- Email: [email protected]
Critical currents and magnetization decay in oxygen deficient YBa2Cu3O7−δ thin films
Phy~icaC 185-189 (1991) 2161-2162 North-Holland
Crilical Currents and Magr~etization Decayin Oxygen I ehclent YBo_2C~O7~6T1K~ F~ms B. M. Lairson 1, I. L Vargas2, S, K. Streffferl, D. C. Larbal,estier2, and J. C. B~mp,a,,al, l~tment ~lmer,
of Materials Science and E.ngineerin~ Stan~rd University, Stanford, CA ~ ; t of Materials Science and Engineering, and A!~lied SupecconductSvity C~tm', Univer~i~ of Wisconsin, Madison, WI 53706, ln-situ grown oaxis YBa2Cu3OT.8 films deposited ohio M ~ single cr~tal subst-ra~ were equ~2na~ecl ~t
Critica, currents, which were l-4x10 A/cruZ before oxygen removal, were reduced ~ m e ~ h a t ~ ox~~rt removal, but ~mained high. An exponential relation between critical ~ r r e n t and ~ p e r a t u r e wa~ o b ~ m t ~ at low temperature for both as-grown and for de,oxygenated film~, Magnefi~flon decay measurements showed stronger relaxation in the oxygen deficient films, inc~icative of weak~ flux pirm~ng, Oxygen content is an important issue in the preparation of YBaCuO superconducting oxides, both for bulk materials and for thin films, and has been studied extensivelyl,2. Oxygen deficient thin films of YBaCuO have been suggested as a model system for the study of localization 1. Here, we consider oxygen deficient YBaCuO thin films as a model system for the investigation of flux pinning, either through changes in the density" of oxygen v,,cancy (pinning) sites, or through the effect of doping on the superconducting properties, such as the anisotropy and the coherence Iengths3. We use an analysis technique developed pre~6ousl)~t to deduce from magnetization d ~ a y the effect of changes in the oxygen content on the pinning potential U(J), where J is the current circulating in the sample. We determine the circulating current using the Bean formula. A large number of epitaxiaI thin films of YBaCuO were deposited in-situ onto MgO single crystal substrates, at an oxygen partial pres-ure of 45mTorr and an argon pressure of t40 reTort, using an off-axis sputter geometry. Films were c-axis oriented and superconducting as removed from the deposition chamber. The transition temperature of these films could be increased to as high as 88.5K for the proper oxygen annealing conditions. As
deposited criticat currents for all of the ; a m p l ~ discussed here were approximately 3x107 A/cm2 at 4.2K in zero applied field. These films were subsequently annealed at 450°C for a minimum of 9 hours a~ v ~ o u s oxygen partial pressures, a time long compared to ~ e oxyger. ,_quilibration time at this temperature (-5 hours), which was in turn much longer than the c o o ! - d o ~ time (several m i n u t e ) for the sample heater.
c
Q
[i ° d
A
2C
<~ O e--
cq O
0
50
100
150
200 250 Temperature (K) Figure i. Resistance vs temperature for films,
subsequently annealed at different oxygen par~a2 pressures. The legend denotes the o×Tg~n anneal pressure at 450°C.
0921-4534/91/$03.50 O 1991 - Elsevier Science Publishers B.V..All rights ~.served.
2162
B.M. Lairson et aL
/ Critical currents and magnetization decay potentials deduced from the magnetization decay measurements, where we have assumed U=0 at 2.6K. As can be seen in the figure, the pinning potentials derived for the de-oxygenated films are much smaller than those for more fully oxygenated films. This is indicative of weaker pinning in the material at strongly reduced levels of oxygen content. This correlates well with the increased broadening which is observed in the resistive transitions of reduced YBaCuO 3.
Figure 1 shows the resistance vs. temperature for the unpatterned films which were annealed at various oxygen partial pressures. As has been shown previously 1,2,s, the transition temperature could be changed dramatically by changing the oxygen treatment,
1 t
I.
-¢ .88K
I "
T¢., SK
I o
_
Tc.70K
70-
TO=21K
1
60 10s 50:>
O
m 40E "1
"~ To=4,5K s
10 0
5
10
" ~ TO-6.SK
e
B 25
30
35
Figure 2. Jc vs temperature for annealed films.
2O
[] 10-
[]
@
o
[]
e
o II
Elo Figure 2 shows the measured critical current vs temperature in an applied field of 5kG. It is found that for reduced oxygen contents, the critical current vs temperature can be approximated by an exponential relation 6 Jc 0~exp(-T/T0). Low temperature magnetization decay measurements were made on the films shown in Figure 2, for T<0.3Tc, Power law fits to the decays were made as discussed in reference 6, and pinning potentials were then constructed using the relation 4 N U(JN) = U(Jo) + i=~dikTi(l__- Ji / Ji-1) (1) where UfJo) is the activation barrier to vortex motion at some known current, n is the power law exponent between the current and the voltage, and T is the temperature at which the decay is carried out. The subscript indexes the isothermal magnetization decays measured at the various temperatures. Equation (1) has been shown to yield activation energy barrier values which are in agreement with those determined from Arrhenius plots 4. Figure 3 shows the resulting pinning
l
O Tc=70 KI II Tc=48 K I [3 Tc=34 K]
e
O
3o-
~ 15 20 T (K)
e
Tc=88'KI
®
o
E [3 ,~.BII"
1 0s
1 0s J ~A/cm2)
O
@ @
1 0r
Figure 3. Low temperature (T<0.3Tc) pinning potentials determined for films with various levels of oxygen content. We have systematically studied the effect of oxygen deficiency on the pinning potentials for YBCO. For relatively large deficiencies (Tc<80K) we observe weaker pinning and reduced critical currents compared to more fully oxygenated material. This indicates that the primary effect of a large reduction in the oxygen content in thin films is to reduce the pinning in the films, either through the effect of doping on the L.t~xiSz~t v z t x u v ~
increased anisotropy. 1. G. Koren, et al., Physica C 162-169, 1021 (1989). 2. M. Ohkubo,et al., Appl. Phys. Lett. 55, 899 (1989). 3. T.T.M. Palstra, Phys Rev B 43, 3756 (1991). 4. B.M. Lairson, et al., Phys. Rev. B 43, 10405 (1991). 5. R.J. Cava, et al., Physica C 165, 419 (1990). 6. B.M. Lairson, et al., Phys. Rev. B 42, 1008 (1990).
Crilical Currents and Magr~etization Decayin Oxygen I ehclent YBo_2C~O7~6T1K~ F~ms B. M. Lairson 1, I. L Vargas2, S, K. Streffferl, D. C. Larbal,estier2, and J. C. B~mp,a,,al, l~tment ~lmer,
of Materials Science and E.ngineerin~ Stan~rd University, Stanford, CA ~ ; t of Materials Science and Engineering, and A!~lied SupecconductSvity C~tm', Univer~i~ of Wisconsin, Madison, WI 53706, ln-situ grown oaxis YBa2Cu3OT.8 films deposited ohio M ~ single cr~tal subst-ra~ were equ~2na~ecl ~t
Critica, currents, which were l-4x10 A/cruZ before oxygen removal, were reduced ~ m e ~ h a t ~ ox~~rt removal, but ~mained high. An exponential relation between critical ~ r r e n t and ~ p e r a t u r e wa~ o b ~ m t ~ at low temperature for both as-grown and for de,oxygenated film~, Magnefi~flon decay measurements showed stronger relaxation in the oxygen deficient films, inc~icative of weak~ flux pirm~ng, Oxygen content is an important issue in the preparation of YBaCuO superconducting oxides, both for bulk materials and for thin films, and has been studied extensivelyl,2. Oxygen deficient thin films of YBaCuO have been suggested as a model system for the study of localization 1. Here, we consider oxygen deficient YBaCuO thin films as a model system for the investigation of flux pinning, either through changes in the density" of oxygen v,,cancy (pinning) sites, or through the effect of doping on the superconducting properties, such as the anisotropy and the coherence Iengths3. We use an analysis technique developed pre~6ousl)~t to deduce from magnetization d ~ a y the effect of changes in the oxygen content on the pinning potential U(J), where J is the current circulating in the sample. We determine the circulating current using the Bean formula. A large number of epitaxiaI thin films of YBaCuO were deposited in-situ onto MgO single crystal substrates, at an oxygen partial pres-ure of 45mTorr and an argon pressure of t40 reTort, using an off-axis sputter geometry. Films were c-axis oriented and superconducting as removed from the deposition chamber. The transition temperature of these films could be increased to as high as 88.5K for the proper oxygen annealing conditions. As
deposited criticat currents for all of the ; a m p l ~ discussed here were approximately 3x107 A/cm2 at 4.2K in zero applied field. These films were subsequently annealed at 450°C for a minimum of 9 hours a~ v ~ o u s oxygen partial pressures, a time long compared to ~ e oxyger. ,_quilibration time at this temperature (-5 hours), which was in turn much longer than the c o o ! - d o ~ time (several m i n u t e ) for the sample heater.
c
Q
[i ° d
A
2C
<~ O e--
cq O
0
50
100
150
200 250 Temperature (K) Figure i. Resistance vs temperature for films,
subsequently annealed at different oxygen par~a2 pressures. The legend denotes the o×Tg~n anneal pressure at 450°C.
0921-4534/91/$03.50 O 1991 - Elsevier Science Publishers B.V..All rights ~.served.
2162
B.M. Lairson et aL
/ Critical currents and magnetization decay potentials deduced from the magnetization decay measurements, where we have assumed U=0 at 2.6K. As can be seen in the figure, the pinning potentials derived for the de-oxygenated films are much smaller than those for more fully oxygenated films. This is indicative of weaker pinning in the material at strongly reduced levels of oxygen content. This correlates well with the increased broadening which is observed in the resistive transitions of reduced YBaCuO 3.
Figure 1 shows the resistance vs. temperature for the unpatterned films which were annealed at various oxygen partial pressures. As has been shown previously 1,2,s, the transition temperature could be changed dramatically by changing the oxygen treatment,
1 t
I.
-¢ .88K
I "
T¢., SK
I o
_
Tc.70K
70-
TO=21K
1
60 10s 50:>
O
m 40E "1
"~ To=4,5K s
10 0
5
10
" ~ TO-6.SK
e
B 25
30
35
Figure 2. Jc vs temperature for annealed films.
2O
[] 10-
[]
@
o
[]
e
o II
Elo Figure 2 shows the measured critical current vs temperature in an applied field of 5kG. It is found that for reduced oxygen contents, the critical current vs temperature can be approximated by an exponential relation 6 Jc 0~exp(-T/T0). Low temperature magnetization decay measurements were made on the films shown in Figure 2, for T<0.3Tc, Power law fits to the decays were made as discussed in reference 6, and pinning potentials were then constructed using the relation 4 N U(JN) = U(Jo) + i=~dikTi(l__- Ji / Ji-1) (1) where UfJo) is the activation barrier to vortex motion at some known current, n is the power law exponent between the current and the voltage, and T is the temperature at which the decay is carried out. The subscript indexes the isothermal magnetization decays measured at the various temperatures. Equation (1) has been shown to yield activation energy barrier values which are in agreement with those determined from Arrhenius plots 4. Figure 3 shows the resulting pinning
l
O Tc=70 KI II Tc=48 K I [3 Tc=34 K]
e
O
3o-
~ 15 20 T (K)
e
Tc=88'KI
®
o
E [3 ,~.BII"
1 0s
1 0s J ~A/cm2)
O
@ @
1 0r
Figure 3. Low temperature (T<0.3Tc) pinning potentials determined for films with various levels of oxygen content. We have systematically studied the effect of oxygen deficiency on the pinning potentials for YBCO. For relatively large deficiencies (Tc<80K) we observe weaker pinning and reduced critical currents compared to more fully oxygenated material. This indicates that the primary effect of a large reduction in the oxygen content in thin films is to reduce the pinning in the films, either through the effect of doping on the L.t~xiSz~t v z t x u v ~
increased anisotropy. 1. G. Koren, et al., Physica C 162-169, 1021 (1989). 2. M. Ohkubo,et al., Appl. Phys. Lett. 55, 899 (1989). 3. T.T.M. Palstra, Phys Rev B 43, 3756 (1991). 4. B.M. Lairson, et al., Phys. Rev. B 43, 10405 (1991). 5. R.J. Cava, et al., Physica C 165, 419 (1990). 6. B.M. Lairson, et al., Phys. Rev. B 42, 1008 (1990).