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PHYSICA

North-Holland

Influence of the Flux Creep on the Irreversibility Line of YBazC%O ,. (6.6

1"

I)

N. Kobayashi ~, K. H1rano ', Y. Mmagawa ', T. Sasaki K. Watanabe t, S. Awaji ~, H. Asaokaz and H. Takei3 •

•

t Institute for Materials Research, Tohoku University, Katahira, Sendai 980-77 Japan 2 JAERI, Tokaimura, lbaraki, 319-11, Japan 3 Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan

The temperature and angular dependence of the irreversibility field of YBa2Cu3Oy (6.6

The magnetization measurements were performed by using a commercial SQUID magnetometer. The sample was rotated around an axis perpendicular to the magnetic field and parallel to the ab-plane of the crystal. The rotating angle 0 was defined by the angle between the c-axis and the magnetic field. The magnetization curve exhibits a large hysteresis indicating a strong pinning of tim.; lines. The characteristic feature in the magnetization curve is an appearance of remarkable "second peak". The irreversibility field Hi, is defined as the lowest magnetic field where the magnetization is reversible. The temperature dependence of Hi~ is shown in Fig 1 as a function of (1-T/To). We find a poxver lax,,,

s°I o

2O

©

,e

1

........ #1

I 6=0.15

o

#2 6=0.1

~'

#3

#4 6 = 0 . 4

o

#5 6 = 0 . 2

.-

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o

'#6' v o ..

o ~ o

8=0.3

"O .-

'

o

¢

~,

~

q

t)

(l-'rl'r~) 2

~ /

(1-'rl'rO"s

/

~

oo

o o

O

o

~¢

0 ~

. "

I =1 I0

I

,

, tit

/

t

i

t

t it,ll

I 0 -z

. ,

i

, , ~ , ,

10-t (l-T/To)

* Present address" Hitachi Cable, Ltd. 0 9 2 1 - 4 5 3 4 / 9 4 1 5 0 7 . 0 0 © 1 9 9 4 - E l s e v i e r S c i e n c e B.V. All r i g h t s r e s e r v e d .

SSD! 0 9 2 1 - 4 5 3 4 ( 9 4 ) 0 1 9 5 8 - 4

Fig. 1. Temperature dependence of H~.

0°

2786

N. Kobayashi et al,/ Physica C 235-240 (1994) 2785-2786

behavior (1-Tfr¢)" for magnetic fields above 10 kOe. The power n increases from 1.5 for y=6,9 to 2 or more for y=6.6. This result suggests that the power n becomes larger with increasing anisotropy as shown later. The angular dependence of H,~ is first attempted to fit Hi,~(0) b~ the effective m a s s model, Hi~(0)=H,~(0)/(cos'0 ~ . z s i n ~ 0) ltZ , w he r e ~," is the effective mass ratio mJmb. The values of H,~(0) and ~ are obtained to be l lkOe and 4.1 for YB~Cu306. 9, respectively. However, the y value obtained is a little smaller than that of literatures[I]. The measured angular dependence shows a steeper change at high tilt angles compared with the effective mass model. This seems to be reasonable if we consider the influence of the flux motion[2]. Here, we consider the temperature dependence and anisotropic behavior of the irreversibility field in the framework of the flux creep and the effective mass models. According to the simple fhux creep model, the critical current density is given by J~--J~o[ 1"(kT/U0) In(v0Hd/E)], (1) where J~0 is the critical current density in the absence of the thermal activation, d the hopping distance, v 0 the attempt frequency for flux lines to escape from the pinning well U0. At high magnetic fields where correlation effect between flux lines becomes significant, we obtain following expression from eq.(1) by a disappearance of J , n,~ln (v0Hi d/Ec)o,:(l-T/T) ~~fl". (2) If the logarithmic term is almost constant, i.e. ln(v0d/E~)>~lnHi~, and T - T , then we obtain a expression Hi~.(t)o~(1.T/T) u~given by Yeshuran and Malozemoff [4]. On the other hand, if the field dependence of the logarithmic term cannot be neglected, eq.(2) is approximately described by H~.InH~°~(1-T/T)t'~/T. Our experimental result is consistent with this expression. Furthermore, if ln(v0H~.d/E ~) is regarded as to be almost independent of the field direction, the value of Hi~lnH~ is proportional t e l/(cos20+3,2sin20) */-'. Figure 2 shows H~ lnH~,~as a ft,aaction of 1/(COS20+~"2 sin20) ~/2 for the YB%Cu30~,s5 sample. As clearly seen in Fig. 2, we get the above linear relation. Similar tendency is found for all samples studied. The "y values then obtained are listed in Table 1.

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i ....'

i

'

i

-"

1

'-"T

_

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YBazCtqO6.~s #1 85.:fl¢,

~ - .

"y= 6.3

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,

I

0

,

1

,

I

2 3 4 1/(cosZ0+-r-2sin20) u2

I

,

I

,

t

5

,

1

6

Fig. 2. Angular dependence of H~,~at 85.5 K. Table 1. Transition temperature T c , oxygen content y and anisotropic parameter ~,. II

II

II

II

s,ample #1 #2 #3 #4

T(K) 89 90 68 58

Y 6.85 6.9 6.7 6.6

V 6.3 6.0 18 26

The 7 value for YBazCu3009 is in good agreement with that obtained from a scaling analysis of the angular dependence of the resistive transition. In conclusion, the angular dependence of H t i T is ., described by a relation, H,,~lnH~. o< 1/(cos20 + 7" sin20) ~I2,which is explained by taking account of the influence of the flux creep on the irreversibility field. The anisotropic parameter ~, of reduced YBa_,Cu3Oy v~'as quantitatively estimated. REFERENCES 1.

Y. lye, A. Fukushima and T. Tamegai, Physica C 185-189 (1991) 297. 2. K. Watanabe, S. Awaji, N. Kobayashi, H. Yamane, T. Hirai and Y. Muto, J. Appl. Phys. 69 (1991) 1543. 3. H. Takei, H. Asaoka, Y. lye and H. Takeya, Jpn. J, Appl. Phys. 30 (1991) L1102 4' . I Id,'~llUl all al~d A.P. Y . "~-" ..... ~l '.,, '1l a i ~. Z. ~.. , .i s l v . c rI , Phys. Rev. Lctt, 60 (1988) 2202.