Reversible magnetization in the (K, Ba)BiO3 high Tc superconductor

PHYSlCA ELSEVIER

Physica C 341-348 (2000) 1067-1068 www.elsevier.nl/locate/physc

Reversible magnetization in the (K, Ba)Bi03 high Tc superconductor I. Joumard a, T. Klein ~, A. Conde-Gallardo ~ *, J. Marcus a and A. Sulpice b aLaboratoire d'Etude des Propri~tes Electroniques des Solides Centre National de la Recherche Scientifique - BP 166, 38042 Grenoble Cedex 9, France bCentre de Recherche sur les Tr~s Basses Temperatures Centre National de la Recherche Scientifique - BP 166, 38042 Grenoble Cedex 9, France Magnetization measurements have been performed on the cubic (K, Ba)BiOa superconductor (To -~ 30K). The reversible part of the magnetization varies linearly with lnH in both liquid and solid phases. The Mr~v curves never cross each other but OM/OIn H strongly deviates from the standard London model at high temperature. Keywords : Reversible Magnetization, Cubic Bismuthates.

1. I N T R O D U C T I O N It is now well established that thermal fluctuations play a significant role in both dynamic and thermodynamic properties of high T~ superconductors. Those fluctuations give rise to a large entropic contribution to the free energy which is probably at the origin of the existence of a field independent crossing point in the reversible magnetization M(T) of strongly anisotropic systems [1]. Alternatively, a crossing point in quasi 2D systems has also. been predicted by Te~anovi6 et al. [2] considering thermal fluctuations of the superconducting order parameter in the Lowest Landau Level approximation. On the other hand, in 3D systems, it has been suggested that 3D-XY critical fluctuations may become predominant leading to a crossing point in the M / v ~ vs T curves (instead of M vs T) at T = T~ [3]. We present here reversible magnetization measurements performed on the perfectly isotropic (i.e. cubic) (K, Ba)Bi03 superconductor. As expected in this 3D system, the reversible magnetization curves M(T, H) never cross each other, however the slope OM/OlnH strongly deviates from the standard London model [4] in the vicinity of To. *Present adresse : D e p a r t e m e n t o de Fisica CINVESTAVINP, Apdo, Po stal 14-740 M4xico, D.F. 07360 MSxico

2. E X P E R I M E N T A L

AND RESULTS

Magnetization loops have been measured on

a (K, Ba)Bi03 single crystal grown by electrochemical crystallisation (To = 30.1 ~ OAK) on a BaBi03 seed using a SQUID magnetometer (from 16K to 29.5K). In (K, Ba)Bi03 the irreversibility is directly related to bulk pinning [5] and the reversible contribution Mre. can thus be measured in both irreversible (solid phase) and reversible (liquid phase) parts of the loop using Mrev = (Mr + M~)/2 where Mr and M 1 are the ascending and descending branches of the loop respectively [6]. As shown in Fig.l, the reversible magnetization is well described by a In H dependence. Above 22K, the slope of the Mrev vs In H curves does not change at the vortex glass transition (see for instance inset of Fig.2 at 28K) but slightly decreases in the liquid phase at low temperature (note that the logarithmic approximation holds down to very small values of the magnetization). The main difference between our data and the one previously obtained in cuprates is that the slope OM/OlnH remains positive on the entire temperature range in (K, Ba)Bi03 and the Mrs,(T, H) curves thus never cross each other. In both positional and order parameter fluctuations theories, the existence of a crossing point is

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L Joumard et al./Physica C 341-348 (2000) 106Z 1068

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directly related to the large value of the Ginzburg number. The absence of any measurable crossing point in our system is thus consistent wi~h the smallness of Gi in our system (Gi ~ ]0-4). However, as shown in Fig.2, the temperature dependence of the slope OM/O In H (which is expected to be directly proportionnal to 1/A 2 in the standard London model) strongly deviates from linearity close to Tc (as an example we have reported on Fig.2 the theoretical prediction assuming a WHH temperature dependence for A(T) with A(O) = 2700)1 [7]). 3. C O N C L U S I O N The reversible magnetization has been deduced from the magnetization loops in the cubic (K, Ba)Bi03 superconductor. We have shown that OM/O In H strongly deviates from the standard London model on a large temperature range below To. We have shown that the disorder induced fluctuations play a major role in the melting process of the vortex solid [8]. In this system, these fluctuations may also contribute to the entropic term in the free energy. However, to our knowledge, no calculation for this contribution has been done yet.

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Figure 1. Field dependence of the reversible magnetization at T = 20, 24, 26, 28, 28.5, 29 and 29.5K (from right to left).

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Figure 2. Slope (gM/O In H vs T in the solid state. The solid line corresponds to the theoretical prediction in the WHH model. In the inset : magnetization loop (plain diamonds) and reversible magnetization (solid line) at T = 22K.

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