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The effect of substituents on magnetic order and superconductivity in RuSr2R2−yCeyCu2O10+δ (R = Eu, Gd)
Current Applied Physics 6 (2006) 515–519 www.elsevier.com/locate/cap www.kps.or.kr
The effect of substituents on magnetic order and superconductivity in RuSr2R2yCeyCu2O10+d (R = Eu, Gd) S.K. Goh a
a,b
, G.V.M. Williams
a,*
, H.K. Lee
c
MacDiarmid Institute for Advanced Materials and Nanotechnology, Industrial Research Limited, P.O. Box 31310, Lower Hutt, New Zealand b School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, New Zealand c Department of Physics, Kangwon National University, Chunchon 200-701, Republic of Korea Available online 4 January 2006
Abstract We have used the partial substitution of Cu by Zn and Ru by Sn to probe the normal, superconducting, and magnetic states in the superconducting and magnetically ordered ruthenocuprate RuSr2R2yCeyCu2O10+d (R = Eu, Gd). The effect of Zn substitution for Cu in the CuO2 planes is to suppress the superconducting transition temperature at a rate similar to that observed in other high temperature superconducting cuprates. Sn substitution for Ru rapidly suppresses the temperature where the ferromagnetic component is observed. This suppression rate depends on the Ce content. We observe that the temperature where the bulk Meissner phase commences is closer to the superconducting transition temperature when the ferromagnetic component is completely suppressed. This provides direct support for the spontaneous vortex phase model that has been applied to RuSr2Gd1.4Ce0.6Cu2O10+d to account for coexisting magnetic and superconducting orders. 2005 Elsevier B.V. All rights reserved. PACS: 74.72.h; 74.62.Dh; 74.25.Ha; 74.25.Fy Keywords: Superconductivity; Magnetism; Ruthenates
1. Introduction The ruthenocuprates RuSr2RCu2O8 and RuSr2R2yCeyCu2O10+d (R is a rare earth) are an interesting group of compounds that are known to exhibit superconductivity and magnetic order [1–11]. They were discovered by Bauernfield et al. [1,2] and are typified by CuO2 and RuO2 planes that are separated by insulating layers. Neutron diffraction measurements on RuSr2RCu2O8 have shown that the magnetic order is antiferromagnetic with a small ferromagnetic component [8,10]. Similar measurements do not exist for RuSr2R2yCeyCu2O10+d. However, magnetization measurements reveal a significantly larger ferromagnetic component [11]. It has been argued that this compound antiferromagnetically orders at temperatures as high as *
Corresponding author. Fax: +64 4 9313 117. E-mail address: [email protected] (G.V.M. Williams).
1567-1739/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2005.11.052
190 K [3]. Superconductivity is observed below 50 K for 0.4 6 y 6 0.8 [11]. It is believed that distortions of the RuO6 octahedra and Dzyaloshinsky–Moriya antisymmetric exchange coupling leads to spin-canting and a small ferromagnetic component below the antiferromagnetic ordering temperature [3]. At a lower temperature, but still above the superconducting transition temperature, the Ru–Ru interactions are assumed to dominate, which leads to a remnant magnetization and a significant increase in the magnetization [3]. Superconductivity and magnetic order can not coexist without some form of accommodation and hence it has been suggested that a spontaneous vortex phase (SVP) exists in RuSr2R2yCeyCu2O10+d [4] and RuSr2RCu2O8 [7]. Support for coexisting superconducting and magnetic order has been provided by zero-field muon spin rotation measurements on RuSr2GdCu2O8 and RuSr2Eu1.4Ce0.6Cu2O10+d where a local field is observed at low
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temperatures in the superconducting state [6,12]. In the SVP model the magnetic field from the ferromagnetic component, BFM, is screened when it is less than the lower critical field, Bc1. Thus, the bulk Meissner state commences for temperatures below TMeissner where Bc1 > BFM and a SVP exists for TMeissner < T < Tc, where Tc is the superconducting transition temperature. Experimental evidence for the SVP in RuSr2RCu2O8 has been provided by recent magnetization measurements on Ru1xSnxSr2RCu2O8 [13] that show a decrease in the low-field ferromagnetic component and hence a decrease in BFM with increasing Sn concentrations. This is accompanied by a corresponding decrease in Tc–TMeissner [13], as expected within the SVP model. Similar measurements are necessary in the lessstudied RuSr2R2yCeyCu2O10+d compounds. Current research on the ruthenocuprates has focused on RuSr2RCu2O8. This compound has two CuO2 planes in close proximity, where it has been argued that exchange interactions between Ru spins and carriers in the CuO2 planes are weak, and hence there is no significant pairbreaking by Ru moments. It has been suggested that the hole concentration in the CuO2 planes of RuSr2RCu2O8 is comparable to that observed in underdoped high temperature superconducting cuprates (HTSCs) [14,15]. There is also evidence of a mixed Ru valence between 4+ and 5+ [16,17] as well as itinerant conduction in the RuO2 planes [18]. The less-studied RuSr2R2yCeyCu2O10+d compound is better compared with single CuO2 layer HTSCs. This is because the two CuO2 layers are further apart, separated by insulating layers, and the unit cell contains a subunit that is similar to that of the single CuO2 plane electrondoped HTSC R2yCeyCuO4. The Ru valence is believed to be near 5+ [19,20], which may suggest that the half-filled e2g orbitals are insulating. However, it has been reported from a X-ray photoemission and absorption study that the RuO2 planes are conducting [21]. Partially substituting atomic species that suppress either the superconducting or magnetically ordered states is an ideal method to probe the normal, superconducting and magnetic states in RuSr2R2yCeyCu2O10+d. We have performed such a study and report the results from magnetization and transport measurements on Ru1xSnxSr2Gd1.4Ce0.6(Cu1zZnz)2O10+d and Ru1xSnxSr2Eu1.2Ce0.8(Cu1zZnz)2O10+d in this paper.
3. Results and analysis We first discuss the effect of Zn substitution for Cu on RuSr2Gd1.4Ce0.6Cu2O10+d and RuSr2Eu1.2Ce0.8Cu2O10+d. Partial Zn substitution for Cu in RuSr2Gd1.4Ce0.6Cu2O10+d leads to a rapid decrease in Tc, which can be seen in Fig. 1 where the normalized resistance is plotted for Zn concentrations increasing from 0% to 3% per CuO2 plane. Similar to previous studies on RuSr2RCu2O8, we take Tc as the temperature where the normalized resistance begins to decrease [15]. The resultant Tc values are plotted in the inset to Fig. 1 (solid circles). In addition, 5%Zn substitution for Cu in RuSr2Eu1.2Ce0.8Cu2O10+d suppresses Tc to 64.2 K, the lowest temperature that we could measure. While Zn substitution rapidly suppresses superconductivity, magnetization measurements (not shown) indicate that Zn has no effect on the temperature where the ferromagnetic component is observed. This is consistent with Zn substituting primarily for Cu in the CuO2 planes and the magnetic order originating in the RuO2 planes. Also shown in the inset to Fig. 1 is the dependence of Tc on Zn concentration that is observed in the HTSCs when the normal-state pseudogap energy is small (solid curve). In this case, the depression in Tc is given by the Abrikosov–Gorkov equation [22]. We find that the initial rate of decrease in Tc for RuSr2Gd1.4Ce0.6Cu2O10+d is dTc/dz = 10.9 K/%Zn. This is comparable to that observed in slightly underdoped Y0.8Ca0.2Ba2(Cu1zZnz)3O7d [23]. Increasing Zn concentrations leads to a semiconductorlike increase in the normalized resistance that has also been observed when impurities are substituted for Cu in the HTSCs [24] and may arise from increased scattering or charge localization. This suggests that the conductivity is dominated by conduction in the CuO2 planes and any contribution from the RuO2 planes is small. The effect of partial Sn substitution for Ru on the resistivity and magnetization data can be seen in Figs. 2 and 3.
2. Experimental details The Ru1xSnxSr2R2yCey(Cu1zZnz)2O10+d samples for this study were made using a method described elsewhere [11]. Electrical resistance measurements were made using a four-terminal technique. Room temperature thermopower measurements were done using the standard differential technique. The magnetization measurements were made using a SQUID magnetometer. The ac magnetization measurements were made with an ac field of 5 · 106 T and at a frequency of 333 Hz.
Fig. 1. Plot of the normalized electrical resistance against temperature for RuSr2Gd1.4Ce0.6(Cu1zZnz)2O10+d with z = 0, 0.01, 0.02 and 0.03. The arrow indicates the increasing Zn concentration. The data have been normalized to the same value at 260 K. Inset: plot of Tc against Zn concentration. The solid curve is a fit to the data as described in the text.
S.K. Goh et al. / Current Applied Physics 6 (2006) 515–519
Fig. 2. Plot of the normalized resistivity against temperature for (a) Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d with x = 0 (solid curve), x = 0.1 (dashed curve) and x = 0.2 (dotted curve), and (b) Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d with x = 0 (solid curve), x = 0.05 (dashed curve) and x = 0.1 (dotted curve). The data were normalized to the same value at 265 K.
Fig. 3. Plot of the real (a and c) and imaginary (b and d) parts of the ac susceptibility against temperature for (a and b) Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d with x = 0 (solid curve), x = 0.1 (dashed curve) and x = 0.2 (dotted curve) (c and d) Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d with x = 0 (solid curve), x = 0.05 (dashed curve) and x = 0.1 (dotted curve). The data have not been corrected for demagnetization effects.
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The normalized resistivity data in Fig. 2 shows that partial Sn substitution leads to a small change in Tc. The upturn in the resistivity curves of Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d (Fig. 2a) before the onset of superconductivity could be attributed to grain boundary effects. This upturn is not observed in Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d (Fig. 2b), which might be due to the fact that Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d has been annealed at 100 bar and the grains are better connected after high pressure annealing. The ferromagnetic component of the magnetic order is only visible in the real and imaginary components of the ac magnetization for temperatures below 90 K in RuSr2Gd1.4Ce0.6Cu2O10+d. Remarkably, there is no evidence of the ferromagnetic component for Sn concentrations of 5% or higher in the ac magnetization data (Figs. 3c and d) in Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d. This can be compared with Ru0.9Sn0.1Sr2Eu1.2Ce0.8Cu2O10+d where the ferromagnetic component is still observable below 60 K. A higher Sn fraction of 0.2 is required for the ferromagnetic component to disappear. The increase in the imaginary part of the magnetization at lower temperatures in all the samples is due to superconductivity. The loss of the ferromagnetic component is accompanied by an initial increase in the temperature where the bulk Meissner state commences, which will be discussed later. A rapid suppression of the ferromagnetic component by Sn in RuSr2Gd1.4Ce0.6Cu2O10+d is not found in RuSr2GdCu2O8, where the antiferromagnetic ordering temperature (132 K) is initially reduced at a rate of 5 K/%Sn [13]. For RuSr2GdCu2O8 the ferromagnetic component appears at the antiferromagnetic ordering temperature. The analysis is more complicated in RuSr2Gd1.4Ce0.6Cu2O10+d because the antiferromagnetic ordering temperature is believed to occur at 180 K, while the significant ferromagnetic component is only evident for temperatures below 100 K [3]. To characterize the loss of the ferromagnetic component, we use the temperature where dM/dT is a minimum. This temperature is 78 K in pure RuSr2Gd1.4Ce0.6Cu2O10+d, 87 K in pure RuSr2Eu1.2Ce0.8Cu2O10+d, and 30 K for Ru0.9Sn0.1Sr2Eu1.2Ce0.8Cu2O10+d. Thus, the rate at which the ferromagnetic component in RuSr2Gd1.4Ce0.6Cu2O10+d disappears is greater than 16 K/%Sn, while the initial suppression rate for RuSr2Eu1.2Ce0.8Cu2O10+d is merely 5.7 K/%Sn and comparable to that observed in Ru1xSnxSr2GdCu2O8. It is not clear why the ferromagnetic component in RuSr2Gd1.4Ce0.6Cu2O10+d is rapidly suppressed in this compound. The dependence of Tc (filled circles) and TMeissner (open circles) on Sn concentration for Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d can be seen in Fig. 4, where similar to a previous study on the pure compound [3], TMeissner is taken as the temperature where the real part of the ac susceptibility is zero. We find a large decrease in Tc–TMeissner for x P 0.05 in Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d and x = 0.2 in Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d, which can be seen from the right and left-hand insets of Fig. 4, respectively. The significantly smaller Tc–TMeissner only occurs for samples
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S.K. Goh et al. / Current Applied Physics 6 (2006) 515–519
Fig. 4. Plot of Tc (filled circles) and TMeissner, the temperature where v 0 (open circles) is zero, against Sn concentration for Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d. Right-hand inset: plot of Tc–TMeissner against Sn concentration for Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d. Left-hand inset: plot of Tc–TMeissner against Sn concentration for Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d. The filled triangles in the insets denote Tc–TMeissner for samples where there is no evidence of the ferromagnetic component.
where there is no evidence of the ferromagnetic component (x P 0.05 for Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d and x P 0.2 for Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d) and Tc–TMeissner is comparable to that found in granular ceramic HTSCs. This result provides direct support for the SVP interpretation as applied to RuSr2R2yCeyCu2O10+d because TMeissner is only significantly below Tc when there is evidence of a ferromagnetic component. Our current and previous results [11] suggest that both RuSr2Gd1.4Ce0.6Cu2O10+d and RuSr2Eu1.2Ce0.8Cu2O10+d are underdoped. This is based on the assumption that the correlation between the room temperature thermopower, SRT, and the number of doped holes per Cu, p, observed in the HTSCs [25] is valid for RuSr2R2yCeyCu2O10+d. Thus, from SRT measured in RuSr2Gd1.4Ce0.6Cu2O10+d (11 lV K1) the value of p estimated from this correlation is 0.117. This decreases to 0.107 (SRT = 17 lV K1) for the x = 0.20 sample. Similarly, for Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d, p is estimated to be 0.102 for x = 0, 0.098 for x = 0.1 and 0.093 for x = 0.2. The value of p implies that both Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d and Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d are underdoped (p < 0.16) for all Sn concentrations and Sn substitution did not significantly change p. The Sn valence is 4+ and previous studies [19,20] indicate that the Ru valence is nearly 5+. Therefore, the partial substitution of Sn for Ru will introduce extra holes. If these holes go into the CuO2 planes and there is no change in the oxygen content, then p will increase by x/2, where x is the Sn concentration in Ru1xSnxSr2R2yCeyCu2O10+d. If we take p = 0.117 for RuSr2Gd1.4Ce0.6Cu2O10+d as deduced from our SRT measurement, then p for the Sn substituted RuSr2Gd1.4Ce0.6Cu2O10+d should be 0.142, 0.167, 0.217 for x = 0.05, 0.10 and 0.20, respectively. Similarly, the expected hole concentrations for Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d should be 0.152, 0.202 for x = 0.10 and x = 0.20, respectively. The expected hole concentra-
tions are clearly larger than those estimated from SRT. Based on the correlation between Tc and p observed in the HTSCs [26], for both RuSr2Gd1.4Ce0.6Cu2O10+d and RuSr2Eu1.2Ce0.8Cu2O10+d, the predicted increase in p with increasing x should lead to an initial increase in Tc that maximizes for the x = 0.10 sample and decreases for the x = 0.20 sample (since p = 0.16 is the optimum doping). In addition, since Tc = 37.8 K for the pure RuSr2Eu1.2Ce0.8Cu2O10+d, using the model proposed in Ref. [26], the calculated Tc of Ru0.9Sn0.1Sr2Eu1.2Ce0.8Cu2O10+d should be 52.1 K. These are clearly not experimentally observed. It is possible that the extra holes induced by Sn substitution might appear predominately in the RuO2 planes. This would have the effect of raising the valence of Ru. For example, simple valence counting indicates that the Ru valence is +5.19 for Ru0.8Sn0.2Sr2Eu1.2Ce0.8Cu2O10+d if we use the Ru valence estimated in the pure compound [20]. A Ru valence greater than +5 is not observed in the known compounds. We suggest a thorough valence study on the Sn-substituted Ru1xSnxSr2R2yCeyCu2O10+d. It is also possible that Sn substitution is accompanied by a decrease in the oxygen content leading to no change in the Ru valence and a small net decrease in the hole concentration in the CuO2 plane. This oxygen compensation effect has already been proposed to explain the nearly Ce-concentration-independent Tc values observed in RuSr2Gd2yCeyCu2O10+d for 0.4 6 y 6 0.8 [11]. 4. Conclusion In conclusion, we have used Zn substitution for Cu and Sn substitution for Ru to probe the normal, superconducting and magnetic states in RuSr2R2yCeyCu2O10+d. We find that the rate of Tc suppression by Zn is similar to that observed in slightly underdoped HTSCs. We also observed a rapid suppression of the ferromagnetic component when Sn is partially substituted for Ru. The rate of suppression for the Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d system is more than 3 times greater than that observed in RuSr2Eu1.2Ce0.8Cu2O10+d and RuSr2RCu2O8. The partial substitution of Ru by Sn has allowed us to test the spontaneous vortex phase model that has been applied to RuSr2Gd1.4Ce0.6Cu2O10+d. Our results show that when the temperature where the ferromagnetic component is observed is suppressed to zero, the onset of the bulk Meissner phase occurs close to Tc, which provides direct support for the spontaneous vortex phase model. Acknowledgements Funding support was provided by the New Zealand Marsden Fund (GVMW, SKG), the Alexander von Humboldt Foundation (GVMW), and the Korea Science and Engineering Foundation under Grant No. R05-2003-00010434-0 (HKL). We acknowledge a fees scholarship provided by Industrial Research Limited (SKG) and useful comments by Jeff Tallon.
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References [1] L. Bauernfeind, W. Widder, H.F. Braun, Physica C 254 (1995) 151. [2] L. Bauernfeind, W. Widder, H.F. Braun, J. Low, Temp. Phys. 105 (1996) 1605. [3] I. Felner, U. Asaf, O. Millo, Phys. Rev. B 55 (1997) 3374. [4] E.B. Sonnin, I. Felner, Phys. Rev. B 57 (1998) 14000. [5] I. Felner, U. Asaf, S. Reich, Y. Tsabba, Physica C 311 (1999) 163. [6] C. Bernhard, J.L. Tallon, Ch. Neidermayer, Th. Blasius, A. Golnik, E. Bru¨cher, D.R. Noakes, C.E. Stonach, E.J. Ansaldo, Phys. Rev. B 59 (1999) 14099. [7] C. Bernhard, J.L. Tallon, E. Bruecher, R.K. Kremer, Phys. Rev. B 61 (2000) 14960. [8] J.W. Lynn, B. Keimer, C. Ulrich, C. Bernhard, J.L. Tallon, Phys. Rev. B 61 (2000) 14964. [9] G.V.M. Williams, S. Kra¨mer, Phys. Rev. B 62 (2000) 4132. [10] J.D. Jorgensen, O. Chmaissem, H. Shaked, S. Short, P.W. Klamut, B. Dabrowski, J.L. Tallon, Phys. Rev. B 63 (2001) 54440. [11] G.V.M. Williams, M. Ryan, Phys. Rev. B 64 (2001) 94515. [12] A. Shengelaya, R. Khasanov, D.G. Eshchenko, I. Felner, U. Asaf, I.M. Savic´, H. Keller, A. Mu¨ller, Phys. Rev. B 69 (2004) 24517. [13] G.V.M. Williams, H.K. Lee, S. Kra¨mer, Phys. Rev. B 67 (2003) 104514. [14] S. Kramer, G.V.M. Williams, Physica C 377 (2002) 282.
519
[15] J.L. Tallon, J.W. Loram, G.V.M. Williams, C. Bernhard, Phys. Rev. B 61 (2000) 6471. [16] R.S. Liu, L.Y. Jang, H.H. Hung, J.L. Tallon, Phys. Rev. B 63 (2001) 212507. [17] A. Butera, A. Fainstein, E. Winkler, J.L. Tallon, Phys. Rev. B 63 (2001) 54442. [18] M. Pozˇek, A. Dulcˇic´, A. Hamzic´, M. Basletic´, E. Tafra, G.V.M. Williams, S. Kra¨mer, Phys. Rev. B 65 (2002) 174514. [19] I. Felner, U. Asaf, C. Godart, E. Alleno, Physica B 259–261 (1999) 703. [20] G.V.M. Williams, L.Y. Jang, R.S. Liu, Phys. Rev. B 65 (2002) 64508. [21] Y. Hirai, I. Zivkovic, B.H. Frazer, A. Reginelli, L. Perfetti, D. Ariosa, G. Margaritondo, M. Prester, D.T. Jiang, Y. Hu, T.K. Sham, I. Felner, M. Pederson, M. Onellion, Phys. Rev. B 65 (2002) 54417. [22] Y. Sun, K. Maki, Phys. Rev. B 51 (1995) 6059. [23] J.L. Tallon, C. Bernhard, G.V.M. Williams, J.W. Loram, Phys. Rev. Lett. 79 (1997) 5294. [24] G.V.M. Williams, J.L. Tallon, R. Dupree, R. Michalak, Phys. Rev. B 54 (1996) 9532. [25] J.L. Tallon, C. Bernhard, R.L. Hitterman, J.D. Jorgensen, Phys. Rev. B 51 (1995) 12911. [26] M.R. Presland, J.L. Tallon, R.G. Buckley, R.S. Liu, N.E. Flower, Physica C 176 (1991) 95.
The effect of substituents on magnetic order and superconductivity in RuSr2R2yCeyCu2O10+d (R = Eu, Gd) S.K. Goh a
a,b
, G.V.M. Williams
a,*
, H.K. Lee
c
MacDiarmid Institute for Advanced Materials and Nanotechnology, Industrial Research Limited, P.O. Box 31310, Lower Hutt, New Zealand b School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, New Zealand c Department of Physics, Kangwon National University, Chunchon 200-701, Republic of Korea Available online 4 January 2006
Abstract We have used the partial substitution of Cu by Zn and Ru by Sn to probe the normal, superconducting, and magnetic states in the superconducting and magnetically ordered ruthenocuprate RuSr2R2yCeyCu2O10+d (R = Eu, Gd). The effect of Zn substitution for Cu in the CuO2 planes is to suppress the superconducting transition temperature at a rate similar to that observed in other high temperature superconducting cuprates. Sn substitution for Ru rapidly suppresses the temperature where the ferromagnetic component is observed. This suppression rate depends on the Ce content. We observe that the temperature where the bulk Meissner phase commences is closer to the superconducting transition temperature when the ferromagnetic component is completely suppressed. This provides direct support for the spontaneous vortex phase model that has been applied to RuSr2Gd1.4Ce0.6Cu2O10+d to account for coexisting magnetic and superconducting orders. 2005 Elsevier B.V. All rights reserved. PACS: 74.72.h; 74.62.Dh; 74.25.Ha; 74.25.Fy Keywords: Superconductivity; Magnetism; Ruthenates
1. Introduction The ruthenocuprates RuSr2RCu2O8 and RuSr2R2yCeyCu2O10+d (R is a rare earth) are an interesting group of compounds that are known to exhibit superconductivity and magnetic order [1–11]. They were discovered by Bauernfield et al. [1,2] and are typified by CuO2 and RuO2 planes that are separated by insulating layers. Neutron diffraction measurements on RuSr2RCu2O8 have shown that the magnetic order is antiferromagnetic with a small ferromagnetic component [8,10]. Similar measurements do not exist for RuSr2R2yCeyCu2O10+d. However, magnetization measurements reveal a significantly larger ferromagnetic component [11]. It has been argued that this compound antiferromagnetically orders at temperatures as high as *
Corresponding author. Fax: +64 4 9313 117. E-mail address: [email protected] (G.V.M. Williams).
1567-1739/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2005.11.052
190 K [3]. Superconductivity is observed below 50 K for 0.4 6 y 6 0.8 [11]. It is believed that distortions of the RuO6 octahedra and Dzyaloshinsky–Moriya antisymmetric exchange coupling leads to spin-canting and a small ferromagnetic component below the antiferromagnetic ordering temperature [3]. At a lower temperature, but still above the superconducting transition temperature, the Ru–Ru interactions are assumed to dominate, which leads to a remnant magnetization and a significant increase in the magnetization [3]. Superconductivity and magnetic order can not coexist without some form of accommodation and hence it has been suggested that a spontaneous vortex phase (SVP) exists in RuSr2R2yCeyCu2O10+d [4] and RuSr2RCu2O8 [7]. Support for coexisting superconducting and magnetic order has been provided by zero-field muon spin rotation measurements on RuSr2GdCu2O8 and RuSr2Eu1.4Ce0.6Cu2O10+d where a local field is observed at low
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temperatures in the superconducting state [6,12]. In the SVP model the magnetic field from the ferromagnetic component, BFM, is screened when it is less than the lower critical field, Bc1. Thus, the bulk Meissner state commences for temperatures below TMeissner where Bc1 > BFM and a SVP exists for TMeissner < T < Tc, where Tc is the superconducting transition temperature. Experimental evidence for the SVP in RuSr2RCu2O8 has been provided by recent magnetization measurements on Ru1xSnxSr2RCu2O8 [13] that show a decrease in the low-field ferromagnetic component and hence a decrease in BFM with increasing Sn concentrations. This is accompanied by a corresponding decrease in Tc–TMeissner [13], as expected within the SVP model. Similar measurements are necessary in the lessstudied RuSr2R2yCeyCu2O10+d compounds. Current research on the ruthenocuprates has focused on RuSr2RCu2O8. This compound has two CuO2 planes in close proximity, where it has been argued that exchange interactions between Ru spins and carriers in the CuO2 planes are weak, and hence there is no significant pairbreaking by Ru moments. It has been suggested that the hole concentration in the CuO2 planes of RuSr2RCu2O8 is comparable to that observed in underdoped high temperature superconducting cuprates (HTSCs) [14,15]. There is also evidence of a mixed Ru valence between 4+ and 5+ [16,17] as well as itinerant conduction in the RuO2 planes [18]. The less-studied RuSr2R2yCeyCu2O10+d compound is better compared with single CuO2 layer HTSCs. This is because the two CuO2 layers are further apart, separated by insulating layers, and the unit cell contains a subunit that is similar to that of the single CuO2 plane electrondoped HTSC R2yCeyCuO4. The Ru valence is believed to be near 5+ [19,20], which may suggest that the half-filled e2g orbitals are insulating. However, it has been reported from a X-ray photoemission and absorption study that the RuO2 planes are conducting [21]. Partially substituting atomic species that suppress either the superconducting or magnetically ordered states is an ideal method to probe the normal, superconducting and magnetic states in RuSr2R2yCeyCu2O10+d. We have performed such a study and report the results from magnetization and transport measurements on Ru1xSnxSr2Gd1.4Ce0.6(Cu1zZnz)2O10+d and Ru1xSnxSr2Eu1.2Ce0.8(Cu1zZnz)2O10+d in this paper.
3. Results and analysis We first discuss the effect of Zn substitution for Cu on RuSr2Gd1.4Ce0.6Cu2O10+d and RuSr2Eu1.2Ce0.8Cu2O10+d. Partial Zn substitution for Cu in RuSr2Gd1.4Ce0.6Cu2O10+d leads to a rapid decrease in Tc, which can be seen in Fig. 1 where the normalized resistance is plotted for Zn concentrations increasing from 0% to 3% per CuO2 plane. Similar to previous studies on RuSr2RCu2O8, we take Tc as the temperature where the normalized resistance begins to decrease [15]. The resultant Tc values are plotted in the inset to Fig. 1 (solid circles). In addition, 5%Zn substitution for Cu in RuSr2Eu1.2Ce0.8Cu2O10+d suppresses Tc to 64.2 K, the lowest temperature that we could measure. While Zn substitution rapidly suppresses superconductivity, magnetization measurements (not shown) indicate that Zn has no effect on the temperature where the ferromagnetic component is observed. This is consistent with Zn substituting primarily for Cu in the CuO2 planes and the magnetic order originating in the RuO2 planes. Also shown in the inset to Fig. 1 is the dependence of Tc on Zn concentration that is observed in the HTSCs when the normal-state pseudogap energy is small (solid curve). In this case, the depression in Tc is given by the Abrikosov–Gorkov equation [22]. We find that the initial rate of decrease in Tc for RuSr2Gd1.4Ce0.6Cu2O10+d is dTc/dz = 10.9 K/%Zn. This is comparable to that observed in slightly underdoped Y0.8Ca0.2Ba2(Cu1zZnz)3O7d [23]. Increasing Zn concentrations leads to a semiconductorlike increase in the normalized resistance that has also been observed when impurities are substituted for Cu in the HTSCs [24] and may arise from increased scattering or charge localization. This suggests that the conductivity is dominated by conduction in the CuO2 planes and any contribution from the RuO2 planes is small. The effect of partial Sn substitution for Ru on the resistivity and magnetization data can be seen in Figs. 2 and 3.
2. Experimental details The Ru1xSnxSr2R2yCey(Cu1zZnz)2O10+d samples for this study were made using a method described elsewhere [11]. Electrical resistance measurements were made using a four-terminal technique. Room temperature thermopower measurements were done using the standard differential technique. The magnetization measurements were made using a SQUID magnetometer. The ac magnetization measurements were made with an ac field of 5 · 106 T and at a frequency of 333 Hz.
Fig. 1. Plot of the normalized electrical resistance against temperature for RuSr2Gd1.4Ce0.6(Cu1zZnz)2O10+d with z = 0, 0.01, 0.02 and 0.03. The arrow indicates the increasing Zn concentration. The data have been normalized to the same value at 260 K. Inset: plot of Tc against Zn concentration. The solid curve is a fit to the data as described in the text.
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Fig. 2. Plot of the normalized resistivity against temperature for (a) Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d with x = 0 (solid curve), x = 0.1 (dashed curve) and x = 0.2 (dotted curve), and (b) Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d with x = 0 (solid curve), x = 0.05 (dashed curve) and x = 0.1 (dotted curve). The data were normalized to the same value at 265 K.
Fig. 3. Plot of the real (a and c) and imaginary (b and d) parts of the ac susceptibility against temperature for (a and b) Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d with x = 0 (solid curve), x = 0.1 (dashed curve) and x = 0.2 (dotted curve) (c and d) Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d with x = 0 (solid curve), x = 0.05 (dashed curve) and x = 0.1 (dotted curve). The data have not been corrected for demagnetization effects.
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The normalized resistivity data in Fig. 2 shows that partial Sn substitution leads to a small change in Tc. The upturn in the resistivity curves of Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d (Fig. 2a) before the onset of superconductivity could be attributed to grain boundary effects. This upturn is not observed in Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d (Fig. 2b), which might be due to the fact that Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d has been annealed at 100 bar and the grains are better connected after high pressure annealing. The ferromagnetic component of the magnetic order is only visible in the real and imaginary components of the ac magnetization for temperatures below 90 K in RuSr2Gd1.4Ce0.6Cu2O10+d. Remarkably, there is no evidence of the ferromagnetic component for Sn concentrations of 5% or higher in the ac magnetization data (Figs. 3c and d) in Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d. This can be compared with Ru0.9Sn0.1Sr2Eu1.2Ce0.8Cu2O10+d where the ferromagnetic component is still observable below 60 K. A higher Sn fraction of 0.2 is required for the ferromagnetic component to disappear. The increase in the imaginary part of the magnetization at lower temperatures in all the samples is due to superconductivity. The loss of the ferromagnetic component is accompanied by an initial increase in the temperature where the bulk Meissner state commences, which will be discussed later. A rapid suppression of the ferromagnetic component by Sn in RuSr2Gd1.4Ce0.6Cu2O10+d is not found in RuSr2GdCu2O8, where the antiferromagnetic ordering temperature (132 K) is initially reduced at a rate of 5 K/%Sn [13]. For RuSr2GdCu2O8 the ferromagnetic component appears at the antiferromagnetic ordering temperature. The analysis is more complicated in RuSr2Gd1.4Ce0.6Cu2O10+d because the antiferromagnetic ordering temperature is believed to occur at 180 K, while the significant ferromagnetic component is only evident for temperatures below 100 K [3]. To characterize the loss of the ferromagnetic component, we use the temperature where dM/dT is a minimum. This temperature is 78 K in pure RuSr2Gd1.4Ce0.6Cu2O10+d, 87 K in pure RuSr2Eu1.2Ce0.8Cu2O10+d, and 30 K for Ru0.9Sn0.1Sr2Eu1.2Ce0.8Cu2O10+d. Thus, the rate at which the ferromagnetic component in RuSr2Gd1.4Ce0.6Cu2O10+d disappears is greater than 16 K/%Sn, while the initial suppression rate for RuSr2Eu1.2Ce0.8Cu2O10+d is merely 5.7 K/%Sn and comparable to that observed in Ru1xSnxSr2GdCu2O8. It is not clear why the ferromagnetic component in RuSr2Gd1.4Ce0.6Cu2O10+d is rapidly suppressed in this compound. The dependence of Tc (filled circles) and TMeissner (open circles) on Sn concentration for Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d can be seen in Fig. 4, where similar to a previous study on the pure compound [3], TMeissner is taken as the temperature where the real part of the ac susceptibility is zero. We find a large decrease in Tc–TMeissner for x P 0.05 in Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d and x = 0.2 in Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d, which can be seen from the right and left-hand insets of Fig. 4, respectively. The significantly smaller Tc–TMeissner only occurs for samples
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Fig. 4. Plot of Tc (filled circles) and TMeissner, the temperature where v 0 (open circles) is zero, against Sn concentration for Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d. Right-hand inset: plot of Tc–TMeissner against Sn concentration for Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d. Left-hand inset: plot of Tc–TMeissner against Sn concentration for Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d. The filled triangles in the insets denote Tc–TMeissner for samples where there is no evidence of the ferromagnetic component.
where there is no evidence of the ferromagnetic component (x P 0.05 for Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d and x P 0.2 for Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d) and Tc–TMeissner is comparable to that found in granular ceramic HTSCs. This result provides direct support for the SVP interpretation as applied to RuSr2R2yCeyCu2O10+d because TMeissner is only significantly below Tc when there is evidence of a ferromagnetic component. Our current and previous results [11] suggest that both RuSr2Gd1.4Ce0.6Cu2O10+d and RuSr2Eu1.2Ce0.8Cu2O10+d are underdoped. This is based on the assumption that the correlation between the room temperature thermopower, SRT, and the number of doped holes per Cu, p, observed in the HTSCs [25] is valid for RuSr2R2yCeyCu2O10+d. Thus, from SRT measured in RuSr2Gd1.4Ce0.6Cu2O10+d (11 lV K1) the value of p estimated from this correlation is 0.117. This decreases to 0.107 (SRT = 17 lV K1) for the x = 0.20 sample. Similarly, for Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d, p is estimated to be 0.102 for x = 0, 0.098 for x = 0.1 and 0.093 for x = 0.2. The value of p implies that both Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d and Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d are underdoped (p < 0.16) for all Sn concentrations and Sn substitution did not significantly change p. The Sn valence is 4+ and previous studies [19,20] indicate that the Ru valence is nearly 5+. Therefore, the partial substitution of Sn for Ru will introduce extra holes. If these holes go into the CuO2 planes and there is no change in the oxygen content, then p will increase by x/2, where x is the Sn concentration in Ru1xSnxSr2R2yCeyCu2O10+d. If we take p = 0.117 for RuSr2Gd1.4Ce0.6Cu2O10+d as deduced from our SRT measurement, then p for the Sn substituted RuSr2Gd1.4Ce0.6Cu2O10+d should be 0.142, 0.167, 0.217 for x = 0.05, 0.10 and 0.20, respectively. Similarly, the expected hole concentrations for Ru1xSnxSr2Eu1.2Ce0.8Cu2O10+d should be 0.152, 0.202 for x = 0.10 and x = 0.20, respectively. The expected hole concentra-
tions are clearly larger than those estimated from SRT. Based on the correlation between Tc and p observed in the HTSCs [26], for both RuSr2Gd1.4Ce0.6Cu2O10+d and RuSr2Eu1.2Ce0.8Cu2O10+d, the predicted increase in p with increasing x should lead to an initial increase in Tc that maximizes for the x = 0.10 sample and decreases for the x = 0.20 sample (since p = 0.16 is the optimum doping). In addition, since Tc = 37.8 K for the pure RuSr2Eu1.2Ce0.8Cu2O10+d, using the model proposed in Ref. [26], the calculated Tc of Ru0.9Sn0.1Sr2Eu1.2Ce0.8Cu2O10+d should be 52.1 K. These are clearly not experimentally observed. It is possible that the extra holes induced by Sn substitution might appear predominately in the RuO2 planes. This would have the effect of raising the valence of Ru. For example, simple valence counting indicates that the Ru valence is +5.19 for Ru0.8Sn0.2Sr2Eu1.2Ce0.8Cu2O10+d if we use the Ru valence estimated in the pure compound [20]. A Ru valence greater than +5 is not observed in the known compounds. We suggest a thorough valence study on the Sn-substituted Ru1xSnxSr2R2yCeyCu2O10+d. It is also possible that Sn substitution is accompanied by a decrease in the oxygen content leading to no change in the Ru valence and a small net decrease in the hole concentration in the CuO2 plane. This oxygen compensation effect has already been proposed to explain the nearly Ce-concentration-independent Tc values observed in RuSr2Gd2yCeyCu2O10+d for 0.4 6 y 6 0.8 [11]. 4. Conclusion In conclusion, we have used Zn substitution for Cu and Sn substitution for Ru to probe the normal, superconducting and magnetic states in RuSr2R2yCeyCu2O10+d. We find that the rate of Tc suppression by Zn is similar to that observed in slightly underdoped HTSCs. We also observed a rapid suppression of the ferromagnetic component when Sn is partially substituted for Ru. The rate of suppression for the Ru1xSnxSr2Gd1.4Ce0.6Cu2O10+d system is more than 3 times greater than that observed in RuSr2Eu1.2Ce0.8Cu2O10+d and RuSr2RCu2O8. The partial substitution of Ru by Sn has allowed us to test the spontaneous vortex phase model that has been applied to RuSr2Gd1.4Ce0.6Cu2O10+d. Our results show that when the temperature where the ferromagnetic component is observed is suppressed to zero, the onset of the bulk Meissner phase occurs close to Tc, which provides direct support for the spontaneous vortex phase model. Acknowledgements Funding support was provided by the New Zealand Marsden Fund (GVMW, SKG), the Alexander von Humboldt Foundation (GVMW), and the Korea Science and Engineering Foundation under Grant No. R05-2003-00010434-0 (HKL). We acknowledge a fees scholarship provided by Industrial Research Limited (SKG) and useful comments by Jeff Tallon.
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References [1] L. Bauernfeind, W. Widder, H.F. Braun, Physica C 254 (1995) 151. [2] L. Bauernfeind, W. Widder, H.F. Braun, J. Low, Temp. Phys. 105 (1996) 1605. [3] I. Felner, U. Asaf, O. Millo, Phys. Rev. B 55 (1997) 3374. [4] E.B. Sonnin, I. Felner, Phys. Rev. B 57 (1998) 14000. [5] I. Felner, U. Asaf, S. Reich, Y. Tsabba, Physica C 311 (1999) 163. [6] C. Bernhard, J.L. Tallon, Ch. Neidermayer, Th. Blasius, A. Golnik, E. Bru¨cher, D.R. Noakes, C.E. Stonach, E.J. Ansaldo, Phys. Rev. B 59 (1999) 14099. [7] C. Bernhard, J.L. Tallon, E. Bruecher, R.K. Kremer, Phys. Rev. B 61 (2000) 14960. [8] J.W. Lynn, B. Keimer, C. Ulrich, C. Bernhard, J.L. Tallon, Phys. Rev. B 61 (2000) 14964. [9] G.V.M. Williams, S. Kra¨mer, Phys. Rev. B 62 (2000) 4132. [10] J.D. Jorgensen, O. Chmaissem, H. Shaked, S. Short, P.W. Klamut, B. Dabrowski, J.L. Tallon, Phys. Rev. B 63 (2001) 54440. [11] G.V.M. Williams, M. Ryan, Phys. Rev. B 64 (2001) 94515. [12] A. Shengelaya, R. Khasanov, D.G. Eshchenko, I. Felner, U. Asaf, I.M. Savic´, H. Keller, A. Mu¨ller, Phys. Rev. B 69 (2004) 24517. [13] G.V.M. Williams, H.K. Lee, S. Kra¨mer, Phys. Rev. B 67 (2003) 104514. [14] S. Kramer, G.V.M. Williams, Physica C 377 (2002) 282.
519
[15] J.L. Tallon, J.W. Loram, G.V.M. Williams, C. Bernhard, Phys. Rev. B 61 (2000) 6471. [16] R.S. Liu, L.Y. Jang, H.H. Hung, J.L. Tallon, Phys. Rev. B 63 (2001) 212507. [17] A. Butera, A. Fainstein, E. Winkler, J.L. Tallon, Phys. Rev. B 63 (2001) 54442. [18] M. Pozˇek, A. Dulcˇic´, A. Hamzic´, M. Basletic´, E. Tafra, G.V.M. Williams, S. Kra¨mer, Phys. Rev. B 65 (2002) 174514. [19] I. Felner, U. Asaf, C. Godart, E. Alleno, Physica B 259–261 (1999) 703. [20] G.V.M. Williams, L.Y. Jang, R.S. Liu, Phys. Rev. B 65 (2002) 64508. [21] Y. Hirai, I. Zivkovic, B.H. Frazer, A. Reginelli, L. Perfetti, D. Ariosa, G. Margaritondo, M. Prester, D.T. Jiang, Y. Hu, T.K. Sham, I. Felner, M. Pederson, M. Onellion, Phys. Rev. B 65 (2002) 54417. [22] Y. Sun, K. Maki, Phys. Rev. B 51 (1995) 6059. [23] J.L. Tallon, C. Bernhard, G.V.M. Williams, J.W. Loram, Phys. Rev. Lett. 79 (1997) 5294. [24] G.V.M. Williams, J.L. Tallon, R. Dupree, R. Michalak, Phys. Rev. B 54 (1996) 9532. [25] J.L. Tallon, C. Bernhard, R.L. Hitterman, J.D. Jorgensen, Phys. Rev. B 51 (1995) 12911. [26] M.R. Presland, J.L. Tallon, R.G. Buckley, R.S. Liu, N.E. Flower, Physica C 176 (1991) 95.