AC susceptibility study of YBaCuO prepared by quench and melt growth process

PhysicaC 166 (1990) 215-220 North-Holland

AC SUSCEPTIBILITY PROCESS S. GOTOH,

STUDY OF YBaCuO PREPARED BY QUENCH AND MELT GROWTH

M. MURAKAMI,

H. FLJJIMOTO,

N. KOSHIZUKA

and S. TANAKA

Superconductivity Research Laboratory, International Superconductivity Technology Center, I-10-13, Shinonome, Koto-ku. Tokyo 135, Japan

Received 12 January 1990

AC susceptibility measurements have been carried out on YBaCuO prepared by the quench and melt growth process. The temperature dependence of the real component, x’, and the imaginary component, x”, of the susceptibility does not indicate the presence of weak-link regions in the sample. The superconducting transition width ofx’ is very narrow and only a single peak of x” is observed as opposed to bulk sintered materials. The observed relationship between x’ and x” is well qualitatively explained in terms of Bean’s critical state model.

1. Introduction Measurements of the AC susceptibility, x=x’ -ix”, have been used to determine accurate critical temperatures of conventional metallic superconductors [ 11, and recently, to determine the onset temperatures for high-T, oxide superconductors [ 21. In highT, bulk sintered superconductors, detailed AC susceptibility measurements as functions of temperature and AC field amplitude typically show two drops in the real component, x’, and two corresponding peaks in the imaginary component, x”, when the AC field amplitude exceeds the threshold value, which depends on the sample quality [ 3-101. This behaviour indicates that the superconducting regions or grains are coupled by weak links or Josephson-type junctions [ 3,5,11], reflecting low values of transport critical current density, J,, typically three orders of magnitude lower than the required level. Extremely short coherence length and low carrier density, which are characteristic of high-T, oxide superconductors, or oxygen deficiency at grain boundaries seem to be the source of the weak-link behaviour [ 12 1. But it has become clear that a low J, is not intrinsic by the fact that very high J, values are obtained in single crystals [ 13 ] and thin films [ 14 1, Recently, large YBaCuO crystals have been fabricated by the quench and melt growth (QMG) pro0921-4534/90/$03.50 0 Elsevier Science Publishers B.V. ( North-Holland )

cess [ 151. They exhibited high J, values that exceeded lx lo4 A/cm2 at 77 K and 1 T. DC magnetization measurements indicated that Bean’s critical state model [ 161 was well established in such samples [ 171. This implied that no weak-link region was present inside the samples. It was also found through microstructural analysis that fine YzBaCuOS inclusions were trapped in a YBa$&O, matrix in the QMG processed samples. They are expected to contribute to flux pinning and thereby high J, values [ 18 1. In this paper, we report the AC susceptibility of the QMG processed YBaCuO and show that the weak-link behaviour is absent according to the temperature and field dependence of AC susceptibility and that the behaviour of the imaginary component is qualitatively explained in terms of the magnetic hysteresis loss based on Bean’s critical state model.

2. Experimental Three different samples of YBaCuO have been studied. Samples A and B were prepared by the quench and melt growth (QMG) process. YBa2Cu30, powders were heated to 1400°C for 5 min and quenched by using copper hammers. The quenched plates were again heated to 1100 oC and held for 20 min, and then cooled to 1000°C at the

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rate of lOO”C/h followed by slow cooling at the rate of 5”C/h. The samples were annealed at 600°C for 1 h and slowly cooled in flowing oxygen. Both samples were cut into 0.4~3~3 and 1 x2.5x2.5 mm3 plates with J, values of 14 000 and 5000 A/cm* at 77 K and 1 T respectively. This difference is attributed to their oxygen contents and distribution of 2 11 inclusions. Details of the QMG process are given in ref. [15]. Sample C, which was employed for comparative measurements, was prepared by mixing fine powders made by a spray-drying method. Reagent grade acetate salts of Y, Ba and Cu (99.9% purity) were mixed to prepare an aqueous solution of the desired composition. Powders produced by this method have proved to be homogeneous, reactive and of high purity. The spray-dried powders were heated in a crucible at 850-950°C for 12 h in air. The calcined powders were pressed into pellets of 20 mm in diameter and 1 mm in thickness. Sintering was carried out in flowing oxygen for 12 h at temperatures of up to 1000” C, followed by cooling to room temperature at a rate of 1 “C/min. Sintered pellets have densities of up to 98% of the theoretical value and the oxygen content, x, of YBa2Cu30x. Sample C was cut into a 1 X 1 X 5 mm3 bar which exhibited a J, value of 600 A/cm* at 77 K and zero field. AC susceptibility was measured by an AC susceptometer (Lake Shore Cryotronics, Inc., Model 7000) as a function of temperature. For samples A and B magnetic field was applied parallel to the c axis, which was parallel to the surface plane of the plate. For sample C a magnetic field was applied parallel to the longitudinal axis. The frequency of the applied AC field, J; was 125 and 133 Hz, and the exciting field amplitude, H,,,, ranged from 0.05 to 10 Oe. No attempt to shield the earth’s magnetic field was made. All values of the measurements were corrected by the demagnetizing factors estimated from their dimensions.

3. Results and discussion Figs. 1 and 2 show the temperature dependence of AC susceptibility for various AC field amplitudes with f= 125 Hz for samples A and B respectively. In both samples, the onset of diamagnetism (x’ ) and

study of YBaCuO

0 0.3 Oe A 0.5 Oe 0 0.7 Oe v 1 Oe * 5 Oe l

Hz

f=125

.

10Oe

;

88.0

89.0

90.0

-0.7 -0.8 -1.0 -0.9 -1.1

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?.

92.0

lb , 0’ :* 8

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(b)

’ 88.0

89.0

90.0

Temperature

91.0

92.0

(K)

Fig. 1. (a) Imaginary component and (b ) real component of the AC susceptibility with f= 125 Hz for sample A. the energy loss peak (x” ) start at the same temperature, 92.0 K, independent of H,. The transition width from the normal to the superconducting state is very narrow with perfect diamagnetism at 88 K, which is in good agreement with the result for DC susceptibility measurements [ 17 1. The x’ curves became slightly broader with increasing H,,, values, but no kink or shoulder in the transition region of the curve appeared, while the transitions of x’ in sample A are sharper than those in sample B (figs. 1b and 2b). For all measured H, values the x” curve exhibits only a single peak as opposed to sintered materials (see fig. 3 ). When H,,, increased, the x” peak became broader and its height increased. However, it did not saturate in the present experimknt for either sample (figs. 1a and 2a). In sample A, the peaks of x” were very sharp and the peak temperatures were almost constant, 91.6 K, for H,,, less’than 5 Oe. As H,,, exceeds 5 Oe the peak temperature shifts slightly to 9 1.5 K. As the transition from the normal to the superconducting state completed, 4xx” reached a constant

S. Gotoh et al. /AC susceptibility study of YBaCuO 0.16 0.14

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30

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91.0 (K)

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Fig. 2. (a) Imaginary component and (b) real component AC susceptibility withy= 125 Hz for sample B.

of the

value of N 0.02 almost independently of H,,,. On the other hand, in sample B the peaks of x” were broader than those in sample A and the peak heights were higher for the same AC field amplitude. The peak temperatures were constant, 9 1.O K, for H,,, less than 1 Oe. As H, exceeds 1 Oe the peak temperature shifts to 90.6 K. As the transition completed, 47rx“ also reached a constant value of N 0.035 almost independently of H,. Fig. 3 shows the results for sample C at H,,, of 0.1, 1 and 10 Oe withy= 133 Hz. All the values were normalized to the value at 4.2 K because the apparent values showed much larger diamagnetism than the expected value of 4nx’ = - 1 for full diamagnetism, for example, 4xx’ = - 1.8 even after a correction was made for the demagnetizing factor. It is thought that the diamagnetic shielding current might flow not only in the superficial layer of the sample, but also partially in the internal layers. Such behaviour was also observed during DC magnetization measurements [ 191 and is considered to be characteristic of bulk

- -0.4 g-o.5 e-O.6 -0.7 -0.8 -0.9 -1.0 -1.1’

. i 0

Fig. 3. AC susceptibility

withf=

100

133 Hz for sample C.

sintered samples and is not observed in the QMG processed samples. For a small field amplitude, H, of 0.1 Oe, a single relatively sharp transition is seen at the onset temperature, 92 K. While for a large H,,, of 10 Oe, the transition width broadens with two drops in the x’ curve and two corresponding peaks of the x” curve. The two-stage transition of the x’ curve and two corresponding x” peaks are observed at a relatively large applied field and are attributed to the presence of weakly-coupled grains [ 2-5,7,9,10,20]. The low temperature peak arises from penetration of the magnetic field along grain boundaries, and the high temperature peak originates from penetration of the magnetic field into the grains. One of the authors [ 191 has also confirmed a similar phenomenon of flux penetration into the weak-link regions by the DC magnetization measurements, in which the initial magnetization curve showed a trough and its slope changed by around 10 Oe at 77 K. However, the OMG processed YBaCuO does not show such behaviour. This indicates that YBaCuO prepared by the QMG process has no weak-link region inside the

S. Gotoh et al. /AC susceptibility study of YBaCuO

218

sample. Therefore, there is no need to use the superconducting multiconnected structure model [ 2 1,22 ] for the analysis of AC susceptibility of the QMG processed samples. Since the oxide superconductors are type II superconductors, the observed x” is related to the magnetic hysteresis loss caused by the irreversible flux motion inside the specimen. The behaviour of the x” curves in samples A and B can be qualitatively understood in terms of Bean’s critical state model. Dubots and Cave [ 81 attributed the x” peaks to the incomplete flux penetration and the complete flux penetration during the transition from normal to superconducting state. Our results can be well interpreted by this model, which could also explain the DC magnetization behaviour [ 17 1, as follows: When the AC field, H= H,,, coswt is applied, magnetic induction, B, inside the sample is expressed by B= B, cos (ot - 19))where w is the angular frequency and 0 is the phase angle. Then the energy losses, W, are related to x” per cycle per unit volume by the following equation (in this equation and all that follow we employ cgs units): W=+C

s

Hm>H*

t$

Hm
M f

’

H

+ Fig. 4. Magnetization curves based on Bean’s critical state model. Hr is the magnetic field strength at which the magnetic field reaches the center of the specimen.

HdB

= xx” H”, .

(1)

Therefore, the pressed by 4rrxcx” = 4 W/P,

imaginary

component,

x~, is exmi

.

Hm>H*

(2)

The averaged hysteresis losses, W, per cycle per unit volume for both incomplete and complete penetration of the magnetic flux (see fig. 4) using Bean’s critical state model are given by W= (1/127t*)H&/(J,d)

’

,

W=J,dH,,,-(4a/3)J,Zd2,

H,,,
(3)

H,,,>P,

(4) where d is the sample thickness and P is the magnetic field strength at which the magnetic field just reaches the center of the specimen. It is also assumed that the J, value is constant and independent of the applied field in this model. Thereby using eqs. (2 )(4) the following relations are obtained: 4xx”=(1/3n2)H,/(J,d), 41q”=4J,djH,-(16x:/3)J~d*/H~,

H,,,
(5) H,>H*. (6)

T Hm3>HmzsHml

>

Fig. 5. Typical behaviour of 4xx” for incomplete flux penetration, and complete flux penetration.

Since the J, value increases as the temperature decreases, the temperature dependence of 4rrxx” using eqs. (5) and (6) is shown in fig. 5. During the transition from the normal to the superconducting state the relation between H,,, and H* is reversed, and then the peak of 47rx“ appears as shown schematically in fig. 6. For the sample with a lower value of J, the peak of 4nx” broadens when H,,, increases, because the J,

S. Gotoh et al. /AC susceptibility study

219

of YBaCuO

T

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

-4lqy’ Fig. 7.4~~ versus 47~~” curves for sample A. (a) Bean’s critical state model. (b ) Superconducting multiconnectcd structure model

T Hm3>Hmz>Hml

1221.

Fig. 6. Typical behaviour of 41t,y” as a function of temperature and various magnetic field amplitudes. (a) Sample with a lower .I, value. (b) Sample with a higher J, value.

Q 0.05 Oe

0.3 t h 0.07 Qe q

values vary strongly with H,,, (fig. 6a). This behaviour can be seen for the two peaks with H,,, of 0.1 and 1 Oe, which indicates the intragrain behaviour of the sintered sample in fig. 3. However, for the sample with a higher J, value the peak height depends much on H,,,, since J, rapidly increases and hence 4xx” also sharply changes when the temperature decreases (fig. 6b). This is in good agreement with the results of the measurements on the QMG processed YBaCuO samples with different J, values, shown in figs. la and 2a. 47~~’can be represented by means of the following equations: 4rq’=H,,,/(41FJcd)-I, 4nx’ = -nJ,d/H,,,

,

H,,,
(7)

H,>P.

(8)

We can therefore show the relationship between 47~~’ and 47rx” using eqs. ( 5)-( 8) as follows: 4Xx”=4/37r(47r~‘+l),

HITlcH*,

4rcx”=-16/3rc(4nx’+3/4)4rcx’,

H,,,>P.

(9) (10)

The curve of x” versus x’ is asymmetric and H,,, in-dependent, thus 47~~” has a maximum value of 0.239 at 4rcx’= -0.375 as shown in figs. 7 and 8. Figs. 7 and 8 show the curves of x’ versus x” for different H, values for samples A and B. The curves based on

0.1 0.3

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l

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1

* 5 00

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\

0

,

,

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -4nx’ Fig. 8. 41rx’ versus 47~~” curves for sample B. (a) Bean’s critical state model. (b ) Superconducting multiconnected structure model

1221. the superconducting multiconnected structure model by Ishida and Masaki [ 221 are also plotted in both figures. The curve expected from their model is symmetric and H, independently has a maximum value of 1In. The curves of x‘ versus x” for sample A with a higher J, value (fig. 7) are asymmetric and the maximum increases with H,,,, which is not consistent with Bean’s model and also far from Ishida and Masaki’s model, although their asymmetric forms are similar to Bean’s model. The curves of x’ versus x” for sample B with a lower J, value, shown in fig. 8, are not asymmetric as expected from Bean’s model. We believe that these deviations from Bean’s model are attributable to the field and temperature depen-

220

S. Gotoh et al. /AC susceptibility study of YBaCuO

dence of J,, which is not taken into account in the model. We will not discuss the curves of x’ and x“ for the sintered sample, because the absolute values of x’ and x“ have not been obtained on account of weak links as mentioned above.

4. Conclusions We have measured the AC susceptibility of YBaCuO prepared by the QMG process, and have shown that no weakly-coupled region is present in the sample. The field amplitude and temperature dependence of the imaginary part of the AC susceptibility were qualitatively well explained by Bean’s critical state model in which most of the energy loss, expressed as x”, is considered to be the magnetic hysteresis loss.

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