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Critical currents and magnetic relaxation in neutron irradiated Y123
PhysicaC 162-164 (1989) 689-690 North-Holland
CRITICAL CURRENTS AND MAGNETIC RELAXATION IN NEUTRON IRRADIATED Y123 Michael E. McHenry, Jeffrey O. Willis, Martin P. Maiey, J. D. Thompson, J. R. Cost and D. E. Peterson Los Aiamos National Laboratory, Los Alamos, NM 87545 USA Critical current densities have been inferred from magnetic hysteresis and volume pinning energies from magnetic relaxation measurements for neutron irradiated and unirradiated polycrystalline Y123 samples. Increases in the critical current densities, by factors of 2-3 in zero field and larger in a field, for the irradiated sample are observed. Large increases in the magnetic relaxation rate arealso observed at H = 1T. From the relaxation data and the amount of irreversible magnetization, pinning energies of 0.01-0.06 eV and 0.01-0.03 eV are inferred for the irradiated and unirradiated samples, respectively. 1. INTRODUCTION Measurements of time dependent magnetization carries information as to the number and energy distribution of flux pinning sites in Type II superconductors1. This is of fundamental importance in determining the persistence of critical current densities at high temperatures where thermally activated flux motion of vortices is a dissipative effect2. Flux creep in the high Tc superconductors has been postulated to be in large part due to the small correlation lengths (~20 A) in these materials 3. The length is the radius of the normal core of a fluxon and signifies one of the pertinent dimensions in pinning theories 4. Pinning centers of this size are desired to optimize Jc. Several authors 5 have now reported that the defects introduced by neutron irradiation (up to a maximum fluence) in Y123 s e r v e to enhance the critical current densities by factors of 2-3 in zero field, and by even larger ratios in a field. Here, we explore the influence of neutron irradiation on pinning energies, comparing the temperature dependence of the flux creep rate for a neutron irradiated Y123 sample with that of a unirradiated control sample. 2. EXPERIMENTAL TECHNIQUES The preparation of the irradiated and unirradiated samples is described in ref. 5. Flux creep measurements on the irradiated Y123 were performed after a total fast neutron (E > 0.1 MeV) fluence of 3 x 1018 n/cm 2 and with Tc depressed to 82 K, as com0921-4534/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland)
pared to 92 K for the unirradiated material. This fluence is larger than that observed to give the maximum enhancement in Jc; however Jc's were still notably enhanced as compared to those of the unirradiated material. Magnetic hysteresis at 7 K indicates a critical current density (inferred from the Bean 6 model using the average 10 p.m grain size) of 9.6 x 106 and 3.6 x 106 A/cm 2 for the irradiated and unirrediated samples, respectively. Jc's at 7 K and 1T were 3.6 x 106 and 1.0 x 106 Ncm 2 respectively. Magnetic relaxation and hysteresis measurements were made using a Quantum Design SQUID magnetometer. The time dependent magnetization was measured on initially zero field cooled samples (ZFC) after having stabilized the temperature and application of the field. Equilibrium magnetization values (Meq(T)) were approximated as the magnetization of a slowly field-cooled (FC) sample. 3. RESULTS AND DISCUSSION Our magnetic relaxation data is accurately described by the empirical relationship: M = MO + A in(t) [1] which is the hallmark of the flux creep model. The strong temperature dependence of the relaxation is manifested in the behavior of the derivative of M(t) with respect to In(t), A, as a function of temperature. In the flux creep model this temperature dependence is a manifestation of the temperature dependence of the volume pinning energy U0 divided by the tern-
690
M.E. McHenry et al. / Neutron irradiated Y123
perature dependent critical current density as given in the following expression: d(M(t)-Meq)
A =
din(t)
= (UIUIU~o) JcoS =(k_~T_)(M0.Meq) 4c
Uo
0.5
H=IT q
E
.~
o.z.
•
0
• O
0.1"
A (unirradiated) A (irradiated)
• O
• O
• °O.
0
o
10
20
30
40
50
60
50'
O o
go o°
• •
irreversible magnetization and R is the grain size. Figure 1 shows the temperature dependence of A for both the irradiated and the unirradiated samples in an applied field of 1T. Apparent from this data is the fact that the relaxation rate is substantially enhanced for the irradiated sample. This enhancement is accompanied by the increased critical current density so that from [2] only modest changes in the volume pinning energy can be inferred.
D& '~ 0.3"
H=IT
60'
[2]
Jc0 is related to the maximum value of the
0.4"
70'
70
T(K) Figure 1: Magnetic relaxation rate P = dM/dln(t) for irradiated and unirradiated polycrystalline Y123. In a previous publication 7 the results of fits of the relaxation data for these samples at 0.1 T showed a small reduction in the pinning energy in the irradiated sample. The data at 1T, where increases in the current density are enhanced to a much greater extent, show increased pinning energies. The zero field critical current densities are less favorably affected by irradiation than the high field values where the hysteresis is most markedly increased. An impediment to a full understanding of irradiation effects are the opposite effects of increased defect density and depressed Tc on pinning in these materials. This can only be understood by a study as a function of neutron fluence. Figure 2 shows the temperature dependence of the pinning energy dedved from relaxation data and magnetic hysteresis measurements using [2]. It is
40' ~-~ o
•
30'
•
20'
o
0
oO
loi e 0
-
0
0 0
g ~.RADIA'~.~ UNIRRADIATED •
10
-
•
•
20
30
-
•
40
-
•
50
-
•
60
70
T(K) Figure 2: Pinning energies for irradiated and unirradiated Y123 samples, as derived from [2]. seen that for all temperatures Uo for the irradiated sample exceeds that of the unirradiated sample at 1T. Values of the pinning energy are very modest, not exceeding .06 eV for the irradiated sample or .03 eV for the unirradiated sample and are certainly underestimates of true activation energies arising from the linear approximation of U(J)8. The observed temperature dependence of Uo is unphysical as noted by Xu et al.9 and reflects the Jc dependence of the pinning potential. The true pinning potential Up (corrected for the Jc dependence of the pinning well) is likely to be larger and will be explored in the future. 4. CONCLUSIONS Large changes in the critical current density and magnetic relaxation of neutron irradiated Y123 materials have been observed. The changes in Jc are accompanied by relatively small changes in the volume pinning energy. The most notable enhancement in both Jc and Uo occur at high fields (1-2 T). 5. REFERENCES 1. M. McHenry et al., Physica B 153-55, 310 t1988). M. McHenry, et al., PRB 39, 4784 (1989). 2. Y. B. Kim, Rev. Mod. Phys.36, 39 (1964). 3. Y. Yeshurun and A. P. Malezemoff, Phys. Rev. Lett. 60, 2202 (1988). 4. M. Tinkham, Phys. Rev. Lett. 61, 1658 (1988). 5. J. O Willis, et al., Mat. Res. Soc. Prec. 99, 391 (1988) and references therein. 6. C. P. Bean, Phys. Rev. Lett. 8, 250 (1962) 7. M. E. McHenry, et al., Prec. of Redstone Arsenal Conf. on High Tc Superconductivity, 1989. 8. M. R. Beasley, et al., Phys. Rev. 181,682 (1969). 9. Y. Xu et al.; prepdnt.
CRITICAL CURRENTS AND MAGNETIC RELAXATION IN NEUTRON IRRADIATED Y123 Michael E. McHenry, Jeffrey O. Willis, Martin P. Maiey, J. D. Thompson, J. R. Cost and D. E. Peterson Los Aiamos National Laboratory, Los Alamos, NM 87545 USA Critical current densities have been inferred from magnetic hysteresis and volume pinning energies from magnetic relaxation measurements for neutron irradiated and unirradiated polycrystalline Y123 samples. Increases in the critical current densities, by factors of 2-3 in zero field and larger in a field, for the irradiated sample are observed. Large increases in the magnetic relaxation rate arealso observed at H = 1T. From the relaxation data and the amount of irreversible magnetization, pinning energies of 0.01-0.06 eV and 0.01-0.03 eV are inferred for the irradiated and unirradiated samples, respectively. 1. INTRODUCTION Measurements of time dependent magnetization carries information as to the number and energy distribution of flux pinning sites in Type II superconductors1. This is of fundamental importance in determining the persistence of critical current densities at high temperatures where thermally activated flux motion of vortices is a dissipative effect2. Flux creep in the high Tc superconductors has been postulated to be in large part due to the small correlation lengths (~20 A) in these materials 3. The length is the radius of the normal core of a fluxon and signifies one of the pertinent dimensions in pinning theories 4. Pinning centers of this size are desired to optimize Jc. Several authors 5 have now reported that the defects introduced by neutron irradiation (up to a maximum fluence) in Y123 s e r v e to enhance the critical current densities by factors of 2-3 in zero field, and by even larger ratios in a field. Here, we explore the influence of neutron irradiation on pinning energies, comparing the temperature dependence of the flux creep rate for a neutron irradiated Y123 sample with that of a unirradiated control sample. 2. EXPERIMENTAL TECHNIQUES The preparation of the irradiated and unirradiated samples is described in ref. 5. Flux creep measurements on the irradiated Y123 were performed after a total fast neutron (E > 0.1 MeV) fluence of 3 x 1018 n/cm 2 and with Tc depressed to 82 K, as com0921-4534/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland)
pared to 92 K for the unirradiated material. This fluence is larger than that observed to give the maximum enhancement in Jc; however Jc's were still notably enhanced as compared to those of the unirradiated material. Magnetic hysteresis at 7 K indicates a critical current density (inferred from the Bean 6 model using the average 10 p.m grain size) of 9.6 x 106 and 3.6 x 106 A/cm 2 for the irradiated and unirrediated samples, respectively. Jc's at 7 K and 1T were 3.6 x 106 and 1.0 x 106 Ncm 2 respectively. Magnetic relaxation and hysteresis measurements were made using a Quantum Design SQUID magnetometer. The time dependent magnetization was measured on initially zero field cooled samples (ZFC) after having stabilized the temperature and application of the field. Equilibrium magnetization values (Meq(T)) were approximated as the magnetization of a slowly field-cooled (FC) sample. 3. RESULTS AND DISCUSSION Our magnetic relaxation data is accurately described by the empirical relationship: M = MO + A in(t) [1] which is the hallmark of the flux creep model. The strong temperature dependence of the relaxation is manifested in the behavior of the derivative of M(t) with respect to In(t), A, as a function of temperature. In the flux creep model this temperature dependence is a manifestation of the temperature dependence of the volume pinning energy U0 divided by the tern-
690
M.E. McHenry et al. / Neutron irradiated Y123
perature dependent critical current density as given in the following expression: d(M(t)-Meq)
A =
din(t)
= (UIUIU~o) JcoS =(k_~T_)(M0.Meq) 4c
Uo
0.5
H=IT q
E
.~
o.z.
•
0
• O
0.1"
A (unirradiated) A (irradiated)
• O
• O
• °O.
0
o
10
20
30
40
50
60
50'
O o
go o°
• •
irreversible magnetization and R is the grain size. Figure 1 shows the temperature dependence of A for both the irradiated and the unirradiated samples in an applied field of 1T. Apparent from this data is the fact that the relaxation rate is substantially enhanced for the irradiated sample. This enhancement is accompanied by the increased critical current density so that from [2] only modest changes in the volume pinning energy can be inferred.
D& '~ 0.3"
H=IT
60'
[2]
Jc0 is related to the maximum value of the
0.4"
70'
70
T(K) Figure 1: Magnetic relaxation rate P = dM/dln(t) for irradiated and unirradiated polycrystalline Y123. In a previous publication 7 the results of fits of the relaxation data for these samples at 0.1 T showed a small reduction in the pinning energy in the irradiated sample. The data at 1T, where increases in the current density are enhanced to a much greater extent, show increased pinning energies. The zero field critical current densities are less favorably affected by irradiation than the high field values where the hysteresis is most markedly increased. An impediment to a full understanding of irradiation effects are the opposite effects of increased defect density and depressed Tc on pinning in these materials. This can only be understood by a study as a function of neutron fluence. Figure 2 shows the temperature dependence of the pinning energy dedved from relaxation data and magnetic hysteresis measurements using [2]. It is
40' ~-~ o
•
30'
•
20'
o
0
oO
loi e 0
-
0
0 0
g ~.RADIA'~.~ UNIRRADIATED •
10
-
•
•
20
30
-
•
40
-
•
50
-
•
60
70
T(K) Figure 2: Pinning energies for irradiated and unirradiated Y123 samples, as derived from [2]. seen that for all temperatures Uo for the irradiated sample exceeds that of the unirradiated sample at 1T. Values of the pinning energy are very modest, not exceeding .06 eV for the irradiated sample or .03 eV for the unirradiated sample and are certainly underestimates of true activation energies arising from the linear approximation of U(J)8. The observed temperature dependence of Uo is unphysical as noted by Xu et al.9 and reflects the Jc dependence of the pinning potential. The true pinning potential Up (corrected for the Jc dependence of the pinning well) is likely to be larger and will be explored in the future. 4. CONCLUSIONS Large changes in the critical current density and magnetic relaxation of neutron irradiated Y123 materials have been observed. The changes in Jc are accompanied by relatively small changes in the volume pinning energy. The most notable enhancement in both Jc and Uo occur at high fields (1-2 T). 5. REFERENCES 1. M. McHenry et al., Physica B 153-55, 310 t1988). M. McHenry, et al., PRB 39, 4784 (1989). 2. Y. B. Kim, Rev. Mod. Phys.36, 39 (1964). 3. Y. Yeshurun and A. P. Malezemoff, Phys. Rev. Lett. 60, 2202 (1988). 4. M. Tinkham, Phys. Rev. Lett. 61, 1658 (1988). 5. J. O Willis, et al., Mat. Res. Soc. Prec. 99, 391 (1988) and references therein. 6. C. P. Bean, Phys. Rev. Lett. 8, 250 (1962) 7. M. E. McHenry, et al., Prec. of Redstone Arsenal Conf. on High Tc Superconductivity, 1989. 8. M. R. Beasley, et al., Phys. Rev. 181,682 (1969). 9. Y. Xu et al.; prepdnt.