassilious, C.W. Chu, M.K. Wu, J.R. Ashburn & C.J. Torng, Phys. Rev. Lett. 58, 911 (1987). R.J. Cava, B. Battogg, R.B. Van Dover, D.W. Murphy, S. Sunshine, T. Siegrist, J.P. Remeka, E.A. Rietman, S. Zahurak & G.P. Espinosa, Phys. Rev. Lett. 58, 1676 (1987). L.C. Porter, R.J. Thorn, U. Geiser, A. Umezawa, H.H. Wang, W.K. Kwok, H.I. Kao, M. Monaghan, G.W. Crobtree, K.D. Carison & J.M. Williams, Inorganic Chemistry (in press). Z. Fish, J.D. Thompson, E. Zirngiebl, J.L. Smith & S.W. Cheong, Solid State Commun. (in press). J.C. Ho, P.H. Hor, R.L. Meng, C.W. Chu & C.Y. Huang, Solid State Commun. 63, 711 (1987). Chuan-xing Zhu, Zhao Guo-meng & Qi-ze Ran, (submitted to Solid State Commun.).
0038-1098/88 $3.00 + .00 Pergamon Press plc
MAGNETISM AND SUPERCONDUCTIVITY IN Ba2GdCu3OT_y* Zhao Guo-meng, Wang Rui-lan and Dong Zi-wen Institute of Physics, Academia Sinica, Beijing, China
(Received 24 October 1987 by IF. Y. Kuan) The structure, resistivity, Meissner effect and magnetization measurements were performed on Ba~GdCu3OT_y. It was found that oxygen contents not only influence drastically on structure and superconductivity but also on magnetic behaviors at low temperatures while magnetic transition temperature remains unchanged. Moreover, paramagnetic susceptibility obeys different Curie-Weiss laws with 0 positive or negative value over different temperature ranges.
THE RECENT discovery of superconductivity above 90K [1, 2] in the compound [3] YBa2CU3OT_y raises many questions about the origin and properties of superconducting electrons. Among the initial studies characterizing these compounds are many involving substitutions of elements on the Y site [4, 5]. Surprisingly, a depression of T~ is rarely observed for full substitutions on the Y sites even if the substituted atom is a rare earth with large magnetic moment and crystal field. The specific heat measurement [6] seemed to indicate that there appears AF ordering at 2.2 K in Ba2GdCu3 O 7 _ y , which awakes other interesting questions of whether superconductivity and AF ordering in this compound could coexist and what is the cause of the AF ordering. In this Communication, we report other experimental results for Ba2GdCu307_y , which strongly suggest that the magnetic transition in Ba2GdCu307_y is not simply correspondent to AF ordering and that the dominant magnetic interaction in these materials is not R K K Y type interaction but dipole-dipole interaction. The samples were prepared by mixing powders of Gd203, CuO, BaCO 3 in appropriate ratios. The powders were ground, reacted in air at 930°C for 16 h and cooled to room temperature within 6h. Then, the powders were reground, pressed into ~b13 mm pellets, sintered at 970°C for 14h and cooled to room temperature within 9 h. Sample 1 was produced by annealing the as-grown sample in vacuum (4 x 10-2 torr.) at 800°C for 10h. Sample 2 was prepared by annealing the as-grown one in air at 700°C for 2 h and cooling it to room temperature within 3 h. Sample 3 is
* The project is supported by National Natural Science Foundation of China.
the as-grown one. Sample 4 was obtained by annealing the as-grown one at 700°C for 15 h and cooling to room temperature within 6 h in flowing oxygen. Sample 5 was produced by annealing the as-grown with nominal composition Ba0.65Gd0.35CuOy, in an almost sealed tube at 800°C for 15 h and cooling to room temperature within 6 h in flowing oxygen. X-ray diffraction was carried using monochromated CuKct radiation. The standard four probe technique was used to measure the resistivity. The magnetization and Meissner effect measurements were made on an extracting magnetometer in which the magnetic field can be continuously varied from 0-8 x 104Oe, the inhomogeneity of the field being less than 10 -4 in the diameter of 60mm. The temperature was computer controlled within the range of 1.5-300 K to be an accuracy of _ 0.01 K. The experimental error of the magnetization was better than 5 x 10-4emu. The temperature dependence of resistivity for sample 5 is shown in Fig. l(a). The sample exhibits a good metal behavior with Q300K~ 750#f~cm and Q300K/QL00k~ 2.5, and a good superconductivity with Tc onset about 98 K, T~ zero-resistance about 94.5 K and AT~ < 1.5 K. The Meissner flux expulsion for the sample (Fig. 1 cross line) reaches about 43% with no paramagnetism correction. Fig. l(b) also shows the susceptibility result for the sample of cooling it in 10 KOe (solid circle). At low temperatures, no kink in susceptibility could be observed down to 1.5 K. It seems to be that no magnetic transition takes place in this sample down to 1.5 K. However, our specific heat measurement [7] does show magnetic transition at 2.23 K in this sample. Figure 2 shows the magnetization curve at 1.5 K for this sample. The curve saturates at about 3.5 tesla indicating that magnetic ordering
1395
1396
MAGNETISM AND SUPERCONDUCTIVITY IN Ba2GdCu3OT_y
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Vol. 65, No. 11
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Fig. 2. The magnetization curve at 1.5 K for sample 5. be lower than warming one at low temperature and there exists kinks at about 2.1 K and 2.4 K respectively. In Fig. 4, we show the susceptibility at tem-peratures between 1.5 to 15K for five samples of cooling them in 10KOe. According to sample preparation conditions, we believe that the oxygen contents should increase with the sample number. For sample 1 showing no superconductivity and tetragonal structure, the oxygen content should be less than 6.5. For the others having orthorhombic structure, the oxygen content should be larger than 6.7. Therefore, one could see that the kinks are lowered and smeared out as oxygen contents are increased. Table 1 lists electrical and magnetic properties for all five samples. The electrical and magnetic properties for these samples are greatly different. However, our specific heat measurements [7] on samples 2 and 5 showed no variation of magnetic transition temperature (2.23 K).
Table 1. Electrical and magnetic properties for different samples No. Q3ooK(QCm) Tcm(K) AT,(K) M.F. (%) O(T < 200K) P~(T < 200K) O(T > 200K) Pe~(T > 200K) T,-(K)
1
2
3
4
5
5.4 6.0K 8.0 6.0K 8.0 2.2
3 × 10 3 96 5.0 35 3.5K 8.0 -18K 7.6 2.3
1.8 X 10 -3 93 3.0 50 4.0K 8.0 -16K 7.6 2.2
9.0 × 10 -4 95.5 1.0 62 2.0K 8.0 -13K 7,7 2.1
7.5 × 10 -4 96.5 1.5 43 3.0K 8.0 -20K 7.6 -
M.F. - - Meissner fraction. * in units of #8. Tr - the temperatures corresponding to the kinks in Fig. 4.
Vol. 65, N o . 11
MAGNETISM
AND
I N Ba~GdCu307_y
SUPERCONDUCTIVITY
15
1397
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T(K) Fig. 3. The temperature dependences of susceptibility for samples 3 and 4. The solid circle presents the susceptibility cooled in 10KOe and open circle presents one warmed in 10KOe for sample 3. The solid triangle and open triangle present the susceptibility cooled and warmed in 10 KOe for sample 4, respectively. The products z T via T for three samples are shown in Fig. 5. For sample 3 (solid triangle) and sample 5 (solid circle), the products tend to increase with temperature between 100 to 200 K and decrease with temperature above 200 K. The peaks at about 210 and 240 K in the curves can not be attributed to experiment fluctuation since all four superconducting samples have almost the same characters. The increase
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Fig. 4. The susceptibility for five samples of cooling them in 10 KOe. Solid circle for sample 1, open circle for sample 2, solid triangle for sample 3, open triangle for sample 4, solid square for sample 5.
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T0 ) Fig. 5. Temperature dependence of the products z T for three samples. Solid circle for sample 5, open circle for sample 1 and triangle for sample 3. of z T with T means paramagnetism followed by Z = C/(T + O) with positive 0 while the decrease of z T with T means paramagnetism followed by g = C/(T + O) with negative 0. As seen from table l, the effective moment of Gd ion for four superconducting samples are all equal to about 8.0#8 in the temperature range between 100 to 200 K. Above 200 K, we neglect those fine structures and obtain fitting parameters of Pefr ~ 7.6/~8/Gd and 0 ,-~ - 1 3 to -20K. The novel behaviors appearing in superconducting samples do not exist in nonsuperconducting samples as seen from Fig. 5 (open circle for sample 1). Above results indicate that oxygen vacancies of these materials play a crucial role in the structure symmetry, electrical properties and magnetic behaviors at low temperatures but they have little effect on magnetic transition temperature [7]. This suggests that the dominant magnetic interaction in these materials cannot be the R K K Y interaction since the R K K Y interaction and superconductivity are both sensitive to the electronic states at Fermi level. Unaffected transition temperature by oxygen vacancies suggest that dipole~tipole interaction may be a main interaction which is responsible for the magnetic transition in these materials. The novel behaviors like irreversibility
1398
MAGNETISM AND SUPERCONDUCTIVITY IN Ba2GdCu307_y
of susceptibility at low temperatures, sign change of 0 with temperature, decrease of effective moment at T > 200 K, inconsistency of transition temperature between magnetic and specific heat measurement, etc, cannot be understood if one thinks that the magnetic transition observed in specific heat is simply correspondent to antiferromagnetic ordering.
2.
3.
4. Acknowledgements - - The authors would like to thank Zhao Yu-ying for X-ray analysis and Dr. Xu Ru-fong for useful discussion.
REFERENCES M.K. Mu, J.R. Ashbun, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang & C.W. Chu, Phys. Rev. Lett. 58, 908 (1987).
5. 6. 7.
Vol. 65, No. 11
P.H. Hor, L. Gao, R.L. Meng, Z.J. Huang, Y.Q. Wang, K. Forster, J. Vassilious, C.W. Chu, M.K. Wu, J.R. Ashburn & C.J. Torng, Phys. Rev. Lett. 58, 911 (1987). R.J. Cava, B. Battogg, R.B. Van Dover, D.W. Murphy, S. Sunshine, T. Siegrist, J.P. Remeka, E.A. Rietman, S. Zahurak & G.P. Espinosa, Phys. Rev. Lett. 58, 1676 (1987). L.C. Porter, R.J. Thorn, U. Geiser, A. Umezawa, H.H. Wang, W.K. Kwok, H.I. Kao, M. Monaghan, G.W. Crobtree, K.D. Carison & J.M. Williams, Inorganic Chemistry (in press). Z. Fish, J.D. Thompson, E. Zirngiebl, J.L. Smith & S.W. Cheong, Solid State Commun. (in press). J.C. Ho, P.H. Hor, R.L. Meng, C.W. Chu & C.Y. Huang, Solid State Commun. 63, 711 (1987). Chuan-xing Zhu, Zhao Guo-meng & Qi-ze Ran, (submitted to Solid State Commun.).