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Dependence of the susceptibility on low magnetic fields for the electron-doped superconductors Ln2−xMxCuO4−y (Ln = Nd, Pr, Sm; M = Ce, Th)
Journal of Magnetism and Magnetic Materials 99 (1991) 239-242 North-Holland
Dependence of the susceptibility on low magnetic fields for the electron-doped superconductors Ln,_,M,CuO,_, (Ln = Nd, Pr, Sm; M = Ce, Th) N.Y. Ayoub Department
of Physics, Yarmouk University, Zrbirl, Jordan
Received 11 February 1991
We report the effect of low magnetic fields (lo-100 Oe) on the zero-field-cooled (ZFC) and field-cooled (FC) susceptibilities, for a systematic series of concentrations x, of the electron-doped copper-oxide superconducting compounds Lnz_,MXCuO.,_,, (Ln = Nd, Pr, Sm; M = Ce) and (Ln = Pr; M = Th). The concentration dependences of the FC susceptibility, measured at 5 K, peak in the range x = 0.15-0.16. Also we found that the FC susceptibility, for the various combinations Ln-M and concentrations x, varies linearly with the natural logarithm of the magnetic field.
1. Introduction The discovery of high temperature superconductivity in the electron-doped compounds, Ln,_,M,CuO.,_, (Ln = Nd, Pr, Sm; M = Ce, Th) [l-4], may hopefully, be helpful to our understanding of the relatively more complicated holedoped high temperature superconductors such as YBa,Cu,O, [5]. The electrical resistivity [6], pressure dependence [3,7], electrical and magnetic transition temperatures [6,8], as well as the zero field cooled (ZFC) susceptibility [8] of the above electron-doped superconductors have been studied experimentally. The effect of applying various low magnetic fields (10-100 Oe) on the susceptibility of these systems needs further investigation. Cronemeyer and Holtzberg, as shown in fig. 2 on p. 78 of ref. [9], have studied two crystals of YBa,Cu,O,_, with different levels of oxygenation. They found that the ZFC susceptibility of both crystals is almost unaffected by very low (less than 10 Oe) and low magnetic fields (10-100 Oe). For the field cooled (FC) susceptibility x, defined as, x = M/H where M is the volume magnetisation and H is the net applied magnetic field, the effect of very low fields (less than 10 Oe) seems to 0304~8853/91/$03.50
be negligible as can be seen from the aforementioned fig. 2 of ref. [9]. However, for low magnetic fields (10-100 Oe), the FC susceptibility, again from fig. 2 of ref. [9], varies almost linearly with log,,[ H(Oe)]. Finally Cronemeyer and Holtzberg seem to find that varying the level of oxygenation of YBa,Cu 307 ._& affects the slope of the straight line in the field range lo-100 Oe. In this paper we shall study the FC susceptibility, x, and the effect of low magnetic fields (lo100 Oe) on x for the electron-doped high temperature superconductors Ln 2_ ,M,O, _y (Ln = Nd, Pr, Sm; M = Ce) and (Ln = Pr; M = Th). Also we make sure that the oxygen deficiency, y, is the same for all the compounds, in order to obtain reliable comparisons of x between the various compounds.
2. Experimental Polycrystalline samples of Ln,_,M,CuO,_, (Ln = Nd, Pr, Sm; M = Ce) and (Ln= Pr; M= Th) were prepared by solid-state reaction of mixtures of high purity (99.99%) of Nd,O,, Pr,O,,, Sm,O,, CeO,, ThOz and CuO. Single-phased
0 1991 - Elsevier Science Publishers B.V. All rights reserved
240
N. Y. Ayoub / Susceptibility
Ln 2_-xMxC~Oq_y powders were synthesised with x = 0.12-0.18 in 0.01 increments. An extensive process of firing and intermediate grindings was carried out in order to make sure that our singlephased polycrystalline samples were indeed homogeneous. The method of preparing these samples is fully described in ref. [8]. It is worth mentioning here, that after pressing these powders into pellets at about 4 kbar pressure, and in order to achieve superconductivity in them, they were reduced and annealed in a flow of He gas at (940 k 5) o C for 24 h, then quickly cooled to room temperature over a period of 2 h. X-ray powder diffraction experiments have shown that all the samples were single-phase compounds with one of the structures of Nd,CuO,, Pr,CuO, or Sm,CuO,. Iodometric titrations performed on the above samples have indicated the same oxygen deficiency of x = 0.03 k 0.02 in each superconducting sample. Magnetic measurements were taken using a vibrating sample magnetometer (VSM) fitted with a continuous flow cryostat covering the range 2 to 500 K. The samples were cut into flat circular discs (diameter = 1 cm, thickness = 1.5 mm). The magnetic field was corrected for demagnetising effects due to the sample shape.
of electron-doped
superconductors
a30 r
0.24 t
0!30 000
0.02
004
a06 Qo8 010 Do~ant concentrat,an
x
Fig. 1. Field-cooled (FC) susceptibility or Meissner fraction -4nx, at 5 K, as determined in a field H =lO Oe, versus dopant concentration x for various Ln-M combinations in the compounds Ln,_,M,CuO,_,.
iced that the ZFC susceptibility slightly decreased with increasing magnetic field but did not follow a well defined behaviour. The story is quite different for the variation of the FC susceptibility with the magnetic field. Figs. 2, 3 and 4 display the variation of FC -4ax (at 5 K) with the natural logarithm of the magnetic field H (i.e. log(H)) for the four types of samples Ln,_,M,CuO,_, (Ln= Nd, Pr, Sm; M = Ce) and (Ln = Pr; M = Th) with x taking the values 0.14, 0.15 and 0.16. It is clear from these figures that the variation of FC -47~ with log H,
3. Results and discussion Fig. 1 shows the field-cooled (FC) susceptibilor diamagnetic fraction (Meissner ity, - 4ax, fraction) for all the Ln,_,M,CuO,_, systems is plotted against the dopant conwhere, -4~x centration x. These measurements were taken in a field of 10 Oe and at 5 K. It is worth noting that the superconducting transition temperatures for these compounds range from 20 to 26 K [6]. The data shown in fig. 1 for the four Ln-M combinations peak in the same x = 0.15-0.16 range and are narrow. This result agrees quite well with our result for the ZFC susceptibility in ref. [8]. We have measured the ZFC susceptibility and the FC susceptibility both at 5 K in six different magnetic fields in the range lo-100 Oe. We not-
ooo23 -,2.5
3.0
3.5
4b
;i
4.5. 4.
lO9lHl
Fig. 2. Variation of FC susceptibility, -4nx (at 5 K) with log(H) for the four compounds Ln,.,,M,.,,CuO,_, (Ln = Nd, Pr, Sm; M = Ce) and (Ln = Pr; M = Th).
N. Y. Ayoub / Susceptibility
of electron-doped
241
superconductors
Table 1 Values of p and 9 of eq. (1) obtained from figs. 2, 3 and 4 using least square fits
(at 5 K) with log(H) Fig. 3. Variation of FC, -4nx four compounds Ln,s,M,,,sCuO,_,.
Sm-Ce
Pr-Ce
Pr-Th
x = 0.14 P= 4=
0.185 0.026
0.155 0.016
0.270 0.029
0.014 0.003
x = 0.15 P= 4=
0.295 0.024
0.257 0.039
0.295 0.030
0.168 0.027
x = 0.16 P= 4=
0.301 0.025
0.130 0.012
0.123 0.014
0.128 0.018
for the
in the range lo-100 Oe, is linear and we can represent it by the relation -4ax=p-4
Nd-Ce
log(H),
(1)
where -4~x is dimensionless and H is in Oe. Table 1 shows the values of p and 4 for all the samples, obtained from the intercepts and slopes of the straight lines in figs. 2, 3 and 4. First, one notes from table 1 the small values of the slope 4 for all our samples, which reflects a relatively slow dependence of - 4nx on the magnetic field H. This seems to be quite different from what Cronemeyer and Holtzberg have shown in their figures on p. 78 of ref. [9] for the well oxygenated YBa,Cu,O, crystal. From table 1 one finds in general that for all the three combinations
Pr-Ce, Pr-Th, Sm-Ce, the maximum effect of H on the FC susceptibility, -4ax, is obtained at x = 0.15. However, for the combination Nd-Ce it seems that the effect is almost the same at the three concentrations, x = 0.14, 0.15 and 0.16. The errors, which are not shown in table 1 due to least square fits, in the p range from 0.002 to 0.004 and the errors in the q range from 0.0004 to 0.001. Finally it will be interesting to compare these results with other experimental results of high temperature polycrystalline superconductors whether they are of the hole-doped type or the electron-doped type.
Acknowledgements I wish to thank Prof. M.B. Maple and his group at UCSD, California, for their help in the techniques of sample preparation.
IC(5K)
References
001
s 3
25
.
30
4.0
4.5
I
4.5
LOO$
(at 5 K) with log(H) Fig. 4. Variation of FC, -4nx four compounds Ln,s,Mo~,,CuO,_,.
for the
[l] Y. Tokura, H. Takagi and S. Uchida, Nature 337 (1989) 345. [2] J.T. Markert and M.B. Maple, Solid State Commun. 70 (1989) 145. [3] J.T. Markert, E.A. Early, T. Bjomholm, S. Ghamaty, B.W. Lee, J.J. Neumeier, R.D. Price+ C.L. Seaman and M.B. Maple, Physica C 158 (1989) 178. [4] E.A. Early, N.Y. Ayoub, J. Beille, J.T. Markert and M.B. Maple, Physica C 160 (1989) 320.
242
N. Y. Ayoub / Susceptibility
[5] M.K. Wu, J.R. Ashburn, C.T. Tomg, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58 (1987) 908. [6] M.B. Maple, N.Y. Ayoub, T. Bjornholm, E.A. Early, S. Ghamaty, B.W. Lee, J.T. Marker?, J.J. Neumeier and C.L. Seaman, Physica C 162-164 (1989) 296. [7] C.L. Seaman, N.Y. Ayoub, T. Bjomholm, E.A. Early, S.
of electron-doped superconductors Ghamaty, B.W. Lee, J.T. Markert, J.J. Neumeier, P.K. Tsai and M.B. Maple, Physica C 159 (1989) 391. [8] N.Y. Ayoub, J.T. Marker%, E.A. Early, C.L. Seaman, L. PauIius and M.B. Maple, Physica C 165 (1990) 469. [9] A.P. MaIozemoff, in: Physical Properties of High Temperature Superconductors I, ed. D.M. Ginsberg (World Scientific, Singapore, 1989) chap. 3.
Dependence of the susceptibility on low magnetic fields for the electron-doped superconductors Ln,_,M,CuO,_, (Ln = Nd, Pr, Sm; M = Ce, Th) N.Y. Ayoub Department
of Physics, Yarmouk University, Zrbirl, Jordan
Received 11 February 1991
We report the effect of low magnetic fields (lo-100 Oe) on the zero-field-cooled (ZFC) and field-cooled (FC) susceptibilities, for a systematic series of concentrations x, of the electron-doped copper-oxide superconducting compounds Lnz_,MXCuO.,_,, (Ln = Nd, Pr, Sm; M = Ce) and (Ln = Pr; M = Th). The concentration dependences of the FC susceptibility, measured at 5 K, peak in the range x = 0.15-0.16. Also we found that the FC susceptibility, for the various combinations Ln-M and concentrations x, varies linearly with the natural logarithm of the magnetic field.
1. Introduction The discovery of high temperature superconductivity in the electron-doped compounds, Ln,_,M,CuO.,_, (Ln = Nd, Pr, Sm; M = Ce, Th) [l-4], may hopefully, be helpful to our understanding of the relatively more complicated holedoped high temperature superconductors such as YBa,Cu,O, [5]. The electrical resistivity [6], pressure dependence [3,7], electrical and magnetic transition temperatures [6,8], as well as the zero field cooled (ZFC) susceptibility [8] of the above electron-doped superconductors have been studied experimentally. The effect of applying various low magnetic fields (10-100 Oe) on the susceptibility of these systems needs further investigation. Cronemeyer and Holtzberg, as shown in fig. 2 on p. 78 of ref. [9], have studied two crystals of YBa,Cu,O,_, with different levels of oxygenation. They found that the ZFC susceptibility of both crystals is almost unaffected by very low (less than 10 Oe) and low magnetic fields (10-100 Oe). For the field cooled (FC) susceptibility x, defined as, x = M/H where M is the volume magnetisation and H is the net applied magnetic field, the effect of very low fields (less than 10 Oe) seems to 0304~8853/91/$03.50
be negligible as can be seen from the aforementioned fig. 2 of ref. [9]. However, for low magnetic fields (10-100 Oe), the FC susceptibility, again from fig. 2 of ref. [9], varies almost linearly with log,,[ H(Oe)]. Finally Cronemeyer and Holtzberg seem to find that varying the level of oxygenation of YBa,Cu 307 ._& affects the slope of the straight line in the field range lo-100 Oe. In this paper we shall study the FC susceptibility, x, and the effect of low magnetic fields (lo100 Oe) on x for the electron-doped high temperature superconductors Ln 2_ ,M,O, _y (Ln = Nd, Pr, Sm; M = Ce) and (Ln = Pr; M = Th). Also we make sure that the oxygen deficiency, y, is the same for all the compounds, in order to obtain reliable comparisons of x between the various compounds.
2. Experimental Polycrystalline samples of Ln,_,M,CuO,_, (Ln = Nd, Pr, Sm; M = Ce) and (Ln= Pr; M= Th) were prepared by solid-state reaction of mixtures of high purity (99.99%) of Nd,O,, Pr,O,,, Sm,O,, CeO,, ThOz and CuO. Single-phased
0 1991 - Elsevier Science Publishers B.V. All rights reserved
240
N. Y. Ayoub / Susceptibility
Ln 2_-xMxC~Oq_y powders were synthesised with x = 0.12-0.18 in 0.01 increments. An extensive process of firing and intermediate grindings was carried out in order to make sure that our singlephased polycrystalline samples were indeed homogeneous. The method of preparing these samples is fully described in ref. [8]. It is worth mentioning here, that after pressing these powders into pellets at about 4 kbar pressure, and in order to achieve superconductivity in them, they were reduced and annealed in a flow of He gas at (940 k 5) o C for 24 h, then quickly cooled to room temperature over a period of 2 h. X-ray powder diffraction experiments have shown that all the samples were single-phase compounds with one of the structures of Nd,CuO,, Pr,CuO, or Sm,CuO,. Iodometric titrations performed on the above samples have indicated the same oxygen deficiency of x = 0.03 k 0.02 in each superconducting sample. Magnetic measurements were taken using a vibrating sample magnetometer (VSM) fitted with a continuous flow cryostat covering the range 2 to 500 K. The samples were cut into flat circular discs (diameter = 1 cm, thickness = 1.5 mm). The magnetic field was corrected for demagnetising effects due to the sample shape.
of electron-doped
superconductors
a30 r
0.24 t
0!30 000
0.02
004
a06 Qo8 010 Do~ant concentrat,an
x
Fig. 1. Field-cooled (FC) susceptibility or Meissner fraction -4nx, at 5 K, as determined in a field H =lO Oe, versus dopant concentration x for various Ln-M combinations in the compounds Ln,_,M,CuO,_,.
iced that the ZFC susceptibility slightly decreased with increasing magnetic field but did not follow a well defined behaviour. The story is quite different for the variation of the FC susceptibility with the magnetic field. Figs. 2, 3 and 4 display the variation of FC -4ax (at 5 K) with the natural logarithm of the magnetic field H (i.e. log(H)) for the four types of samples Ln,_,M,CuO,_, (Ln= Nd, Pr, Sm; M = Ce) and (Ln = Pr; M = Th) with x taking the values 0.14, 0.15 and 0.16. It is clear from these figures that the variation of FC -47~ with log H,
3. Results and discussion Fig. 1 shows the field-cooled (FC) susceptibilor diamagnetic fraction (Meissner ity, - 4ax, fraction) for all the Ln,_,M,CuO,_, systems is plotted against the dopant conwhere, -4~x centration x. These measurements were taken in a field of 10 Oe and at 5 K. It is worth noting that the superconducting transition temperatures for these compounds range from 20 to 26 K [6]. The data shown in fig. 1 for the four Ln-M combinations peak in the same x = 0.15-0.16 range and are narrow. This result agrees quite well with our result for the ZFC susceptibility in ref. [8]. We have measured the ZFC susceptibility and the FC susceptibility both at 5 K in six different magnetic fields in the range lo-100 Oe. We not-
ooo23 -,2.5
3.0
3.5
4b
;i
4.5. 4.
lO9lHl
Fig. 2. Variation of FC susceptibility, -4nx (at 5 K) with log(H) for the four compounds Ln,.,,M,.,,CuO,_, (Ln = Nd, Pr, Sm; M = Ce) and (Ln = Pr; M = Th).
N. Y. Ayoub / Susceptibility
of electron-doped
241
superconductors
Table 1 Values of p and 9 of eq. (1) obtained from figs. 2, 3 and 4 using least square fits
(at 5 K) with log(H) Fig. 3. Variation of FC, -4nx four compounds Ln,s,M,,,sCuO,_,.
Sm-Ce
Pr-Ce
Pr-Th
x = 0.14 P= 4=
0.185 0.026
0.155 0.016
0.270 0.029
0.014 0.003
x = 0.15 P= 4=
0.295 0.024
0.257 0.039
0.295 0.030
0.168 0.027
x = 0.16 P= 4=
0.301 0.025
0.130 0.012
0.123 0.014
0.128 0.018
for the
in the range lo-100 Oe, is linear and we can represent it by the relation -4ax=p-4
Nd-Ce
log(H),
(1)
where -4~x is dimensionless and H is in Oe. Table 1 shows the values of p and 4 for all the samples, obtained from the intercepts and slopes of the straight lines in figs. 2, 3 and 4. First, one notes from table 1 the small values of the slope 4 for all our samples, which reflects a relatively slow dependence of - 4nx on the magnetic field H. This seems to be quite different from what Cronemeyer and Holtzberg have shown in their figures on p. 78 of ref. [9] for the well oxygenated YBa,Cu,O, crystal. From table 1 one finds in general that for all the three combinations
Pr-Ce, Pr-Th, Sm-Ce, the maximum effect of H on the FC susceptibility, -4ax, is obtained at x = 0.15. However, for the combination Nd-Ce it seems that the effect is almost the same at the three concentrations, x = 0.14, 0.15 and 0.16. The errors, which are not shown in table 1 due to least square fits, in the p range from 0.002 to 0.004 and the errors in the q range from 0.0004 to 0.001. Finally it will be interesting to compare these results with other experimental results of high temperature polycrystalline superconductors whether they are of the hole-doped type or the electron-doped type.
Acknowledgements I wish to thank Prof. M.B. Maple and his group at UCSD, California, for their help in the techniques of sample preparation.
IC(5K)
References
001
s 3
25
.
30
4.0
4.5
I
4.5
LOO$
(at 5 K) with log(H) Fig. 4. Variation of FC, -4nx four compounds Ln,s,Mo~,,CuO,_,.
for the
[l] Y. Tokura, H. Takagi and S. Uchida, Nature 337 (1989) 345. [2] J.T. Markert and M.B. Maple, Solid State Commun. 70 (1989) 145. [3] J.T. Markert, E.A. Early, T. Bjomholm, S. Ghamaty, B.W. Lee, J.J. Neumeier, R.D. Price+ C.L. Seaman and M.B. Maple, Physica C 158 (1989) 178. [4] E.A. Early, N.Y. Ayoub, J. Beille, J.T. Markert and M.B. Maple, Physica C 160 (1989) 320.
242
N. Y. Ayoub / Susceptibility
[5] M.K. Wu, J.R. Ashburn, C.T. Tomg, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58 (1987) 908. [6] M.B. Maple, N.Y. Ayoub, T. Bjornholm, E.A. Early, S. Ghamaty, B.W. Lee, J.T. Marker?, J.J. Neumeier and C.L. Seaman, Physica C 162-164 (1989) 296. [7] C.L. Seaman, N.Y. Ayoub, T. Bjomholm, E.A. Early, S.
of electron-doped superconductors Ghamaty, B.W. Lee, J.T. Markert, J.J. Neumeier, P.K. Tsai and M.B. Maple, Physica C 159 (1989) 391. [8] N.Y. Ayoub, J.T. Marker%, E.A. Early, C.L. Seaman, L. PauIius and M.B. Maple, Physica C 165 (1990) 469. [9] A.P. MaIozemoff, in: Physical Properties of High Temperature Superconductors I, ed. D.M. Ginsberg (World Scientific, Singapore, 1989) chap. 3.