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Critical scaling and vortex glass transition in a-axis oriented EuBa2Cu3O7 thin films
PHYSICA
Physica C 235-240 (1994)3171-3172 North-Holland
C r i t i c a l s c a l i n g a n d v o r t e x g l a s s t r a n s i t i o n in a - a x i s o r i e n t e d EuBa,2Cu30 7 thin f i l m s M.Velez, J.I.Martin and J.L.Vieent Dpto. Fisiea de Materiales, Faeultad de Ciencias Fisieas, Universidad Complutense, 28040 Madrid, Spain. The I-V characteristics and the magnetoresistive transition of a-axis oriented EuBa2Cu307thin films grown by DC magnetron sputtering have been measured as a function of temperature. Evidence is found of a second order transition with critical exponents t)=l.1 and z=8.5, greater than those reported in literature for e-axis films. The temperature dependence of the magnetoresistivity slightly above Tg can be fitted to a sealing behavior p ~ (T-Tg)' with s ~ 8. These high values of the critical exponents may be attributed to a softening of the vortex lattice due to sample microstructure. Flux dynamics in the mixed state of high temperature superconductors have revealed a number of new interesting features. One of the most important is the existence of a second order phase transition from a truly superconducting glassy state into a liquid like state at a temperature Tg, in the presence of microscopic disorder [1]. However the critical exponents associated to this transition, have been reported to be dependent on the applied magnetic field [2], and sample geometry [3]. Most of the experimental studies presented up to now in YBa2Cu307 have been done on e-axis oriented films with the magnetic field applied parallel to the e-axis. Recent single crystal results with Hllab have suggested the existence of a new second order transition from a vortex phase with eaxis periodicity induced by intrinsic pinning by the layered structure [4]. In this paper we report on the critical behavior of a-axis oriented EuBa2Cu30 7 thin films, with the magnetic field parallel to the ~ plane. The influence of sample microstructure, consisting of small (~,200 nm) domains separated by 90 o boundaries [5], will be discussed. Pure a-axis oriented EuBa2Cu307 thin films have been grown by DC magnetron sputtering on (100) SrTiO 3 substrates [6]. The films had normal state resisivity of 1000 ~tD,cm, thickness 300 nm and critical temperature T~0=80-84 K. They were photolitographycally patterned into 100 pm bridges. I-V characteristics were recorded at a constant temperature with stability better than 10 mK. De current was supplied in short 1 s pulses by a Keithley 220 current source, and reversed to
minimize thermal offsets. An external magnetic field of 1T was applied perpendicular to sample plane, that is, parallel to the ab planes. Figure 1 shows I-V characteristics from 55 K to 75 K. A linear region at low currents is clearly seen in the higher temperature curves, while a much stronger current dependence appears in the low temperature region, where I-V characteristics are approximately linear in this Iogarithmic plot. An
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10 5
Figure l: I-V characteristics at 1T from 55K to 75K, with 1K interval. initial estimation of the critical temperature Tg and dynamic exponent z was taken from one of these low temperature power law curves, using the relation E ~ j~z+l)/(dq) [1]. We have assumed the dynamics of our system to be 3D. To obtain an independent estimation of the dynamic exponent o
0921-4534/94/$07.00 © 1994 - ElsevierScience B.V. All rights reserved. SSDI 0921-4534(94)02151-1
3172
M. Velez et al./P/o,sica C 235240 (1994) 3171 3172
we have also recorded the magnetoresistive transition in the same applied field at the lowest current density employed J = 1 A/cm 2. This should vanish at the transition as p~(T-Tg)' where s=u(z-1). Figure 2 shows a logarithmic plot of p vs T-Tg, where we have also included the linear resistivity extracted from the I-V characteristics. The solid line is a fit with s=8. This is radically different from single crystal results with Hilab, s=l.4 [4]. In our samples in-plane orientation of the c-axis is distributed at random along the two principal directions of the substrate, and then the vortex glass c-axis periodicity is destroyed. I 0 -2 10
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(,I/T) (1--T/T)V(1-d) Figure 3: Data in figure 1 scaled down with d=3, u=l.1, z=8.5, Tg=61.8 K.
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Figure 2: Logarithmic plot of p vs (T/Tg-I): C) measured at 1 A/cm 2, • obtained from the I-V curves. Solid line,is a fit to p ~(T-Tg)', s=8 Then we proceed to adjust these exponents to obtain a good scaling of the I-V characteristics. The results of such a procedure yield z=8.5 o=l.1 and Tg=61.8 K (see figure 3). The dynamic exponent z obtained is much higher than typical values for c-axis oriented films, and is out of the range predicted by the vortex glass model z=4-6. However there have already been reports o f anomalously high values of this exponent in certain particular conditions, for example Roberts e t al [2] found an increase in z at low fields and obtain z=8.5, s=8 and u=l.1 at 10 mT, and Ando et al [3] observe the same effect on very thin microbridges (width ~ 0.8 gm). They interpretate this increase of the slope of the critical I-V characteristic as a softening of the vortex interaction due to the decrease of the elastic moduli of the
vortex lattice at low fields, in the first case [2], and to the reduction of the elastic energy of the lattice in a distance )~, the London penetration depth, from the edge of the sample, in the second case [3]. In a-axis films we may obtain a similar effect due to the domain microstructure of the sample, of a size comparable to k. Also the number of vortices per domain (~ 20 at 1T) is in the same range as found in [2-3] for similar values of z, In summary, we have measured the current voltage characteristics of a-axis oriented EuBa2Cu~O 7 thin films, and found evidence of the existence of a second order transition, with critical exponents z=8.5 and u = 1.1. The high value of the dynamic exponent z may be due to a softening of the vortex lattice caused by the disordered microstructure.
A CKNOWLED GEMENTS This work has been supported by Spanish CICYT grant MAT92-0388. REFERENCES
[1] R.H.Koch et al Phys.Rev.Lett. 63 (1989) 1511; D.S.Fisher et al Phys.Rev.B 43 (1991) 130. [2] J.M.Roberts et al Phys. Rev. B 49 (1994) 6890. [3] Y.Ando et al Phys. Rev. Lett. 69 (1992) 2851. [4] W.K.Kwok et al Phys.Rev.Lett. 72 (1994) 1088. [5] C.B.Eom et al Science 249 (1990) 1549. [6] LColino et al Phys. Rev. B 49 (1994) 3496.
Physica C 235-240 (1994)3171-3172 North-Holland
C r i t i c a l s c a l i n g a n d v o r t e x g l a s s t r a n s i t i o n in a - a x i s o r i e n t e d EuBa,2Cu30 7 thin f i l m s M.Velez, J.I.Martin and J.L.Vieent Dpto. Fisiea de Materiales, Faeultad de Ciencias Fisieas, Universidad Complutense, 28040 Madrid, Spain. The I-V characteristics and the magnetoresistive transition of a-axis oriented EuBa2Cu307thin films grown by DC magnetron sputtering have been measured as a function of temperature. Evidence is found of a second order transition with critical exponents t)=l.1 and z=8.5, greater than those reported in literature for e-axis films. The temperature dependence of the magnetoresistivity slightly above Tg can be fitted to a sealing behavior p ~ (T-Tg)' with s ~ 8. These high values of the critical exponents may be attributed to a softening of the vortex lattice due to sample microstructure. Flux dynamics in the mixed state of high temperature superconductors have revealed a number of new interesting features. One of the most important is the existence of a second order phase transition from a truly superconducting glassy state into a liquid like state at a temperature Tg, in the presence of microscopic disorder [1]. However the critical exponents associated to this transition, have been reported to be dependent on the applied magnetic field [2], and sample geometry [3]. Most of the experimental studies presented up to now in YBa2Cu307 have been done on e-axis oriented films with the magnetic field applied parallel to the e-axis. Recent single crystal results with Hllab have suggested the existence of a new second order transition from a vortex phase with eaxis periodicity induced by intrinsic pinning by the layered structure [4]. In this paper we report on the critical behavior of a-axis oriented EuBa2Cu30 7 thin films, with the magnetic field parallel to the ~ plane. The influence of sample microstructure, consisting of small (~,200 nm) domains separated by 90 o boundaries [5], will be discussed. Pure a-axis oriented EuBa2Cu307 thin films have been grown by DC magnetron sputtering on (100) SrTiO 3 substrates [6]. The films had normal state resisivity of 1000 ~tD,cm, thickness 300 nm and critical temperature T~0=80-84 K. They were photolitographycally patterned into 100 pm bridges. I-V characteristics were recorded at a constant temperature with stability better than 10 mK. De current was supplied in short 1 s pulses by a Keithley 220 current source, and reversed to
minimize thermal offsets. An external magnetic field of 1T was applied perpendicular to sample plane, that is, parallel to the ab planes. Figure 1 shows I-V characteristics from 55 K to 75 K. A linear region at low currents is clearly seen in the higher temperature curves, while a much stronger current dependence appears in the low temperature region, where I-V characteristics are approximately linear in this Iogarithmic plot. An
10 - a
I
I
i
I
i
10 - s
IO._,L 10 - 8
10 -n 10 -x
I
I
I
I
I
10 0
101
10"
10 a
10 4
10 5
Figure l: I-V characteristics at 1T from 55K to 75K, with 1K interval. initial estimation of the critical temperature Tg and dynamic exponent z was taken from one of these low temperature power law curves, using the relation E ~ j~z+l)/(dq) [1]. We have assumed the dynamics of our system to be 3D. To obtain an independent estimation of the dynamic exponent o
0921-4534/94/$07.00 © 1994 - ElsevierScience B.V. All rights reserved. SSDI 0921-4534(94)02151-1
3172
M. Velez et al./P/o,sica C 235240 (1994) 3171 3172
we have also recorded the magnetoresistive transition in the same applied field at the lowest current density employed J = 1 A/cm 2. This should vanish at the transition as p~(T-Tg)' where s=u(z-1). Figure 2 shows a logarithmic plot of p vs T-Tg, where we have also included the linear resistivity extracted from the I-V characteristics. The solid line is a fit with s=8. This is radically different from single crystal results with Hilab, s=l.4 [4]. In our samples in-plane orientation of the c-axis is distributed at random along the two principal directions of the substrate, and then the vortex glass c-axis periodicity is destroyed. I 0 -2 10
.-~
I
l
I
I
lO s
-2 lo"
~II
~:
_._j/
!
:
I 10 -~
,
i
,
10 o
,
J
J
10 a
, 10 e
(,I/T) (1--T/T)V(1-d) Figure 3: Data in figure 1 scaled down with d=3, u=l.1, z=8.5, Tg=61.8 K.
-3
I0 -+
10 .8
10 . 7 1 0 -a
I
I
1 0 -5 ~k
I
io"
0.1
.
.
T/T i- i
Figure 2: Logarithmic plot of p vs (T/Tg-I): C) measured at 1 A/cm 2, • obtained from the I-V curves. Solid line,is a fit to p ~(T-Tg)', s=8 Then we proceed to adjust these exponents to obtain a good scaling of the I-V characteristics. The results of such a procedure yield z=8.5 o=l.1 and Tg=61.8 K (see figure 3). The dynamic exponent z obtained is much higher than typical values for c-axis oriented films, and is out of the range predicted by the vortex glass model z=4-6. However there have already been reports o f anomalously high values of this exponent in certain particular conditions, for example Roberts e t al [2] found an increase in z at low fields and obtain z=8.5, s=8 and u=l.1 at 10 mT, and Ando et al [3] observe the same effect on very thin microbridges (width ~ 0.8 gm). They interpretate this increase of the slope of the critical I-V characteristic as a softening of the vortex interaction due to the decrease of the elastic moduli of the
vortex lattice at low fields, in the first case [2], and to the reduction of the elastic energy of the lattice in a distance )~, the London penetration depth, from the edge of the sample, in the second case [3]. In a-axis films we may obtain a similar effect due to the domain microstructure of the sample, of a size comparable to k. Also the number of vortices per domain (~ 20 at 1T) is in the same range as found in [2-3] for similar values of z, In summary, we have measured the current voltage characteristics of a-axis oriented EuBa2Cu~O 7 thin films, and found evidence of the existence of a second order transition, with critical exponents z=8.5 and u = 1.1. The high value of the dynamic exponent z may be due to a softening of the vortex lattice caused by the disordered microstructure.
A CKNOWLED GEMENTS This work has been supported by Spanish CICYT grant MAT92-0388. REFERENCES
[1] R.H.Koch et al Phys.Rev.Lett. 63 (1989) 1511; D.S.Fisher et al Phys.Rev.B 43 (1991) 130. [2] J.M.Roberts et al Phys. Rev. B 49 (1994) 6890. [3] Y.Ando et al Phys. Rev. Lett. 69 (1992) 2851. [4] W.K.Kwok et al Phys.Rev.Lett. 72 (1994) 1088. [5] C.B.Eom et al Science 249 (1990) 1549. [6] LColino et al Phys. Rev. B 49 (1994) 3496.