Pinning centers in YBa2Cu3O7−δ thick films prepared on Y2BaCuO5 substrates by the painting-on method

ELSEVIER

Physica C 233 (1994) 263-272

Pinning centers in YBa2Cu307_-6thick films prepared on Y2BaCuOS substrates by the painting-on method N.V. Vuong a9b9*, E.V. Raspopina b, V.V. Scugar b, N.M. Vladimirova N.A. Yakovenko b, I.A. Stepanova b

c,

aLaboratory of Neutron Physics, Joint Institutefor Nuclear Research, Dubna 141980, Russian Federation b Department ofSuperconductivity, Institute ofphysics and Technical Problems, PO.Box 39, Dubna 141980, Russian Federation ’ Laboratory ofHigh Energy Physics, Joint Institutefor Nuclear Research, Dubna 141980, Russian Federation Received 15 February 1994

Abstract The preparation technology of YBa2Cu307_-b (Se 1) thick films on YzBaCuOS substrates with micrometer inclusions of Y,BaCu05 particles as pinning centers of magnetic vortex is presented. The pinning abilities of these inclusions have been analyzed by investigating the temperature and field dependences of AC magnetic susceptibilities of the films. It was found, that in the films with the inclusions educed in the three-minute partial melting of the YBa2Cu307_-b phase at 985°C followed by the cooling to 920°C with the rate of 1 “C/min the pinning force is increased by half of one order of magnitude in comparison with the films without ones.

1. Introduction High-temperature superconducting thick films (with a thickness of more than the London penetration depth) have been the subject of intensive investigations because of the flexibility in their preparation. However, the application of these films is limited by low values of the critical current J, (J, N 105-lo6 A/m* at 77 K) and is suppressed by an order of magnitude in fields of some hundreds Oe. For most applications J, N 108- 1O9A/m* at 77 K in fields of few T is needed. Recent achievements in thin-film synthesis allow for the possibility of obtaining quasimonocrystal epitaxial films composed of large monocrystal blocks with a small angle orientation relative to each other and good contacts between them. This structure increases the value of J, up to 10” A/m* at * Corresponding author. 0921~4534/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO921-4534(94)00466-8

T= 4.2 K, B= 5 T [ 11. How to increase J, in ceramic samples can be solved by improving the sample texture and introducing pinning centers into the YBa2Cu30,_6 matrix [ 21. In such ceramic samples the J, value reaches lo*-lo9 A/m* at 77 K in magnetic fields of few T. In a previous report [ 31 we presented a technique for preparing thick films of YBa2Cu@_S (6~ 1) (Y 123 ) on a Y,BaCuO, (Y2 11) substrate using the painting method. As the substrate is one of the three components in the reaction to prepare the film, the film can be regarded as an extension of the substrate. Together they can withstand thermal treatment, at least at the thermal conditions of synthesis. From this fact it obviously follows that the high-temperature process for Y211 phase eduction in a Y 123 matrix can be applied to our films. They slowly cool down after melting and go through the peritectic temperature. These Y211 inclusions in the Y 123 are pinning

264

N. K Vuonget al. /Physica

centers and increase J, [ 2 ] by an order of magnitude. In this paper a technique for preparing MG (meltgrowth) thick Y123 films on the Y2 I 1 substrate via a painting method is reported. The effect of Y2 11 inclusions in the Y 123 matrix on sample properties was determined by studying the imaginary and real parts of the magnetic susceptibility, measured by an AC magnetometer [4]. It should be emphasized that as the painting method enables one to easily prepare a Y123 film of any shape (copying the shape of the substrate), including a cylinder, it is possible to control the weak-link properties of MG thick films, which is of great importance for SQUID applications.

2. Technology Thick Y I23 films prepared on a Y2 11 substrate are obtained through a reaction between the Y211 substrate and the ( 3BaCu02 + 2CuO) mixture covering it. The technological details of this process were reported in Ref. [3]. Unlike the thermal regime of Y 123~films synthesis described in Ref. [ 31, the thermal process shown in Fig. 1 for Y 123 MG film preparation on a Y2 11 substrate is used. This process may be divided into two main stages. The first stage is the preparation of the YBazCu306_, film which is completed with the cooling of the sample from 985°C to 920°C at a rate of 1 “C/min and stabilizing it at 920°C for 3 h. The MG film is prepared in the second stage. First a sample with the YBa2Cu306.5 phase is rapidly heated from 920°C to 985°C and held at this temperature for to. After this it is again cooled at a rate of 1 “C/min down to 920°C. During these two

,

;,ooor-;?F,;;:

5 g

:

i

C 233 (1994) 263-272

stages as well as during the modified MG process [ 21 the YBa2Cuj06.5 phase is re-formed with inclusions of the Y2 1I phase whose size, volume and distribution depend on to and the cooling rate. Unlike ceramic MG samples which have a certain porosity, the films under consideration, at the end of the first synthesis stage, have a high density due to the impregnations of the ( 3BaCuOz + 2CuO) mixture into the porous Y2 11 substrate. In this way the porosity of the MG films is reduced to a minimum. The YBa2CuJ06.S phase with Y211 inclusions is stabilized at 920°C for 3 h and is then cooled to 400°C at a rate of 0.5”C/ min in an oxygen atmosphere (P(0,) = 1 atm). In such cooling conditions the oxygen content in the unit cell takes up to the equilibrium value (7-6~6.93 [ 5 ] ). The technological parameters and characteristics of the investigated samples are presented in Table 1.

3. Results and discussion 3.1. Phase analysis In order to prove the eduction of Y211 particles into the Y 123 matrix during the time (to) at a temperature of 985’ C an experiment with a 2 g Y 123 ceramic pellet was performed. The temperature regime was identical to the second stage of a temperature diagram of Fig. 1 with the time to= 3 min. Fig. 2 shows a neutron-diffraction spectrum of this sample taken with the DN-2 diffractometer installed on the IBR-2 pulse reactor [ 61. The spectrum was processed by the Rietveld method and the two phases, Y 123 and Y2 11, were taken for the theoretical curve. It can be seen in Fig. 2 that there is a good agreement between the exTable 1 Some parameters of the investigated samples. Sample N6 is a standard thick Y 123 film without Y2 11 inclusions.

:

//)

5ooi

:li,i;i~~l_~~~;~i;_,

520

ii,

Tlme(min)

Fig. 1. Temperature diagram of thick YBa2CuJ0,_-6 MC film synthesis.

N

to (min)

Thickness (pm)

fW W)

T: (K)

~(300 K) (mficm)

1 2 3 4 5 6

0 1 2 3 4 -

70 130 70 42 140 56

77 78 61 13 75 77

87.9 86.2 84.9 81.9 82.9 87.5

10.4 19.3 20.6 11.3 -

N. K Vuonget al. / PhysicaC 233 (1993) 263-272

i

lo : ybca3

nax

: 904

ntn : -269

-

265

9

:I

ll,‘l”“‘.“““““” 1.675

1.071

2.280

2.885

3.489

4.09)

Fig. 2. Neutron-diffraction spectrum of a bulk sample prepared at the thermal regime shown in Fig. 1. Points are the experimental data. The upper curve has been evaluated with the assumution of two chases, Y 123 and Y2 11, in the sample. The lower curve is the difference between them. The arrows mark the Y2 11 phase peaks.

perimental and the theoretical curves and, therefore, we can conclude the presence of the Y2 11 inclusions in the Y 123 matrix. A phase analysis of the films was performed using X-ray diffraction with the DRON-3 diffractometer. Typical spectra are given in Fig. 3. All of them have peaks characteristic for the Y 123 phase. The average

for all sample unit-cell parameters is: a= 3.825 (5), b=c/3=3.879(5) A, with the oxygen content (7-6) determined by assuming the parameter c is equal x 7. The X-ray diagram data confirm that superconducting Y 123-films may be obtained via the preparation process described above despite the fact that their synthesis time is considerably shorter than that of standard solid-phase techniques of ceramic sample preparation. Fig. 3 also shows the Y211 and the BaCuOz phase peaks. The latter (2& 23.8’ ) can be explained by the presence of the rest of not reacted mixture on the surface of the substrate. We identify the 28= 29.8” and 30.7” peaks as the (040) and ( 13 1) peaks of the Y2 11 phase. 3.2. Morphology

1'8

27 2 theta

36 (degree)

45

Fig. 3. X-ray diffraction spectra of the ordinary sample N6 and the samples subjected to eduction of Y2 11 particles. Instead of the Y 123 phase peaks, the peaks of the BaCuO, phase ( * ) and the Y2 11 phase ( o ) are marked.

The film morphology (a surface and a cross-section) was studied with the JSM-840 scanning electronic microscope. Fig. 4 shows a micrograph of part of the cross-section of film N6. On the left-hand side of the figure a porous substrate can be seen with the film of a thickness - 100 urn adjacent to this substrate on the right-hand side. It can be seen that the

266

N. V. bong et al. / Physica C 233 (1994) 263-272

Fig. 4. Micrograph of the cross section of the film No. 6. The dense region belongs to the film, the less dense one to the substrate.

film has a higher density than the substrate. In Fig. 5 a typical view of the Y 123 (Fig. 5 (a) ) and MG-Y 123 film (Fig. 5 (b) ) surfaces are given. The grains have a plate-like shape and are closely adjoined to each other. The long axes of the grains are almost parallel to the cylinder axis, though they are arranged with a low degree of order respective to each other. The cross-sections of the obtained films are shown in Fig. 6. These pictures also state the long-axis orientation of the grains parallel to the cylinder axis. In these figures we can see only the cross-sections of the grains as the elongated crystals are not seen as they are in Fig. 5. Unlike the Y 123-film (Fig. 6 (a) ) the N4 film (Fig. 6 (b ) ) underwent a partial melting for to = 3 min; the cross- section in Fig. 6 (b ) shows more monolithic crystal structure and micrometer-sized particles which appear to be the Y2 11 -phase. The above results of the neutron-diffraction and scanning electronic microscope studies permit us to claim that by using our thermal regime (see Fig. 1) particles of the Y2 11 phase were educed into the superconducting MG-Y 123 films. The effects of these inclusions on the superconducting properties of the films were studied through an examination of the p(T), x’ ( T, H) and x” ( T, H) dependences.

3.3. Resistance temperature dependences The temperature dependences, R ( T) /R (300 K), of the samples obtained as measured by the fourprobe method at a direct current are presented in Fig. 7. For samples with to < 4 min all the curves at T> T, have a metal-like behavior. The values of the specific resistance p( T,,,,) of these samples are close and lie in the range of tens of mR cm (see Table 1). For samples with to2 4 min at T> T, the p ( T) curves have a semiconducting behavior and the transition width, AT, changes over a wide range. Obviously such behavior is connected with the fact that with increasing t,,, the volume of the Y211 inclusions in the MGY 123-films increases, as well. According to the R(T) measurements the MGY 123 films with to<4 min were selected for further investigations with the AC magnetometer [ 41. 3.4. Temperature and field dependences of magnetic susceptibility Signals proportional to the real (x’ ) and imaginary (x” ) parts of the AC magnetic susceptibility of the sample were read from two pick-up coils, one of them containing the sample. These pick-up coils were

N. V. Vuong et al. /Physica C 233 (1993) 263-272

261

Fig. 5. Micrographs ofN6(a) and N4(b) film surfaces.

switched on in series anti-phase and placed inside the exciting coil. Direct and alternating currents flow through this coil inducing direct fZnc (O-2.5 x 1O4A/ m) and alternating & sin 2xff cf= 68 Hz, HAc = O3.5 X lo3 A/m) magnetic fields, respectively, inside the coil. The system was balanced at a sample temperature above T, with a compensating coil which was

near the pick-up coil. The susceptibility calibration was carried out at 77 K with a high-quality Y13a2CuS0,_6 sample. Temperature (77-100 K) and the field dependences x’ and x” were controlled with an IBM PC/AT computer. All the x’ ( T, HAc, HE 1, x” ( T, %c, fbc 1 dewdences were normalized according to the sample vol-

268

N. K bong et al. / Physica C 233 (1994) 263-272

Fig. 6. Micrographs of the cross-sections of N6(a) and N4( b) films.

ume L’, and HAc measurement-field amplitude. All samples under investigation were cylindrically shaped: 0.8 mm long, about 0.5 mm in diameter and had 42- 140 urn thick walls and contained inside the cylinder the Y211 phase. Such a sample geometry makes the demagnetization factor rather small and it was negligible during the result processing. The effect

of the Y2 11 phase on the measurement was not detected. Measurements, as a rule, were performed under a ZFC regime, followed by heating at a rate of about 1.5 ’ C/min. In field-dependence measurements H,, changed jump-wise with a step of 0.6 Oe; the signal was measured 0.5 s later than the field jumped.

N. V. Vuong et al. / Physica C 233 (1993) 263-272

77

177 Temperoture

277 (K)

Fig. 7. Reduced resistances (R/R300 K) vs. temperature of the investigated samples.

-I

TD

I

of Fig. 8 corresponds to a weak-link part (weak region) connecting the grains which is penetrated by the external field in this temperature range. The third part corresponds to a system of strong superconducting grains isolated from each other (strong region) with a typical value of the transition temperature into the superconducting state, Tcg= 90 K for a Y 123 system. We should point out that this picture differs from a similar one taken for a bulk ceramic sample by the value of the x’ plateau. This discrepancy is connected with the geometry of the investigated samples. The field “sees” the sample in the first part of Fig. 8, as a solid cylinder with a radius R and in the third part as a hollow cylinder with a wall thickness d. In the second part, the field “sees” the sample as a hollow cylinder for x” but as a solid cylinder for x’ . Due to the small value of the film thickness i.e. the small total volume of the grains the second peak of x”, corresponding to the field penetration into the grain system, cannot be seen with this AC magnetometer. For the end of the second part of Fig. 8 the following formula is true: .L Ix: I = lx;, I ~R2/Q2-r2)

82 87 Temperature

92 CK‘)

Fig. 8. Typical temperature dependences of the real x’ and imaginary x” parts of the AC magnetic susceptibility of the investigated samples.

Fig. 8 is a typical view of the temperature dependence of the real x’ ( T) and imaginary x” ( T) parts of the samples in the range T= 77-95 K measured in a zero Hnc field at a measurement field HAc= 15 A/m. In the first part of Fig. 8 the sample is at a low temperature (77 K) and in a weak field HAc= 15 A/m. The values x‘ = - 1 and 1” =O mean that at this moment the film is completely in the Meissner state and the inducing superconducting current flows on the surface of the cylinder and completely screens the external field. In the second part when the temperature increases, x’ increases reaching a plateau (x,, N - 0.1)) and x” has its peak. In the third part of Fig. 8, x’ reaches zero and x” stays at a zero level. Within the frame of a conventional explanation, the second part

269

>

(1)

where R and r are the outer- and inner-cylinder radii, XLis the real part of the magnetic susceptibility of the strong region, andf, is the volume of the strong region. The coefficient ~cR*/R(R’- r2) was introduced into formula ( 1) because the measured signal, x’, was normalized to a xR* volume. Over this second part all the grains stay fully in the Meissner state and x’ has a value of - 1. To determine the volumes of the strong and weak regions offw for the case of r cc R we have the simple formulae: fs = Ixh I Rl2d

fw=l-fs.

>

(2) (3)

The fwvalues for the investigated samples are given in Table 1. The described behavior of the samples can be observed in the susceptibility field dependences, x’ (H,,) and xU ( (H,,); a typical view of such dependences is given in Fig. 9. At 77 K and a measurement signal HAc = 15 A/cm, x’ = - 1 and x” = 0 if field H,,=O. When the direct external field increases, x’ = - 1 remains at - 1 and x” at zero until the mo-

270

N. V. Vuong et al. / Physica C 233 (1994) 263-272

0

10000

20000

Hdc(A/m)

10000

20000

Hdc(A/m)

0.0

o”““‘i~ddd”“‘;~ddd”’

Hdc(A/m) Fig. 9. (a) Field dependences of the real part x’ (I&) of sample N3 measured at 77 K at different AC values. From bottom to top H,,= 15, 75, 150, 300, 900, 3825 A/m, respectively. (b) Field dependences of the imaginary part x” ( HDc) of sample N7 measured at 77 K at different HAc values. From the right-hand side to the left one HAc= 15,75,150,300,900,3825 A/m, respectively.

ment when H,, becomes greater than the first critical field of the weak region H:i . When H,, > &‘, , the HACfield begins to penetrate the weak region and drives it up to the critical state. Then 1x’ 1 decreases and x” has a peak in this region. With further increase of H,, field until it is smaller than the first critical field value of the grains HEi, x’ and x” remain constant. If not, then Ix’ 1 decreases and the second peak of x” appears only when Hx > HE, and the field penetrates to the core of the grains. Such a peak in this case is observed only on the curve measured at H,,=3.8 k A/m and can occur at H,= 17 K A/m. The existence of the second peak x” (H,,) and the second raising of 1’ ( T) (see Fig. 8) means that the first peak of the curve x” ( T) be-

longs to the weak region. An interpretation of the results of x’, x” (T, HAc, H,,) measurements of the given films can be described as a mosaic composed of weak and strong regions built into each other. As we wanted to determine the effect of Y211 inclusions in the Y 123 matrix on the critical current, J,, we will give a detailed analysis of x” in the peak region below. Fig. 10 presents the x” ( T) dependences measured at various values of the HAc in zero H,, field. It was noticed that at given conditions (HAc, T) a time dependence of sample magnetization was not observed, thus we will analyze them with the model of the critical state [ 7 1. According to this model the profile of the nonequilibrium distribution of the induced field Hi inside the type-II superconductor standing in a mixed state is attained by two forces of equilibrium: a Lorentz force, which allows the field to penetrate into the sample in the form of vortices, and a pinning force of this superconductor (Y( T, Hi ). It is the pinning force which gives the maximum value of the superconducting current, called the critical J,( T, Hi), induced in a sample by an external field. Until now most authors [ 81 have explained their results with various forms of dependences of J,( T, Hi). Here we would simply assume that J, can depend or not depend on Hi. In the first case, according to Bean’s assumption, a( T, Hi) grows proportionally to the local field Hi, which leads to the independence of J, from Hi. In the second case, CX(T, Hi) does not depend on Hi and J, NH; ’ (Kim model). The other dependences of

1 02

Hdc

=

0

x 0.1

0.0 78

a3 Temperature

88

93 (K)

Fig. IO. Temperature dependences x” (T) of sample N2 measured with different HAc values in a zero Hm field. From the right-hand side to the left one HAc= 15,30,45, 60, 90, 120,225, 300 A/m, respectively.

N. V. Vuong et al. / Physica C 233 (1993) 263-272

271

Jc(Hi) are a combination of the above two cases. Moreover they are usually determined by comparing the magnetic-moment experimental data with a theoretical evaluation, where an integration over the profile of the time-dependent local field, Hi (x, t ), is performed. As a result the precision of the determination Of J,( Hi) is not high. Let us assume that, in general, J, can be written: Jc=aFi(T)J’I(fi)

>

(4)

where a is a coefficient directly proportional to the pinning force, F, ( T) is a J, temperature dependence, Fz (Hi) is a J, field dependence. According to Ampere’s law we have: dHi =-sgn(J)aF,(T)FZ(Hi) dx

.

Fig. 11. Peaked field H’& vs. T,/T: for the investigated samples.Legend: (O)Nl,(+)N2,(*)N3,(d)N4,(O)N5,(*) N6.

(5)

In the case of cylindrical samples the peak field, to the peak of x’ (T= T,) is approximately equal to the first penetration field HP [lo], which sets the critical state in the entire sample. The expression relating Hrc (or HP), with the peak temperature, T,, and the film thickness, d, is

Hf&-, that corresponds

(6) definition we take F,(T) = ( T,- T)“, F2 (Hi ) = ( 1Hi I+ Ho) -s The F2 (Hi ) dependences are described by the Bean and Kim models at p=O and P= 1, respectively. The power law of F, (T) was observed, for example in Ref. [ 9 1. Within the frame of the conception of “ideal superconductivity” one can adopt Ho x 0 which means that in a very weak field H, <
-dH”

2

a

=,llta+l),d’/‘B+l)(p+l)-a/ts+l)

dTP x(T,_T,)(“-(8+‘))/(8+1)

(7)

The Hrc dependences on the temperature T, reduced to the transition temperature of the weak region T: are plotted in Fig. 11. The constant slope of these dependences for all samples investigated gives

to(min) Fig. 12. Dependences of the pinning force on the duration to of the educed Y2 11 inclusions in Bean (a, ( 0 ) ) and Kim ( aK ( * ) ) models. ( 0 ) and ( A ) correspond to the values of aK and aB for sample N6, respectively.

the relation n=p+ 1, i.e. n= 1, /3=0 for the Bean model and n = 2, /3= 1 for the Kim model. To compare the pinning abilities of the educed Y211 inclusions in different samples a geometrical factor must be taken into account. In this case, it is a thickness of the film. Such an influence in the Bean model is in direct proportion with d and in the Kim model with d’j2. Thus the pinning force, a, determined using the Bean model is au = 1dH$/dT,, 1/d, and ak = I dHzC/dTp /*/2d using the Kim model. Fig. 12 shows how the pinning abilities of Y2 11 inclusions in the Y 123 matrix change with the inclusion-forming time, to. Similar data for the starting sample without Y211 inclusions are marked with rectangles for aB and triangles for ak in the same figure. We can conclude that ( 1) the choice of either the Bean or Kim model does

272

N. K Vuonget al. / PhysicaC 233 (1994) 263-272

not affect the estimates of the pinning-center forming process; and (2) by increasing the time to the pinning force grows, reaching a maximum at to= 3 min and drops at to= 4 min. Most papers [ 2,1 O-l 5 ] report using the Y2 11 inclusions in the Y 123 matrix as pinning centers. In some papers [ 2, lo- 12 ] the authors concluded these inclusions had a positive role in increasing the pinning force, in others a negative role [ 13- 15 1. As is emphasized in Ref. [ 141 a pinning ability of Y211 inclusions in the Y 123 matrix depends on their volume fraction and on the size of the Y2 11 particles. In a sample with micrometer-sized Y2 11 inclusions at a relatively low volume fraction, J, increases with increasing Y2 11 inclusion size. In contrast, if the Y2 11 volume fraction is large, J, decreases when the Y2 11 inclusion size increases. Regulation of the critical current, J,, by means of Y2 11 inclusions involves two processes: ( 1) magnetic flux pinning on the interface between the Y211 inclusions and the surrounding Y 123 matrix; and (2 ) degradation of superconducting properties of the Y 123 matrix due to the inclusion of non-superconducting Y2 11 phase (superconducting phase volume degrades, percolation path becomes shorter, etc. ) The results obtained (see Figs. 7 and 12 ) for these films illustrate the above described behavior of Y2 11 inclusions in the Y 123 matrix. Because we used the temperature 985 “C instead of 1100°C [ 21 for partially melting the Y 123 phase with a constant cooling rate for passing the peritectic point in all the investigated samples, the Y211 inclusion sizes in all the samples remain approximately constant but the volume fractions of Y2 11 inclusions grow as the time to increases. As to increases a decrease of transition temperature of the weak region T: and the increase of p( 300 K) are observed (see Fig. 4). Due to the above mentioned processes regulating J,, the dependence of pinning forces on to is observed as shown in Fig. 12. 4. Conclusion In this paper we discuss a technological process of forming pinning centers in a YBa2Cu307--6 (8~ 1) thick film prepared by the painting method. Particles of the Y,BaCuOs phase are educed into the Y 123

matrix during its melting at 985°C and subsequent cooling, which passes through the peritectic point. These particles are pinning centers. The pinning abilities of these inclusions were tested by the AC susceptibility measurements ofx’ (T, HAc) andx” (T, HAc) An optimum regime for these pinning centers forming has been found: a 3 min stand at 985°C 1 “C/ min cooling rate from 985°C down to 920°C for a Y 123 film. Films prepared using this method can be considered as a composition of two systems systematically built into each other: a strong region of superconducting grains and a weak region of connecting grains. The weak region can be considered a superconductor of the second type. Its properties can be changed via the above process of forming pinning centers, at least to the discussed optimum regime of a live-fold increase in the pinning force. It would be interesting to apply such cylindrical films with changeable weak-link properties to SQUID preparation. Acknowledgements We wish to thank B.V. Vasiliev for his support and useful discussions, V.M. Dropin for his assistance in carrying out the R(T) measurements, V.K. Semina for the X-ray diffraction measurements and A.M. Balagurov and L.C. Quy for their help in carrying out and analyzing the neutron-diffraction spectra. References [ 1] I.V. Khoroshun

et al., Supercond. Sci. Technol. 3 (1990) 493. [2] M. Murakami, Supercond. Sci. Technol. 5 ( 1992) 185. [ 31 N.V. Vuong, E.V. Raspopina and B.T. Huy, Supercond. Sci. Technol. 6 (1993) 453. [ 41 Yu.V. Obukhov et al., Preprint of the IPTP, Dubna, Russia 93-5-5 (1993). [ 51 J. Goodenough, Int. J. Mod. Phys. B2 ( 1988) 379. [ 61A. Novae et al., Preprint of the Institute of Atomic Energy, Rumania, Bukharest CS-21-1991 (1991). [7] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. [ 81 A. Sanchez et al., Physica C I75 ( 1991) 133. [ 91 F. Ludvig et al., Supercond. Sci. Technol. 5 ( 1992) 196. [lo] M. Morita et al., Physica C 172 (1990) 383.

[ 111 Ch. Neumann

et al., Z. Phys. B 84 ( 199 1) 37. [12]M,Staskietal.,PhysicaC185-189 (1991)2495. [13]W,Winetal.,PhysicaCl72(1990)217. [ 141 P.Mc Ginn et al., Physica C 176 (1991) 203. [ 151 A.A. El-Abbar et al., Physica C 198 (1992) 81.