Angular scaling of critical current measurements on laser-ablated. YBa2Cu307_ thin films. G. Ravi Kumar a b M. R. Koblischka , J. C. Martinez ~, R. Griessen , B.
Physica C 235-240 (1994) 3053-3054 North-Holland
PRYSICA
(~
Angular scaling of critical current measurements on laser-ablated YBa2Cu307_ thin films. G. Ravi K u m a r a b M. R. Koblischka ~, J. C. Martinez ~, R. Griessen ~, B. D a m ~ and J. Rector ~ ~Free University, Faculty of Physics and Astronomy, De Boelelaan 1081, 1081 HV A m s t e r d a m , The Netherlands. bSolid State Physics Division, B. A. R. C. Bombay-400085, India Orientation dependent measurements of torque hysteresis loops on high quality laser-ablated YBa2Cu3OT-6 thin films were performed at a temperature of 30 K in magnetic fields up to 7 T. From these measurements we extract the field and orientation dependence of the in-plane critical current density J~(H, ®), where ® is the angle between the applied external magnetic field H and the plane of the film. Following the scaling arguments of Blatter, Geshkenbein and Larkin based on the anisotropic Ginzburg-Landau theory, we have been able to scale the measured Js(H,®)into a form f(e(®)H), where e(®) = (sin2 O + cos2 ®/ 2)~/2 with ~' = 3.4-4-0.3. The obtained value of the anisotropy parameter 3, is much smaller than the values reported earlier for single crystals close to T~.
High-T~ superconductors are characterized by a large anisotropy originating from their layered structure. A measure of anisotropy is the effective mass ratio 72 :- mc/ma where ma and me are the Ginzburg-Landau superconducting effective masses for pair motion in the ab-plane and along the c-direction respectively. In a truly 2dimensional superconductor 7 = cx~ while it is 1 for an isotropic superconductor. The present case of YBa2Cu307-~ is intermediate between the two. Farrell el al. [1] have presented an experimental method based on torque magnetometry, to estimate the value of 7- They infer a value of 7.9+0.2 for 7. Their analysis is valid only in the reversible regime of the superconductor i. e. above the irreversibility line. In the case of YBa2Cu307-~ it is therefore applicable only close to To. In this paper we present an experimental method, based on the scaling analysis of Blatter, Geshkenbein and Larkin [2], to estimate the value of 3' from measurements in the irreversible regime. Recently Blatter et hi. [2] developed a scaling approach, based on the anisotropic GinzburgLandau theory. It allows to generalize the resuits for isotropic superconductors to the case of anisotropic superconductors, including the case of an arbitrary direction of the applied magnetic
field with respect to the sample. Of particular importance is the behaviour of the in plane critical current density as a function of magnetic field and its orientation with respect to the sample. In their article they indicate that the in-plane critical current density Js measured in an external magnetic field H making an angle O with respect to the (a,b)-plane should vary as J,(O, H) = f(((O)H)
,
(1)
with
= (8i 20 + cos20/72) 1/2
(2)
In this paper we present torque measurements on a high critical current density YBa2Cu3OT-6 thin film patterned into a ring of diameter (2R) 1 m m with an annular width (w) of 0.1 m m and thickness (D) 400 am. The films are prepared by laser ablation. The torque hysteresis loops are measured at 30 K as a function of (9, which is varied upto 70 °. The magnetic field is swept at a rate of 20 m T / s upto 7 T. In thin films we know that the induced currents are confined to the plane of the film and the magnetic m o m e n t r~ is always perpendicular to the film. We measured the torque = #0rfi x /~ by means of a capacitance torque m a g n e t o m e t e r [3]. In our ring shaped samples the
0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)02092-2
3054
G. Ravi Kumar et al./Plwsica C 235-240 (1994) 3053 3054 20
.
J
o
15
~
., 1 5 . 7
4
.~..
I.
~
I.
~.~
¢.
[
ll).4
i
2f)
,.
i . . . .
+--,~-
I
- - 4
F
D 22.7
,. ,.
I
~ . . . .
+1
36.7 22.7
I ()
t
t ()
! ....
()
~ ....
I
! ....
2
~
I'
. . . .
4
-,i
'''I
~
....
(,
l
(
7
)
0
t
~
,
.
I
l
....
I
.?
....
~ 4
:
~
i
....
I , ,-'--'-+ ¢) v
bt()H c(O) (T)
~ H (T~ Figure 1. Critical current density as a function of external magnetic field measured at T = 3OK, f o r d(fferent angles. The values of 0 are 10.4 °, 15.70, 22.7 °, 36.7 °, and 68.3 °. ['or clarity of presentation the data f o r six other values of 0 have been excluded.
magnetic moment is given by r r R 2 w D J , where ,1, is the induced current density in the film. From the torque hysteresis loops we obtain the field and angle dependence of the critical current density using the relation J~(H, O) = A r ( H , O ) / 2 ~ R 2 w D # o H c o s O
t
....
(3)
where A v ( H , O) is the hysteresis in the measured torque at a field H for a fixed angle O. In fig. 1 we present the field dependence of the current density for various values of O. Farrell et al. [1] argued, at temperatures below 70K, that the coherence length along c-direction becomes much smaller than the CuO2 interlayer spacing. Therefore, YBa2CuaO7-~ should behave as a two dimensional superconductor. In such a case J~ should be determined solely by #0H sin O. However we did not find any evidence for such a 2D behaviour. Following Blatter et al. [2], we scale the magnetic field axis in fig. 1 with the angle dependent anisotropy factor e(O). We obtain a remarkably good scaling of the J~ vs H data for various angles, when the magnetic field axis is scaled by e(O) with 7 = 3.4±0.3. This is significantly smaller than the values between 5 and 8 reported earlier for YBa2CuaO7_t single crys-
Figure 2. Same data as in fig. I but now plotted as a function of e ( O ) p o H with ~ = 3.4 i~ eq. (2).
tals [1], but consistent with the values obtained in anisotropic H~I measurements [4]. The origin of this difference is not known. The major differenc(, between our samples and single crystals is found in their critical currents. For the present laser ablated films Ja is at. least one order of magnitude higher than in single crystals. In conclusion, we presented an experimental method to estimate the anisotropic effective mass ratio in the irreversible regime of the superconductor. From the torque hysteresis loop measurements at different sample orientations wilh respect to field, we established that the inplane critical current density depends solely on H ( s i n ' O + c o s 20/7~') 1/2. Excellent scaling is obtained for the anisotropy parameter 7 = 3.4±0.a. This work is subsidized by FOM. GRK is grateful to the European community for granting a fellowship under the indo-EC agreement. REFERENCES
1. 2. 3. 4.
D.E. Farrell el al., Phys. Rev. Let~. 61 (1988) 2805; ibid. 64 (1990) 1573. G. Blatter ct al., Phys. Rev. Lett. 68 (1992) 875. M. Qvarford et al., Re~. ,5'ci. lnslrum. 63 (1992) 5726. L. Krusin-Elbauln el al., Phys. Rev. B 39
(1989) 2936.
