Coexistence of magnetism and high-Tc superconductivity in GdBa2Cu3O7

Coexistence of magnetism and high-Tc superconductivity in GdBa2Cu3O7

Physica 145B (1987) 260-266 North-Holland, Amsterdam COEXISTENCE OF MAGNETISM AND HIGH-T c SUPERCONDUCTIVITY IN GdBazCu307 K. KADOWAKI, H.P. VAN DER...

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Physica 145B (1987) 260-266 North-Holland, Amsterdam

COEXISTENCE OF MAGNETISM AND HIGH-T c SUPERCONDUCTIVITY IN GdBazCu307

K. KADOWAKI, H.P. VAN DER MEULEN, J.C.P. KLAASSE, M. VAN SPRANG, J.Q.A. KOSTER, L.W. ROELAND, F.R. DE BOER, Y.K. HUANG, A.A. MENOVSKY and J.J.M. FRANSE Natuurkundig Laboratorium der Universiteit van Amsterdam, Valckenierstraat 65, 1018 X E Amsterdam, The Netherlands Received 30 June 1987

Specific heat, ac-resistance, ac- and dc-susceptibilities and high-field magnetization have been measured on one of the 90 K-class high-Tc superconductors: GdBa2CU3OT. No significant influence was observed of the Gd magnetic moments on the superconducting transition temperature Tc = 94.8 K, which is essentially the same as in YBa2Cu30 7. A sharp peak in the specific heat, found at 2.23 K, clearly indicates a magnetic-ordering phenomenon in this compound in the superconducting state. This observation suggests that superconductivity coexists with the magnetically-ordered state, probably of antiferromagnetic nature in the GdBa2Cu30 7 system.

I. Introduction

After the discovery of high-Tc superconductivity around 90 K in a Y - B a - C u mixed-oxide system by Wu et al. [1], there has been an exploding amount of activities in the field of high-Tc superconductivity. Shortly after this discovery, phase and structural analyses have been performed by several groups [2-7]. These investigations have revealed as an important fact that the orthorhombic YBazCu30 7 phase, which is a distorted Perovskite-related ABO 3 structure with ordered Y and Ba at the A site and with ordered oxygen vacancies, is responsible for the high-T~ superconductivity. In this orthorhombic structure, an important role is played by the oxygen deficiency that is directl3; related to the formation of the high-To superconducting state [3, 8-9]. According to the detailed crystallographic analysis [4-7], substitutions of Y and Ba by other rare-earth elements, as commonly seen in a Perovskite-type structure are possible. Following these crystallographic data, many substitution studies have been tried not only at the Y and Ba sites but also at the Cu site. The attempts of replacements at the Cu site have not been successful, so far; a degradation of the supercon-

ductivity is always observed. On the other hand, it was recently shown by several groups [10-16] that replacement of Y by most of the rare-earth elements does not influence Tc. In some cases Tc is even slightly higher than that of YBa2Cu30 7, irrespective of the fact that most of the rareearth ions possess their own magnetic moments. We have substituted Y in YBa2Cu30 7 by other rare-earths like Sc, La, Sm, Gd, Dy, Ho, Er, Yb and Lu. All of these compounds, except for Sc and La, showed the orthorhombic structure and the high-T~ superconductivity around 90 K [17]. Among them, a magnetic-ordering phenomenon in the case of Gd substitution was observed in specific-heat measurements at low temperatures. In this paper, we focus on the experimental results of the specific heat and its magnetic-field dependence, ac- and dc-susceptibilities, highfield magnetization and ac-resistance of GdBa2Cu30 7. The specific-heat measurements were performed in an adiabatic cell between 1.4 K and 35 K in fields up to 5 T. AC-resistivity and susceptibility at 90 Hz were measured by conventional equipments from room temperature down to 1.4K. Susceptibilities above Tc were measured in a pendulum-type magnetometer. The magnetization at liquid helium temperatures in fields up to 35 T was measured in the

0378-4363/87/$03.50 (~) Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

K. Kadowaki et al. / Coexistence of magnetism and high-T c superconductivity in GdBa2Cu 307

261

High Magnetic Fields Installation at the University of Amsterdam.

2. Sample preparation The samples used for the present experiments were prepared by the powder-ceramics technique described before [2-3]. Stoichiometric amounts of the fine oxide powders of G d 2 0 3 and CuO, and BaCO 3 were well mixed and pressed into pellets prior to the reaction. These pellets were solid-state-reacted at 980°C for 12 h in air, and subsequently milled and pressed at a pressure of 5 tons/cm 2. The tablets were sintered at 980°C for 12 h in air and next, heat-treated at 940°C for 48 h in a flowing oxygen atmosphere. After this process, the furnace was cooled slowly at room temperature in order to avoid degradation of the samples at passing the high-temperature phase transformation. Properly heat-treated samples, as described above, showed high-T c superconductivity at 94.8 K as determined by the mid-point of the resistive transition as well as by the large diamagnetic signal. These characteristics are indicative for a high quality of the samples. Unidentified lines in the X-ray powder patterns often appear in improperly heat-treated samples. This implies that the formation of single-phase G d B a 2 C u 3 0 7 is very sensitive to the conditions during the reaction and sintering processes. Its details have not yet been understood completely. The lattice parameters, obtained by fitting the X-ray powder patterns, are a = 3 . 8 6 1 / ~ , b = 3.912/~ and c = 11.715/~. The atomic arrangements of G d B a 2 C u 3 0 7 in a unit cell are given in fig. 1, assuming they are the same as for Y B a z f u 3 0 7 [4-5, 7]. Gd and Ba atoms are located at the centers of the cubes whose corners are occupied by CuO 6 octahedrons. This unit cell can be understood as the stacking of three simple perovskite structures in an ordered manner with highly-ordered oxygen vacancies that give rise to an orthorhombic distortion. As a result of the missing oxygen layer in the plane at the c / 2 position, the actual atomic distances shrink significantly towards this missing

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b:3.8863 Fig. 1. Crystal structure of GdBa2Cu307. The shifts of the atoms due to the oxygen vacancies are indicated by arrows. The displacements of the atoms are exaggerated by a factor of two from the real scale. The positions and the number of the vacancies of oxygen are assumed to be the same as those of YBa2Cu307. Distances between specific atoms have been taken from ref. [5]. oxygen layer. The displacements of the atoms are indicated by arrows in fig. 1. It is interesting to note that the 0 4 atoms move towards C u l , resulting in a shorter distance than that of C u l O1. At the center of the unit cell, the gadolinium ion is located and surrounded by eight oxygen atoms, four 0 2 and four 0 3 , forming an approximately tetragonal local symmetry. All of these oxygen atoms are expected to have a strong covalent bonding with Cu2 atoms so that the center of the unit cell where Gd 3÷ is located is rather isolated. The Ba site is also influenced by the missing

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oxygen layer at the c/2 position. The displacement of the Ba atoms with respect to the CuO 1 layer results in a large vacant space at the center in the ab plane. It has been shown that the essential properties of the high-Tc superconductivity are strongly related to the number of vacancies and their positions in the CuO 1 plane between two BaO layers [2-3, 8-9]. Because the oxygen atoms are missing along the b axis in an ordered manner, the CuO 1 layer in the ab plane forms a chain-like structure along the b axis. The importance of this structure for the formation of the high-T~ superconductivity has been emphasized [8-9].

3. Electrical resistance

An example of the temperature dependence of the resistance of GdBa2Cu307 below room temperature is shown in fig. 2. The resistance shows an almost linear temperature dependence down to about 130 K. Below this temperature, it starts to decrease more rapidly. It is worthwhile stressing that a well heat-treated sample has a steeper temperature dependence and lower resistance 25

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values. If this resistance is caused by the ordinary electron-phonon interaction as in an usual metal, this large variation of the coefficient of the linear term in the resistivity cannot be understood. Deviations from the linear temperature dependence of the resistance occurring well above the superconducting transition temperature, are also very peculiar and unusual for an ordinary superconducting transition. The precursor effect of the superconductivity on the normal state resistance in ordinary metals is, in general, very small. Short-range correlations of the superconducting Cooper pairs does not occur above the transition temperature. These facts support the suggestions that the high-T c superconductivity in these oxides compounds may be driven by a different mechanism. The observed superconducting transition temperature Tc of 94.8 K is determined by the midpoint of the resistive transition which has a width of 1.2 K as defined by the 90% and 10% points. It is remarkable that the transition temperature in GdBa2Cu307 is even slightly higher than that of YBazCu307, for which Tc is equal to 92.8 K. Since the dominating factor for high-To super-

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conductivity primarily depends upon the structural stability, this indicates that the Gd atoms fit better into this structure than the Y atoms. The resistance ratio defined by R(300K)/ R(100 K) is 2.4, a typical value for the high-To superconducting compounds.

ceptibility is clearly seen above the superconducting transition temperature Tc. The curve has been fitted by using the following modified Curie-Weiss formula:

4. Magnetic properties

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X0 = - 2.31 x 10 -8 m3/mol, C = 1.06 x Km3/mol and O = - 10.2K. The effective magnetic moment is estimated to be 8.2/za. High-field magnetization measurements on GdBa2Cu307 were performed above (4.2 K) and below (1.6K) the N6el temperature. As an example the recording of the magnetic moment at 1.6 K in a stepwise decreasing field with a maximum value of 35 T is shown in fig. 5. As may be expected in the case of Gd, there is a clear tendency towards saturation in the highest fields. The incomplete Meissner effect and the flux trapping associated with the superconducting state, which still exists at 35 T, hamper to extract from the experimental data in a simple way the precise magnetic response of the Gd ions to the applied field. The full data together with an analysis will be published elsewhere. 10 -4

AC-susceptibility in GdBa2Cu307 is shown in fig. 3. The sharp drop in the susceptibility is characteristic for a good-quality sample. In acsusceptibility, there is no sign of the anomaly at 2.23 K, which is found in the specific-heat measurements (see next section). In improperly heattreated samples which are not fully bulk superconductors, a broad transition in the ac-susceptibility has been observed. This implies that the screening due to the supercurrent in the superconducting state is completely established in properly prepared samples. DC-susceptibility of GdBa2Cu30 7 up to B = 1.3 T was measured above Tc. The susceptibility together with its inverse are plotted in fig. 4. A paramagnetic Curie-Weiss behaviour in the sus-

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5. Specific heat The specific heat in GdBa2Cu30 7 has been measured at various magnetic fields in the temperature range between 1.4 K and 35 K. The results are shown in fig. 6 and confirm previous results [18]. A large and sharp anomaly at 2.23 K in the

specific heat clearly indicates that a magnetically ordered state is realized below this temperature [18]. Since the electrical resistance measured down to 1.4K is zero and, moreover, the diamagnetic ac-susceptibility is not anomalous around this temperature, this sharp magnetic phase transition indicates the coexistence of the magnetically ordered state and the high-Tc superconducting state. This coexisting phenomenon can be compared with Chevrel phase compounds like (RE)Mo6S s (where RE = Gd, Dy, Er, Tb, for instance) [19] and Ho(Rhl_xIrx)4a 4 [20]. A large magnetic field dependence of the specific heat is obvious from the data in fig. 6. The sharp peak at 2.23 K at zero field is completely suppressed by the field at B = 5 T. Instead of this peak, there appears a broad hump in a wide temperature range as seen in fig. 6. This can be understood by a Schottky-type of specific heat contribution due to the Gd 3÷ paramagnetic moments in a magnetic field. A small peak in the inset in fig. 6 is seen at 11.5 K. By X-ray powder analysis, any impurity phase was hardly visible. This small peak has a strong magnetic field dependence and is absent at B --5 T. This anomaly is probably due to the presence of a small amount of an impurity phase which magnetically orders around this tempera-

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Fig. 6. Specificheat of GdBa2Cu307 between 1.4 K and 5 K at various magnetic fields, B = 0 T(@), 1 T(II), 2 T(@), 3 T(&), 4 T ( , ) and 5 T('k) in a plot of C / T vs. T. The inset shows C/T data as a function of T2 in fields at B = 0 T(O) and B = 5 T(it). ture. In the case of the YBa2CusO 7 compound, two impurity phases YzBaCuO5 and BaCuO 2 are known to occur. By this analogy, it may be the case that in GdBazCu307 one of the two compounds, either Gd2BaCuO 5 or BaCuO2, orders magnetically at this temperature. The magnetic field dependence of the specific heat at low temperatures is shown in detail in fig. 6 at various magnetic fields up to ST. With increasing the magnetic field, the transition temperature shifts towards lower temperatures and the transition becomes broader. At B = 1 T, the transition temperature is at 2.05 K and above B = 2 T , it is below 1.4 K, which is the lowest attainable temperature in our equipment. The entropy involved in this phase transition above 1.4 K is calculated to be l l . 9 J / m o l K in zero field and 14.4 J/tool K in B = 5 T. If the entropy below 1.4 K is taken into account, the observed entropy is comparable to the entropy loss due to the ordering of the G d 3+ magnetic moments which is 1 7 . 2 9 J l m o l K . This confirms that the phase transition is caused by magnetic ordering of the G d ~+ moments. The Debye temperature has been obtained from the coefficient of the T 3 term in the specific heat. Assuming a three-dimensional lattice a

value for ~9o of 313 K is obtained. This value is comparable with the value in ref. [18].

6. Discussion According to the detailed analyses of the crystal structure and oxygen content of YBa2Cu3OT, a degradation of the linear chain-like structure along the b direction easily destroys superconductivity. This sensitive character is a common feature in all the rare-earth substituted compounds [17]. The remarkable feature in this GdBa2Cu307 system is the coexistence of the magnetic and the high-T¢ superconducting states. Essentially, no reduction of the superconducting transition temperature of 94.8 K by the Gd 3+ magnetic moment is noted. In some samples, even higher values than that of YBa2Cu30 7 are observed. This coexistence suggests that the magnetic order is antiferromagnetic. A tentative explanation of the insensitivity of the superconducting temperature to magnetism can be found as follows. As seen in the crystal structure of this compound, the Gd 3÷ ions are well isolated from the CuO1 layers which are situated between two BaO

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layers. Only this C u O 1 layer is thought to be conductive, and the charge carrier in this layer is thought to be responsible for the high-T c superconductivity. It is reasonable to expect that in this situation there is only weak interaction between the Gd 3÷ moment and the superconducting layer, resulting essentially in no interference of the magnetism with the superconductivity. On the other hand, the Gd 3+ ions are surrounded by eight oxygen atoms. These distances are also large (2.4 A), resulting in a weak hybridization betweenoxygen p orbits and Gd f orbits. As a consequence, a relatively weak exchange interaction can be expected, as indicated by the relatively small Weiss temperature. The Cu ions occupy two different sites in this structure. The corresponding local atomic arrangements of the ligand oxygen are either of pyramidal or of a distorted square-plane type. If a Cu 2÷ ion is realized in both arrangements, the 3d 9 state of Cu z÷ should be magnetic. In effect, the dc magnetic susceptibility of YnaaCu307 has been found to show paramagnetic behaviour above Tc. This paramagnetism, however, sensitively depends upon heat treatment, suggesting that the ordering of the oxygen vacancies in the ab plane plays a role in controlling the magnetism in this crystal. This strange relation between magnetism and high-Tc superconductivity has to be further studied.

Acknowledgements This work is part of the research program of the "Stichting voor Fundamenteel Onderzoek der Materie (F.O.M.)", which is financially supported by the "Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek (ZWO)". The authors wish to acknowledge Drs. H. Bakker, and P.F. de Ch~tel at the University of Amsterdam.

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