MAGNETIC FIELD PENETRATION IN SINGLE CRYSTAL AND POWDER DyBa2Cu3O7-x AND GdBa2Cu3O7-x

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MAGNETIC FIELD PENETRATION IN SINGLE

CRYSTAL AND POWDER DyBa2Cu3O7-x AND

GdBa2Cu3O7-x

E. Vincent, J. Hammann, J. Hodges, H. Noel, M. Potel, J. Levet, P. Gougeon

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Supplement au no 12, Tome 49, decembre 1988

MAGNETIC FIELD PENETRATION IN SINGLE CRYSTAL AND POWDER

DyBa2Cu307-x AND GdBa2Cu307-,

E. Vincent (I), J. Hammann ( I ) , J. A. Hodges ( I ) , H. Noel ("), M. Potel (2), J. C. Levet ( 2 )

and P. Gougeon (2)

( I ) Service de Physique du Solide et de Rksonance MagnLtique, C.E.N. Saclay, 91191 Gif Cedez, France

(2) Laboratoire de Chirnie Mine'rale B, Universite' de Rennes, Av. du G6n6ral Leclerc, 35042 Rennes Cedex, France

A b s t r a c t . - From magnetisation measurements on superconducting DyBa2Cu307-, and G ~ B ~ ~ L ~ C U ~ O ~ - ~ , we show that t h e average field penetration a t the rare earth site is much lower than expected from Meissner effect estimates. This preferential field rejection is strongest in single crystals, where the rare earth magnetic ordering cannot be detected.

parison for DyBa2Cu307-, and GdBa2Cu307-,, with

.

Average at the oy sites powder and single crystal samples.

o Average over the whole Replacing Y by a magnetic rare earth in l ' ' ' ' I ' . ' ~ I . ' . '

,

- sample

Field p e n e t r a t i o n i n a DyBaaCu307-, powder

1

YBazCu307-,has very little influence on the superconducting properties [I]. We can take advantage of the presence of magnetic ions to test the magnetic flux penetration a t the rare earth site, in a way which is complementary to the bulk penetration deduced from

₂

the Meissner effect. We have performed this com-

>

0.5

I We have compared magnetisation measurements 'on 0o 1 7 " '5 1 ' ' ' 10 ' 1 ~ A 15 ' ~ 1 J zo orthorhombic superconducting (SC) DyBa2Cus07-,, ~ ! ~ ( k C e )

...,. ...o... ... ... o 0 6 b - - - - I

and its tetragonal non superconducting (NS) oxygen deficient counterpart. This study is detailed in [2], we only recall briefly here the main results. Above 90 K, both the Sc and NS samples show the same param- agnetic behaviour, with an effective moment equal t o that of the D ~ ~ ' free ion. Near 1 K, well defined peaks in the thermal variation of the susce~tibilitv of both samples identify magnetic ordering of the Dy sublat- tice. For the field cooled SC compound, the peak is superimposed on a diamagnetic background, the total magnetisation remaining opposite t o the applied fieId (100 Oe).

These field cooled susceptibilities have been measured in.fields from 0.1 t o 20 kOe. Both in the SC and NS samples, the low temperature D ~ ~ + contribution t o the magnetisation is measured through the increase AM observed between 20 and 4.2 K. This allows the definition of an effective average field penetration He@ at the Dy sites in the SC sample, in the following way: when an external field

Ha

produces a given A M in the (partially shielded) SC sample, he^ is defined as the field which is required to produce the same A M in the NS sample. Figure 1 presents the ratio H e ~ / H a as a function Ha. The bulk sample field penetration obtained from the Meissner effect is also plotted.

The comparison of both kinds of results is striking. For low fields, the bulk sample penetration is more

Fig. 1.

-

Ratio of the average value of the effective field

He5 at the Dy sites of t h e D Y B % C U ~ O ~ - ~ powder t o the applied field Ha, as a function of Ha (triangles). The circles represent t h e level of the macroscopic field penetration as deduced from the Meissner effect.

than two times higher than that a t the Dy sites. A Ha near 20 kOe is required before H e f f approaches Ha. Thus, the Dy sites appear t o be preferentially shielded. C o m p a r i s o n of powder a n d single c r y s t a l G d B a 2 C ~ ~ 0 7 - ~

We have performed low-field (3 t o 30 Oe) susceptibility measurements on two single crystals and a sintered powder of G d B a 2 C ~ ~ 0 7 - ~ . The three samples have been studied under the same conditions, using a r.f. SQUID magnetometer.

At 4.2 K and under 3 Oe, taking into account the de- magnetising factors, the Meissner diamagnetic susceptibility of the crystal 0.45 x 0.4 x 0.4 mm3 is 2.5 times greater when the field is parallel t o the c-axis than when it is parallel t o the (a, b) plane (6 times under 30 Oe). In the sintered sample under 3 Oe, the Meiss- ner susceptibility is about the same (- 30 %) as in the single crystal when the field is parallel to the c-axis.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888975

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C8

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2176 _{JOURNAL DE PHYSIQUE}

But the very striking result is that, whatever the direc- tion of the field, the Gd ordering peak, which appears very clearly on the diamagnetic background slightly above 2.2 K in the sintered sample, is not visible in the single crystals, neither under 3 nor 30 Oe.

This is shown in figure 2, which sketches the low temperature magnetisation variations of the sintered and single crystal (1.4 x 1.2 x 0.3 mm3) samples (the two crystals show similar magnetisation variations). The curves are obtained by directly recording the variation of the SQUID signal with temperature. The data are thus referred to arbitrary base lines. The signals are not corrected for the sample holder and possible SQUID drifts. If the slight difference between the single crystal responses parallel and perpendicular t o the c-axis reflects the antiferromagnetism of the Gd ions (which are known to align along the c-axis [3]), then the corresponding peak cannot exceed 25 % of that obtained in the powder. Thus, the Gd ions are even more strongly shielded from the external field in the single crystals than in the powder.

Discussion

Our comparison of superconducting (z 0) and non-superconducting (a: N 1) DyB%Cu3 07-, powders

shows that the average field penetration at the D ~ ~ + sites is weaker than the average penetration over the

whole sample (as deduced from the Meissner effect). In the two G d B a ~ C u 3 0 7 - ~ single crystals studied, no susceptibility peak was detected at the rare earth magnetic ordering temperature, in contrast to the case of a sintered powder. With the reasonable assumption that the Gd properties are the same in the' powder and in the single crystals, then the larger rare earth contribution observed in the powder sample may be attributed to the higher proportion of rare earth ions which lie close to the surface within the London penetration depth.

The observed preferential field rejection at the rare earth (Gd, Dy) site suggests that an important spatial modulation of the field takes place inside the sample. It is known that in these matrices the rare earth site is located between two CuO planes, 3.9

A

apart; this distance being smaller than the usual estimates of the coherence length perpendicular to the planes (N 4-7

-4,

see e.g. [4]), we can consider that the 4

A

region de-

1. H = 3 0 & Gd Ba, Cu, 07+ .**.

c.0.008

sintered powder

00

.

Fig. 2.

-

Low temperature variation of the field-cooled magnetisation (H = 30 Oe) in single crystal and powder

GdBa2Cu307-=. The magnetisation curves (which cor- respond to the same number of moles) have been shifted vertically by an arbitrary amount t o compensate for the diamagnetic background.

limited by these planes behaves like a superconducting sheet. Two such regions are separated by slightly more than one coherence length, and the whole system can therefore be described as a set of weakly coupled superconducting sheets. It seems likely that the main flux penetration takes place in between these sheets. Acknowledgments

We thank M. Ocio for stimulating discussions, R. Gerard-Deneuville for technical assistance, C. JBhanno and D. Luzet for sample characterization.

[I] Cava, R. J., et al., Phys. Rev. Lett. 58 (1987) 1676.

[2] Hodges, J. A., et a{., Physica C 156 (1988) 143.. [3] Chattopadhyay, T., et al., preprint ILL (Greno-

ble) SP.88CH5015.

[4] Worthington, T. K., et al., Phys. Rev. Lett. 59