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A method for doping Tl-based cuprate superconductors with holes
PHYSICA
Physica C 197 (1992) 385-388 North-Holland
A method for doping Tl-based cuprate superconductors with holes T e t s u y u k i K a n e k o , K a z u y u k i H a m a d a , Seiji A d a c h i a n d H. Y a m a u c h i Superconductivity Research Laboratory, International Superconductivity Technology Center, 10-13 Shinonome 1-chome, Koto-ku, Tokyo 135, Japan Received 10 April 1992
We previously demonstrated that the Tc for a TI-2223 superconductor was increased to above 125 K when the sample was encapsulated in an evacuated quartz tube and post-annealed at 750°C for 250 h. It was indicated by thermo-electric power measurements that the post-annealed TI-2223 sample had a hole concentration higher than an as-sintered TI-2223 sample. Moreover, when the same encapsulation-and-post-annealing (ECPA) technique was applied to a TL-2212 phase and a T1-2234sample, the Tc decreased in the case of TI-2212 while it increased in the case of TL-2234 phase. This demonstrated that the ECPA technique was effective in increasing the hole concentration in both over-doped (i.e. TI-2212) and under-doped (i.e. T1-2223 and TI-2234) phases. Judging from the changes in the lattice constants and the chemical composition, it was likely that the increases in hole concentration and the subsequent changes in T¢were due to decreases in the TI content during ECPA processes.
1. Introduction In 1988, Sheng et al. [1 ] discovered superconductivity in the T 1 - B a - C a - C u - O system and, subsequently, Parkin et al. [2] confirmed that the T1"2223" phase, i.e. T12Ba2Ca2Cu3OIo, had a zero-resistance temperature o f 125 K. Adachi et al. [3] reported that a long-period sample annealing in a closed system was effective in increasing Tc for the T1-2223 phase. Recently, we successfully synthesized T]-2223 superconductors with Tc higher than 125 K [4]. The highest values for zero-resistance temperature ( T~n=° ) and diamagnetic onset temperature ( T mag ) were 127 and 130 K, respectively. Liu et al. [5] also obtained Tc higher than 125 K for a T1-2223 superconductor by means o f a synthesis technique parallel to ours. In this work we investigate the reason for the "higher temperaturization" (or the increase in To) o f the T1-2223 phase by the encapsulation-and-postannealing (ECPA) technique and apply this technique to Tl-based superconductors o f other types.
2. Experimental
T1203, BaO2, CaO and CuO as starting materials. The powders were mixed to a nominal composition of Tll.TBa2Ca2.3Cu3Oz. First, samples were sintered at 890°C for 5 h. Next, the sintered samples were encapsulated in an evacuated quartz tube and post-annealed at 750°C for various periods of time ( 2 400 h). Details o f the sample preparation process were given elsewhere [4]. For the T1-2212 and T12234 phases, the starting powders were mixed to nominal compositions o f T12Ba2CaCu2Oz and Tll.7Ba2Ca3.3Cu4Oz, respectively. The sample o f T12212 phase was sintered at 900 °C for 1 h in flowing oxygen gas, while that of T1-2234 phase at 860°C for 2.5 h in air [ 6 ]. Structural properties o f the samples were studied by X-ray powder diffraction ( X R D ) using Cu Kit radiation. The DC magnetic susceptibility was measured using a S Q U I D magnetometer (Quantum Design: Model MPMS ) in the field-cooled mode with a DC magnetic field or 10 Oe. Electrical resistivity was measured using a standard four-probe method. The thermo-electric power ( T E P ) was measured by a DC differential method in which the temperature gradient across the sample was determined using two pairs o f copper-constantan thermocouples [71.
The samples o f the T1-2223 phase were prepared by solid state reaction using high-purity powders o f 0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
T. Kaneko et al. / Hole doping of Tl-based superconductors
386
3. Results and discussion Figure 1 shows the relationship between the squareroot of post-annealing time (t) and the diamagnetic onset-temperature ( T ~ a~) for the T1-2223 samples which were encapsulated during post-annealing. As shown in fig. 1, when t was shorter than 250 h, the T mag increased as t increased. The sample which was post-annealed for 250 h had the highest zero-resistance temperature ( T R=°) of 127 K. When t exceeded 250 h, T mag leveled offaround 130 K. In the range of t shorter than 250 h, the increase in T mag was linearly proportional to the square-root of t. This suggests that a certain atomic diffusion process was involved during post-annealing. Figure 2 shows the temperature dependences of 132 2'5 Oh .i............
130
'
128 lOh
126
80 h
124
F-
40011
2h
122 121 0
4t
i
t
i
5
10
15
(t: P o s t - a n n e a l i n g
20
Time)
Fig. 1. Relationship between T ~ "8 and the square-root of the postannealing time for the samples which were encapsulated during post-annealing.
6
• •
4 =I. I,I.I I,-
as-sintered
o
A
2
• o
•
o
o
o
o
o
• o
•
•
•
nn~t rvv-- a n n e a l e d ° ° o o o o • ,
for 250 h.
° o
0
-2 I01
150
200
250
300
Temperature (K) Fig. 2. Temperature dependence ofthermo-electric power for the as-sintered 2223 sample and the 2223 sample encapsulated and post-annealed for 250 h.
thermo-electric power (TEP) for an as-sintered sample ( T ~ a g-- 122 K) and for a sample encapsulated and post-annealed for 250 h (Tmag= 130 K). The major carriers in both samples were considered to be holes, because both sets of TEP data were positive. When the two sets of data were compared, the magnitude of the TEP signal for the post-annealed sample was smaller than that for the as-sintered sample. It is generally considered that the magnitude of the TEP signal decreases with increasing carrier (hole) concentration. Thus, it was demonstrated that the combination of sample encapsulation in an evacuated tube with post-annealing at 750°C, i.e., the "ECPA" process, was effective in increasing the carrier concentration in the sample. The effects of the ECPA process on other Tl-based cuprate superconductors were also investigated. One of the Tl-based cuprate superconductor samples was of the T1-2212 phase and another was of the T1-2234 phase, both phases of the T 1 - B a - C a - C u - O system. A T1-2212 sample sintered in 1 atm oxygen gas has been considered to be in an over-doped state [8,9]. The present as-sintered 2212 sample was also in an over-doped state since the Tc increased as the oxygen content decreased. On the other hand, for the T1-2234 phase, there have been only a few studies on the relationship between Tc and hole concentration. Presland et al. [ 10] reported that the T1-2234 samples, which were synthesized from off-stoichiometric nominal compositions of Tlz_xBazCa4+:,CusOz, were in under-doped states. We previously reported that Tc for a T1-2234 sample was increased by post-annealing in high-pressure oxygen gas [ 6 ]. So, it was considered that the T1-2234 sample was in an underdoped state. The 2212 and 2234 samples were encapsulated in evacuated quartz tubes and annealed under the same conditions (i.e., at 750°C for 250 h) as those employed for obtaining the 2223 sample with the highest To-value. The temperature dependences of electrical resistivity for the as-sintered and postannealed 2212 samples are shown in fig. 3. Those for the as-sintered and post-annealed 2234 samples are given in fig. 4. In the case of the 2212 phase, Tc decreased after the ECPA process. In contrast, that for the 2234 phase increased after the same process. These results are consistent with the idea that the ECPA process works in Tl-based superconductors to increase that carrier (hole) concentration. That is,
T. Kaneko et al. /Hole doping of Tl-based superconductors
What then was the origin of the doped carriers? There are two possibilities. One is a change in the oxygen content and the other a variation in the T1 content. The relationships between the duration of post-annealing and the lattice constants are shown in fig. 5. The a-axis changed little with post-annealing. On the other hand, the c-axis was elongated as the post-annealing time became long. That is, the higher the value of To, the longer the length of the c-axis. For the double T I - O layered superconductors, it was reported that, when the oxygen content in the sample increased, the length of the c-axis rather decreased [ 9,11 ]. Thus, it is unlikely that the increase in the carrier concentration was due to an increase in the oxygen content of the sample, because the caxis was rather elongated in the present case. The chemical composition of the sample was analyzed by an energy-dispersive X-ray analysis technique (EDX). The analyzed local chemical compositions were, respectively, T12.oBaL9Cal.7Cu3Oz and Tl,.9Bax.9Cal.7Cu3Ozfor the as-sintered 2223 sample and for the 2223 sample encapsulated and post-annealed for 250 h. Only the T1 among the cation elements was reduced during post-annealing. I f the oxygen content in the sample did not increase after postannealing, the increase in the carrier concentration should be attributed to a deficiency in Tl. The reduction in T1 content by 0.1 out of 2.0 may correspond to creating 0. l holes per Cu-O2. Such an increment in the hole concentration made Tc of an under-doped superconductor, i.e., the as-sintered sample, increase. Thus, one of the major reasons for the increase in Tc of the T1-2223 phase was thought to be the hole doping caused by T1 vacancies created by T1 evaporation. Such Tl vacancies may be filled with Cu or Ca atoms. However, at present we have
0.003
as-s~
....
E
~
o.oo2
•:
"~,
0.001
=: 0.00, 100
o ~ o 105
j,
, 110
115
120
Temperature (K)
Fig. 3. Temperature dependence of resistivity for the as-sintered 2212 sample and the 2212 sample encapsulated and post-annealed for 250 h.
0.002
•
...
as-sintered
_~ o.ool Z~
•
o° post-annealed
=
o
,m
•
o
a-
f
0.000 100
,~ 110
o
.~=~ o , 120 130 Temperature (K)
38 7
140
Fig. 4. Temperature dependence of resistivity for the as-sintered 2234 sample and the 2234 sample encapsulated and post-annealed for 250 h.
for an over-doped 2212 sample, Tc decreased and, for an under-doped 2234 sample, T~ was raised as the carrier concentration increased. 3.050
35.72
3.~9j
•
•
35.70
3.848
A
o<
3.847
0
3.846
3s.sa
35.66 35.64
3.845 0
I
I
100
200
Post-annealing Time (h)
3OO
35.62
0
100
200
300
Post-annealing Time (h)
Fig. 5. Relationship between the lattice constants, a and c, and the post-annealing time for the samples which were encapsulated during post-annealing.
388
T. Kaneko et al. ~Hole doping of Tl-based superconductors
no experimental d a t a to discuss this possibility. Moreover, it is not quite sure at present whether T~ d e p e n d s only on the carrier concentration. There might be other factors that control Tc, as for example, an ordering o f cations. More detailed studies on this m e c h a n i s m are required for further enhancem e n t o f To.
4. Conclusions It was revealed that the ECPA process was effective in increasing Tc for Tl-based superconductors. in the case o f the 2223 phase, the increase in Tc was f o u n d to fit to a root-square function o f the post-annealing time in the range o f 2 to 250 h. The highest values for T ~ =° a n d Tcmag were recorded at 127 K a n d 130 K, respectively. Thermo-electric power m e a s u r e m e n t s revealed that the carriers were holes and that the carrier concentration was increased by the ECPA process. Applying the same ECPA process, Tc decreased for the 2212 sample which was overd o p e d and increased for the 2234 sample which was under-doped. The change o f the lattice constants in terms o f post-annealing time and the analysis o f chemical c o m p o s i t i o n i n d i c a t e d that the increase in carrier concentration was not due to an increase in the oxygen content but to a reduction in the T1 content.
Acknowledgements The authors are grateful to H. Takei a n d K. T a d a o f S u m i t o m o Electric Industries, Ltd., R.S. Liu and W.Y. Liang o f I R C in Superconductivity, U n i v e r s i t y o f Cambridge, and R. Beyers, o f IBM A l m a d e n Research Center for their helpful discussions. This work was s u p p o r t e d by the New Energy a n d Industrial Technology D e v e l o p m e n t Organization in the Program for R & D o f Basic Technology for Future Industries.
References [1 ] Z.Z. Sheng and A.M. Hermann, Nature 332 (1988 ) 138. [2] S.S. Parkin, V.Y. Lee, E.M. Engler, A.I. Nazzel, T.C. Huang, G. Gormann, R. Savoy and R. Beyers, Phys. Rev. Lett. 60 (1988) 2539. [3] S. Adacbi, K. Mizuno, K. Setsune and K. Wasa, Physica C 171 (1990) 543. [4] T. Kaneko, H, Yamauchi and S. Tanaka, Physica C 178 (1991) 377. [5] R.S. Liu, J.L. Tallon and P.P. Edwards, Physica C 182 (1991) 119. [6] T. Kaneko, K. Hamada, S. Adachi, H. Yamauchi and S. Tanaka, J. Appl. Phys. 71 (1992) 2347. [7] K. Matsuura, T. Wada, Y. Yaegashi, S. Tajima and H. Yamauchi, in preparation for publication. [8] C. Martin, A. Maignan, J. Provost, C. Michel, M. Hervieu, R. Tournier and B. Raveau, Physica C 168 (1990) 8. [9] M.R. Presland, J.L. Tallon, R.G. Buckly, R.S. Liu and N.E. Flower, Physica C 176 ( 1991 ) 95. [10] M.R. Presland, J.L. Tallon, P.W. Gilberd and R.S. Liu, Physica C 191 (1992) 307. [ 11 ] Y, Shimakawa, Y. Kubo, T. Manako and H. Igarashi, Phys. Rev. B40 (1989) 11400.
Physica C 197 (1992) 385-388 North-Holland
A method for doping Tl-based cuprate superconductors with holes T e t s u y u k i K a n e k o , K a z u y u k i H a m a d a , Seiji A d a c h i a n d H. Y a m a u c h i Superconductivity Research Laboratory, International Superconductivity Technology Center, 10-13 Shinonome 1-chome, Koto-ku, Tokyo 135, Japan Received 10 April 1992
We previously demonstrated that the Tc for a TI-2223 superconductor was increased to above 125 K when the sample was encapsulated in an evacuated quartz tube and post-annealed at 750°C for 250 h. It was indicated by thermo-electric power measurements that the post-annealed TI-2223 sample had a hole concentration higher than an as-sintered TI-2223 sample. Moreover, when the same encapsulation-and-post-annealing (ECPA) technique was applied to a TL-2212 phase and a T1-2234sample, the Tc decreased in the case of TI-2212 while it increased in the case of TL-2234 phase. This demonstrated that the ECPA technique was effective in increasing the hole concentration in both over-doped (i.e. TI-2212) and under-doped (i.e. T1-2223 and TI-2234) phases. Judging from the changes in the lattice constants and the chemical composition, it was likely that the increases in hole concentration and the subsequent changes in T¢were due to decreases in the TI content during ECPA processes.
1. Introduction In 1988, Sheng et al. [1 ] discovered superconductivity in the T 1 - B a - C a - C u - O system and, subsequently, Parkin et al. [2] confirmed that the T1"2223" phase, i.e. T12Ba2Ca2Cu3OIo, had a zero-resistance temperature o f 125 K. Adachi et al. [3] reported that a long-period sample annealing in a closed system was effective in increasing Tc for the T1-2223 phase. Recently, we successfully synthesized T]-2223 superconductors with Tc higher than 125 K [4]. The highest values for zero-resistance temperature ( T~n=° ) and diamagnetic onset temperature ( T mag ) were 127 and 130 K, respectively. Liu et al. [5] also obtained Tc higher than 125 K for a T1-2223 superconductor by means o f a synthesis technique parallel to ours. In this work we investigate the reason for the "higher temperaturization" (or the increase in To) o f the T1-2223 phase by the encapsulation-and-postannealing (ECPA) technique and apply this technique to Tl-based superconductors o f other types.
2. Experimental
T1203, BaO2, CaO and CuO as starting materials. The powders were mixed to a nominal composition of Tll.TBa2Ca2.3Cu3Oz. First, samples were sintered at 890°C for 5 h. Next, the sintered samples were encapsulated in an evacuated quartz tube and post-annealed at 750°C for various periods of time ( 2 400 h). Details o f the sample preparation process were given elsewhere [4]. For the T1-2212 and T12234 phases, the starting powders were mixed to nominal compositions o f T12Ba2CaCu2Oz and Tll.7Ba2Ca3.3Cu4Oz, respectively. The sample o f T12212 phase was sintered at 900 °C for 1 h in flowing oxygen gas, while that of T1-2234 phase at 860°C for 2.5 h in air [ 6 ]. Structural properties o f the samples were studied by X-ray powder diffraction ( X R D ) using Cu Kit radiation. The DC magnetic susceptibility was measured using a S Q U I D magnetometer (Quantum Design: Model MPMS ) in the field-cooled mode with a DC magnetic field or 10 Oe. Electrical resistivity was measured using a standard four-probe method. The thermo-electric power ( T E P ) was measured by a DC differential method in which the temperature gradient across the sample was determined using two pairs o f copper-constantan thermocouples [71.
The samples o f the T1-2223 phase were prepared by solid state reaction using high-purity powders o f 0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
T. Kaneko et al. / Hole doping of Tl-based superconductors
386
3. Results and discussion Figure 1 shows the relationship between the squareroot of post-annealing time (t) and the diamagnetic onset-temperature ( T ~ a~) for the T1-2223 samples which were encapsulated during post-annealing. As shown in fig. 1, when t was shorter than 250 h, the T mag increased as t increased. The sample which was post-annealed for 250 h had the highest zero-resistance temperature ( T R=°) of 127 K. When t exceeded 250 h, T mag leveled offaround 130 K. In the range of t shorter than 250 h, the increase in T mag was linearly proportional to the square-root of t. This suggests that a certain atomic diffusion process was involved during post-annealing. Figure 2 shows the temperature dependences of 132 2'5 Oh .i............
130
'
128 lOh
126
80 h
124
F-
40011
2h
122 121 0
4t
i
t
i
5
10
15
(t: P o s t - a n n e a l i n g
20
Time)
Fig. 1. Relationship between T ~ "8 and the square-root of the postannealing time for the samples which were encapsulated during post-annealing.
6
• •
4 =I. I,I.I I,-
as-sintered
o
A
2
• o
•
o
o
o
o
o
• o
•
•
•
nn~t rvv-- a n n e a l e d ° ° o o o o • ,
for 250 h.
° o
0
-2 I01
150
200
250
300
Temperature (K) Fig. 2. Temperature dependence ofthermo-electric power for the as-sintered 2223 sample and the 2223 sample encapsulated and post-annealed for 250 h.
thermo-electric power (TEP) for an as-sintered sample ( T ~ a g-- 122 K) and for a sample encapsulated and post-annealed for 250 h (Tmag= 130 K). The major carriers in both samples were considered to be holes, because both sets of TEP data were positive. When the two sets of data were compared, the magnitude of the TEP signal for the post-annealed sample was smaller than that for the as-sintered sample. It is generally considered that the magnitude of the TEP signal decreases with increasing carrier (hole) concentration. Thus, it was demonstrated that the combination of sample encapsulation in an evacuated tube with post-annealing at 750°C, i.e., the "ECPA" process, was effective in increasing the carrier concentration in the sample. The effects of the ECPA process on other Tl-based cuprate superconductors were also investigated. One of the Tl-based cuprate superconductor samples was of the T1-2212 phase and another was of the T1-2234 phase, both phases of the T 1 - B a - C a - C u - O system. A T1-2212 sample sintered in 1 atm oxygen gas has been considered to be in an over-doped state [8,9]. The present as-sintered 2212 sample was also in an over-doped state since the Tc increased as the oxygen content decreased. On the other hand, for the T1-2234 phase, there have been only a few studies on the relationship between Tc and hole concentration. Presland et al. [ 10] reported that the T1-2234 samples, which were synthesized from off-stoichiometric nominal compositions of Tlz_xBazCa4+:,CusOz, were in under-doped states. We previously reported that Tc for a T1-2234 sample was increased by post-annealing in high-pressure oxygen gas [ 6 ]. So, it was considered that the T1-2234 sample was in an underdoped state. The 2212 and 2234 samples were encapsulated in evacuated quartz tubes and annealed under the same conditions (i.e., at 750°C for 250 h) as those employed for obtaining the 2223 sample with the highest To-value. The temperature dependences of electrical resistivity for the as-sintered and postannealed 2212 samples are shown in fig. 3. Those for the as-sintered and post-annealed 2234 samples are given in fig. 4. In the case of the 2212 phase, Tc decreased after the ECPA process. In contrast, that for the 2234 phase increased after the same process. These results are consistent with the idea that the ECPA process works in Tl-based superconductors to increase that carrier (hole) concentration. That is,
T. Kaneko et al. /Hole doping of Tl-based superconductors
What then was the origin of the doped carriers? There are two possibilities. One is a change in the oxygen content and the other a variation in the T1 content. The relationships between the duration of post-annealing and the lattice constants are shown in fig. 5. The a-axis changed little with post-annealing. On the other hand, the c-axis was elongated as the post-annealing time became long. That is, the higher the value of To, the longer the length of the c-axis. For the double T I - O layered superconductors, it was reported that, when the oxygen content in the sample increased, the length of the c-axis rather decreased [ 9,11 ]. Thus, it is unlikely that the increase in the carrier concentration was due to an increase in the oxygen content of the sample, because the caxis was rather elongated in the present case. The chemical composition of the sample was analyzed by an energy-dispersive X-ray analysis technique (EDX). The analyzed local chemical compositions were, respectively, T12.oBaL9Cal.7Cu3Oz and Tl,.9Bax.9Cal.7Cu3Ozfor the as-sintered 2223 sample and for the 2223 sample encapsulated and post-annealed for 250 h. Only the T1 among the cation elements was reduced during post-annealing. I f the oxygen content in the sample did not increase after postannealing, the increase in the carrier concentration should be attributed to a deficiency in Tl. The reduction in T1 content by 0.1 out of 2.0 may correspond to creating 0. l holes per Cu-O2. Such an increment in the hole concentration made Tc of an under-doped superconductor, i.e., the as-sintered sample, increase. Thus, one of the major reasons for the increase in Tc of the T1-2223 phase was thought to be the hole doping caused by T1 vacancies created by T1 evaporation. Such Tl vacancies may be filled with Cu or Ca atoms. However, at present we have
0.003
as-s~
....
E
~
o.oo2
•:
"~,
0.001
=: 0.00, 100
o ~ o 105
j,
, 110
115
120
Temperature (K)
Fig. 3. Temperature dependence of resistivity for the as-sintered 2212 sample and the 2212 sample encapsulated and post-annealed for 250 h.
0.002
•
...
as-sintered
_~ o.ool Z~
•
o° post-annealed
=
o
,m
•
o
a-
f
0.000 100
,~ 110
o
.~=~ o , 120 130 Temperature (K)
38 7
140
Fig. 4. Temperature dependence of resistivity for the as-sintered 2234 sample and the 2234 sample encapsulated and post-annealed for 250 h.
for an over-doped 2212 sample, Tc decreased and, for an under-doped 2234 sample, T~ was raised as the carrier concentration increased. 3.050
35.72
3.~9j
•
•
35.70
3.848
A
o<
3.847
0
3.846
3s.sa
35.66 35.64
3.845 0
I
I
100
200
Post-annealing Time (h)
3OO
35.62
0
100
200
300
Post-annealing Time (h)
Fig. 5. Relationship between the lattice constants, a and c, and the post-annealing time for the samples which were encapsulated during post-annealing.
388
T. Kaneko et al. ~Hole doping of Tl-based superconductors
no experimental d a t a to discuss this possibility. Moreover, it is not quite sure at present whether T~ d e p e n d s only on the carrier concentration. There might be other factors that control Tc, as for example, an ordering o f cations. More detailed studies on this m e c h a n i s m are required for further enhancem e n t o f To.
4. Conclusions It was revealed that the ECPA process was effective in increasing Tc for Tl-based superconductors. in the case o f the 2223 phase, the increase in Tc was f o u n d to fit to a root-square function o f the post-annealing time in the range o f 2 to 250 h. The highest values for T ~ =° a n d Tcmag were recorded at 127 K a n d 130 K, respectively. Thermo-electric power m e a s u r e m e n t s revealed that the carriers were holes and that the carrier concentration was increased by the ECPA process. Applying the same ECPA process, Tc decreased for the 2212 sample which was overd o p e d and increased for the 2234 sample which was under-doped. The change o f the lattice constants in terms o f post-annealing time and the analysis o f chemical c o m p o s i t i o n i n d i c a t e d that the increase in carrier concentration was not due to an increase in the oxygen content but to a reduction in the T1 content.
Acknowledgements The authors are grateful to H. Takei a n d K. T a d a o f S u m i t o m o Electric Industries, Ltd., R.S. Liu and W.Y. Liang o f I R C in Superconductivity, U n i v e r s i t y o f Cambridge, and R. Beyers, o f IBM A l m a d e n Research Center for their helpful discussions. This work was s u p p o r t e d by the New Energy a n d Industrial Technology D e v e l o p m e n t Organization in the Program for R & D o f Basic Technology for Future Industries.
References [1 ] Z.Z. Sheng and A.M. Hermann, Nature 332 (1988 ) 138. [2] S.S. Parkin, V.Y. Lee, E.M. Engler, A.I. Nazzel, T.C. Huang, G. Gormann, R. Savoy and R. Beyers, Phys. Rev. Lett. 60 (1988) 2539. [3] S. Adacbi, K. Mizuno, K. Setsune and K. Wasa, Physica C 171 (1990) 543. [4] T. Kaneko, H, Yamauchi and S. Tanaka, Physica C 178 (1991) 377. [5] R.S. Liu, J.L. Tallon and P.P. Edwards, Physica C 182 (1991) 119. [6] T. Kaneko, K. Hamada, S. Adachi, H. Yamauchi and S. Tanaka, J. Appl. Phys. 71 (1992) 2347. [7] K. Matsuura, T. Wada, Y. Yaegashi, S. Tajima and H. Yamauchi, in preparation for publication. [8] C. Martin, A. Maignan, J. Provost, C. Michel, M. Hervieu, R. Tournier and B. Raveau, Physica C 168 (1990) 8. [9] M.R. Presland, J.L. Tallon, R.G. Buckly, R.S. Liu and N.E. Flower, Physica C 176 ( 1991 ) 95. [10] M.R. Presland, J.L. Tallon, P.W. Gilberd and R.S. Liu, Physica C 191 (1992) 307. [ 11 ] Y, Shimakawa, Y. Kubo, T. Manako and H. Igarashi, Phys. Rev. B40 (1989) 11400.