Angular dependence of magnetoresistance and critical current density in YBa2Cu3O7−δ thin films

Angular dependence of magnetoresistance and critical current density in YBa2Cu3O7−δ thin films

Solid State Communications, Vol. 83, No. 9, pp. 695-699, 1992. Printed in Great Britain. 0038-1098/92 $5.00 + .00 Pergamon Press Ltd A N G U L A R D...

395KB Sizes 0 Downloads 89 Views

Solid State Communications, Vol. 83, No. 9, pp. 695-699, 1992. Printed in Great Britain.

0038-1098/92 $5.00 + .00 Pergamon Press Ltd

A N G U L A R DEPENDENCE OF MAGNETORESISTANCE AND CRITICAL C U R R E N T DENSITY IN YBa2Cu30~_6 THIN FILMS K-H. Yoo* and J.C..Park Korea Research Institute of Standards and Science, P.O. Box 3, Taedok Science Town, Taejon 305-606, Republic of Korea and H.G. Lee Korea Atomic Energy Research Institute, P.O. Box 7, Taedok Science Town, Taejon 302-353, Republic of Korea

(Received 27 April 1992 by G. Fasol) We have investigated the angular dependence Of the resistivity and the critical current density in the magnetic field for YBa2Cu307_6 thin films. Angular dependence of the magnetoresistivity is found to follow a sin 2 ~ form expected for the Lorentz-force-driven flux motion model, although when the magnetic field is applied parallel to the direction of grain or twin boundaries, the resistivity drops are observed depending on the sample. In the angular dependence of the critical current density, asymmetry is observed with respect to ~b= 180°, which may be due to surface pinning at the interface between the film and the substrate. However, its angular dependence is explained by the simple flux motion model in spite of asymmetry. THE ANOMALOUS 'broadening of the resistive transition in magnetic fields has been commonly observed in high Tc superconductors. This behaviour has been quite successfully explained by thermally activated magnetic flux creep or flux flow phenomena [1]. However, some recent experiments [2-5], which measure the magnetoresistivity for different orientations of transport current and magnetic field, particularly on thin films of high Tc superconductors, have raised a question of the interpretation based on flux motion. A difference of the resistivity for H III and H 1 1 has not been detected, indicating that the Lorentz-force does not play a role in this loss mechanism. In contrast, similar experiments on YBa2Cu3OT_~ single crystals. [6] show the presence of flux motion effects in a form of an excess angular dependent resistivity superimposed on an angular independent resistivity. In this paper, we present careful measurements of angular dependences of the resistivity and the effective pinning energy Ueff on the relative orientation of transport current and magnetic field in two YBa2Cu3OT_6 thin films. We have carded out these measurements to see whether the angular

dependent resistivity is observed even in thin films of YBa2Cu3OT_6. The results show a distinct angular dependence of the resistivity on the relative orientations of current and magnetic field, supporting the Lorentz-force-driven flux motion model. Additionally, we have also measured the angular dependence of the critical current density in the magnetic field to investigate the effects of flux motion on the critical current density. The YBa2Cn3OT_ 6 thin films were prepared on (100) SrTiO3 substrates by the chemical vapor deposition method described elsewhere [7]. The films were predominantly oriented with the c axis normal to the substrate plane and the film thickness was about 0.5 #m. The resistivity and the critical current density were measured by using a four-probe d.c. technique on a bridge 100#m wide and 500pro long, which was patterned by photolithography and chemical etching using phosphoric acid. The measuring current of the resistivity was 100 ~A and Jc was determined using an electric field criterion of 1.0 #V/ cm. The transport current was applied parallel to the a(b) axis and the magnetic field generated by a superconducting solenoid was applied perpendicular * Author to whom all correspondence should be to the c axis. The misalignment of the ab plane with addressed. respect to the magnetic field was less than 0.5 °, which 695

CRITICAL C U R R E N T DENSITY IN yBa2Cu3OT_6 THIN FILMS

696

s[

100 H=20 kG il lib pi0ne

,,,~

80 E

0

,pO#q~,..,.

&&&,l.+~ .(. 4J 4. 4a& ~+

~. 40

.so°

2O

8

89

90

(a)

4



&A ~4. 4.'1' • +

60

Vol. 83, No. 9

D n

.oo



n

• H : 0 kG

0

' 91

'

0

92

0

90

T(K)

270

360

270

360

# (deg)

Fig. 1. Resistive transition curves in the zero field and in the magnetic field of 20 kG at 0 = 0 and 90 °. was measured using the coordinate measuring machine (UMM 500, Zeiss). The orientation of the transport current with respect to the magnetic field was varied by rotating the film about its c axis for study of the angular dependence of the resistivity and the critical current density in the magnetic field. The sample temperature was measured with a calibrated carbon-glass thermometer and regulated to better than +5inK during angular dependence measuremerits of the resistivity and the critical current density at a fixed temperature. Figure 1 shows the resistive transition curves in zero field and in the magnetic field of 20 kG at ~ = 0 and 90 ° for the YBa2Cu3OT_6 film, where ~ is the angle between the current and the magnetic field. In the zero field, the zero resistance temperature is 90.3K and the resistive transition width A T c (10-90%) is about 0.8K. In the magnetic field of 20 kG, the resistivity at 0 = 90 ° has a higher value than that at ~ = 0 °, although the resistive transition curves are broadened by the applied magnetic field for both ~ = 0 and 90 °. The broadening of the resistive transition in the magnetic field observed in the absence of Lorentz-force is discussed below. The magnetoresistive measurements were also performed in the magnetic field of 50 kG for ~ = 0 and 90 °. The difference between the transition curves measured at two angles was more pronounced than that in the 20 kG curves. This result agrees with the Lorentzforce-driven flux motion model. Figures 2(a) and (b) show the measured angular dependence of the resistivity in the magnetic field of 20kG at the temperature of 88.5K for two YBa2Cu307_6 films. Different angular dependences of the resistivity are observed for two films prepared in similar conditions. In Fig. 2(a), the data follow the sin 2 ~ dependence expected for a resistive component

180

(b) 6

O2

0

0

90

180 • #

((leg)

Fig. 2. Angular dependence of the resistivity in the magnetic field of 20kG at T = 88.5K for (a) a YBa2Cu3OT_ 6 film and (b) a different YBa2Cu307_ 6 film from (a). The solid curves represent a sin2 ~b dependence. due t o Lorentz-force-driven flux motion. It is consistent with the results reported by Kwok et al. [6] except that the resistivity drop is not observed in Fig. 2(a) when the magnetic field is aligned along the direction of the twin boundaries. In Fig. 2(b), however, the resistivity takes minima at 0 = 0, 90, 45, and 135°, although the resistivity except for the resistivity drops seems to follow the sin20 dependence represented by the solid curve. Similar resistivity minima at ~ = 0, 90, 45, and 135° have also been reported in thin films of YBa2Cu3OT_~ by Iye et al. [8], though the sin20 dependence is not seen in their measurement. To find the difference between two films, they were investigated by X-ray diffraction and scanning electron microscopy (SEM), but no difference was found. Although the reason to induce different angular dependence of the resistivity for two samples is not clear, the observed difference is estimated to be ascribed to the microscopic difference which cannot be detected by SEM.

Vol. 83, No. 9

CRITICAL CURRENT DENSITY IN YBa2Cu307_ 6 THIN FILMS

2.3

>

1.7

i

0

9O # (deg)

180

Fig. 3. Angular dependence Of the effective pinning energy U~ for the YBa2Cu307_ ~ film used in Fig. 2(b). In order to explain the observed resistivity drops at ~b= 0, 45, 90, and 135°, the effective pinning energy Ucffhas been obtained from the linear portion (10- < p < 10°#f~-cm) of the Arrhenius plot a t various angles as presented in Fig. 3. The peaks are seen at $ = 0, 45, 90, and 135°. Th~ peaks at ~b= 45 and 135° may be considered to be due to flux pinning at twin boundaries, whereas the peaks at $ = 0 and 90 ° to be due to flux pinning at grain boundaries since the grain boundaries are formed parallel to the a and b axes. The peak positions of Ucff coincide with the minimum positions in i'esistivity as shown in Figs. 2(b) and 3. It implies that the drops in resistivity may be ascribed to pinning of the flux lines at twin or grain boundaries when the applied magnetic field is aligned along twin or grain boundaries. However, the sample dependence shown in Figs. 2(a) and (b) suggests that flux pinning centers may be impurities or atomic defects such as an oxygen disorder located in the planes containing twin or grain boundaries rather than twin or grain boundaries themselves. Otherwise, the resistivity drop may be also observed in Fig. 2(a) since the film used in Fig. 2(a) is not single-crystalline, and has also twin and grain boundaries. Several measurements [2'5] of the resistivity in the magnetic field have been reported in thin films of high Tc superconductors for both H III and H _1_/. They have shown that the data for both oridentations are nearly the same, and it has been suggested that the Lorentz-force is not the origin of the resistive loss in the high Tc superconductors. However, the above results indicate that the resistive transition curves in the magnetic field can be almost the same for H [[ I and H _l_ I depending on the sample, if impurities or defects are located in the planes containing grain boundaries and play the role of pinning centers as in

697

Fig. 2(b). Hence, in order to test the applicability of the magnetic flux motion picture for the high Tc superconductors, the whole angular dependence should be examined. The measured magnetoresistivity has been explained quite well by the Lorentz-force-driven flux motion model except that the broadening of the resitive transition in the magnetic field is observed even in the absence of the macroscopic Lorentz-force as shown in Fig. 1. Similar broadening [2-6] has been also reported a n d proposed to be due to Josephsonjunction tunneling [9], flux line melting [10], or an unknown mechanism [6]. However, although it is not conclusive, it may be suggested that the Lorentz-force may not be actually zero even for HI] I since the current and/or flux line directions may deviate from the macroscopic average due to defects contained in the sample. If this is the case, the broadened resistive transition for H HI may be explained without introducing another mechanism. Figures 4(a) and (b) show the critical current densities as a function of the angle between the current and the magnetic field of 20 kG at temperatures of 70 and 80 K for the same film as in Fig. 2(a). As expected from the Lorentz-force-driven flux motion model, the maximum values of the critical current density are observed at ~b= 0, 180, and 360 °, while the minimum ones at ~b=90 and 270 °. However, they are not symmetric with respect to ~b= 180°; the minimum value at 90° is lower than that at 270 °. Since the only difference between $ = 90 and 270 ° is the direction of the Lorentz-force on the vortices, the current direction was reversed to change the direction of the force and then the critical current density measured. Indeed, the value of "reverse" critical current density at $ = 90° was almost the same as that of "forward" critical current density at = 270 °. It implies that the observed asymmetry in Figs. 4(a) and (b) may be due to surface pinning at the interface between the film and the substrate. In other words, if the force on the flux lines is directed from the film surface to the substrate (downward), the flux lines are pinned at the interface, while if the force is directed from the substrate to the film surface (upward), the flux lines cannot be pinned at the interface. So, a higher critical current density is obtained for the downward direction than for the upward one. For comparison with low Tc superconductors, similar critical current density measurements in a magnetic field of 1 kG at a temperature of 4.2 K were repeated for a 3700 A-thick Nb film and the result is presented in Fig. 4(c). The Nb film was prepared by r.f..sputtering on a Si substrate and patterened to the

698

CRITICAL C U R R E N T DENSITY IN YBa2Cu3OT_ 6 THIN FILMS

Vol. 83, No. 9

10

R' E u

(a) YBCO

(a) YBCO

1.2 ~k0

000 O

<

o

V

o o

m

m

0 x

n

o

o 0

o m

io

1.1

o

• o

o 0ooo°

o

U

o o

o

gooo

4

1

0

I

"00

m

I

90

l

I

180

m

270

8

360

0.0

0.2

0.4

g (dog)

I

3.5 . (b) YBCO

!0j

m o

m

,

E

2.2

0

90

180

270

2.5 0.0

360

0.6

0.8

1.0

|

0.2

no o 0

8

Uq

.

g

0.4

sin fl

(c) Nb

• 0

~ 7

O O

U 0

<

1.0

m°~ mm

(c) Nb u

0.8

3.0

m

2,6 2,4

0.6

3.5

M~ I

(deg)

,-,.

1.0

4.0

j

z,

m

am mm • m

2.5

.l

m

0.8

sin #

.°o

O

i

06

U OUO00

0

0

2,0 O

x

1.8

u ~

1,6

O

O

• O

g,

O 0

O

UnooO 1.4

|

0

9O

180

m

270

3

360

0.0

fi (deg)

0.2

0.4

sin ft

Fig. 4. Angular dependence of the critical current density in the magnetic field of 20 kG at (a) T = 70 K and (h) T = 80 K for the same YBa2Cu307-6 film as in Fig. 2(a), and (c) T = 4.2 K for a Nb film.

Fig. 5. The reciprocal of the critical current density as a function of the angle between the current and the magnetic field at (a) T = 70 K and (b) T = 80 K for a YBa2Cu3OT± 6 film, and (c) T = 4.2 K for a Nb film.

strip 30 ~m wide and 500/~m long. A similar angular dependence is observed. In the simple flux motion picture, the relation between the critical current density and the magnetic field is given by [11]

where 0 is the angle between the current and the magnetic field, and c~ and B0 are constants depending on the temperature and the materials. In Figs. 5(a), (b), and (c), the critical current density is replotted in the form 1/Jc vs sin ~ . Straight lines are obtained which show the agreement of the data with equation (1). The values of a and B0 derived from these curves

Jc = a / ( H s i n ~ + B0),

(i)

Vol. 83, No. 9

CRITICAL CURRENT DENSITY IN YBa2Cu3OT_6 THIN FILMS

699

Table 1. The values of a and Bo for YBa2Cu3OT_6 and Nb films

YBCO (70 K) c~(106 kG A/cm 2) ¢=0-180

YBCO (80 K)

Nb (4.2 K)

13.2

1.6

60.2

106.9

41.6

22.0

20.8

2.5

261.1

170.5

69.6

97.4

o

B0 (kG) o (106 kG A/era 2) = 180-360 ° Bo (kG)

are given in Table 1. The values of a and B0 for the downward direction are larger than those for the upward one. In summary, angular dependences of the magnetoresistivity have been studied in two YBa2Cu307_6 thin films on the relative orientation of transport current and magnetic field. As expected for Lorentzforce dissipation, the angular dependence of the resistivity follows a sin2~ form, although when the applied magnetic field is aligned parallel to grain o r twin boundaries the resistivity drops are observed for one of the two films, while they are not observed for the other. The sample dependence suggests that the resistivity drops observed only for one of the two samples may be ascribed to flux pinning at impurities or defects located in the plane containing twin or grain boundaries. In addition , we have also measured the critical current density as a function of the angle between the current and the magnetic field. The data can be explained by the simple flux motion model. However, because of surface pinning at the interface between the film and the substrate, the critical current density is higher when the Lorentz-force is directed from the film surface to the substrate than when it is directed vice versa. Acknowledgements - This study is supported by the Ministry of Science and Technology, Republic of Korea. We would like to thank Dr Soon-Gul Lee for his helpful discussion, Dr Y.H. Lee for providing a Nb film, and Mr C.M. Ihm and Y.K. Song for their technical assistance.

REFERENCES 1.

2. 3. 4. 5. 6. 7. 8.

9. 10. 11.

For a review, see A.P. Malozemoff, T.K. Worthington, E. Zeldov, N.C. Yeh, M.W. McElfresh & F. Holtzberg, in Strong Correlations and Superconductivity, Springer Series in Solid State Sciences Vol. 89 (Edited by H. Fukuyama, S. Maekawa & A.P. Malozemoff), p. 349. Spinger-Verlag, Heidelberg (1989). K.C. Woo, K.E. Gray, R.T. Kampwirth, J.H. Kang, S.J. Stein, R. East & D.H. McKay, Phys. Rev. Lett. 63, 1877 (1989). A.A.A. Youssef, T. Fukami & S. Mase, Solid State Commun. 74, 257 (1990). Y. Iye, S. Nakamura & T. Tamegai, Physica C159, 433 (1989). R.C. Budhani, D.O. Welch, M. Suenaga & R.L. Sabatini, Phys. Rev. Lett. 64, 1666 (1990). W.K. Kwok, U. Welp, G.W. Crabtree, K.G. Vandervoot, R. Hulscher & J.Z. Liu, Phys. Rev. Lett. 64, 966 (1990). H.G. Lee, J.S. Park, C.J. Kim, J.C. Lee, G.W. Hong, I.S. Chang & D.Y. Won (to be published). Y. Iye, S. Nakamura, T. Tamegai, T. Terashima & Y. Bando, in Proc. M R S '89 Fall Meeting, Boston, 27 November-1 December, p. 871. Materials Research Society, Pittsburgh (1989). D.H. Kim, K.E. Grey, R.T. Kampwirth, K.C. Woo, D.M. McKay & J. Stein, Phys. Rev. B41, 11642 (1990). D.R. Nelson, Phys. Rev. Lett. 60, 1973 (1988), P.L. Gammel, L.F. Schneerneyer, J.V. Waszczak & D.J. Bishop, Phys. Rev. Lett. 61, 1666 (1988). G.W. Cullen, G.D. Cody & J.P. McEvoy Jr, Phys. Rev. 132, 577 (1963).