Diamagnetism in granular La3Ba3CaCu7O16 weak-link effects

Physica C 249 (1995) 377-386

ELSEVIER

Diamagnetism in granular La3Ba3CaCu7016weak-link effects M. Nicolas *, B. Mettout, K. Sauv Laboratoire de Physique du Solide ESPCI-CNRS, 10, rue Vauquelin, 75231 Paris cedex 05, France

Received 13 March 1995

Abstract We show that for the La3Ba3CaCu7016 ceramic, the effective grain size a, which plays a part in the "granular" phenomenon, is much smaller than the observed microstructural grain size. This fact induces strong differences between AC and DC susceptibilities due to the difficulty for the grains to adjust their phase in a static magnetic field because a is smaller than Aj. The phase can be adjusted provided the amplitude of the alternative field remains low. As long as h o is lower than a given value, the AC field does not penetrate the grains and no difference is observed between ZFC and FC experiments. When h o increases, the AC field goes into the grains and hysteresis occurs. The intergranular critical current density increases quickly with decreasing temperature; a crossover is observed between two regimes for the flux-line movements on increasing the static magnetic field, an isolated and a collective behavior, respectively.

1. Introduction In the superconducting ceramics, the transition temperature, the problems connected to reversibility and granularity, can be analyzed by various methods, among them AC susceptibility, DC magnetization and resistivity measurements. The complex susceptibility is given by XAc = ( a M / O H ) r with X(tO)= X'(to) + i x " ( t o ) where X'(to), the in-phase component, corresponds to screening effects and X"(to), the out-of-phase component, is related to energy dissipation in the material. Normally, at very low fields, if the superconductor is type-l, pure and homogeneous without cracks and dislocations, magnetic losses might not exist at the temperature at which the field is expelled. Nevertheless, in classical superconductors, losses have been observed in ilia-

* Corresponding author.

mentary or inhomogeneous materials, which has been found evidence for by a maximum in the x " ( T ) variations. In these materials, the existence of this maximum has been explained by the increase of the electrical conductivity in the normal parts of the sample, which allows eddy currents to develop [1,2]. In high-Tc ceramics, at very low field, one also always observes a dissipation peak at a temperature Tp equal to or lower than To, the true transition one. Such a phenomenon has been associated to the granular aspect of the material: the intrinsic (intragranular) superconductivity nucleates at T¢ and the coupling between the grains by Josephson links which leads to the macroscopic screening of the sample, occurs at the same or at a lower temperature. A weak-link network would behave as a superconductor of type-II for which it is possible to define a penetration depth Aj, a first critical field hclj, a decoupling field hc2J and a critical current density J~j (for a review of all these problems, see Senoussi, Ref. [3].

0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00325-8

378

M. Nicolas et al. /Physica C 249 (1995) 377-386

In AC susceptibility measurements, the intergranular magnetic properties will be very sensitive to the amplitude h 0 of the alternative field [4] and to the magnitude of a superimposed static field H [5]. For amplitudes h 0 < hclJ, the field sees an homogeneous effective medium and the sample behaves as in a Meissner state. For h 0 > hclJ, the field penetrates the intergranular region and the X" peak can be connected to the "irreversibility line" of the (h0, T) plane; with a superimposed static field H, the problems are similar but in another order of magnitude and imply also the grains themselves in a (H, T) diagram. Usually the irreversibility line (IL), which has been observed originally in spin glass systems [6], is determined by DC magnetization measurements with Xoc = ( A M / A H)r. The temperature of irreversibility T * is that at which the ZFC and FC magnetization curves become identical at the applied magnetic field H. It has been suggested that the temperature of the X" maximum in AC susceptibility measurements could also be connected to this temperature T * provided the measuring frequency is small (i.e. tends to zero) and the amplitude h 0 not too high [7,8]. However, the T * parameter is an intrinsic property of the material whereas the temperature of the X" maximum depends on the sample size and it is better to take the temperature of the upper part of the X" peak as equal to T*. Various 2-Theta

'

models have tried to explain the irreversibility-to-reversibility transition: giant flux creep and flux-lattice melting, which are models independent of the pinning mechanisms, melting of a vortex glass towards a vortex liquid or simple depinning [9-12]. Up to now, it is difficult to choose the best model since many parameters interfere: microstructure, granularity, homogeneity, weak-link nature and so on... Normally, the susceptibility given either by an AC or by a DC technique might give similar results. Nevertheless in certain circumstances, different values can be obtained, especially when the grain size is small compared to the London depth Ao leading to an important play of the intergranular network in the AC technique. Moreover, it is possible to calculate an intergranular critical current J~j from the X" peak (T, h 0, H) variations [13], according to the Bean model and assuming that the magnetic losses are maximum when the field leaves the center of the sample, i.e. when the shielding currents become everywhere equal to the critical current density. In these granular materials, two important parameters will be set in action: on one hand the intragranular condensation energy E G = (H2/8rt)Vo (Vo is the volume of the grains) and on the other hand, the Josephson coupling energy Ej = ( h / 2 e ) I o (I o is the critical current of the junction). As shown by Clem [14], the ratio e = E j / E G plays a leading part: when

Scale ,

~

~cq

cq

c~

ca

r~

o

g

....

~'0 . . . .

i~ ....

2'0 . . . .

Aa ....

3'0

" 3~ ....

4'0 . . . .

4~ ....

5'0 . . . .

5~3 . . . .

6b ....

Fig. 1. XRD pattern for the La3Ba3CaCu7016 compound.

¢5 ....

¢0 ....

7'5' ' "

M. Nicolas et al. / Physica C 249 (1995)377-386 z ,~ 1, the intragranular order parameter is not modified by intergranular currents and the effect of granularity is very pronounced; when E < 1, the Josephson coupling energy becomes primordial and granular effects are strongly reduced. This energy Ej also has to be compared to the thermal energy kBT since the phase coherence will be maintained between the grains only for temperatures lower than To , i.e. for Ej >_ksT. In the present work, we report a systematic study of the x ' ( T ) and x " ( T ) signal variations as a function (1) of the amplitude h 0 of the alternative field, (2) of the magnitude of a superimposed static field, for a polycrystalline La3Ba3CaCu7016 sample with a mean grain size of 1 to 5 I~m (small grains). In such a sample, the weak-link effects will be predominant compared to the intrinsic granular superconductivity. The AC susceptibilities have been compared to DC data obtained on the same sample by magnetization experiments carried out with a SQUID magnetometer. The La3Ba3CaCu7016 structure is derived from the La3Ba3Cu6014 one which is insulating [15,16], Addition of one amount [CaO + CuO] to this stoi-

379

chiometry leads to a superconducting material with T~ ~ 80 K [17]

2. Experimental details The sample has been prepared by the usual solidstate reaction by sintering pure BaCO3, La203, CaCO 3 and CuO powders at 950°C in air and then, after a new grinding, by annealing the pellet at 950°C under oxygen atmosphere, followed by a slow cooling. The resulting compound is single phased, isomorphous of YBa2Cu306. 5 [18] as shown in Fi~. 1. Its crystal structure is tetragonal with a = 3.87 A and c = 11.61 ~, in good agreement with Ref. [19]. The oxygen chemical analysis gives a content y = 19.05 which means that the material contains an excess of oxygen compared to that required for the charge neutrality which is y = 15.5. Besides, it is very stable as regards oxygen since DTA and DTG experiments carded out between room temperature and 1000°C show reversible losses lower than 1% and only above 400°C. The superconducting transition temperature at zero field is T¢ ffi 78.6 K. The AC susceptibility measurements have been carded out at a frequency f = 392 Hz in an alternative field hAc =

0.0

~.....~ -1.0 I

I

I

I

65

70

75

80

'2' ( K )

Fig. 2. X' vs. T variationsfor differentamplitudesof the AC field. The dottedline representsthe positionof the X" peak(h o = 0.0180 mT (1), 0.0360 mT (2), 0.0540 mT (3), 0.0720 mT (4), 0.180 mT (5), 0.360 mT (6), 0.540 mT (7), 0.720 naT (8), 0.90 mT (9).

M. Nicolas et al./ Physica C 249 (1995)377-386

380

h o cos tot with 0.0180 m T < h o < 0 . 9 mT in the absence or in the presence of a static field H, parallel to h0, increasing up to ~ 0.7 T as previously described [5,20]. The sample shape is a bar with (0.5 X 0.1 X 0.1) cm 3 dimensions.

degrees below To, i.e. nearly at 74 K. The position of the maximum on the temperature scale is indicated in Fig. 2 by a dotted line. At low field, if we consider that this temperature corresponds to the Josephson coupling temperature between the grains T~j, as previously described by Clem [14], we can estimate the actual size of the grains involved in the "granular phenomenon," following Jardin et al. [21] in their percolating model. First we calculate the normal resistance R of the junction by using the relationship [22] T~j/Tc = 1/[1 + a(R/R~)] where a is a dimensionless parameter, a = 0.12 and R~ = h//e 2= 4114 ~ . In our case T~j/T~ = 0.86, taking the lowest value of h 0, since the model is established in zero magnetic field, and one obtains R = 26.3 1"1. Following Ref. [20], we assume that R = Pn/a with Pn the resistivity just above T~j, a being the grain size. The resistivity of our sample, measured just above T% is p ~ 600 IXfl cm [23] which leads to a = 200 A, a value a little lower than that found in a material such as SmE_xCexCuOg_y for instance, where a = 600/~ [21]. Thus the a value calculated in that way is much smaller than the grain size measured by a microscopic observation and would represent the size along which the order parameter does not vary which implies the existence of Joseph-

3. Results and discussion

3.1. AC susceptibility at zero H field: h o influence At low h 0 values, the X' transition is very sharp with AT ~ 1 K for 90% of the screening effect. As h o is increased, a slight broadening occurs (Fig. 2), but Tc is unaffected by the h 0 variations. The screening is complete at 60 K, in the whole field range. Usually, for ceramics, the broadening is observed in the lowest part of the transition; but here, the whole transition is broadened meaning that it is mainly due to intergrain couplings, the diamagnetism of the grains themselves being negligible. Indeed, x ' ( T ) measurements carried out on powders of the same sample, with the same h 0 conditions give a very small signal as shown in Fig. 3. The temperature of the X" maximum, Tp, does not vary very much as h 0 is increased and it saturates at a T value, some

0,001

0

F

f

® -0,001

Q

-0,002

/

-0,003

-0,004

-0,006 60

i

6s

~0

,

75

r

ao

~

85

,

9o

g5

Fig. 3. ComparativeX' signal for powdersand bar of La3Ba3CaCu7016compound.

381

M. Nicolas et al. / Physica C 249 (1995) 377-386

son junctions inside the grains. This very small length explains the importance of the junction network compared to that of the grains themselves. We have plotted h 0 as a function of (1 - t) m with t = T p / T c according to a flux-creep model for instance; in that case two behaviors are observed as shown in Fig. 4(a): a low-field range (h 0 < 1.8 G) with n ~ 1.20 and a higher-field domain with m ~ 7. As previously mentioned, we can consider that Tp -T*, and the power law can be explained with the flux-creep model. However, the m-=-7 value is too high for a flux-creep mechanism. Thus it seems better to plot the (h 0, Tp) variations in a logarithmic form, since this is valid in the whole studied h 0 range (Fig. 4(b)): log h o = A - B T p with B = 1.25 K -1 or h 0 ot exp(-BTp). In the framework of the Bean model, these (h 0, Tp) variations are connected to the intergranular critical current density since we have JcJ = h o / r , r being the dimension of the sample [13]. Consequently, for our material, JcJ increases exponentially with decreasing temperature (in the studied field range) contrary to intergranular behaviors previously observed in other ceramics; indeed the shift of the X" maximum towards low temperatures is small ( < 5 K) compared to YBCO [13] or to a Hg 1-2-23 polycrystal [24] where a shift A T p ~ 1 0 - 1 5 K is observed for Ah 0 ~ 1 0 G, at frequencies of the same order as ours. Such a result means also a larger intergranular pinning force for the vortices, as shown by Muller [25]. This weak AC field dependence of the X" maximum would also agree with a melting model of a frozen glass provided Tp ~ T* [25].

even though the intrinsic Tc does not vary as proved by DC measurements carried out on the same sample (Fig. 7); (3) at 60 K the X' signal decreases as H increases; one can estimate the first critical field Hcl for the

(a) ho 3) +1.0,

8

6 4 +0.5. 2

~p

75 0.0'

-05

log (1 - ~Tp )

-210

- 115

(b)

i0

•

3.2. A C susceptibility: influence of a superimposed field H 3.2.1. L o w h o The X measurements have been made at h 0 = 0.0180 mT which is lower than hcl J in nearly the whole temperature range• The Figs. 5 and 6 give the x ' ( T ) and X " ( T ) variations for various amplitudes of the field H. As concerns the x ' ( T ) curves, one observes: (1) the transition remains abrupt even at H = 0.55 T, at least in its upper part; (2) the temperature Tons of the transition, as defined in Fig. 5, decreases when increasing the field H

1•0

o.z

I

7s

7s

t

77

7a

Tp (z)

Fig. 4. (a) Log h0 variations as a function of log (1- t) with t = Tp/ Tc. (b) Exponentialdecrease of ho vs. Tp.

382

M. Nicolas et al. /Physica C 249 (1995) 377-386 o

"effective" medium formed by the "composite syst e m " junctions + grains. We obtain He1J ~ 200 G, which is a relatively high value for a pure system of Josephson junctions and a low value for a single grain. The extrapolation to X' = 0 of the linear parts of the x ' ( T ) curves of Fig. 5 will give the H~2J parameter. From the H~2 values, we have estimated a coherence length for this medium, assuming the validity of the Ginsburg-Landau model. We obtain at T = 75.5 K, H~2 = 286 G, ffj.--- 1517 A, at T = 70.5 K, H~2 = 5540 G, ~j -- 345 A, and at T = 60 K, o

He2 = 63 kG, ~j = 102 A. These lengths, much higher than those found for the intrinsic superconductivity of cuprates, are of the same order of magnitude as those of the more classical materials. As regards Tons, we see that it is equal to the irreversibility temperature T * ( H ) at the field H (H* =napplied) since, as we shall see below, it corresponds to the upper part of the X" peak ( X" = 0). Plotting log H versus log(1 - t) with t = Tons/Tc gives a power law H ~ (1 - t)" with n = 1.20 in the low-field range ( H < 280 G) and n = 3 in the higher-field range (Fig. 8). Thus there is a crossover

0.0 ?c=78.6K

d

v

-0.5

-1.0 I

I

I

I

65

70

75

80

J T

(X)

Fig. 5. The x'(T, H) signal. H = 10 G (1), 17 G (2), 138 G (3), 193 G (4), 286 G (5), 455 G (6), 662 G (7), 856 G (8), 1540 G (9), 2240 G (10), 2900 G (11), 3450 G (12), 4100 G (13), 4750 G (14), 5540 G (15).

M. Nicolas et al. / Physica C 249 (1995) 377-386

383

0.5 Tm 1&131211 I0 9 1__7 | 5

&

2

]

/

(D

0.1

|

!

|

|

!

65

70

75

80

T

(K)

Fig. 6. X" vs. T and H. Same fields as in Fig. 5.

at H ~ 200 G and T ~ 76 K between two vortex regimes. This crossover has been explained recently by Galand et al. [26] who consider that at low fields, the flux spacing exceeds a value a* and the flux lines are weakly correlated; in such a case, the depinning concerns individual vortices and n ~_23 whereas at higher fields, the correlations are stronger and collective depinning is involved which leads to an exponent n = 3, this latter value agrees very well with our results. In that model, the crossover would

occur at a critical field HeR with an intervortex spacing a equal to •

a

2~bo

[

=

1/2

l (3),/2Hc R

which gives a* = 346 ,~ for HeR = 200 G, a length which is just a little greater than the above calculated grain size. Other models, not based on a simple depinning mechanism lead also to power laws be'1

Too

0

J

OI | O

e.4 v

8.6k

-1

0.1T

l

60

70

l

80

T

Fig. 7. The D C signal for the same sample as for Fig. 5.0.1 T < H < 1 T.

(K)

M. Nicolas et a l . / Physica C 249 (1995) 377-386

384

t) n with n = 4, meaning that the critical current density JcJ increases strongly as T decreases.

(1 log H

3.2.2. h o influence

3.5,

3.0.

2.5.

• "

-2.0

.~.s

Fig. 8. Log H-log (1 -

t)

Tc

tog plots with t =

Tons / Tc.

tween H and ( 1 - t) with an exponent n = 1.33 (vortex glass melting) or n = 2 (lattice melting). As concerns the X"(T, H) variations, we have to consider two temperatures as T decreases: the temperature T *, when X" begins to behave different from zero and Tm, the temperature of the maximum. Indeed, in our mind, strictly speaking, the irreversibility temperature T* is the temperature of the upper part of the X" peak when X" = 0, i.e. when J~s = 0 rather than that of the X" maximum itself as it is usually taken, following thus the observations of Sun et al. [27,8]. This T * temperature is also that of Tons of the X' signals as we have mentioned above. The temperature of the X" maximum corresponds also to the moment when the field H reaches the center of the sample; the critical current density is then J~j ~ H */r, r being here the dimension of the grains. If the amplitude h 0 and frequency f are low, both temperatures can be considered as nearly equal since the peak is sharp. Indeed, the Tons(H) and Tm(H) variations are similar as shown in Fig. 9 (a shift of 1 or 2 K) but not identical. We find again the two behaviors previously observed, clearly shown in Fig. 5 where the dotted line indicates the position of the X" maximum on the x'(T, H) curves: at low H values the maximum is situated in the lowest part of the transition whereas at H > 286 G, it is situated in the middle of the straight part of the X' transition. The log H versus ( 1 - t) log variations with t = Tm/Tc and 286 < H c < 5540 obey the relation H at

In the above experiment h 0 is very small ( = 0.0180 mT), << H. In that case, it seems that at low temperature, when h 0 is very small, on the one hand it does not destroy the coherence of the grains which are still coupled at the field H, on the other hand, it does not penetrate the grains up to their center and the Bean scenario would become invalid except near Tc. And we observe that the x(T, H) curves are strictly reversible, i.e. the ZFC and FC experiments give same results. As increasing the amplitude h0, a small hysteresis appears between ZFC and FC experiments (Fig. 10), because vortices penetrate the intergranular domain (h 0 > hclJ); in that case, the ZFC curve is always above the FC one. We must note here that DC and AC experiments do

H(G)

o Tm

(~')

5000,

4000.

2000.

1000¸

65

70

75

T(K)

Fig. 9. "Irreversibflity l i n e " in the H, T diagram.

M. Nicolas et al. / Physica C 249 (1995) 377-386

385

both techniques which usually must give the same results arises probably from the fact that the grain size is small compared to the London penetration depth. It appears that in a static field H, which represents an equilibrium state (DC measurements), the grain phase does not achieve a long-range coherence and this situation could lead to a "frustrated" system in which the frustration is produced by the phase factors. Such a problem has been theoretically studied some years ago by Ebner and Stroud [28]; their model, developed for a system of weakly linked superconducting dusters, predicts a large difference between AC and DC susceptibilities at low temperature. In such a structure, at the equilibrium, the grains cannot adjust simultaneously their phase and remain decoupled. Superimposing an AC field favors the setting up of a long-range coherence between the grains thus leading to an AC signal bigger than the DC one in absolute value. In that sense the system of weakly coupled superconducting small particles would be analogous to a "spin glass". But we must remark that, in our case, the difference is strong even at high temperature. Finally one can consider that the decoupling tem-

not give same results about this question since in DC measurements the ZFC curve is always below the FC one. These discrepancies between both behaviors can perhaps be understood if we remark that the technique of AC susceptibility explores only the intergranular network whereas the technique of DC susceptibility can usually reach both inter- and intragranular domains except in particular situations as we shall see below. In AC measurements, the magnetic field due to the flux exclusion from the grains in the intergranular area is larger in a ZFC situation than in a FC one. Then the effective field in the intergranular zone is larger in a ZFC run than in a FC one and consequently the X' signal is smaller [271.

3.3. Comparison between AC and DC susceptibilities From the data shown in Figs. 5 and 7, it is clear that the AC susceptibility and DC susceptibility give different results, the DC diamagnetic response being smaller than the AC diamagnetic one. The DC signal is rather similar to the AC signal of the powder (decoupled grains). This strong difference between

Teo

0

M

0.5

ZFC FC 1.0

H(G) ho(G) I

i

i

i

60

65

70

75

A 10

B 2240

C 224O

o.xs

o.xs

x.s

T

(K)

Fig. 10. Influence of the amplitude h 0 of the AC field and of the magnitude H of the static field on the ZFC and FC curves.

386

M. Nicolas et aL / Physica C 249 (1995) 377-386

perature T~j would also correspond to the vanishing of the intergranular vortex lattice which may exist in an homogeneous system of coupled grains and we can assume that Tcj = T *. From that point of view, T* would really correspond to a melting of the lattice (or glass) of vortices with a breaking of the flux lines arising at the level of the junctions. [By a similar argument, we suggest that the intrinsic irreversibility temperature found for the grains themselves by other authors in higher-field ranges corresponds to the breaking of the vortex lattice at the level of the interplane domains, i.e. corresponds to the decoupling of the superconducting CuO 2 monoplane or multiplanes (see for instance Ref. [5]).]

4. C o n c l u s i o n In this work, we have shown that for the La3Ba3CaCu7016 ceramic, the actual grain size a, which plays a part in the " g r a n u l a r " phenomenon, is much smaller than the observed microstructural grain size. This fact induces strong differences between AC and DC susceptibilities due to the difficulty for the grains to adjust their phase in a static magnetic field because a is much smaller than Aj. The phase can be adjusted provided the amplitude of the alternative field remains low. As long as h 0 is lower than a given value, the AC field does not penetrate the grains and no difference is observed between ZFC and FC experiments. When h 0 increases, the AC field goes into the grains and hysteresis occurs. The intergranular critical current density increases quickly with decreasing temperature. On increasing the static field, a crossover is observed between two regimes for the flux-line movements: an isolated and a collective behavior, respectively.

References [1] T. Ishida and H. Mazaki, Phys. Rev. B 20 (1979) 131. [2] A.F. Khoder and M. Couach, in: Magnetic susceptibilityof

Superconductors and other spin systems, eds. R.A. Hein, T.L. Francavillaand D.H. Liebenberg(Plenum, New York, 1992). [3] S. Senoussi,J. Phys. III (Paris) 2 (1992) 1041. [4] T. lshida and H. Mazaki, Jpn J. Appl. Phys. 26 (1987) L1296. [5] M. Nicolas, J. Negri and J.P. Burger, J. Less Comm. Met. 164&165 (1990) 1076. [6] I. Morgenstern, K.A. Miiller and J.G. Bednorz, Physica C 153 (1988) 59. [7] S. Samaraduli, A. Schillinf, M.A. Chernikov and H.R. Ott, Physica C 201 (1992) 159. [8] K. Sauv, M. Nicolas, C. Nguyen Van Huong, A. Dubon, P. Legeay and L. Kandels, Supercond. Sci. Techn. 6 (1993) 327. [9] Y. Yeshurun and A.P. Malozemoff, Phys. Rev. Lett. 60 (1988) 2202. [10] K.A. Miiller, M. Takashige and J.G. Bednorz, Phys. Rev. Lett. 58 (1987) 1143. [11] D.R. Nelson and H.S. Seung, Phys. Rev. B 39 (1989) 9153. [12] J.H.P.M. Emmen, G.M. Stollman and W.J.M. De Jonge, Physica C 169 (1990) 418. [13] N. Savvides, A. Katsaros, C. Andrikidis and K.H. Miiller, Physica C 197 (1992) 267. [14] J.R. Clem, Physica C 153 (1988) 50. [15] B. Domenges, M. Hervieu, C. Michel, A. Maignan and B. Raveau, Phys. Status Solidi A 107 (1988) 73. [16] J. Provost, F. Studer, C. Michel and B. Raveau, Synth. Met. 4 (1981) 47. [17] D.S. Wu, H. Cikao, M.K. Wu and C.M. Wang, Physica C 214 (1993) 261. [18] D.M. De Leeuw, C.A. Mutsaers, H.A.M. Vanhal, H. Verweij, A.H. Carim and H.C. Smoorenburg, Physica C 156 (1988) 126. [19] S. Engelsberg,Physica C 176 (1991) 451. [20] Z. Zanoun, M. Rabii, G. Alquie, J.P. Burger, M. Nicolas, Mater. Chem. Phys. 32 (1992) 183. [21] R.F. Jardim, L. Ben-Dor, D. Stroud and M.B. Maple, Phys. Rev. B 50 (1994) 10080. [22] D.C. Harris, S.T. Herbert, D. Stoud and J.C. Garlani, Phys. Rev. Lett. 67 (1991) 3606. [23] B. Mettout et al., to be published. [24] D. Berling, E.V. Antipov, J.J. Capponi, M.F. Gorius, B. Loegel, A. Mehdaoui and J.L. Tholence, Physica C 225 (1994) 212. [25] K.H. Miiller, Physica C 159 (1989) 717. [26] J.C. Galand, C.C. Almasan and M.B. Maple, Physica C 191 (1992) 158. [27] H.B. Sun, K.N.R. Taylor and G.J. Russel, Physica C 227 (1994) 55. [28] C. Ebner and D. Stroud, Phys. Rev. B 31 (1985) 165.