Magnetic and superconducting behaviour of the oxides, Pr1−xGdxBa2Cu3Oy

PhysicaC 173 (1991) 331-336 North-Holland

Magnetic and superconducting behaviour of the oxides, Pr, _,Gd,Ba,Cu,O, I. Das, E.V. Sampathkumaran

and R. Vijayaraghavan

Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400 005, India

Y. Nakazawa

and M. Ishikawa

The Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan

Received 28 September 1990 Revised manuscript received 5 December 1990

The results of our investigation on the superconducting behaviour of the oxides Pr, _XGdXBaZCu30,by resistivity and AC susceptibility measurements suggest that the superconducting transition temperature (T,) increases monotonically with x for x> 0.6. The concentration of Pr required to depress T, in this Cd series appears to be smaller than in the corresponding Y series, possibly due to volume dependent hole trapping effects. Specific heat measurements were performed in the temperature range 0.4-20 K in order to understand the magnetism of the rare-earth ions. Interestingly, Pr and Cd ions undergo magnetic ordering independently though both are presumably present in the same magnetic sub-lattice and the observed ordering temperatures rule out any direct intersite Pr-Gd magnetic interaction in this series of compounds.

One of the interesting observations made during the course of investigations of high temperature supercond;cting oxides is the fact that the compound PrBazCu30, (hereafter referred to as Pr123 ) is not even a metal whereas for many other REBa2Cu30, (RE = Rare earths) compounds superconductivity is observed below about 90 K [ l-5 1. This anomalous behaviour of Pr123, was initially attributed to possible tetravalency of Pr [ 2,5 1. This idea was later questioned by the lattice constants, solid solution investigations [ 6 ] and spectroscopic data [ 7 1. The hybridization of Pr 4f orbitals with Cu 3d/02P valence band was proposed to be the origin of the suppression of superconductivity in PrBa2Cu307 [ 8,9]. While we believe [ 6-81 that the valency of Pr, a measure of the 4f occupation number, is close to 3, there appears to be no consensus on the value of valency of Pr in Pr 123 (see also the refs. [ 10-131). It is also important to note that Pr in PrBa2Cu30, orders antiferromagnetically at about ( TN= 17 K) [ 141 which is two orders of magnitude larger than the value expected, if one scales TN for GdBa2Cu307( T,=2.3 K, ref. [ 151) assuming dipolar

interactions or Rudermann-Kittel-Kasuya-Yosida (RKKY) exchange. In order to understand the magnetism and nonsuperconductivity of PrBa2Cu30,, based on solution studies several solid Pr, _XYXBa2Cu30, were reported [3-l 91. The results show that the superconducting transition temperature ( T,) monotonically increases for x> 0.5 and the TN of Pr sublattice decreases with increasing x reaching almost zero for x= 0.4. Available data [ 141 in the literature indicate the dominance of superexchange mechanism involving Cu-0 sheets in determining Pr magnetism. It is worth noting that even in Gd123 for which TN is low, the relative roles of superexchange mechanism and dipolar interactions are being debated in the current literature [ 20-241. In this respect, we thought it worthwhile to investigate the series Prl_,GdXBazCu30, to see whether Pr and Gd moments influence the magnetism of each other in addition to studying the insulator-superconductor transition. The results presented here indicate that Pr is more effective in depressing superconductivity in Gd123 than in Y 123. The specific heat (C) data clearly show that, as far as magnetic

0921-4534/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

332

I. Das et al. /Magnetic and superconducting behaviour of the oxides, Pr,_,GdXBalCuJO,

ordering temperatures of Pr and Gd are concerned, Pr and Gd ions do not influence each other and thus any mechanism incorporating direct intersite 4f-4f interaction is not favoured. The samples, Pr, _,Gd,Ba2Cu30, (x= 0, 0.1, 0.2, 0.3,0.4,0.5,0.6,0.7,0.8,0.9and 1.0) wereprepared following the procedure described in ref. [ 191, by a conventional solid-state reaction technique. Stoichiometric amounts of high purity compounds, BaCO,, Y,03, Pr601r and CuO were thoroughly mixed and pelletised. The samples were calcined several times. For x30.5, the calcination temperature was 950°C and for ~~0.5, it was 900°C. It was noted [ 181 that the lower calcination temperature for Pr rich end oxides avoid the formation of other undesirable phases. Final heat treatment was given in a flowing oxygen atmosphere at the respective calcination temperatures and the pellets were allowed to stay at 500°C for 12 h while cooling. X-ray diffraction patterns confirm the single phase nature of all these specimens; these patterns show orthorhombic distortion for ~-0.4, though for Pr rich end the distortion is not clearly resolvable in the X-ray patterns. Resistivity (p) and AC susceptibility (x) measurements were performed in the temperature interval 12-300 K employing a closed cycle refrigerator (Sumitomo Industries Model SCR 204T). Specific heat (C) measurements were performed on samples with selected concentrations (x= 0.1, 0.2, 0.4 and 0.7) by an adiabatic heat pulse method. No attempt has been made to determine the oxygen content. Considering that the superconducting transition width is rather small and that T, is close to 90 K (see below) for our Gd 123 specimen, we believe that the oxygen content (y) is close to 7 and hence we replace y by 7 in the chemical formula throughout this paper. The results of resistivity (p) and AC susceptibility (x) measurements are shown in figs. 1 and 2. The value of p monotonically increases marginally with x at 300 K for x= 1.O to 0.6 and the samples become superconducting at lower temperatures. There is a drastic increase of p at 300 K for sample with x< 0.5 which are found to be non-metallic (not shown in figure). This observation is consistent with the idea that the high-T= superconductivity occurs near insulator to metal transitions. The temperature at which p goes to zero for x> 0.5 agrees with that at which x starts becoming negative. The width of the transition

marginally increases as the Gd is replaced by Pr. T, values are plotted as a function of x for x> 0.5 in the inset of fig. 2 (denoted by circles ). T, monotonically increase with x. For the sake of comparison, T, versus x for the Pr, _,Y,Ba2Cu30, series, as reported in refs. [ 17 ] and [ 19 1, are also shown by squares in the inset of fig. 2. Clearly, Pr is more effective in depressing T, in a Gd123 matrix in a Y123 matrix. At the stage of finalizing this manuscript, another publication by Iwasaki et al. [ 25 ] reporting similar results appeared in print. It is known that Pr in the Eu123 matrix behaves in a fashion similar to that in Cdl 23 [ 25 1. It is therefore obvious that the critical concentration of Pr required to depress superconductivity is related to the ionic size of the rare-earth ion of the matrix. Our recent 0 1S edge studies reveal that the holes in the 0 2p band are present for all concentrations [ 261. Therefore, the mobility of the carriers is being influenced by the substitution by Pr for Gd and this could be a sensitive function of the unit cell dimensions. The measurement of Cu-K-edge shift may also throw some light on this point. The results of specific heat (C) measurements below 20 K on selected compositions are shown in fig. 3 in order to investigate the question of Pr/Gd magnetism in these oxides. Since, for x= 0, several groups have reported C versus T data, we have not reproduced it here. The main observation in the specific heat data is that there are two peaks for x=0.1, 0.2 and 0.4. From the knowledge of TN of Gd123 and Pr 123, reported in the literature, the peak at the high temperature side is attributed to the magnetic ordering of Pr ions and the one at low temperature to Gd ions. Thus TN of Pr decreases with x(x=0, 17 K, 0.1, 12 K, 0.2, 10 K and 0.4, - 8 K). This variation of TN with x is very similar to that of Pr,_,Y,Ba2Cu307 [ 17,191. Also, TN of Gd ions as obtained from the peak position of C versus T data also increases monotonously with Gd concentration (x=0.7, 1.6 K; 0.4, K, 0.2, K, and 0.1, 0.6 It is quite instructive note that Gd exhibits magnetic ordering in the dilute of our , thereby proving of the magnetic of the Gd sub lattice Gd123; this observation of super-exchange interactions in Cdl 23 and might is that x depen-

I. Das et al. /Magnetic and superconducting behaviour of the oxides. Pr,_,GdXBa2CuJOy

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dence of TN of Gd ions in a Pr matrix is almost the same as that known in Y and Eu matrices [ 27-29 1, which are nonmagnetic. The observed variations of TN versus x of both types of rare-earth ions belonging to the same sublattice show that Pr and Gd ions do not influence the magnetism of each other. In other words, direct intersite 4f-4f interaction appears to be weak, as otherwise one would have naively expected that the (large moment-containing) Gd ion would have somehow influenced Pr4f ordering and also larger spatial extension of Pr 4f orbitals would have interacted with neighbouring rareearths. The present experimental findings - persistence of magnetic order for dilute concentrations for Gd, and faster depression of TN of Pr sublattice with dilution - suggest different mechanisms for Pr and Gd magnetic ordering. The results are consistent with a short-range (superexchange) mechanism in Pr123 and of long range (dipolar) interactions in Gd123. An argument in favour of this conclusion is that TN of Gd 123 is not significantly dependent on the oxygen content [ 29 1, whereas for Pr 123, a small variation in TN [ 301 is known. We cannot derive any meaningful information regarding the x dependence of the linear coefficient of

(0.4-20

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specific heat (y) due to the dominance of the specific heat by magnetic ordering anomalies in the temperature range of this investigation. Three points are noteworthy: (i) there is a shoulder around 1 K for x=0.7 in the specific heat data, as known for x= 1.0; this feature is clearly discernable in a C/T versus T2 plot (see fig. 4). It was earlier discussed in terms of single magnetic ordering [ 3 1 ] of Gd ions. (ii) The shape of the curve of the heat capacity versus temperature plot above TN is indicative of the existence of short range order effects. (iii) The magnetic contribution (C,,,) to the specific heat, obtained after subtracting the nonmagnetic part (assuming reasonable values for y( - 10 mJ/mol K2) and 6, ( - 330 K), refs. [ 161 and [ 321) shows (fig. 5 ) a rise above 15 K, the magnitude of which roughly scales with the concentration of Pr. It is possible that the choice of values for y and 8p to derive C, may not be correct and hence we do not attach much significance to the absolute values of C,. However, the trend in the x dependence of C,,, above 15 K is practically the same for various values of y and BDknown for Y 123 in the literature. It is worth noting that Pr in Y 123 matrix also shows a similar behaviour, but the magnitude of the concentration dependence is

I. Das et al. /Magnetic

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336

I. Das et al. /Magnetic

and superconducting behaviour of the oxides, Prl_,GdXBa~Cu~Oy

tion in this Gd series; the rise in C,,, may signal a peak due to schottky specific heat of Pr3+ ion. To conclude, Pr doping is more effective in depressing superconductivity in a GdBa2Cu30, matrix than in a Y-based matrix, possibly due to the unit cell volume dependent carrier mobility effects. Pr and Gd ions order independently in Pr, _,Gd,Ba2Cu30,, thereby indicating negligible direct 4f-4f interaction effects.

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