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Nitroxyl radical adsorption on superconducting yttrium ceramics increases its critical current
N.S. Belousov et al. / Nitroxyl radical adsorption on YBa~CujOr_6
extracted a core of 3 mm in diameter, which later was used for the AC susceptibility measurements. The critical transport current was determined from the magnetic hysteresis after one magnetization cycle in a range of +20 Oe [6-8]. The ring magnetization was calculated as the difference between maximal magnetization and that provided by the granules. The AC magnetic susceptibility as a function of temperature was measured with an AC susceptometer (home made). The frequency of the applied magnetic field, o9, was 400 Hz, and the exiting field amplitude, Hm, was 0.35 Oe. Then we analyzed the influence of pure chloroform on the SC properties of yttrium ceramics. After the SC-transition temperature, To, and the magnitude of the critical transport current, I¢, were measured the bulk sample of yttrium ceramics was dipped into pure chloroform and kept there for a period of 1 to 30 days. The values of Tc and lc measured afterwards practically coincided with the previous ones even after 30 days treatment. Moreover, the control samples kept in air for this time period appeared to degrade and Tc and I~ decreased by 0.4% and 7%, respectively.
3. R e s u l t s and d i s c u s s i o n
Figure I shows the critical transport current (below the word "transport" will be omitted) as a function of temperature. Apparently, the adsorbed radical provides a significant effect on the critical current ]c,A 1.0
171
value in the temperature range 77.4-88.0 K. Figure 1 shows some peculiarities of this effect. Firstly, if the radical concentration is 1.7 X 10-z wt.% then the increase of the critical current is observed within the whole temperature interval under study. To quantitatively estimate the observed effect let us introduce the value cYx=Ic/l,.o determined by the ratio of the critical current in the treated sample to that in the initial one. Index x (in wt.%) corresponds to the radical concentration in the sample. One can see that ao.o~7 is equal to 1.05, 2.8, and 3.4 at 77.4, 81.0 and 83.0 K, respectively. Thus, with increasing temperature the critical current in the treated sample becomes higher than in the initial sample. Secondly, a further increase of the radical concentration in the sample decreases the critical current in the sample. In this case at 77.4 K the critical current value of the treated sample is less than that of the initial sample, i.e. coefficient ax becomes less than 1 :ao.o42=0.91, ao.0a5 = 0.85 and ao.t 7= 0.8 I. However, with increasing temperature the critical current in the treated sample begins to exceed that in the initial one and the c~ exceeds 1 once again. The temperature at which a x = 1 is called the threshold temperature Tth. Figure 2 shows the dependence of the threshold temperature on the radical concentration in the sample. Tt~ for the sample with 1.7X 10 - 2 WL% or radical was obtained via extrapolation of the corresponding curves to lower temperatures (see fig. 1 ). The radical influence on the superconducting properties of HTSC ceramics was further studied by registering the real part of AC magnetic susceptibility Z'. Figures 3 and 4 show the typical temperature dependences of the relative magnetic susceptibility (real part) for the initial sample and for that treated by 0.2 M chloroform solution of the stable radical.
O.8 06
Tth,K
O.4 02 75
r
79
.I
80
85
90 T.K
Fig. 1. Dependence of the critical current I= in the composite material TEMPO~-YBa2Cu3OT_s o n temperature T a t various concentrations x (wt.%) of the stable radical in the sample. ( Q ) X=0; (X) x = 1.7× 10-2; ( B ) x--4.2X 10-2; ( O ) X=8.5 X 10-2; ( A ) X= 1.7X 1O-L
78 77
. . 0,02
.
. 0.10
0,18 X,wt %
Fig. 2. Dependence of threshold temperature Tta on the concentration x (wt.%) of the TEMPO radical in the ceramics YBa,Cu30~_a.
172
N.S. Belousov et aL / Nitroxyl radical adsorption on YBazCuj07_~
,X/Xo
X/X°
O0
0.9
1 0 ~ !
~ 82K
02 Q4 06
0 7 0.5
1
~
08
0.0~
1.0
70 ~
78
82
86
0.3
90 T.K
Fig. 3. Dependenceof the real part of AC relative magnetic susceptibilityon the sampletemperature Tbcfore its treatment with the TEMPO radical al various values of the external magnetic field.
Fig. 5. Dependence of the real part of AC relative raagnetic susceptibility on the external magnetic field H=, at 82 K at various concentrations x (wt.%) of the TEMPO radical in the SC sample.
•/•O 0.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0A 0.6 0.8 1.0
0.2
70 74
78
87
86
90 T,K
Fig. 4. Dependence of the real part of AC relative magnetic susceptibility on the sample temperature T after its treatment with a 0.2 M solution of the TEMPO radical in chloroformat various values ofthe external magnetic field. The external constant magnetic field was switched on after sample cooling (ZFC). The plots show that at low temperatures where intergranular contacts are known to play a predominant role, a noticeable difference is observed between the curves of initial and treated samples. At the beginning of the SC transition ( " h o t " region) there is practically no difference. This fact indicates that there is no radical effect on the granules. Similar dependences of magnetic susceptibility on temperature were registered for other radical concentrations in the sample. The plots of figs. 5 and 6 represent the dependences of relative magnetic susceptibility at 82 and 70 K, respectively,
0.4[ 0
171¢ 40
80
120
H,Oe
Fig. 6. Dependence of the real part of AC relative magnetic susceptibility on the external maanetic field Hm at 70 K at various concentrations x (wt.%) of the TEMPO radical in the SC sample. on the external constant magnetic field at various radical concentrations. One can see that the greatest effect is observed when the radical concentration in the sample is 1.7× 10 -2 wt.% (0.2 M). Paper [10] shows that the residual sample magnetization M ( H ) is practically a linear function of the magnitude of the critical current in a sample and it does not depend on the behavior of the critical current in the external magnetic field. This result is in good agreement with the original Bean model [ 11 ]. Since the measured magnetic susceptibility z ' = M ( H ) / H is proportional to the magnitude of the critical current in the sample, the obtained dependences (figs. 3-6) reflect the behavior of the critical
N.S. Belousov et al. / Nitroxyl radical adsorption on YBa~Cu~OT_s
current with respect to the treatment and the external magnetic field. Indeed (figs. 5 and 6), the treatment of SC ceramics by a 0.2 M chloroform solution of the radical leads to a rise in the z ' ( H ) and thus to an increase of the critical current. A characteristic feature of these curves (figs. 5 and 6) is the decrease of the radical effect with the rise of the external magnetic field. When the external magnetic field exceeds 70 Oe the influence of the radical becomes unnoticeable. Note, that in such a field the disruption ofintergranular contacts is known to occur. Such a correlation indicates that the adsorbed radical mainly affects the intergranular contacts. The maximal magnetic susceptibility (Z' magnitude in zero external magnetic field at a temperature of 70 K) appears to be independent of sample treatment with radical. Hence, the content of the SC phase in the sample does not change and there is no chemical interaction between the adsorbed radical and the sample material. Evidently the influence of adsorbed molecules with paramagnetic moment on the electromagnetic properties of HTSC ceramics is rather complex. To reveal this influence in "pure form" we investigated a HTSC sample synthesized exactly as the previous one but treated by a 0.2 M chloroform solution of a non-paramagnetic analog of the TEMPO radical. We used the non-radical 2,2,6,6-tetramethylpiperidine-4-oxy compound. The direct measurement of the critical current in this sample showed that adsorption of diamagnetic molecules had no effect on the value of the critical current. Thus, the observed influence of the adsorbed stable radical on the critical current can be related mainly to its paramagnetic properties. Note, however, that the radical and its diamagnetic analog are distinguished by their different donor-acceptor properties with respect to oxide. That is why we do not exclude the possibility that the observed effect does occur due to the donor or acceptor properties of the adsorbing compound as well. The observed dependence of the critical current on the concentration of adsorbed radical has a complex character. Thus, the initial treatment of HTSC ceramics by a 0.2 M radical solution noticeably increases the critical current but a subsequent increase of the radical concentration is followed by a critical current "drop" to lower than its initial value (nontreated sample) at T < T,h. Usually this "drop" is ex-
173
plained by the influence of local magnetic fields on the coherent condition of SC electrons in the material, see, e.g. ref. [ 9 ]. However, it is rather difficult to explain the effect of a rise of the critical current in the treated sample. But using our data we can build up a reasonable hypothesis explaining the "rise" effect. Polycrystalline HTSC ceramics may be considered as two weakly interacting SC systems differing by their current-carrying properties by an order of magnitude: these are granules and intergranular interlayers. The external influence should first affect the "weak link" in the material under study, that is the intergranular contacts in the given case. The observed increase of the critical current at T > Tth in the radical-treated superconductor is evidently related to the creation of magnetic centres at the intergranular contacts. These centres additionally stabilize the Josephson vortices. In this case the dependence of the threshold temperature Tth on the radical concentration in the sample (fig. 2) is the analog of a phase equilibrium diagram which divides all the diagram space "temperature-radical concentration" into two regions characterized by a different influence of the stable radical on the current-carrying ability of HTSC ceramics. The composite material with a critical current higher than that in the initial HTSC ceramics is placed above the border, while the material with the lower critical current stays below the border. We consider the influence to be provided by the interaction of local magnetic moments of the paramagnetic molecules with SC currents. One could imagine that similar magnetic centres may be produced by the adsorbed oxygen whose molecules are also paramagnetic. But our preliminary experiments showed that the presence of adsorbed oxygen in the ceramics practically does not affect its current-carrying properties. Probably it is because of a very short time of spin-lattice relaxation of the magnetic moment of the 02 molecule as compared to the molecules of stable nitroxyl radicals. Further investigations of the influence of magnetic centres on HTSC ceramics will elucidate this interesting peculiarity of oxygen, i.e. the mechanism of radical interaction with the ceramic surface and will explain the unexpected effect of the rise of the critical current in these composite materials.
174
N.S. Belousov et al. / Nitroxyl radical adsorption on YBazCuj07_6
4. Conclusions It is ascertained that adsorption of the stable nitroxyl radical on HTSC ceramics changes its currentcarrying ability in the temperature range 70.0-88.0 K. It is shown that at some concentration of the adsorbed radical a considerable increase of the critical current takes place. The value of the magnetic susceptibility in zero external magnetic field at 70.0 K is shown to be independent of the radical concentration in the treated sample. This fact indicates that there is no chemical interaction between the stable radical and the sample material, while the same amount of SC phase is retained.
Acknowledgements The authors are grateful to O.A. Grigor'ev (The Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences) for supplying the radicals and their diamagnetic analog for the experiment. This work was done in the framework of project No. 90522 ofthe National Program of the Russian
Federation: High Temperature Superconductivity. References [ 1 ] B.M. Hoffman, F.R. Gamble and H.M. McConnell, J. Am. Chem. Soc. 89 (1967) 27. [2]N.S. Belousov, L.L. MIk~r~hln and V.N. Parmon, Supercond. Phys. Chem. Tech. 4 ( 1991 ) 1614. [3] V.M. Koshkin, V.D. Zaporozhsky, ICV. Savchenko, E.E. Ovechk/na and A.A. Makarov, Supercond. Phys. Chem. Tech. 3 (1990) 2772. [4] D.N. Matthew's, A. Baily and R.A. Vaile, Nature 328 (1987) 786. [5]O.V. Misochko, U.A. Osip'an, O.V. Zharikov, R.K. Nikolaev, N.S. Sidorov, V.I. Kulakov and A.M. Gromov, Supercond. Phys. Chem. Tech. 4 ( 1991 ) 954. [6]A. Takeoka, M. Hasanuma, S. Sakaiya, Y. Kishi and Y. Kuwano, Jpn. J. Appl. Phys. 27 (1988) 2260. [7] M.R. Gimberle, C. Ferdezhini, G.L. Nicchioti, M. Puni, S. Sirit, C. Rizzuto, C.A. Costa, M. F ~ , L. Olcese and F.C. Mutacotta, Supereond. Sci. Technol. I (1988) 30. [ 8 ] F.J. Eberha~t, A.D. Hibbs and A.M. Campbell, IEEE Trans. Magn. 25 (1989) 2146. [9] W. Buckel, Supraleitung, Grundlagen and Anwendungen (Weinheim/Bergsm, 1972) p. 305. [ 10] W.J. Yen, Z.Q. Yu, S. Labroo and J.Y. Park, Physica C 194 (1992) 141. [ 11 ] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250.
extracted a core of 3 mm in diameter, which later was used for the AC susceptibility measurements. The critical transport current was determined from the magnetic hysteresis after one magnetization cycle in a range of +20 Oe [6-8]. The ring magnetization was calculated as the difference between maximal magnetization and that provided by the granules. The AC magnetic susceptibility as a function of temperature was measured with an AC susceptometer (home made). The frequency of the applied magnetic field, o9, was 400 Hz, and the exiting field amplitude, Hm, was 0.35 Oe. Then we analyzed the influence of pure chloroform on the SC properties of yttrium ceramics. After the SC-transition temperature, To, and the magnitude of the critical transport current, I¢, were measured the bulk sample of yttrium ceramics was dipped into pure chloroform and kept there for a period of 1 to 30 days. The values of Tc and lc measured afterwards practically coincided with the previous ones even after 30 days treatment. Moreover, the control samples kept in air for this time period appeared to degrade and Tc and I~ decreased by 0.4% and 7%, respectively.
3. R e s u l t s and d i s c u s s i o n
Figure I shows the critical transport current (below the word "transport" will be omitted) as a function of temperature. Apparently, the adsorbed radical provides a significant effect on the critical current ]c,A 1.0
171
value in the temperature range 77.4-88.0 K. Figure 1 shows some peculiarities of this effect. Firstly, if the radical concentration is 1.7 X 10-z wt.% then the increase of the critical current is observed within the whole temperature interval under study. To quantitatively estimate the observed effect let us introduce the value cYx=Ic/l,.o determined by the ratio of the critical current in the treated sample to that in the initial one. Index x (in wt.%) corresponds to the radical concentration in the sample. One can see that ao.o~7 is equal to 1.05, 2.8, and 3.4 at 77.4, 81.0 and 83.0 K, respectively. Thus, with increasing temperature the critical current in the treated sample becomes higher than in the initial sample. Secondly, a further increase of the radical concentration in the sample decreases the critical current in the sample. In this case at 77.4 K the critical current value of the treated sample is less than that of the initial sample, i.e. coefficient ax becomes less than 1 :ao.o42=0.91, ao.0a5 = 0.85 and ao.t 7= 0.8 I. However, with increasing temperature the critical current in the treated sample begins to exceed that in the initial one and the c~ exceeds 1 once again. The temperature at which a x = 1 is called the threshold temperature Tth. Figure 2 shows the dependence of the threshold temperature on the radical concentration in the sample. Tt~ for the sample with 1.7X 10 - 2 WL% or radical was obtained via extrapolation of the corresponding curves to lower temperatures (see fig. 1 ). The radical influence on the superconducting properties of HTSC ceramics was further studied by registering the real part of AC magnetic susceptibility Z'. Figures 3 and 4 show the typical temperature dependences of the relative magnetic susceptibility (real part) for the initial sample and for that treated by 0.2 M chloroform solution of the stable radical.
O.8 06
Tth,K
O.4 02 75
r
79
.I
80
85
90 T.K
Fig. 1. Dependence of the critical current I= in the composite material TEMPO~-YBa2Cu3OT_s o n temperature T a t various concentrations x (wt.%) of the stable radical in the sample. ( Q ) X=0; (X) x = 1.7× 10-2; ( B ) x--4.2X 10-2; ( O ) X=8.5 X 10-2; ( A ) X= 1.7X 1O-L
78 77
. . 0,02
.
. 0.10
0,18 X,wt %
Fig. 2. Dependence of threshold temperature Tta on the concentration x (wt.%) of the TEMPO radical in the ceramics YBa,Cu30~_a.
172
N.S. Belousov et aL / Nitroxyl radical adsorption on YBazCuj07_~
,X/Xo
X/X°
O0
0.9
1 0 ~ !
~ 82K
02 Q4 06
0 7 0.5
1
~
08
0.0~
1.0
70 ~
78
82
86
0.3
90 T.K
Fig. 3. Dependenceof the real part of AC relative magnetic susceptibilityon the sampletemperature Tbcfore its treatment with the TEMPO radical al various values of the external magnetic field.
Fig. 5. Dependence of the real part of AC relative raagnetic susceptibility on the external magnetic field H=, at 82 K at various concentrations x (wt.%) of the TEMPO radical in the SC sample.
•/•O 0.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0A 0.6 0.8 1.0
0.2
70 74
78
87
86
90 T,K
Fig. 4. Dependence of the real part of AC relative magnetic susceptibility on the sample temperature T after its treatment with a 0.2 M solution of the TEMPO radical in chloroformat various values ofthe external magnetic field. The external constant magnetic field was switched on after sample cooling (ZFC). The plots show that at low temperatures where intergranular contacts are known to play a predominant role, a noticeable difference is observed between the curves of initial and treated samples. At the beginning of the SC transition ( " h o t " region) there is practically no difference. This fact indicates that there is no radical effect on the granules. Similar dependences of magnetic susceptibility on temperature were registered for other radical concentrations in the sample. The plots of figs. 5 and 6 represent the dependences of relative magnetic susceptibility at 82 and 70 K, respectively,
0.4[ 0
171¢ 40
80
120
H,Oe
Fig. 6. Dependence of the real part of AC relative magnetic susceptibility on the external maanetic field Hm at 70 K at various concentrations x (wt.%) of the TEMPO radical in the SC sample. on the external constant magnetic field at various radical concentrations. One can see that the greatest effect is observed when the radical concentration in the sample is 1.7× 10 -2 wt.% (0.2 M). Paper [10] shows that the residual sample magnetization M ( H ) is practically a linear function of the magnitude of the critical current in a sample and it does not depend on the behavior of the critical current in the external magnetic field. This result is in good agreement with the original Bean model [ 11 ]. Since the measured magnetic susceptibility z ' = M ( H ) / H is proportional to the magnitude of the critical current in the sample, the obtained dependences (figs. 3-6) reflect the behavior of the critical
N.S. Belousov et al. / Nitroxyl radical adsorption on YBa~Cu~OT_s
current with respect to the treatment and the external magnetic field. Indeed (figs. 5 and 6), the treatment of SC ceramics by a 0.2 M chloroform solution of the radical leads to a rise in the z ' ( H ) and thus to an increase of the critical current. A characteristic feature of these curves (figs. 5 and 6) is the decrease of the radical effect with the rise of the external magnetic field. When the external magnetic field exceeds 70 Oe the influence of the radical becomes unnoticeable. Note, that in such a field the disruption ofintergranular contacts is known to occur. Such a correlation indicates that the adsorbed radical mainly affects the intergranular contacts. The maximal magnetic susceptibility (Z' magnitude in zero external magnetic field at a temperature of 70 K) appears to be independent of sample treatment with radical. Hence, the content of the SC phase in the sample does not change and there is no chemical interaction between the adsorbed radical and the sample material. Evidently the influence of adsorbed molecules with paramagnetic moment on the electromagnetic properties of HTSC ceramics is rather complex. To reveal this influence in "pure form" we investigated a HTSC sample synthesized exactly as the previous one but treated by a 0.2 M chloroform solution of a non-paramagnetic analog of the TEMPO radical. We used the non-radical 2,2,6,6-tetramethylpiperidine-4-oxy compound. The direct measurement of the critical current in this sample showed that adsorption of diamagnetic molecules had no effect on the value of the critical current. Thus, the observed influence of the adsorbed stable radical on the critical current can be related mainly to its paramagnetic properties. Note, however, that the radical and its diamagnetic analog are distinguished by their different donor-acceptor properties with respect to oxide. That is why we do not exclude the possibility that the observed effect does occur due to the donor or acceptor properties of the adsorbing compound as well. The observed dependence of the critical current on the concentration of adsorbed radical has a complex character. Thus, the initial treatment of HTSC ceramics by a 0.2 M radical solution noticeably increases the critical current but a subsequent increase of the radical concentration is followed by a critical current "drop" to lower than its initial value (nontreated sample) at T < T,h. Usually this "drop" is ex-
173
plained by the influence of local magnetic fields on the coherent condition of SC electrons in the material, see, e.g. ref. [ 9 ]. However, it is rather difficult to explain the effect of a rise of the critical current in the treated sample. But using our data we can build up a reasonable hypothesis explaining the "rise" effect. Polycrystalline HTSC ceramics may be considered as two weakly interacting SC systems differing by their current-carrying properties by an order of magnitude: these are granules and intergranular interlayers. The external influence should first affect the "weak link" in the material under study, that is the intergranular contacts in the given case. The observed increase of the critical current at T > Tth in the radical-treated superconductor is evidently related to the creation of magnetic centres at the intergranular contacts. These centres additionally stabilize the Josephson vortices. In this case the dependence of the threshold temperature Tth on the radical concentration in the sample (fig. 2) is the analog of a phase equilibrium diagram which divides all the diagram space "temperature-radical concentration" into two regions characterized by a different influence of the stable radical on the current-carrying ability of HTSC ceramics. The composite material with a critical current higher than that in the initial HTSC ceramics is placed above the border, while the material with the lower critical current stays below the border. We consider the influence to be provided by the interaction of local magnetic moments of the paramagnetic molecules with SC currents. One could imagine that similar magnetic centres may be produced by the adsorbed oxygen whose molecules are also paramagnetic. But our preliminary experiments showed that the presence of adsorbed oxygen in the ceramics practically does not affect its current-carrying properties. Probably it is because of a very short time of spin-lattice relaxation of the magnetic moment of the 02 molecule as compared to the molecules of stable nitroxyl radicals. Further investigations of the influence of magnetic centres on HTSC ceramics will elucidate this interesting peculiarity of oxygen, i.e. the mechanism of radical interaction with the ceramic surface and will explain the unexpected effect of the rise of the critical current in these composite materials.
174
N.S. Belousov et al. / Nitroxyl radical adsorption on YBazCuj07_6
4. Conclusions It is ascertained that adsorption of the stable nitroxyl radical on HTSC ceramics changes its currentcarrying ability in the temperature range 70.0-88.0 K. It is shown that at some concentration of the adsorbed radical a considerable increase of the critical current takes place. The value of the magnetic susceptibility in zero external magnetic field at 70.0 K is shown to be independent of the radical concentration in the treated sample. This fact indicates that there is no chemical interaction between the stable radical and the sample material, while the same amount of SC phase is retained.
Acknowledgements The authors are grateful to O.A. Grigor'ev (The Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences) for supplying the radicals and their diamagnetic analog for the experiment. This work was done in the framework of project No. 90522 ofthe National Program of the Russian
Federation: High Temperature Superconductivity. References [ 1 ] B.M. Hoffman, F.R. Gamble and H.M. McConnell, J. Am. Chem. Soc. 89 (1967) 27. [2]N.S. Belousov, L.L. MIk~r~hln and V.N. Parmon, Supercond. Phys. Chem. Tech. 4 ( 1991 ) 1614. [3] V.M. Koshkin, V.D. Zaporozhsky, ICV. Savchenko, E.E. Ovechk/na and A.A. Makarov, Supercond. Phys. Chem. Tech. 3 (1990) 2772. [4] D.N. Matthew's, A. Baily and R.A. Vaile, Nature 328 (1987) 786. [5]O.V. Misochko, U.A. Osip'an, O.V. Zharikov, R.K. Nikolaev, N.S. Sidorov, V.I. Kulakov and A.M. Gromov, Supercond. Phys. Chem. Tech. 4 ( 1991 ) 954. [6]A. Takeoka, M. Hasanuma, S. Sakaiya, Y. Kishi and Y. Kuwano, Jpn. J. Appl. Phys. 27 (1988) 2260. [7] M.R. Gimberle, C. Ferdezhini, G.L. Nicchioti, M. Puni, S. Sirit, C. Rizzuto, C.A. Costa, M. F ~ , L. Olcese and F.C. Mutacotta, Supereond. Sci. Technol. I (1988) 30. [ 8 ] F.J. Eberha~t, A.D. Hibbs and A.M. Campbell, IEEE Trans. Magn. 25 (1989) 2146. [9] W. Buckel, Supraleitung, Grundlagen and Anwendungen (Weinheim/Bergsm, 1972) p. 305. [ 10] W.J. Yen, Z.Q. Yu, S. Labroo and J.Y. Park, Physica C 194 (1992) 141. [ 11 ] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250.