Magnetic hysteresis effects in the ac susceptibility of YBaCuO ceramics

PHYSICA

Physica C 235-240 (1994)3189-3190 North-Holland

Magnetic Hysteresis Effects in the ac Susceptibility of YBaCuO Ceramics L. A. Angurel, F. Lera, C. Rillo, R. Navarro Instituto de Ciencia de Materiales de Arag6n (Universidad de Zaragoza-C.S.I.C.) Centro Polit6cnico Superior, Maria de Luna 3, E-50015 Zaragoza, Spain

Dc field magnetic hysteresis effects in ac susceptibility measurements have been measured on YBaCuO ceramic samples at different temperatures. The onset of hysteresis takes place at a field for which flux begins to penetrate inside the grains. Two regimes have been found. At low fields, hysteresis is weak and can be explained considering the evolution of the local intergranular field. At higher fields, a second regime, associated with intragranular effects, is observed. 1.-INTRODUCTION The intergranular magnetic properties of YBaCuO ceramics can be well understood using the Critical State Model (CSM) and introducing the granular nature of the sample in an effective permeability,//cer [1, 2]. However, the observed dc field magnetic hysteresis effects on ac susceptibility, Zac(Hdc), [3-7] are not predicted by the CSM itself. These phenomena, similar to those observed in transport critical currents measurements, have been explained considering the differences in the local intergranular field associated with the penetration inside the grains [5, 8]. Should this be the case, some relation between the onset of the hysteresis in the susceptibility and the field of first penetration inside the grains, Hfp, should be found. To demonstrate this, dc magnetization, M(Hdc) and Zac (Hdc) measurements are discussed in this paper.

inset of this Figure shows AM, the deviation of 4~3/ from linearity, giving Hfp (30 K)~-l15 Oe and Hfp (75 K)-.~-40Oe. Note that at low fields (below 40 Oe at 30 K and below 10 Oe at 75 K) the intergranular hysteresis loop is observed. The slopes of the intraganular linear region are related with -(1//cer), and give #cer(30 K)=0.52 and //cer(75 K)=0.60. The hysteresis in Zac(Hdc) has been measured scanning up to different +Hmax values ranging betwen 10 and 10000 Oe and the results are presented in Fig. 2. The curves show a shoulder that separates

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(Oe) ,

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......................... ,......................

-50 ~ -I00

z ;f1~i..............

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~

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2.- RESULTS AND DISCUSSION M(Hdc ) and Zac(Hdc) ( h 0 = 0 . 5 0 e ,

f=100 Hz)

measurements have been performed in a SQUID magnetometer (Quantum Design) on an YBCO ceramic cylinder with a neglegible demagnetizing factor (9.70 mm in length, 1.75 mm in diameter and 121.1 mg). Experiments have been performed under ZFC conditions at two temperatures: 30 and 75 K. First, the initial magnetization curve has been measured in order to determine Hfp (Fig. 1). The

-250

T=

-300 . . . . . . . . . I . . . . . . . . . t . . . . . . . . . 0 500 1000 1500 H

de

(Oe)

Figure 1: Initial magnetization isotherms at 30 and 75 K. Deviation from linearity is presented in the inset. The arrows indicate the Hfp values.

0921-4534D4/$07.00 © 1994 - ElsevierScience B.V. All rights reserved. SSDI0921-4534(94)02160-0

L.A. Angurel et al./Physica C 235 -240 (1994) 3189 3190

3190

4

7rZ'

0.0

........

,

........

,

........ I T=75K

........

-0.2

4 :rr Z " ( l l a ) 0.08 . . . .

,

H max

0.06

-0.4

~

o

100

Oe

¢

300

Oe

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0.04

Oe

H

-0.6 -0.8

......................

-1.0 1

........ ' 10

"- "

=

........ ' ........ ' ...... 100 1000 10000 H dc ( O e )

Figure 2: Virgin curves (full symbols) and descending branch (open) of Zac(Hdc) measurements at 30 and 75 K with Hrnax (Oe) = 100 (circles), 300 (rhombus), 10000 (triangles).

inter and intragranular regions, yielding /.Icer values similar to those above. In both cases, with HmaxHfp, different regimes have been observed, which can be better identified in a Z" vs Z' plot (Fig. 3). In the virgin curve two peaks in Z" are expected: one at low fields, associated with grain's junctions and other with the grains themselves. If the value of Hmax does not allow to reach the Z" minimum, the hysteresis is weak (see Fig. 2). In this situation, upon field decreases, the shielding ability of the junctions improves, increasing Iz'l. At low fields, IZ'I shows a maximun and its zero field value becomes lower due to the remanent flux trapped inside the grains. At least qualitatively, it can be explained analysing the hysteresis of the intergranular local field [8]. Similar effects have been observed in other ceramic samples [3-6] as well as in transport measurements. If this were the reason of the hysteresis, it could only be observed in a Zac(Hdc) plot, being negligible in an Z" vs Z' one, as can be observed in the curve of 100 Oe in Fig. 3. For fields above the Z" minimum, the behaviour changes qualitatively. If the field or the temperature is high enough, Ix'l reduces in the descending branch. This fact has

0.02

0.00

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'

-0.75

'

'

I

'

I

-0.50

4

-0.25

z'

0.00

(%?

Figure 3: Z" vs ;If' plot of the ac susceptibility at 75 K with different Hrnax- Symbols are the same as in Fig. 2 while lines are eye-guides.

an intragranular origin because it has been observed in single crystals [7] and has been associated with oscillations of vortices around their equilibrium positions. In conclusion, the onset of the hysteresis in ac measurements has been related with Hfp, and two regimes have been observed, one associated with the hysteresis in the local intergranular field and the other associated with intragranular effects.

ACKNOWLEDGEMENTS This work has been supported by CICYT (MAT 92-0896-C02-01 and -02) and by MIDAS Program (93/2332).

REFERENCES

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