Lower critical field, remanent magnetization and irreversibility line of ceramic (Bi,Pb)-2223-HTSC

PhysicaC 172 (1991) North-Holland

391-399

Lower critical field, remanent magnetization and irreversibility line of ceramic (Bi,Pb ) -2223-HTSC R. Job and M. Rosenberg Instituteftir Experimentalphysik VI, Ruhr-Universitiit Bochum, Postfach 10 21 48, 4630 Bochum 1, Germany Received 16 July 1990 Revised manuscript received

7 November

1990

A detailed magnetic study of the (Bi, Pb)-2223 superconductors was carried out over the temperature range 4-120 K. At temperatures below 25 K the lower critical field H,, was drastically enhanced. This enhancement can probably be explained either by a theory based on an intrinsic proximity effect in the normal layers of the Bi-2223 unit cell or by a blocking of the flux penetration because of a surface barrier effect. An analysis of the anomalies in the irreversibility line has been undertaken in the framework of the proximity effect allowing us to obtain also plausible values for 6, I, and the Fermi velocities in the superconducting and normal layers. The temperature dependence of the large remanent magnetization at 4 K with a drastic decrease above 25 K offered evidence for a strong depinning of the vortex lattice around 25 K. Below this temperature the active pinning centers have a rather small pinning energy. Above 25 K a smaller group of pinning centers with larger activation energy gives rise to a small and weakly temperaturedependent remanent magnetization. A scaling behavior of the magnetization curves was obeyed in the temperature range between 40 and 60 K in good agreement with the predictions of an extended Bean model.

netic relaxation study of the 2223 ceramic presented and discussed.

1. Introduction Many of the extrinsic properties of the high-temperature superconductors such as lower critical field H,, , remanent magnetization and irreversibility line depend on the preparation conditions, phase purity, presence of different kind of defects and so on. This is true especially in the case of ceramic samples intrinsically polycrystalline, isotropic or textured. The aim of our study was, by using an extensive program of magnetic measurements, to find out how the magnetic properties change over the whole temperature range between 4 K and T, and what kind of physical mechanisms are involved in the case of ceramic samples of the practically single phase (Bi, Pb)-2223HTSC with a critical temperature of about 107 K prepared under well determined conditions. Our results are first of all specific for the samples we studied but they are also relevant for a more general better understanding of the magnetic properties of ceramic samples of high-T= superconductors. In a forthcoming publication the results of a mag-

0921-4534/91/%03.50

0 1991 - Elsevier Science Publishers

will be

2. Sample preparation and experimental details Ceramic Bi-HTSC of the 2223-type partially substituted with Pb for Bi were prepared by the conventional powder technique. Starting from the nominal composition Bi,.84Pbo,s4Srz.03Ca,.~~Cu~.o~O~ [ 1 ] and a-special procedure of heat treatments one can obtain almost single-phase polycrystalline samples. The annealing of the samples was carried out in two steps. Firstly the samples were annealed for one week in a pure oxygen atmosphere at 855°C and afterwards a second annealing in order to stabilize the 2223 phase was carried out in air under normal pressure at 855°C for one and about two weeks, respectively. The details of the preparation of samples containing mainly the Bi-2223 phase have been described in an earlier publication by Job et al. [ 21. The final sample had a low density of only 40% of the theoretical density and examination by an electron microscope showed that it consisted of well developed

B.V. (North-Holland)

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R. Job, M. Rosenberg /Magnetic study of (Bi, Pb)-2223.HTSC

platelet-like crystals in a conglomerate with a low packing factor. X-ray analysis and temperature-dependent susceptibility measurements in lower fields ( 1 mT) indicate that the samples were almost single phase of the 2223 type with a critical temperature T, of about 107 K. For comparison, Bi-2212 superconductors have also been prepared without Pb substitutions for Bi. In this case a similar heat treatment was carried out, firstly annealing for one week in pure oxygen at 855°C and secondly for ten days in air at 855°C. The single-phase 22 12 sample exhibited a T, of about 83 K. The samples were prepared in a cylindric shape with a diameter of 4.4 mm (4.45 mm) and a height of 2.9 mm (2.55 mm) in the 2223 (2212) case. Magnetic measurements were carried out with the applied field parallel to the plane of the cylinders. The demagnetization factors are 0.296 and 0.227 for the 2223- and 2212-sample, respectively. The magnetization of the studied samples was measured with a vibrating sample magnetometer in fields of up to 2 T in the temperature range between 4 and 120 K. Susceptibility measurements were carried out at various fixed fields as a function of the temperature in the zero-field-cooled (zfc) and field-cooled (fc) regime. The data were taken by warming up from 4 K with heating rates of about 2 Kmin-‘. In addition, several magnetization curves were measured at fixed temperatures after zero-field cooling in fields up to 1.9 T. Using an automated temperature control, the fluctuation of the temperature during the measurements was less than 50 mK. Studies of the remanent behavior of the Bi-2223 superconductors were undertaken by cooling the samples in a fixed field down to 4 K and switching off the field after the temperature had been stabilized. Then the samples were slowly heated to about 120 K, taking the remanent magnetization data (in zero field) as a function of temperature.

3. Temperature dependence of the critical field HC1 Figure 1 exhibits some magnetization curves at various fixed temperatures. At T=4 K the remanence of the Bi-2223 sample is rather large. From the remanent magnetization, one can estimate the crit-

50

25 7 ol 4 ; ai I

o-

-25 -

-50

1

-2

I

-1

0

1

2

p'.H I T I

Fig. 1. Magnetization curves of (Bi, Pb)-2223 and 50 K (from outer to inner curve).

ical current

at T=4 K, 35 K

density using the simple Bean formula (in practical units). Because of the platelet-like shape of the grains, one can take for R the average thickness which is of the order 10e4-1 O-’ cm, as can be seen with the electron microscope. Then one reaches at 4 K a critical current density of the order 106-10’ A cmp2, i.e. values probably over- or underestimated by one order of magnitude. The critical field H,, was determined from the magnetization curve, defining H,, as the field at which the virgin magnetization curve starts to deviate from linearity. The H,, values obtained in this way are in good agreement with those determined from magnetic relaxation measurements, when H,, can be defined as the field at which the magnetic relaxation starts [ 3 1. The virgin magnetization curves of the (Bi, Pb)-2223 sample at several temperatures are shown in fig. 2. Although the uncertainty reaches about 5- 10% in the estimation of H,, because the deviation of the virgin curve from linearity is very smooth, one can observe some remarkable trends in the dependence of H,, on temperature (fig. 3 ) . At 4 K the values of I&, of the Bi-2223 samples were close to 0.03 T. With increasing temperature up to about 25 K, H,, decreased rapidly to a value of about 0.01 T. Around 25 K the H,, (T) curve exhibited a kink followed by a smoother, almost linear, decrease with increasing temperature than in the temperature region below 25 K.

J,=30M,,/R

R. Job, M. Rosenberg /Magnetic study of (Bi, Pb)-2223-HTSC

0

-1

7

-2 0) ;E

-3

II) E

-4

-5

-51

0

10

20

p,H

I

mT

30

)

Fig. 2. Virgin magnetization curves of (Bi, Pb)-2223 for temperatures between 4 K (lower curve) and 80 K (upper curve) in steps of about 5 K. H,, can be determined from the deviation from linearity.

350 I

393

ported for single crystals of Y,BazCu30, by Adrian et al. [ 41 and McElfresh et al. [ 5 1. Batlogg et al. [ 6 ] reported in the case of a Bi-22 12 single crystal an H,, dependence on T similar to that of our ceramic 22 12 sample. Since the polycrystalline ceramic samples exhibit a random orientation of grains, the values of H,, can be regarded as an estimation for the case when the applied magnetic field is parallel to the (a, b)-plane of the unit cell, since in this case H,, is a factor of 5 or 6 smaller than with the applied field parallel to the c-axis, because of the strong anisotropy of the penetration depth (see, for instance, ref. [ 4 ] ) . The value of about 30 mT for H,, at 4 K in the case of the Bi2223 sample is a reasonable one, compared with a value of H,, = 45 mT for a Bi-22 12 single crystal at 1.5 K in the field geometry with H parallel to the (a, b)-plane [ 7 1. This result is plausible because of the microstructure of the 2223 sample with a very low density and well developed, rather loosely packed grains which could behave as more or less separate crystals at low temperatures. At this point it is necessary to mention that when taking for H,, the form given by the Ginzburg-Landau theory, i.e.

(1)

0

20

40

60 T

20

100

(K)

Fig. 3. Temperature dependence of the lower critical field H,, for a (Bi, Pb)-2223 and Bi-2212 sample. The kink in the H,,(T)plot of the 110 K phase occurs at about 25 K and of the 85 K phase at about I2 K.

The critical field H,, of the Bi-22 12 samples (85 K phase) behaves in a similar way. But in this case H,, reaches only a value of about 0.01 T at T=4 K and the kink after the strong decrease in its temperature dependence appears at about 12 K. An anomalous behavior of H,, similar to that shown in fig. 3 for our 2223 sample has been re-

with 1 the London penetration depth and < the coherence length, a much smoother form for H,, has to be expected because of the temperature dependence of A2 which appears to have no anomalies in the case of the Bi-HTSC. There are at least three possible ways to explain the observed temperature dependence of H,,. Recently Koyama et al. [8] presented a theory which explained the anomalous behavior as an intrinsic one resulting from the layered structure of the HTSC. The model used basically the fact that the structure of the HTSC is composed of arrays of superconducting and normal layers (i.e. BiO) where superconductivity can be induced by the proximity effect [ 9 1. From their calculation, performed for the field geometry with the applied magnetic field parallel to the c-axis of the unit cell, one can derive two different temperature regimes. At higher temperatures the superconducting order in the normal layers is strongly suppressed and the superconducting state is maintained mainly by the superconducting layers (i.e. CuO planes). At

394

R. Job, hf. Rosenberg /Magnetic study of (Bi, Pb)-2223-HTSC

lower temperatures the superconducting order in the normal layers is induced by a proximity effect, so that the order extends over both superconducting and normal layers resulting in a strong enhancement of H,, . The anomalous temperature dependence of H,, arises from the crossover between the two temperature regimes. Koyama et al. also presented a plausible argument for a similar behavior in a field geometry with the applied field parallel to the (a, b)plane without a detailed calculation. Another explanation would be to take into account the existence of intergranular regions and/or different types of defects inside the grains, which facilitate the flux penetration at higher temperatures but become superconducting at lower temperatures, again because of the proximity effect or simply because of some distribution of lower T, values. The above mentioned circumstance, that the anomalous temperature dependence of H,, has also been found in the case of single crystals, makes such an explanation less plausible [ 8 1. Very recently McElfresh et al. [ 5 ] tried to explain the anomalous increase of the onset field for flux penetration at lower temperatures, found in their study of the isothermal remanent magnetization of a Y ,BazCu30, crystal, by incorporating surface barriers in the extended Bean model. They claim that the appearance of surface barriers at low rather than high temperatures is plausible because of the higher the temperature the more easily the surface barrier can be overcome by thermal activation. The possible influence of the surface on the flux penetration in type-II superconductors was firstly proposed and treated for a simple model of a flux line interacting with the magnetic field by Bean and Livingstone [ lo]. According to their model, in order to generate a flux thread at the surface of the superconductor one has to overcome an energy barrier which can persist in applied fields much higher than H,,. The more perfect the surface, the higher the applied field has to be in order to let flux threads penetrate the crystal. In the case of our sample, because of the low density and well developed grains, surface effects cannot be excluded. That would mean that the anomalous rise of H,, at low temperatures does not reflect the behavior of the intrinsic H,, but that of the blocking effect of the surface barriers.

4. Temperature dependence of the remanent magnetization An anomalous behavior of the remanent magnetization was observed in the low temperature region. A Bi-2223 sample was cooled in several fixed fields (2-50 mT) down to 4 K. After stabilization of the temperature, the field was switched off and a remanent magnetization appeared as expected for hard type-II superconductors. At 4 K the remanent moment satisfies the relation M,,, =-A4rc-MZr,, but when slowly heating up the sample remanent magnetization decreased rapidly until about 25 K and the relation no longer held. From 25 K up to T, the remanent signal decreases slowly to zero. In fig. 4 the temperature-dependence behavior of M,, is shown for several (former applied) fields between 20 and 50 mT. The large values of the remanent magnetization below 25 K show that in this temperature range a large number of pinning centers are active, in contrast to the temperature range between 25 and 80 K where the remanent magnetization reaches quite low values with only a weak dependence on temperature and magnetic field. The extremely sharp decay of the remanent moment with increasing temperature in a narrow temperature range below 25 K can be interpreted in terms of strong depinning of the flux vortices, i.e. in terms

T

(Kl

Fig. 4. Remanent magnetization as a function of temperature for several (former) applied fields up to 50 mT (see text): (a) 50 mT,(b)40mT,(c)30mT,(d)25mT,(e)20mT,(f)15mT, (g) 10 mT, (h) 5 mT, (i) 2 mT.

395

R. Job, M. Rosenberg /Magnetic study of (Bi, Pb)-2223-HTSC

of a low value of the activation energy U, for the pinning centers responsible for the high remanent magnetization at low temperatures. The dramatic decrease of the remanence up to about 25 K means also a sharp decrease of the critical current density because of depinning. On the other hand, the very low remanence and its weak temperature dependence above 25 K means that another type of pinning mechanism is present, involving deeper lying pinning centers, i.e. pinning with a higher activation energy UO. In fact, recent measurements of the magnetic relaxation rate of a ceramic Bi-2223 sample by Mittag et al. [ 3 ] have shown that the activation energy U,, exhibited a relatively sharp transition around 25 K, reaching only 25 meV for T-c 25 K but 150 meV for 25 K< T<80 K. The different activation energies lead to different temperature dependences of the critical current below and above 25 K. In this case, an anomalous transition behavior can again be observed indicating a change in the structure of the vortex lattice and leading to the conclusion that a strong depinning of the flux line lattice might occur in the characteristic temperature range between 20 and 30 K for the Bi-2223 samples.

5. Scaling behavior of the magnetization In the temperature range between about 40 and 60 K one can describe the magnetization curves according to a scaling behavior which can be understood in an extended Bean model [ 111 for a superconducting slab of an infinite extension and a thickness D. The applied magnetic field is directed along the surface plane of the slab. The critical current density J, is field dependent and follows the power law J,=J,,(H,,lh)“,

h>H,,;

(2)

where J,, is the maximal critical current density at a fixed temperature, h the local field at the position x (distance from the surface of the slab) and n a phenomenological power of the order 1. According to Yeshurun et al. [ 111 a universal scaling behavior can be described by the equation 47tM=PF,

(H/P),

(3)

where + , - indicates the field region above and below a scaling field H* (above H,, ). The scaling behavior is intrinsic for the extended Bean model. The scaling equations F? (H/H*) holding for two field ranges are

(I)=_ H CI.
> (4)

&f)=_(g) +

~[(!L)““_(($~“_ ly”+2)‘(n+1)] )

H*
(5)

where the scaling field P

can be described

H*=[(CD/2)-H,“:‘]“(n+1),

x =x,=D/2

and C is defined

by

;

(6)

by

C=(471/10)(n+l)J,,H;,

.

(7)

For our purposes we neglect H,, in eq. (6), which can be done if H,, is small enough or H large enough. In the Bean model H* is proportional to H,,,,,, the field at which the magnetization of the virgin magnetization curve has a maximum (negative), and the condition H> H,,,,, is fulfilled. Making the estimation P _NH,,, and plotting &f/M,,,,, versus H/H,,, we observed a scaling behavior for our samples in the temperature range between about 40 and 60 K (fig. 5 ). In our case we used data taken in fields larger than H,,,,, (H x=-H,, ), because H,, is still quite large in the observed temperature range. In addition one observes for H* ( = H,,,,, and M* ( EM,,,) an inverse dependence on T (fig. 6 ) which was also found by other authors [ 121. One sees that despite its simplifying assumptions the extended Bean model can satisfactorily describe the observed behavior of the virgin magnetization curve in fields above H,,,,, and in the temperature range between about 40 and 60 K. The values of the power n in eq. (2) were calculated by fitting eq. (5) with a nonlinear least squares fit. We found n=0.65 for the Bi-22 12 and

R. Job, M. Rosenberg /Magnetic study of (Bi, Pb)-2223-HTSC

396

manent magnetization in the low temperature range. In this case it is plausible that J, may change in an anomalous way so that the simplifying assumptions of the extended Bean model could not hold anymore.

2

0.6

I

6. Irreversibility line and fc susceptibility 0.4

IL---J 0

2

4

6

8

10

"'L

Fig. 5. Scaling behavior of the magnetization rangebetween40and60K:(-)40K,(+)45K,(*)50K,(O) 55 K, (x) 60 K, (s) 70 K, (#) 80 K.

in the temperature

Shortly after the discovery of the high temperature superconductivity in the BaLaCuO system, Miiller et al. [ 131 reported the existence of an irreversibility line and considered it to be evidence for a superconducting glass state, since the irreversibility line follows the law P=Ho(

1 - T/Tcj312 ,

(8)

in analogy to the Almeida-Thouless line for spin glasses [ 14 1. In the H-T plane the irreversibility line separates the region where the magnetization M is reversible from the region where M is dependent on the previous path in the H-T-plane and on time. Similar results have been observed in the YBaCuO system [ 151. Yeshurun et al. have shown that the irreversibility line following the same power law with the power n= 2/3 can also be explained by thermally activated flux creep, i.e. in the framework of a more or less conventional type-II superconducting theory

[lb].

-8 1 0.010

030° I 0.015

0.020 T-’

Fig. 6. Linear dependence temperature T-‘.

I K-’

0.025

0.

1

of M,,,,, and H,,

on the reciprocal

(T>, 70 K) which was also observed for the Tl-sample [ 121 may originate froman anomalous behavior of the critical current density, i.e. a change in the flux line lattice. Studies of the irreversibility line, which will be reported in the next section, also indicate a transition at temperatures closer to T,. On the other hand, deviations at lower temperatures may be correlated with anomalies observed in the temperature dependence of H,, and re-

The irreversibility line was determined from zfc and fc susceptibility measurements at several applied magnetic fields. Cooling the sample in zero field down to 4 K and turning on the field after the stabilization of the temperature yields the zfc flux exclusion or shielding measurement. Cooling in an applied field through T, down to 4 K yields the fc flux expulsion measurement (Meissner signal). At the temperature T * both temperature-dependent susceptibility curves (zfc, fc) converge and the corresponding value of the magnetic field yields a point on the irreversibility line. Figure 7 shows the irreversibility line of a Bi-2223 sample in a In-ln plot. The irreversibility line has a low field (high temperature) region with a rather large slope and a high field (low temperature) region. The transition in the irreversibility line occurs at about 25 mT (respectively 90 K). From the slope of the logarithmic plot we obtain the power n of the relation

397

R. Job, M. Rosenberg /Magnetic studyof (Bi, Pb)-2223-HTSC 10

6

6* 2 4-

T’ ,

-6

I

12

I

I

3

4

1

K ,

I

5

6

7

6

9

01 50

10

60

70

1n H

Fig. 7. Irreversibility line of the (Bi, Pb)-2223 sample. In the low field range up to about 25 mT the irreversibility line is linear in a double logarithmic plot. In the high field range (below =zOAT,) it deviates from this trend. The inset shows the H( T*) plot.

I - T*/T,

=aH”,

(9)

where T,= 107.8 K, the exponent n=0.89 in the low field case (H-c20 mT) and a= 1.37~ 10d3 Oe-0,89. Recently de Rango et al. [ 171 presented their reof a ceramic sults of a magnetic study Bi2Pb0&2Ca2Cu30y sample and proposed an explanation of the behavior of the irreversibility line far from T, based on the proximity effect [ 91, when for lower temperatures (higher fields) the irreversibility line follows an exponential decrease as a function of the temperature. De Rango et al. interpreted this effect by the existence of normal zones (BiO, SrO planes) and superconducting CuO zones in the layered structure of the Bi-HTSC. Superconductivity might be induced into the normal regions of BiO and SrO blocks by the proximity effect. Then according to Deutscher and de Gennes [ 91 one can define a breakdown field (the field at which the induced superconductivity in the normal regions of the unit cell breaks down) which follows an exponential temperature law Hb=Hoexp[

where almost in the normal

- ‘22’1

60

90

100

110

T*(K)

(10)

d, is the thickness of the normal region, i.e. equal to the distance between the CuO layers unit cell, and 2s~ is the Fermi velocity in the region. Figure 8 shows our results, indicating

Fig. 8. Linear dependence of the irreversibility line for magnetic fields above 25 mT in a In(H) vs. T* plot (see text).

a linear dependence of In H on T* between T * x 60 K and about 90 K. From the plot of the irreversibility line shown in fig. 8 one can deduce the fit function y=lnH=a-(l/b)T*,

(11)

with the fit parameters 6= 8.80 K and a= 15.96 Oe. By identifying H with H,, as de Rango et al. did [ 17 1, one sees that a comparison with eq. ( 10) yields bE

v,zt 25cd,,kB ’

Ho =exp(a)

(12a) .

(12b)

With d, z 12 x 1O-8 cm we get for the Fermi velocity in the normal regions v,= 8.7~ lo5 cm s-’ and Ho= 8.5 x lo6 Oe. Following the arguments of de Rango et al. [ 171 one can get more information by making use of the dependence of the reversible magnetization on field, i.e. in the region of the susceptibility where there is no difference between the zfc and fc curves (near T,). For H,, c H<< Hcz and K>> 1 the magnetization in equilibrium follows the equation [ 18 ] 4Eil4=

ln(H-H,,)-ln-

90

4l@

1’

(13)

when the superconductor has an ideal flux line lattice. On the other hand, the reversible magnetization data from measurements in several applied fields up to 1.3 T show an almost linear scaling behavior de-

398

R. Job, M. Rosenberg /Magnetic study of (Bi, Pb)-2223-HTSC

scribed by the phenomenological

equation

M/(1-T/T,)=a(lnH+c),

In addition, one can calculate the Fermi velocity of the superconducting regions from ref. [ 17 ] (14)

where a and c are constants (fig. 9). The validity of eq. ( 14) would mean that according to eq. ( 13) one has to adopt for the penetration depth the form which is a result of the London and BCS theories: A(T)=A(O)(l-T/T,)-l.2.

(15)

In fact this form of the temperature dependence for II is only valid close to T,, but it is in good agreement with the very recent results of a magnetic study of the Tl-equivalent of our sample, i.e. T1,BazCa,Cu,O,, providing evidence for the validity of eq. ( 15 ) down to Tw 0.7 T, [ 19 1. Another practical argument in favor of this choice is the temperature dependence of H,, above 40 K which is almost linear in T, as can be seen in fig. 3 for the 2223 sample. Since H,, is givenbyeq. (l),eq. (15)hastobevalidforA(T). H,, can be neglected in eq. ( 13) because in our temperature range H,, CKH and because of the weak temperature dependence of In [go/ (4x(*) ] the equivalency between eqs. ( 13) and ( 14) can be established. From a linear lit to the data in fig. 9 (solid line) the values of To and n(O) can be extracted, but because of the polycrystallinity of our samples only as average values. We obtain (A( 0) ) = 1950 8, and (to) = 3 1.1 8, and a Landau-Ginzburg parameter rc=62.7.

OC I

I

‘0 ;

-1

i .,

-3

Fig.

9.

Scaling

behavior

of

the

reversible magnetization magnetization vs. In

(T* < Tc T,) in a plot of the normalized (HI.

50=o.18*, KB~C

(16)

with T,=107.8 K and lo=31.1 8, one has ez= 1.76~ 10’ cm s-‘. This value is by about a factor of 10 larger than the Fermi velocity of the normal regions. Recently Rossel et al. [ 20 ] presented some magnetic measurements on PbMo,& single crystals, which show that this classic isotropic superconductor ( T,= 14.5 K) exhibits surprisingly similar irreversibility and relaxation properties as the layered HTSC. In particular, an irreversibility line with ( 1 - T*/T,) - H”.‘j6 was observed for Hc0.1 T. PbMo6Ss has also a very short coherence length (
7. Conclusions An anomalous dependence of both lower critical field and remanent magnetization on temperature was found in a magnetic study of ceramic samples nominal with the composition Bil.84Pbo,~4Sr,,o,Ca,.90Cu3.060y and a critical temperature of about 107 K. Because of the low packing densities of our samples with well developed platelet-like crystals, their behavior at least at lower temperatures has to be closer to that of single crystals. The sharp increase of H,, below 25 K found also in the case of YBaCuO single crystals [4] could tentatively be explained by a model proposed by Koyama et al. [ 8 ] as an intrinsic effect due to the crossover between two temperature regimes. In the low temperature range, superconducting order is induced into the normal layers by the proximity effect and gives rise to a strong enhancement of H,,. Certainly the opposite view of McElfresh et al. [ 5 1, that the anomalous increase is not intrinsic but arises be-

R. Job, M. Rosenberg /Magnetic study of (Bi, Pb)-2223-HTSC

of a surface barrier effect for flux penetration at low temperatures, is also plausible. Further experimental and theoretical work is necessary in order to elucidate this problem. The large values and drastic decrease of the remanence below 25 K can be explained by the presence of many pinning centers with low activation energies giving rise to a rapid depinning of the vortex lattice at rather low temperatures. Above 25 K the presence of deeper lying pinning centers give rise only to a small and weakly temperature-dependent remanence. In the temperature range between 40 and 60 K the magnetization curves obey a scaling relation which can be derived from an extended Bean model. Whereas in the low field (high temperature) region the irreversibility line behaves in a manner close to that expected either for a superconducting glass [ 13 ] or the flux pinning model [ 16 1, in the high field (low temperature) region an exponential dependence for H(T) gives a better fit to the experimental data. Considering in this case the field as a breakdown field for the induced superconductivity in the normal layers [ 17 1, one can find plausible values for the characteristic parameters &,, lo, v,s and 2s~.

cause

Acknowledgements The financial support from the Bundesministerium fur Forschung und Technologie (project 13 N 57 13 ) is gratefully acknowledged. We would like to thank W. Oswald for the microprobe analysis and M. Mittag, P. Stauche and H. Bach for helpful discussions.

399

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Ll90.

[ 21 R. Job, M. Rosenberg and H. Bach, Proc. ICMC’90 Topical Conf. on High-Temperature Superconductors: Material Aspects, May 1990, Garmisch-Partenkirchen, Germany. [3] M. Mittag, R. Job and M. Rosenberg, preprint ( 1990), to be published in Physica C. [4] H. Adrian, W. Assmus, A. Hohr, J. Kowalewski, H. Spille and F. Steglich, Physica C 162-164 (1989) 329. [S] M.W. McElfresh, Y. Yeshurun, A.P. Malozemoff and F. Hohzberg, Physica A 168 ( 1990) 308. [6] B. Batlogg, T.T.M. Palstra, L.F. Schneemeyer, R.B. van Dover and A.R. Cava, Physica C 153-155 (1988) 1062. [ 71 Chang-geng Ciu, Hong Yin, Jing-long Zhang, Shan-lin Li, Ke-then Wu, Ji-hong Wang and Yi-feng Yan, Solid State Commun. 68 (1988) 331. [S] T. Koyama, N. Takezawa and M. Tachiki, Physica C 168 (1990) 69. [ 91 G. Deutscher and P.G. de Gennes, in: Superconductivity, vol. 2, ed. R.J. Parks (Marcel Dekker, New York, 1969) p. 1005. [ IO] C.P. Bean and J.D. Livingston, Phys. Rev. Lett. 12 ( 1964) 14. [ 111 Y. Yeshurun, A.P. Malozemoff, F. Holtzberg and T.R. Dinger, Phys. Rev. B40 ( 1988) 11828. [ 121 Y. Wolfus, Y. Yeshurun, I. Felner and H. Sompolinsky, Phys. Rev. B40 (1989) 2701. [ 131 K.A. Miiller, M. Takashige and J.G. Bednorz, Phys. Rev. Lett. 58 (1987) 1143. [ 141 J.R.L. de Almeida and D.J. Thouless, J. Phys. Al 1 (1978) 983. [ 151 Y. Yeshurun, I. Felner and H. Sompolinsky Phys. Rev. B36 (1987) 840. [ 16]Y. Yeshurun and A.P. Malozemoff, Phys. Rev. Lett. 60 (1988) 2202. [ 171 P. de Rango, B. Giordanengo, R. Toumier, A. Sulpice, J. Chaussi, G. Deutscher, J.L. Genicon, P. Lejay, R. Retoux and B. Raveau, J. Phys. (Paris) 50 ( 1989) 2857. [ 18 ] A.L. Fetter and P.C. Hohenberg, in: Superconductivity, vol. 2, ed. R.J. Parks (Marcel Dekker, New York, 1969) p. 8 17. [ 191 J.R. Thompson, D.K. Christen, H.A. Deeds, Y.C. Kim, J. Brynestad, S.T. Sekula and J. Budai, Phys. Rev. B4 1 ( 1990) 7293. [20] C. Rossel, E. Sandvold, M. Sergent, R. Chevrel and M. Potel, Physica C 165 ( 1990) 233.