MAGNETIC BEHAVIOUR OF THE HIGH-Tc OXIDE SUPERCONDUCTORS EuBa2Cu3Oy AND GdBa2Cu3Oy

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MAGNETIC BEHAVIOUR OF THE HIGH-Tc OXIDE

SUPERCONDUCTORS EuBa2Cu3Oy AND

GdBa2Cu3Oy

J. Schaf, P. Pureur, J. Kunzler

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment au no 12, Tome 49, dkcembre 1988

MAGNETIC BEHAVIOUR OF

THE HIGH-Tc OXIDE SUPERCONDUCTORS

EuBa2Cu30y AND GdBa2Cu30y

J. Schaf, P. Pureur and J. V. Kunzler

Instituto de Fisica da UFRGS, C.P. 15051, 915UU Porto Alegre, RS, Brasil

Abstract. - We perform DC magnetic susceptibility measurements on the ceramic superconductors EuBa2Cu307 and GdBa2Cu307 as a function of temperature, which reveal broken ergodicity phenomena analog to those found in spin- glasses.

The irreversible magnetic behaviour of the new high-T, oxide superconductors has evidenced the in- homogeneous character of their superconducting state since the early stages of the experimental work [l]. A superconducting granular network, where the overall phase coherence is established through weak links, is now the mostly accepted picture. Although a clear identification of the superconducting islands and the exact nature of the weak links has still t o be determi- ned, the inhomogeneous character of the supercondu- ting state arises clearly from the physical properties revealed in electrical resistance [2], magnetization [l] and microwave absorption [3]. For a granular super- conducting network frustration arises with the appli- cation of magnetic fields [4], if closed loops of .coupled islands are present, leading t o a non-ergodic behaviour -

[5] below the freezing temperature, very much as in spin-glass systems [6].

Our magnetic measurements in sintered EuBa2Cu307 and GdBa2Cu307 ceramic samples tend to systematically corroborate these assumptions and to characterize their superconducting state as an inho- mogeneous and frustrated islands network. Detailed description of our sample preparation and measuring techniques can be found in reference [7].

Our DC diamagnetic susceptibility data were obtai- ned, after cooling the samples t o 4.2 K in zero field (ZFC) or in an external field (FC) and then heating. Below a limiting temperature Tg (H), which marks the onset of irreversibility as a function of the field, the ZFC and FC curves split into very distinct branches, which is the signature of a non-ergodic behaviour. While the FC curve (Meissner effect) is reversible wi- thin our experimental precision, representing equili- brium states and amounting t o 20 % of the ideal dia- magnetism ( - 1 / 4 ~ ) a t 50 Oe and 4.2 K, the ZFC curve (shielding effect) is irreversible and metastable, amou- ting t o 70 % of the ideal value a t 50 Oe and 4.2 K.

Cycling the samples with a magnetic field a t 4.2 K after ZFC, gives broad diamagnetic hysteresis cycles, see figure 1, representing a path through metastable states, as the magnetization is time dependent a t all the points. By cycling at 4.2 K just after field cooling, the samples start a t an equilibrium state and join gra- dually t o the ZFC cycles by following the dashed lines in figure 1. By increasing the temperature the hystere-

Fig. 1. - ZFC magnetic hysteresis cycles. The dashed lines show the path after field cooling.

sis cycles gain progressively the form of those usually observed in type I1 superconductors [8]. For the Gd based compound, there'is of course a considerable pa- ramagnetic contribution.

In order t o get the detailed behaviour of

T,

(H), which marks the limit between ergodic and non- ergodic regimes, we have performed systematic mea- surements as a function of the applied field. Figure 2 displays a set of ZFC and FC susceptibility curves in the temperature region, where irreversibility sets in, obtained for the EuBa2CusO7 sample under the fields indicated in the figure. Almost identical data were ob- tained for the GdBa2Cu307 sample. In figure 3 we display the measured Tg (H) values in a H-T plane. The dashed lines are a guide to the eye. The data curves for both compounds are closely alike and are also remarkably similar t o the analog data curve for YBazCu307, reported in reference [g]. A de Almeida Thouless (AT) line [10], N (1

-

Tg (H) IT,), fits

well t o our low field data. The AT line has been de- rived for a system of Ising spins by using mean-field theory and represents a limit between ergodic and non- ergodic regimes in the H- T plane. This line is generally found for spin-glasses (SG) and other glass-like sys- tems [Ill. For higher fields, however, our data clearly depart from the AT line. This however is not comvle- tely surprising as such departures have been observed

JOURNAL DE PHYSIQUE

It may be remarked that the Hamiltonian

proposed [5] t o describe the physical properties of "glassy" granular superconductors, is the supercon- ducting analog of the frustrated

XY

spin model. The- refore we may conjecture that the superconducting state of our ceramic superconductors, which seems Ising-like for low fields, undergoes an Ising-XY cros- sover for higher fields. Correspondingly the irrever- sibility line changes from AT-like t o GT-like in ana- logy with SG transitions [12]. The anisotropy forces, necessary t o stabilize a low-field Ising regime, could be related t o the very anisotropic character of the bulk superconducting state of these oxide systems. At high enough fields this superconducting state might be do- minated by an XY-like regime.

In conclusion, our magnetic data in the high-T, -

Fig. 2. - FC (open circles) and ZFC (crosses) magneti- _{EuBazCu307 and GdBazCus07 superconductors re-} zations about T, in the indicated measuring fields. The _{veal systematic features, consistent with a description} vertical arrows indicate the splitting of the FC and ZFC

magnetizations. in terms of a grain superconducting network, where disorder and frustration play a major role.

Fig. 3. - Experimental points defining a line, which sepa- rates ergodic and non-ergodic regions in the H T plane, for EuBa2Cu307 (open circles) and G d B a z C ~ ~ 0 . r (closed circles). The full line represents an AT-like fitting, while the dashed line is a guide to the eye.

0

in SG systems [K?]. For SG systems they have been ascribed t o a crossover from a low-field Ising regime, induced by random anisotropy forces [13], to a high- field Heisenberg regime. For Heisenberg and

XY

spins, mean field theory of SG systems predicts a phase tran- sition at the much less field dependent Gabay-Toulouse

,

I

I l l ,

(GT) line, .H2 (1 - Tg ( H ) / T = ) [14]. The ZFC-FC

irreversibility has been interpreted here as resulting from a "glassy" superconducting state. Other expla- nations in terms of pinned flux may also be proposed [15]. It seems however clear that, although a simple flux-pinning model may account for single crystal data, it can hardly explain our high field data in ceramics, where systematic departures from the AT line occur.

80 85 90 " 95

TEMPERATURE (K)

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