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Effect of annealing and granularity on the physical properties of GdSr2RuCu2O8
Physica C 443 (2006) 69–76 www.elsevier.com/locate/physc
Effect of annealing and granularity on the physical properties of GdSr2RuCu2O8 T. Geetha Kumary *, J. Janaki, V.S. Sastry, Y. Hariharan, M.C. Valsakumar Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India Received 20 November 2005; received in revised form 3 February 2006; accepted 2 May 2006 Available online 21 June 2006
Abstract The variation of crystallite size (coherent domains as measured from X-ray analysis) and the physical properties of the ferromagnetic superconductor GdSr2RuCu2O8 on increasing the oxygen annealing time has been studied by X-ray analysis, SEM, resistivity and ac susceptibility measurements. Prolonged annealing in oxygen atmosphere at 1060 C leads to an increase in grain and crystallite sizes and a decrease in the superconducting transition width DTc. The derivative of resistivity with respect to temperature, dq/dT, shows two peaks at T1 and T2 (with T1 > T2), which can be attributed to the onset of intra-granular and inter-granular superconductivity, respectively. T2 increases drastically on increasing the annealing time, whereas only a marginal increase is observed in T1. On the contrary, there is no apparent change in the thermodynamic transition temperature Tth. The temperature dependence of the normal state resistivity, q(T), showed a gradual change from semiconducting-like to metallic behavior on increasing the annealing time. The variation in resistivity and the reduction in superconducting transition width can be attributed to the increase in grain size and the better intergrain coupling achieved by increasing the annealing time. A large reduction in the diamagnetic signal on powdering and a broadening of the susceptibility transition on increasing the measuring field are observed in this compound. This can be attributed to the large penetration depth compared to the particle size and the small inter-granular lower critical field Hc1J, respectively. 2006 Elsevier B.V. All rights reserved. PACS: 74.72.h; 74.25.Ha; 74.25.Fy Keywords: Ruthenate cuprates; Granularity; Transport properties
1. Introduction The observation of long range ferromagnetism (with magnetic ordering temperature, Tm 132 K) and superconductivity (Tc 46 K) well within the ferromagnetic state with relatively high value for the superconducting transition temperature, in the hybrid rutheno-cuprate GdSr2RuCu2O8 has initiated intense activities to study the nature of superconductivity, magnetism and their interrelation in the family of compounds RSr2RuCu2O8 and R1+xCe1xSr2RuC u2O10 (R = Sm, Eu and Gd) [1,2].
*
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0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.05.057
GdSr2RuCu2O8 (Gd1212) forms in a triple perovskite structure very similar to that of the well known high temperature superconductor YBa2Cu3O7 (YBCO), with Y and Ba being completely replaced by Gd and Sr, respectively, and the CuO chain replaced by RuO2 planes. Many of the physical properties of this compound are similar to those of the underdoped cuprates. It is also observed that as in the case of YBCO, hole doping to an optimal value can lead to an increase in Tc in Gd1212 [3,4]. It is thus reasonable to infer that superconductivity in Gd1212 arises in the CuO2 planes and ferromagnetism occurs in the RuO2 planes. However, unlike in the case of YBCO, the superconducting properties of Gd1212 are extremely sensitive to the details of sample preparation. A range of values (15–50 K) has been reported in the literature [2,5–11] for Tc and there have also been reports on non-occurrence of
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superconductivity in this compound [12,13]. However, appearance of bulk superconductivity has been confirmed from specific heat measurements [14,15] subsequently. All the above mentioned works have been on sintered samples. There is a solitary report of onset of superconducting transition at 60 K for single crystals of Gd1212 from Magnetization measurements [16]. Zero Field lSR experiments [5] provided the first clear evidence for a spatially homogeneous magnetically ordered phase in GdSr2RuCu2O8 with an ordered moment of 1 Bohr magneton per unit cell. This would suggest presence of an internal magnetic (dipolar) field of the order of 700 Oe (at the site of the muon) with concomitant consequences on the superconducting order. However, ZF– lSR experiments cannot provide direct information about the type of magnetic order and its direction [5]. Subsequent neutron diffraction studies [17,18] showed the magnetic order (that develops at 132 K) to be G-type antiferromagnetism involving Ru moments which are oriented along the c-axis of the unit cell. This study provided an upper bound of 0.1 lB (per unit cell) for the ferromagnetic component of the ordered moment. This is consistent with the observation of a low temperature remnant magnetization Mr of 0.05–0.1 lB (and hence a corresponding internal magnetic field of the order of 35–70 Oe) from the dc magnetization measurements [19,20]. While these studies clearly establish that GdSr2RuCu2O8 is a weak ferromagnet, they also show that the elucidation of the details of the magnetic order in this compound is still an open problem. This information is a necessary input for theoretical studies aimed at understanding how the superconducting and magnetic orders accommodate each other in this compound. Under the assumption of ferromagnetic order with an ordered moment of 1 lB, Pickett et al. [21] suggested that the superconducting order may be of the Fulde–Ferrell– Larkin–Ovchinnikov type. However, as discussed earlier, the internal field is much smaller, and the spontaneous vortex phase scenario may be more pertinent. Clues to understand the reasons for the contradictory results on superconductivity in this compound can be obtained by having a closer look at the structure and the microstructure of the sintered samples. It is well known that the grains that adhere together to form the sintered samples are themselves made of several crystallites (X-ray coherent regions). The XRD studies reported in literature show that Gd1212 adopts a tetragonal structure with ˚ and c 11.5735 A ˚ , and space group P4/ a 3.8396 A mmm [22,23]. However, electron diffraction and neutron diffraction studies reveal some nuances in the structure of this compound. High resolution transmission electron microscopy [24] of Gd1212 shows formation of several small domains with their c-axis oriented along the three crystallographic axes. This is a consequence of the fact that the lattice parameters a and b are almost equal to c/3 in this compound. Electron diffractionp[24] ffiffiffi and pffiffiffi powder neutron diffraction studies [17] show a 2a 2a c superstructure resulting from coherent rotations of the RuO6 octahe-
˚ within the above dra within subdomains of size 50–200A mentioned domains. The mismatch in the lengths of the in-plane Ru–O and Cu–O bonds results in rotations of 13 the RuO6 octahedra pffiffiffi by p ffiffiffi about the c-axis and this in turn causes the 2a 2a c superstructure. Thus the Gd1212 samples prepared by the solid state reaction route can show granularity at two length scales – the grains of size of the order of microns as measured by SEM and the intra-grain domains, where the RuO6 octahedra are coherently rotated around the c-axis, being separated by sharp antiphase boundaries [24]. This implies that as the temperature is decreased, superconductivity should first develop in the domains at some temperature Tth, phase coherence should emerge between the domains within a grain (i.e. intra-grain superconducting transition) at a lower temperature T1, and then at a still lower temperature T2, the inter-granular transition should occur. Thus the superconducting properties of GdSr2RuCu2O8 prepared via the solid state reaction route is expected to be intimately related to the microstructure. Hence the heating schedule must be playing an important role in deciding the superconducting properties of this compound, and this aspect has received some attention in past [14,25]. In this paper, the results of the study carried out on the grain/crystallite sizes and on the superconducting and normal state properties of Gd1212 on annealing it at 1060 C in oxygen atmosphere for different time durations [25] and the effects of granularity are presented. A large reduction in the diamagnetic signal on powdering and a broadening of the susceptibility transition on increasing the measuring field are observed in this compound. These results are discussed in terms of a simple model that incorporates a penetration depth, k, large compared to the particle size, and a small inter-granular lower critical field Hc1J. 2. Sample preparation and characterization Polycrystalline samples of GdSr2RuCu2O8 were prepared by the solid state reaction technique. Stoichiometric quantities of Gd2O3, SrCO3, CuO and Ru powder, all better than 99.99% pure, were mixed and ground well. This was given an initial heat treatment at 600 C for 48 h, and then at 950 C for 48 h in air. The intermediate product was mixed and ground well, and was then heated at 1000 C in air for 3 days. The reacted powder was again ground and mixed and was pelletized. The pellets were again given a heat treatment of 1000 C for 3 days. Two pellets each were taken from these as-prepared samples and were ground and re-pelletized and oxygen annealed at 1060 C for different time durations of 15 h, 3 days and one week and were slow cooled. The structure and microstructure of all the samples were characterized by XRD and SEM measurements. The XRD measurements were carried out using Cu Ka radiation using the STOE X-ray diffractometer. Fig. 1 depicts the XRD pattern (after stripping the K a2 contribution) for the as-prepared sample and the ones that are oxygen
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Fig. 1. XRD patterns for the as-prepared and annealed samples of Gd1212. There is no pronounced difference between the XRD patterns.
annealed at 1060 C for different time durations. As can be seen from the figure, there are no conspicuous differences between the XRD patterns of the as-prepared and annealed samples. On the other hand, there are significant differences in the superconducting properties. This and similar results already available in the literature suggest a vital link between superconducting properties and microstructure in this compound, and the evolution of the latter with annealing. Scanning electron micrographs of the samples were taken using XL30 SEM, which are shown in Fig. 2. The SEM pictures show that, roughly, the grain size
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increases from fraction of a micron for the as-prepared to a few microns for the well annealed samples. The crystallite size distribution and its variation with annealing time have been studied using WINFIT package of Stefan Krumm [26]. Applicability of this package for crystallite size determination was ascertained by analyzing a standard sample of nanophasic CeO2. The computed sizes matched well with that obtained by other methods, and the distribution fitted well to a log-normal distribution, as expected. For the samples of Gd1212, the [1 0 1] reflection at 2H = 24.4 was used for profile analysis as most of the other peaks were overlapping. The [1 1 1] reflection of NBS Si powder with large and strain free crystallites was taken as the standard for deconvolution of the instrumental contribution to the total line profile. The average crystallite sizes were determined using the integral breadth method, the method of Dehlez et al. [27] and the Fourier method. Fig. 3 shows the frequency distribution of the particle size, obtained using the Fourier method, for the asprepared and one week annealed samples. Tendency for the particles to coarsen is evident from the figure- the aver˚ for the as-prepared age particle size increases from 300 A ˚ for the one week annealed sample. It sample to 390 A should be mentioned that all these methods yield different estimates for the average particle size in view of the differences in the averaging methods that they employ. For example, the Scherrer method gives volume averaged crystallite size, whereas the Fourier method gives area weighted average crystallite size. However, the results of the analysis
Fig. 2. SEM photographs of Gd1212 (a) as-prepared and the samples oxygen annealed for (b) 15 h (c) 3 days and (d) one week (scale – 5 lm). An increase in grain size can be noticed with increase in annealing time.
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C C C
Fig. 3. Frequency distribution of particle size for the as-prepared and one week annealed samples. The coarsening of the crystallites as a functions of annealing can be clearly seen.
using each of the above mentioned methods indicate that the average crystallite size increases upon annealing. The analysis based on integral method shows an increase in ˚ for the as-prepared the average particle size from 500 A ˚ for the sample annealed for one week. sample to 1000 A SEM and X-ray studies show that the samples are highly granular in nature and the X-ray coherent domain size is much smaller than the grain size seen by SEM. An increase in grain size as well as crystallite size is observed on prolonged annealing. 3. Resistivity measurements The resistivity of all the samples were measured in the temperature range 4.2–300 K, using an excitation current of 1 mA, on disk shaped samples of approximately 10 mm diameter and 1.5 mm thickness. Fig. 4 depicts the variation of resistivity as a function of temperature for the as-prepared as well as the annealed samples. The asprepared sample shows a semiconducting like temperature dependence for the normal state resistivity q(T). A reduction in resistivity corresponding to superconducting transition is seen, but, the resistivity did not fall to zero even at 4.2 K. The semiconducting like behaviour in q(T) decreases for the 15 h annealed sample and turns metallic when the annealing time is increased to 3 days. The resistivity also decreased drastically on increasing the annealing time as
Fig. 4. The variation of resistivity as a function of temperature for the asprepared and annealed samples of Gd1212.
can be seen from Fig. 4. A small increase in T onset and a c large increase in Tc(0), and hence a reduction in the superconducting transition width, DTc, is observed (see Table 1) on prolonged annealing. The temperature corresponding to the peak in the resistivity is taken as T onset and the temperc ature at which the resistance becomes zero, as Tc(0). Annealing for one week did not lead to further improvement in the superconducting properties. In fact, a slight deterioration in properties is seen on increasing the annealing time to one week. The reduced superconducting transition temperature for the one week annealed sample must be due to a slight decomposition or due to a small oxygen deficiency [28]. A pertinent result in this context is the observation of decomposition of GdSr2RuCu2O8 at 1060 C at 0.85 bar of oxygen pressure due to sublimation of RuO2 in thermogravimetry measurements [29]. It is important to note that a semiconducting like upturn in resistivity is observed below 100 K even in the well annealed samples. This observation is consistent with the earlier study which showed that GdSr2RuCu2O8 is a typical underdoped superconducting cuprate in which the pseudogap dominates normal state transport, thermodynamic and substitutional properties [2]. Furthermore, the doping state does not change with annealing [2]. It then follows that the larger increase in Tc(0) compared to T onset and the decrease c in semiconducting like behavior on increasing the annealing time must be due to the better inter-granular coupling
Table 1 0 00 The values, in K, corresponding to Tth, T onset , Tc(0), DTc, T1 and T2 estimated from the resistivity transition and T vc , T v and Tm, obtained from the ac c susceptibility measurements, for the as-prepared and annealed samples of GdSr2RuCu2O8 00
Sample
Tth ± 0.5
T onset 0:2 c
Tc(0) ± 0.2
DTc ± 0.4
T1 ± 0.5
T2 ± 0.2
T vc 0:2
T v 0:2
Tm ± 0.2
As-prepared 1060 C, 15 h. 1060 C, 3 days 1060 C, 1 week
47.3 47.5 47.5 47.8
35.8 40 41.5 40
<4.2 8.4 19.12 17.28
>31.5 31.6 22.38 22.72
25.07 27.06 30.18 28.45
<4.2 13.8 23.1 21.47
<4.2 7 17.8 15.12
<4.2 4.42 14.23 11.56
137.6 136.2 134.8 135.8
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achieved by prolonged annealing. The solid state reaction method of sample preparation in general leads to formation of stoichiometric grains separated by off-stoichiometric material. The conspicuous semiconducting like behaviour in the resistivity (at low temperatures) suggests that the material at the grain boundaries must be insulating. The properties of such granular materials can be modelled in terms of array of SIS type Josephson junctions [30]. In granular superconductors [31], one expects the intragranular superconductivity to set in at a higher temperature, T1 and the inter-granular superconductivity, which requires phase coherence between the grains, to occur at a lower temperature, T2. As mentioned in the introduction, the additional complexity in the present case is the fact that the individual grains themselves behave as Josephson junction arrays due to the presence of coherent domains separated by antiphase boundaries which act as weak links. In order to obtain a better understanding of what annealing does to the inter-granular and intra-granular superconducting properties, the derivative of resistivity with respect to temperature (dq/dT) is calculated (using conventional numerical methods [32]) in the temperature range 4.2– 60 K. In Fig. 5, the variation of dq/dT, normalized with respect to the maximum value, is plotted as a function of temperature. Two peaks are observed in the derivative curves. The peak at the higher temperature, T1, corresponds to the intra-granular and that at the lower temperature, T2, corresponds to inter-granular superconducting transitions. It can be seen from the Fig. 5, that for the as-prepared sample inter-granular coupling does not occur above 4.2 K. The grain size, as seen from SEM, of the asprepared and the sample annealed for 15 h are very small (of the order of fraction of a micron) and hence the semiconducting like behavior in these samples must be due to the small grain size and the weak coupling between the
Fig. 5. The variation of dq/dT (normalized with respect to the maximum value) as a function of temperature for the as-prepared and annealed samples of GdSr2RuCu2O8.
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grains. The grain size and the coupling between the grains increase on increasing the annealing time, which leads to a metallic behaviour in resistivity in the well annealed samples. T2 increases drastically upon increasing the annealing time whereas only marginal increase is observed in T1 (Table 1). Following Garcia et al. [33], if the temperature at which the derivative curves deviate from the high temperature base line is defined as the thermodynamic transition temperature Tth (the temperature at which the individual domains become superconducting), it can be seen that all the samples show approximately the same Tth 48 K. Thus annealing leads to better grain boundary properties and inter-granular coupling but the intra-grain superconducting properties change only marginally on increasing the annealing time. It should be mentioned that the values of T1 and T2 depend on the magnitude of the external magnetic field and measuring current density [33]. The results reported in the present paper correspond to zero external field and a fixed current density 20 mA/cm2. The sample annealed at 1060 C for 3 days have comparable normal state resistivity properties with that of Lorentz et al. [34], where they have given extra intermediate heat treatments before the final annealing. However, the superconducting transition is slightly broader in the present case. The intermediate heat treatment may lead to a decrease in the transition width. It is reported by Lorentz et al. [34], that the introduction of additional heat treatment steps raises T2 but not T1. High pressure oxygen annealing or texturing by directional solidification should be tried to obtain a higher value for T1.
4. Ac susceptibility measurements Ac susceptibility measurements were carried out from 4.2–300 K using a home made dipstick. The real (v 0 ) and imaginary (v00 ) parts of ac susceptibility measured on the bulk sample of the as-prepared and annealed Gd1212 are shown in Figs. 6 and 7, respectively. The temperatures cor0 responding to the onset of diamagnetic signal (T vc ) and the v00 00 peak ðT Þ in v shift to higher values upon increasing the annealing time (Table 1). No diamagnetic signal is observed in the as-prepared sample. The transition associated with the onset of intra-granular or inter-granular superconductivity is not observed in the magnetic susceptibility measurements. Diamagnetic signal is seen only when zero resistivity is reached. This must be due to the small grain size compared to the large penetration depth [35] and the weak inter-granular coupling between the grains in this compound. The intra-granular and inter-granular penetration depths of Gd1212 are reported to be 2.5– 6 lm and 30 lm, respectively, from ac susceptibility measurements [35], which are very large compared with the penetration depth of most of the superconductors. In order to study the effect of granularity on the superconducting properties, the ac susceptibility measurements were carried
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Fig. 6. The ac v as a function of temperature for the as-prepared as well as the annealed samples of GdSr2RuCu2O8. The temperature is plotted on log scale so that the superconducting transitions are clearly seen. The region where the magnetic transition occurs is expanded and is shown in the inset.
Fig. 7. The imaginary part of ac v, normalized with respect to the peak value, vs temperature is plotted for the as-prepared and annealed GdSr2RuCu2O8.
out on samples with different particle size and also in different measuring fields. A drastic reduction in diamagnetic signal is seen [36] on powdering Gd1212. Fig. 8 represents the ac susceptibility data of the bulk (A) (pieces of 1 · 2 · 4 mm), coarse powder B (radius fraction of an mm) and powder samples (C and D) of Gd1212 annealed for one week. The average particle size of the samples C and D are estimated to be 55 and 8 lm respectively, using Malvern particle size analyzer. The diamagnetic signal corresponding to superconductivity decreases drastically on powdering the sample and an upturn is seen for particles with an average diame-
Fig. 8. The ac v vs temperature for the bulk and powder samples of GdSr2RuCu2O8 (annealed for one week).
ter of 8 lm. A reduction in diamagnetic signal can arise if the particle size is small compared to the penetration depth. The typical grain size of the oxygen annealed samples are of the order of a few microns, comparable to the intragranular penetration depth. Thus the diamagnetic susceptibility of the isolated grains is expected to be small. When these grains are in contact with each other the effective particle size is larger than the penetration depth and hence the diamagnetic signal is large in magnitude. Upon powdering, the agglomerates of grains break into individual particles causing incomplete shielding and hence a smaller diamagnetic signal. In addition to this the Gd moments in the flux penetrated region will also contribute to magnetization causing an upturn in the susceptibility data at low temperature. Calculation of susceptibility assuming spherical shape for the particles gives results in qualitative agreement with the experimental observation. The susceptibility of a spherical superconducting particle, containing paramagnetic moments, is given by [37], vðT Þ ¼ vpara ðT Þð1 þ vsc ðT ÞÞ þ vsc ðT Þ 2 kðT Þ kðT Þ vsc ðT Þ ¼ 1 þ 3 CothðR=kðT ÞÞ 3 R R
ð1Þ ð2Þ
where, k(T) is the penetration depth and R is the radius of the particle. The paramagnetic susceptibility vpara(T) due to Gd ions varies as vGd/(T + hN). The Curie constant vGd is calculated to be 0.9786 SI units and hN 4.4 K is obtained by fitting the susceptibility data. The v calculated using Eq. (1) is shown in Fig. 9 for various values of R/k(0). It is clear that the superconducting signal can be completely masked by the paramagnetic contribution when the particle size is small. Thus absence of diamagnetic signal and also a rise in susceptibility signal at low temperatures as seen in Fig. 8 can arise when the particle size is of the order of k. The calculations were carried out assuming a fixed value for Tc. But in the actual case there is a distribution of Tc,
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ac
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Fig. 9. The curves from top to bottom correspond to v calculated using Eq. (1) and are shown as a function of temperature for R/k(0) = 0.5, 1, 2, 5, 10, 20 and 50,q respectively. ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiThe temperature dependence of k(T) was
Fig. 10. The ac v measured as a function of temperature under different measuring fields for the one week annealed GdSr2RuCu2O8 is plotted. The diamagnetic signal is maximum when the measuring field is the lowest.
taken to be kð0Þ= 1 ðT =T c Þ4 in the calculation.
as is evident from the broad superconducting transition (Fig. 8). Hence, for a quantitative description of the observed data, the distribution in Tc, the corresponding variation in k and the particle size distribution are to be incorporated in the calculation. However, the present analysis gives a qualitative understanding of the observed decrease in Meissner signal on powdering the sample. The observed reduction in diamagnetic signal on powdering the sample cannot be explained even if we consider the phase separation to superconducting Ru1xCuxSr2GdCu2O8x and ferromagnetic RuSr2GdCu2O8 reported recently [38]. In Fig. 10, the ac v measured for different measuring fields (.35, .25, .15 and .075 Oe) for the sample annealed for one weak is plotted. The susceptibility transitions become broader on increasing the measuring field. Such a large variation in the diamagnetic response with a very small change in the measuring field in a material with internal magnetic field of the order of 35–70 Oe is indeed puzzling. This and some other anomalies like absence of diamagnetic response while signature of superconductivity is manifest in resistivity can, however, be understood as follows. As mentioned earlier, GdSr2RuCu2O8 prepared via the solid state reaction route is made up of grains each of which containing coherent domains separated by antiphase boundaries. Below the magnetic transition temperature, each of these grains is ferromagnetic with an internal magnetic field of the order of 35–70 Oe. Since the intra-granular penetration depth of GdSr2RuCu2O8 is 5–10 times larger than that of well annealed YBCO [35], the lower critical field Hc1 is expected to be 25–100 times smaller than that of YBCO. In view of its sensitivity to disorder, we expect Hc1 to have a distribution with its typical value to be of the order of a few Oe. As a matter of fact, DC magnetization studies [10] of a Gd1212 sample with Tc 40 K
shows an Hc1 < 1 Oe at 5 K. It is well known that Hc1J, the field at which first fluxon enters the Josephson junction array, is usually much smaller than Hc1 [30]. We expect Hc1J also to have a distribution with its typical value to be of the order of a fraction of an Oe. Hence a broadening in ac susceptibility signal for such small variations in the measuring field and absence of diamagnetic signal in much higher fields can be expected due to the highly granular nature of GdSr2RuCu2O8. The magnetic transition temperature, Tm, decreased monotonically when the annealing time is increased to 3 days (Table 1). This result can be understood by considering the effect of annealing on the Ru electrons, as the ferromagnetism in Gd1212 is due to the Ru moments. It appears that this may be due to dilution of the magnetic sublattice by the evaporation of a small amount of Ru [29]. But the one week annealed sample showed a small increase in Tm compared to the sample annealed for 3 days. A depletion in oxygen in GdSr2RuCu2O8 is reported to cause an increase in Tm, due to change in the valence state of Ruthenium, and a decrease in Tc [28]. It is to be seen whether there is an oxygen deficiency in the one week annealed sample which may cause an increase in Tm with a concomitant reduction in Tc. 5. Conclusions The granular nature of the ferromagnetic superconductor GdSr2RuCu2O8 is clearly brought out in the present study. Superconductivity develops first in the coherent domains within the grains at Tth, followed by emergence of phase coherence among the domains within a grain leading to intra-granular superconductivity at a temperature T1 < Tth. Inter-granular phase coherence occurs at a still lower temperature T2. Oxygen annealing of Gd1212 at 1060 C lead to an increase in the grain as well as crystallite
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sizes. Upon increasing the annealing time, the temperature dependence of the normal state resistance became metallic in nature from the semiconducting like behaviour of the asprepared sample. Study of the variation of dq(T)/dT provides a way of understanding the variation Tth, T1 and T2. A drastic increase is observed for the inter-granular transition temperature, T2, whereas the intra-granular transition temperature, T1 increased only marginally. This shows that the grain boundary properties improve on annealing leading to better inter-granular coupling. It is argued that the anomalous magnetic response of GdSr2RuCu2O8 is intimately related to the size of the crystallites and the large penetration depth. A large reduction in the Meissner signal is observed on powdering these compounds which can be attributed to the large penetration depth which is comparable in magnitude to the grain size. Plausibility of such a large penetration depth in GdSr2RuCu2O8, in comparison with those of the conventional superconductors, has been a topic of debate. It may be noted that there is, as yet, no microscopic theory that describes the precise manner in which superconductivity is accommodated in an otherwise ferromagnetic material. Since ferromagnetism is the dominant phenomenon in GdSr2RuCu2O8, it is plausible that the superconducting order parameter allows a gradual magnetic flux penetration, spread over a larger length scale, so that the magnetic energy is not significantly altered. Broadening in the susceptibility transition when the measuring field is increased by a fraction of an Oersted confirms that the Hc1J in these compounds should be very low. The decrease in Tm on annealing can be due to sublimation of a small amount of Ru. More experiments are required to pinpoint the connection between the microstructure and the superconducting properties in Gd1212. Acknowledgements The authors acknowledge Mrs. M. Radhika and Dr. P. Parameshwaran, Materials Development Division, IGCAR for providing the SEM micrographs of the samples. References [1] L. Bauernfeind, W. Widder, H.F. Braun, Physica C 254 (1995) 151. [2] J. Tallon, C. Bernhard, M. Bowden, P. Gilberd, T. Stoto, D. Pringle, IEEE Trans. Appl. Supercond. 9 (1999) 1696. [3] J. Janaki, T. Geetha Kumary, R. Nagarajan, T.A. Mary, M.C. Valsakumar, V.S. Sastry, Y. Hariharan, T.S. Radhakrishnan, Mat. Chem. Phys. 75 (2002) 110. [4] A.C. McLaughlin, J.P. Attfield, Phys. Rev. B 60 (1999) 14605. [5] C. Bernhard, J.L. Tallon, Ch. Niedermayer, Th. Blasius, A. Golnik, E. Bru¨cher, R.K. Kremer, D.R. Noakes, C.E. Stronach, E.J. Ansaldo, Phys. Rev. B 59 (1999) 14099. [6] A. Fainstein, E. Winkler, A. Butera, J.L. Tallon, Phys. Rev. B 60 (1999) R12597.
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Effect of annealing and granularity on the physical properties of GdSr2RuCu2O8 T. Geetha Kumary *, J. Janaki, V.S. Sastry, Y. Hariharan, M.C. Valsakumar Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India Received 20 November 2005; received in revised form 3 February 2006; accepted 2 May 2006 Available online 21 June 2006
Abstract The variation of crystallite size (coherent domains as measured from X-ray analysis) and the physical properties of the ferromagnetic superconductor GdSr2RuCu2O8 on increasing the oxygen annealing time has been studied by X-ray analysis, SEM, resistivity and ac susceptibility measurements. Prolonged annealing in oxygen atmosphere at 1060 C leads to an increase in grain and crystallite sizes and a decrease in the superconducting transition width DTc. The derivative of resistivity with respect to temperature, dq/dT, shows two peaks at T1 and T2 (with T1 > T2), which can be attributed to the onset of intra-granular and inter-granular superconductivity, respectively. T2 increases drastically on increasing the annealing time, whereas only a marginal increase is observed in T1. On the contrary, there is no apparent change in the thermodynamic transition temperature Tth. The temperature dependence of the normal state resistivity, q(T), showed a gradual change from semiconducting-like to metallic behavior on increasing the annealing time. The variation in resistivity and the reduction in superconducting transition width can be attributed to the increase in grain size and the better intergrain coupling achieved by increasing the annealing time. A large reduction in the diamagnetic signal on powdering and a broadening of the susceptibility transition on increasing the measuring field are observed in this compound. This can be attributed to the large penetration depth compared to the particle size and the small inter-granular lower critical field Hc1J, respectively. 2006 Elsevier B.V. All rights reserved. PACS: 74.72.h; 74.25.Ha; 74.25.Fy Keywords: Ruthenate cuprates; Granularity; Transport properties
1. Introduction The observation of long range ferromagnetism (with magnetic ordering temperature, Tm 132 K) and superconductivity (Tc 46 K) well within the ferromagnetic state with relatively high value for the superconducting transition temperature, in the hybrid rutheno-cuprate GdSr2RuCu2O8 has initiated intense activities to study the nature of superconductivity, magnetism and their interrelation in the family of compounds RSr2RuCu2O8 and R1+xCe1xSr2RuC u2O10 (R = Sm, Eu and Gd) [1,2].
*
Corresponding author. Tel.: +91 44 274280081. E-mail address: [email protected] (T. Geetha Kumary).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.05.057
GdSr2RuCu2O8 (Gd1212) forms in a triple perovskite structure very similar to that of the well known high temperature superconductor YBa2Cu3O7 (YBCO), with Y and Ba being completely replaced by Gd and Sr, respectively, and the CuO chain replaced by RuO2 planes. Many of the physical properties of this compound are similar to those of the underdoped cuprates. It is also observed that as in the case of YBCO, hole doping to an optimal value can lead to an increase in Tc in Gd1212 [3,4]. It is thus reasonable to infer that superconductivity in Gd1212 arises in the CuO2 planes and ferromagnetism occurs in the RuO2 planes. However, unlike in the case of YBCO, the superconducting properties of Gd1212 are extremely sensitive to the details of sample preparation. A range of values (15–50 K) has been reported in the literature [2,5–11] for Tc and there have also been reports on non-occurrence of
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superconductivity in this compound [12,13]. However, appearance of bulk superconductivity has been confirmed from specific heat measurements [14,15] subsequently. All the above mentioned works have been on sintered samples. There is a solitary report of onset of superconducting transition at 60 K for single crystals of Gd1212 from Magnetization measurements [16]. Zero Field lSR experiments [5] provided the first clear evidence for a spatially homogeneous magnetically ordered phase in GdSr2RuCu2O8 with an ordered moment of 1 Bohr magneton per unit cell. This would suggest presence of an internal magnetic (dipolar) field of the order of 700 Oe (at the site of the muon) with concomitant consequences on the superconducting order. However, ZF– lSR experiments cannot provide direct information about the type of magnetic order and its direction [5]. Subsequent neutron diffraction studies [17,18] showed the magnetic order (that develops at 132 K) to be G-type antiferromagnetism involving Ru moments which are oriented along the c-axis of the unit cell. This study provided an upper bound of 0.1 lB (per unit cell) for the ferromagnetic component of the ordered moment. This is consistent with the observation of a low temperature remnant magnetization Mr of 0.05–0.1 lB (and hence a corresponding internal magnetic field of the order of 35–70 Oe) from the dc magnetization measurements [19,20]. While these studies clearly establish that GdSr2RuCu2O8 is a weak ferromagnet, they also show that the elucidation of the details of the magnetic order in this compound is still an open problem. This information is a necessary input for theoretical studies aimed at understanding how the superconducting and magnetic orders accommodate each other in this compound. Under the assumption of ferromagnetic order with an ordered moment of 1 lB, Pickett et al. [21] suggested that the superconducting order may be of the Fulde–Ferrell– Larkin–Ovchinnikov type. However, as discussed earlier, the internal field is much smaller, and the spontaneous vortex phase scenario may be more pertinent. Clues to understand the reasons for the contradictory results on superconductivity in this compound can be obtained by having a closer look at the structure and the microstructure of the sintered samples. It is well known that the grains that adhere together to form the sintered samples are themselves made of several crystallites (X-ray coherent regions). The XRD studies reported in literature show that Gd1212 adopts a tetragonal structure with ˚ and c 11.5735 A ˚ , and space group P4/ a 3.8396 A mmm [22,23]. However, electron diffraction and neutron diffraction studies reveal some nuances in the structure of this compound. High resolution transmission electron microscopy [24] of Gd1212 shows formation of several small domains with their c-axis oriented along the three crystallographic axes. This is a consequence of the fact that the lattice parameters a and b are almost equal to c/3 in this compound. Electron diffractionp[24] ffiffiffi and pffiffiffi powder neutron diffraction studies [17] show a 2a 2a c superstructure resulting from coherent rotations of the RuO6 octahe-
˚ within the above dra within subdomains of size 50–200A mentioned domains. The mismatch in the lengths of the in-plane Ru–O and Cu–O bonds results in rotations of 13 the RuO6 octahedra pffiffiffi by p ffiffiffi about the c-axis and this in turn causes the 2a 2a c superstructure. Thus the Gd1212 samples prepared by the solid state reaction route can show granularity at two length scales – the grains of size of the order of microns as measured by SEM and the intra-grain domains, where the RuO6 octahedra are coherently rotated around the c-axis, being separated by sharp antiphase boundaries [24]. This implies that as the temperature is decreased, superconductivity should first develop in the domains at some temperature Tth, phase coherence should emerge between the domains within a grain (i.e. intra-grain superconducting transition) at a lower temperature T1, and then at a still lower temperature T2, the inter-granular transition should occur. Thus the superconducting properties of GdSr2RuCu2O8 prepared via the solid state reaction route is expected to be intimately related to the microstructure. Hence the heating schedule must be playing an important role in deciding the superconducting properties of this compound, and this aspect has received some attention in past [14,25]. In this paper, the results of the study carried out on the grain/crystallite sizes and on the superconducting and normal state properties of Gd1212 on annealing it at 1060 C in oxygen atmosphere for different time durations [25] and the effects of granularity are presented. A large reduction in the diamagnetic signal on powdering and a broadening of the susceptibility transition on increasing the measuring field are observed in this compound. These results are discussed in terms of a simple model that incorporates a penetration depth, k, large compared to the particle size, and a small inter-granular lower critical field Hc1J. 2. Sample preparation and characterization Polycrystalline samples of GdSr2RuCu2O8 were prepared by the solid state reaction technique. Stoichiometric quantities of Gd2O3, SrCO3, CuO and Ru powder, all better than 99.99% pure, were mixed and ground well. This was given an initial heat treatment at 600 C for 48 h, and then at 950 C for 48 h in air. The intermediate product was mixed and ground well, and was then heated at 1000 C in air for 3 days. The reacted powder was again ground and mixed and was pelletized. The pellets were again given a heat treatment of 1000 C for 3 days. Two pellets each were taken from these as-prepared samples and were ground and re-pelletized and oxygen annealed at 1060 C for different time durations of 15 h, 3 days and one week and were slow cooled. The structure and microstructure of all the samples were characterized by XRD and SEM measurements. The XRD measurements were carried out using Cu Ka radiation using the STOE X-ray diffractometer. Fig. 1 depicts the XRD pattern (after stripping the K a2 contribution) for the as-prepared sample and the ones that are oxygen
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Fig. 1. XRD patterns for the as-prepared and annealed samples of Gd1212. There is no pronounced difference between the XRD patterns.
annealed at 1060 C for different time durations. As can be seen from the figure, there are no conspicuous differences between the XRD patterns of the as-prepared and annealed samples. On the other hand, there are significant differences in the superconducting properties. This and similar results already available in the literature suggest a vital link between superconducting properties and microstructure in this compound, and the evolution of the latter with annealing. Scanning electron micrographs of the samples were taken using XL30 SEM, which are shown in Fig. 2. The SEM pictures show that, roughly, the grain size
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increases from fraction of a micron for the as-prepared to a few microns for the well annealed samples. The crystallite size distribution and its variation with annealing time have been studied using WINFIT package of Stefan Krumm [26]. Applicability of this package for crystallite size determination was ascertained by analyzing a standard sample of nanophasic CeO2. The computed sizes matched well with that obtained by other methods, and the distribution fitted well to a log-normal distribution, as expected. For the samples of Gd1212, the [1 0 1] reflection at 2H = 24.4 was used for profile analysis as most of the other peaks were overlapping. The [1 1 1] reflection of NBS Si powder with large and strain free crystallites was taken as the standard for deconvolution of the instrumental contribution to the total line profile. The average crystallite sizes were determined using the integral breadth method, the method of Dehlez et al. [27] and the Fourier method. Fig. 3 shows the frequency distribution of the particle size, obtained using the Fourier method, for the asprepared and one week annealed samples. Tendency for the particles to coarsen is evident from the figure- the aver˚ for the as-prepared age particle size increases from 300 A ˚ for the one week annealed sample. It sample to 390 A should be mentioned that all these methods yield different estimates for the average particle size in view of the differences in the averaging methods that they employ. For example, the Scherrer method gives volume averaged crystallite size, whereas the Fourier method gives area weighted average crystallite size. However, the results of the analysis
Fig. 2. SEM photographs of Gd1212 (a) as-prepared and the samples oxygen annealed for (b) 15 h (c) 3 days and (d) one week (scale – 5 lm). An increase in grain size can be noticed with increase in annealing time.
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C C C
Fig. 3. Frequency distribution of particle size for the as-prepared and one week annealed samples. The coarsening of the crystallites as a functions of annealing can be clearly seen.
using each of the above mentioned methods indicate that the average crystallite size increases upon annealing. The analysis based on integral method shows an increase in ˚ for the as-prepared the average particle size from 500 A ˚ for the sample annealed for one week. sample to 1000 A SEM and X-ray studies show that the samples are highly granular in nature and the X-ray coherent domain size is much smaller than the grain size seen by SEM. An increase in grain size as well as crystallite size is observed on prolonged annealing. 3. Resistivity measurements The resistivity of all the samples were measured in the temperature range 4.2–300 K, using an excitation current of 1 mA, on disk shaped samples of approximately 10 mm diameter and 1.5 mm thickness. Fig. 4 depicts the variation of resistivity as a function of temperature for the as-prepared as well as the annealed samples. The asprepared sample shows a semiconducting like temperature dependence for the normal state resistivity q(T). A reduction in resistivity corresponding to superconducting transition is seen, but, the resistivity did not fall to zero even at 4.2 K. The semiconducting like behaviour in q(T) decreases for the 15 h annealed sample and turns metallic when the annealing time is increased to 3 days. The resistivity also decreased drastically on increasing the annealing time as
Fig. 4. The variation of resistivity as a function of temperature for the asprepared and annealed samples of Gd1212.
can be seen from Fig. 4. A small increase in T onset and a c large increase in Tc(0), and hence a reduction in the superconducting transition width, DTc, is observed (see Table 1) on prolonged annealing. The temperature corresponding to the peak in the resistivity is taken as T onset and the temperc ature at which the resistance becomes zero, as Tc(0). Annealing for one week did not lead to further improvement in the superconducting properties. In fact, a slight deterioration in properties is seen on increasing the annealing time to one week. The reduced superconducting transition temperature for the one week annealed sample must be due to a slight decomposition or due to a small oxygen deficiency [28]. A pertinent result in this context is the observation of decomposition of GdSr2RuCu2O8 at 1060 C at 0.85 bar of oxygen pressure due to sublimation of RuO2 in thermogravimetry measurements [29]. It is important to note that a semiconducting like upturn in resistivity is observed below 100 K even in the well annealed samples. This observation is consistent with the earlier study which showed that GdSr2RuCu2O8 is a typical underdoped superconducting cuprate in which the pseudogap dominates normal state transport, thermodynamic and substitutional properties [2]. Furthermore, the doping state does not change with annealing [2]. It then follows that the larger increase in Tc(0) compared to T onset and the decrease c in semiconducting like behavior on increasing the annealing time must be due to the better inter-granular coupling
Table 1 0 00 The values, in K, corresponding to Tth, T onset , Tc(0), DTc, T1 and T2 estimated from the resistivity transition and T vc , T v and Tm, obtained from the ac c susceptibility measurements, for the as-prepared and annealed samples of GdSr2RuCu2O8 00
Sample
Tth ± 0.5
T onset 0:2 c
Tc(0) ± 0.2
DTc ± 0.4
T1 ± 0.5
T2 ± 0.2
T vc 0:2
T v 0:2
Tm ± 0.2
As-prepared 1060 C, 15 h. 1060 C, 3 days 1060 C, 1 week
47.3 47.5 47.5 47.8
35.8 40 41.5 40
<4.2 8.4 19.12 17.28
>31.5 31.6 22.38 22.72
25.07 27.06 30.18 28.45
<4.2 13.8 23.1 21.47
<4.2 7 17.8 15.12
<4.2 4.42 14.23 11.56
137.6 136.2 134.8 135.8
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achieved by prolonged annealing. The solid state reaction method of sample preparation in general leads to formation of stoichiometric grains separated by off-stoichiometric material. The conspicuous semiconducting like behaviour in the resistivity (at low temperatures) suggests that the material at the grain boundaries must be insulating. The properties of such granular materials can be modelled in terms of array of SIS type Josephson junctions [30]. In granular superconductors [31], one expects the intragranular superconductivity to set in at a higher temperature, T1 and the inter-granular superconductivity, which requires phase coherence between the grains, to occur at a lower temperature, T2. As mentioned in the introduction, the additional complexity in the present case is the fact that the individual grains themselves behave as Josephson junction arrays due to the presence of coherent domains separated by antiphase boundaries which act as weak links. In order to obtain a better understanding of what annealing does to the inter-granular and intra-granular superconducting properties, the derivative of resistivity with respect to temperature (dq/dT) is calculated (using conventional numerical methods [32]) in the temperature range 4.2– 60 K. In Fig. 5, the variation of dq/dT, normalized with respect to the maximum value, is plotted as a function of temperature. Two peaks are observed in the derivative curves. The peak at the higher temperature, T1, corresponds to the intra-granular and that at the lower temperature, T2, corresponds to inter-granular superconducting transitions. It can be seen from the Fig. 5, that for the as-prepared sample inter-granular coupling does not occur above 4.2 K. The grain size, as seen from SEM, of the asprepared and the sample annealed for 15 h are very small (of the order of fraction of a micron) and hence the semiconducting like behavior in these samples must be due to the small grain size and the weak coupling between the
Fig. 5. The variation of dq/dT (normalized with respect to the maximum value) as a function of temperature for the as-prepared and annealed samples of GdSr2RuCu2O8.
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grains. The grain size and the coupling between the grains increase on increasing the annealing time, which leads to a metallic behaviour in resistivity in the well annealed samples. T2 increases drastically upon increasing the annealing time whereas only marginal increase is observed in T1 (Table 1). Following Garcia et al. [33], if the temperature at which the derivative curves deviate from the high temperature base line is defined as the thermodynamic transition temperature Tth (the temperature at which the individual domains become superconducting), it can be seen that all the samples show approximately the same Tth 48 K. Thus annealing leads to better grain boundary properties and inter-granular coupling but the intra-grain superconducting properties change only marginally on increasing the annealing time. It should be mentioned that the values of T1 and T2 depend on the magnitude of the external magnetic field and measuring current density [33]. The results reported in the present paper correspond to zero external field and a fixed current density 20 mA/cm2. The sample annealed at 1060 C for 3 days have comparable normal state resistivity properties with that of Lorentz et al. [34], where they have given extra intermediate heat treatments before the final annealing. However, the superconducting transition is slightly broader in the present case. The intermediate heat treatment may lead to a decrease in the transition width. It is reported by Lorentz et al. [34], that the introduction of additional heat treatment steps raises T2 but not T1. High pressure oxygen annealing or texturing by directional solidification should be tried to obtain a higher value for T1.
4. Ac susceptibility measurements Ac susceptibility measurements were carried out from 4.2–300 K using a home made dipstick. The real (v 0 ) and imaginary (v00 ) parts of ac susceptibility measured on the bulk sample of the as-prepared and annealed Gd1212 are shown in Figs. 6 and 7, respectively. The temperatures cor0 responding to the onset of diamagnetic signal (T vc ) and the v00 00 peak ðT Þ in v shift to higher values upon increasing the annealing time (Table 1). No diamagnetic signal is observed in the as-prepared sample. The transition associated with the onset of intra-granular or inter-granular superconductivity is not observed in the magnetic susceptibility measurements. Diamagnetic signal is seen only when zero resistivity is reached. This must be due to the small grain size compared to the large penetration depth [35] and the weak inter-granular coupling between the grains in this compound. The intra-granular and inter-granular penetration depths of Gd1212 are reported to be 2.5– 6 lm and 30 lm, respectively, from ac susceptibility measurements [35], which are very large compared with the penetration depth of most of the superconductors. In order to study the effect of granularity on the superconducting properties, the ac susceptibility measurements were carried
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Fig. 6. The ac v as a function of temperature for the as-prepared as well as the annealed samples of GdSr2RuCu2O8. The temperature is plotted on log scale so that the superconducting transitions are clearly seen. The region where the magnetic transition occurs is expanded and is shown in the inset.
Fig. 7. The imaginary part of ac v, normalized with respect to the peak value, vs temperature is plotted for the as-prepared and annealed GdSr2RuCu2O8.
out on samples with different particle size and also in different measuring fields. A drastic reduction in diamagnetic signal is seen [36] on powdering Gd1212. Fig. 8 represents the ac susceptibility data of the bulk (A) (pieces of 1 · 2 · 4 mm), coarse powder B (radius fraction of an mm) and powder samples (C and D) of Gd1212 annealed for one week. The average particle size of the samples C and D are estimated to be 55 and 8 lm respectively, using Malvern particle size analyzer. The diamagnetic signal corresponding to superconductivity decreases drastically on powdering the sample and an upturn is seen for particles with an average diame-
Fig. 8. The ac v vs temperature for the bulk and powder samples of GdSr2RuCu2O8 (annealed for one week).
ter of 8 lm. A reduction in diamagnetic signal can arise if the particle size is small compared to the penetration depth. The typical grain size of the oxygen annealed samples are of the order of a few microns, comparable to the intragranular penetration depth. Thus the diamagnetic susceptibility of the isolated grains is expected to be small. When these grains are in contact with each other the effective particle size is larger than the penetration depth and hence the diamagnetic signal is large in magnitude. Upon powdering, the agglomerates of grains break into individual particles causing incomplete shielding and hence a smaller diamagnetic signal. In addition to this the Gd moments in the flux penetrated region will also contribute to magnetization causing an upturn in the susceptibility data at low temperature. Calculation of susceptibility assuming spherical shape for the particles gives results in qualitative agreement with the experimental observation. The susceptibility of a spherical superconducting particle, containing paramagnetic moments, is given by [37], vðT Þ ¼ vpara ðT Þð1 þ vsc ðT ÞÞ þ vsc ðT Þ 2 kðT Þ kðT Þ vsc ðT Þ ¼ 1 þ 3 CothðR=kðT ÞÞ 3 R R
ð1Þ ð2Þ
where, k(T) is the penetration depth and R is the radius of the particle. The paramagnetic susceptibility vpara(T) due to Gd ions varies as vGd/(T + hN). The Curie constant vGd is calculated to be 0.9786 SI units and hN 4.4 K is obtained by fitting the susceptibility data. The v calculated using Eq. (1) is shown in Fig. 9 for various values of R/k(0). It is clear that the superconducting signal can be completely masked by the paramagnetic contribution when the particle size is small. Thus absence of diamagnetic signal and also a rise in susceptibility signal at low temperatures as seen in Fig. 8 can arise when the particle size is of the order of k. The calculations were carried out assuming a fixed value for Tc. But in the actual case there is a distribution of Tc,
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ac
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Fig. 9. The curves from top to bottom correspond to v calculated using Eq. (1) and are shown as a function of temperature for R/k(0) = 0.5, 1, 2, 5, 10, 20 and 50,q respectively. ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiThe temperature dependence of k(T) was
Fig. 10. The ac v measured as a function of temperature under different measuring fields for the one week annealed GdSr2RuCu2O8 is plotted. The diamagnetic signal is maximum when the measuring field is the lowest.
taken to be kð0Þ= 1 ðT =T c Þ4 in the calculation.
as is evident from the broad superconducting transition (Fig. 8). Hence, for a quantitative description of the observed data, the distribution in Tc, the corresponding variation in k and the particle size distribution are to be incorporated in the calculation. However, the present analysis gives a qualitative understanding of the observed decrease in Meissner signal on powdering the sample. The observed reduction in diamagnetic signal on powdering the sample cannot be explained even if we consider the phase separation to superconducting Ru1xCuxSr2GdCu2O8x and ferromagnetic RuSr2GdCu2O8 reported recently [38]. In Fig. 10, the ac v measured for different measuring fields (.35, .25, .15 and .075 Oe) for the sample annealed for one weak is plotted. The susceptibility transitions become broader on increasing the measuring field. Such a large variation in the diamagnetic response with a very small change in the measuring field in a material with internal magnetic field of the order of 35–70 Oe is indeed puzzling. This and some other anomalies like absence of diamagnetic response while signature of superconductivity is manifest in resistivity can, however, be understood as follows. As mentioned earlier, GdSr2RuCu2O8 prepared via the solid state reaction route is made up of grains each of which containing coherent domains separated by antiphase boundaries. Below the magnetic transition temperature, each of these grains is ferromagnetic with an internal magnetic field of the order of 35–70 Oe. Since the intra-granular penetration depth of GdSr2RuCu2O8 is 5–10 times larger than that of well annealed YBCO [35], the lower critical field Hc1 is expected to be 25–100 times smaller than that of YBCO. In view of its sensitivity to disorder, we expect Hc1 to have a distribution with its typical value to be of the order of a few Oe. As a matter of fact, DC magnetization studies [10] of a Gd1212 sample with Tc 40 K
shows an Hc1 < 1 Oe at 5 K. It is well known that Hc1J, the field at which first fluxon enters the Josephson junction array, is usually much smaller than Hc1 [30]. We expect Hc1J also to have a distribution with its typical value to be of the order of a fraction of an Oe. Hence a broadening in ac susceptibility signal for such small variations in the measuring field and absence of diamagnetic signal in much higher fields can be expected due to the highly granular nature of GdSr2RuCu2O8. The magnetic transition temperature, Tm, decreased monotonically when the annealing time is increased to 3 days (Table 1). This result can be understood by considering the effect of annealing on the Ru electrons, as the ferromagnetism in Gd1212 is due to the Ru moments. It appears that this may be due to dilution of the magnetic sublattice by the evaporation of a small amount of Ru [29]. But the one week annealed sample showed a small increase in Tm compared to the sample annealed for 3 days. A depletion in oxygen in GdSr2RuCu2O8 is reported to cause an increase in Tm, due to change in the valence state of Ruthenium, and a decrease in Tc [28]. It is to be seen whether there is an oxygen deficiency in the one week annealed sample which may cause an increase in Tm with a concomitant reduction in Tc. 5. Conclusions The granular nature of the ferromagnetic superconductor GdSr2RuCu2O8 is clearly brought out in the present study. Superconductivity develops first in the coherent domains within the grains at Tth, followed by emergence of phase coherence among the domains within a grain leading to intra-granular superconductivity at a temperature T1 < Tth. Inter-granular phase coherence occurs at a still lower temperature T2. Oxygen annealing of Gd1212 at 1060 C lead to an increase in the grain as well as crystallite
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sizes. Upon increasing the annealing time, the temperature dependence of the normal state resistance became metallic in nature from the semiconducting like behaviour of the asprepared sample. Study of the variation of dq(T)/dT provides a way of understanding the variation Tth, T1 and T2. A drastic increase is observed for the inter-granular transition temperature, T2, whereas the intra-granular transition temperature, T1 increased only marginally. This shows that the grain boundary properties improve on annealing leading to better inter-granular coupling. It is argued that the anomalous magnetic response of GdSr2RuCu2O8 is intimately related to the size of the crystallites and the large penetration depth. A large reduction in the Meissner signal is observed on powdering these compounds which can be attributed to the large penetration depth which is comparable in magnitude to the grain size. Plausibility of such a large penetration depth in GdSr2RuCu2O8, in comparison with those of the conventional superconductors, has been a topic of debate. It may be noted that there is, as yet, no microscopic theory that describes the precise manner in which superconductivity is accommodated in an otherwise ferromagnetic material. Since ferromagnetism is the dominant phenomenon in GdSr2RuCu2O8, it is plausible that the superconducting order parameter allows a gradual magnetic flux penetration, spread over a larger length scale, so that the magnetic energy is not significantly altered. Broadening in the susceptibility transition when the measuring field is increased by a fraction of an Oersted confirms that the Hc1J in these compounds should be very low. The decrease in Tm on annealing can be due to sublimation of a small amount of Ru. More experiments are required to pinpoint the connection between the microstructure and the superconducting properties in Gd1212. Acknowledgements The authors acknowledge Mrs. M. Radhika and Dr. P. Parameshwaran, Materials Development Division, IGCAR for providing the SEM micrographs of the samples. References [1] L. Bauernfeind, W. Widder, H.F. Braun, Physica C 254 (1995) 151. [2] J. Tallon, C. Bernhard, M. Bowden, P. Gilberd, T. Stoto, D. Pringle, IEEE Trans. Appl. Supercond. 9 (1999) 1696. [3] J. Janaki, T. Geetha Kumary, R. Nagarajan, T.A. Mary, M.C. Valsakumar, V.S. Sastry, Y. Hariharan, T.S. Radhakrishnan, Mat. Chem. Phys. 75 (2002) 110. [4] A.C. McLaughlin, J.P. Attfield, Phys. Rev. B 60 (1999) 14605. [5] C. Bernhard, J.L. Tallon, Ch. Niedermayer, Th. Blasius, A. Golnik, E. Bru¨cher, R.K. Kremer, D.R. Noakes, C.E. Stronach, E.J. Ansaldo, Phys. Rev. B 59 (1999) 14099. [6] A. Fainstein, E. Winkler, A. Butera, J.L. Tallon, Phys. Rev. B 60 (1999) R12597.
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