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Lanthanide substitution by high pressure in the RuSr2GdCu2O8 magnetic superconductor
Physica C 382 (2002) 395–400 www.elsevier.com/locate/physc
Lanthanide substitution by high pressure in the RuSr2GdCu2O8 magnetic superconductor R. Ruiz-Bustos, J.M. Gallardo-Amores, R. S aez-Puche, * E. Moran, M.A. Alario-Franco Facultad de Ciencias Quımicas, Laboratorio Complutense de Altas Presiones and Departamento de Quımica Inorg anica I, Universidad Complutense de Madrid, 28040 Madrid, Spain Received 6 August 2001; received in revised form 3 January 2002; accepted 16 January 2002
Abstract A systematic study of the structural and magnetic properties of the family RuSr2 RECu2 O8 (RE ¼ Er, Ho, Y, Dy, Tb, Gd and Eu) has been performed. All these materials appear to be tetragonal (P4/mmm) and the unit cell volume decreases along with the lanthanide ion dimension. Differences are observed in the magnetic behaviour of these compounds. High pressure and high temperature are needed to synthesize most of the members of this family. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.72.)h; 74.25.Ha; 74.62.)c Keywords: Ruthenates–cuprates; High pressure; Superconductors magnetic materials; Lanthanides
1. Introduction Recent reports claiming the coexistence of superconductivity and magnetic order have focused attention on the properties of the so-called ruthenate–cuprate RuSr2 GdCu2 O8 [1,2]. This material has a tetragonal ‘‘1212’’ type structure (space group: P4/mmm) similar to that of YBa2 Cu3 O7 with [Cu–O2 ] chains being replaced by [Ru–O6 ] octahedral layers. The ruthenium atoms, which seem to be in an oxidation state intermediate between IV and V, order magnetically below a Curie temperature of 130–140 K while the ma-
terial becomes superconducting below a critical temperature (Tc 15–40 K), that seems to depend on the synthesis conditions and on the sample microstructure [3–6]. At room pressure, Sm, Eu and Gd are the only RE elements that accept to enter into the structure of RuSr2 GdCu2 O8 [3]. However by high pressure and high temperature we have performed several substitutions in the gadolinium site in order to extend the knowledge of the RuGd1212 family. In this way, we have been successful in replacing Gd by Tb, Dy, Y, Ho and Er.
2. Synthesis and characterization *
Corresponding author. Tel.: +34-1-394-4338; fax: +34-1394-4352. E-mail address: [email protected] (M.A. Alario-Franco).
A ‘‘belt-type’’ apparatus was used to carry out all experiments in the 20–80 Kbar and 1073–1473 K
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 2 3 3 - 9
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pressure and temperature ranges, respectively. Samples were prepared by reacting stoichiometric RuO2 , SrCuO2 and RE2 O3 (previously calcined between 1173 and 1273 K), according to RuO2 þ SrCuO2 þ RE2 O3 () RuSr2 RECu2 O8 :
ð1Þ
SrCuO2 was previously prepared by reacting strontium carbonate and copper oxide at 1323 K for three days in air. This step aids to minimize the formation of the SrRuO3 impurity phase, which once formed appears to be very stable. Sr2 RERuO6 is a useful precursor to avoid the formation of SrRuO3 for the room pressure synthesis. Nevertheless, these phases seem to be very stable under high pressure, therefore the use of SrCuO2 as precursor is to be preferred in our case. The reactants were placed in sealed gold capsules for experiments at or below 1373 K while platinum capsules were used for the higher temperatures. Best results were obtained for experiments performed at 60 Kbar and 1373 K. All samples have been characterized by means of XRD power diffraction performed either in a high resolution STOE transmission diffractometer using CuKa1 radiation (Y, Dy and Ho substitutions) or/and a Siemens D-500 diffractometer also equipped with a CuKa source (Fig. 1).
Fig. 1. Rietveld refinement fit of the CuKa X-ray diffraction data for (i) RuSr2 DyCu2 O8 , (ii) SrRuO3 and (iii) Cu2 DyO4 impurities are identified.
The X-ray diffraction patterns were all fitted by the Rietveld method [7] using a GSAS program [8] with the starting model previously reported for the RuSr2 GdCu2 O8 , see Fig. 1. The average crystal structure of RuSr2 RECu2 O8 is tetragonal with P4/ mmm space group symmetry. Backgrounds were fitted using a linear interpolation and the peak shapes were modelled using a pseudo-Voight function. Perovskite-type SrRuO3 appears in all samples, its amount being relatively low ( K 6%) in the Y, Dy and Ho samples. However, other members of the family present variable amounts of the SrRuO3 impurity in an extent between 8% and 60%. Magnetic properties were measured with a Quantum Design MPMS-XL SQUID magnetometer. The temperature dependence of the magnetic susceptibility was measured under an applied magnetic field of H ¼ 100 Oe in the temperature range from T ¼ 2 to 300 K.
3. Results and discussion High pressure and high temperatures are then needed for the synthesis of the Er, Ho, Y, Dy, Tb, compounds, while the Gd, Eu, Sm can be synthesized at ambient pressure: it then seems that high pressure stabilizes this family of phases when small cations are present. However, all samples are biphasic and the XRD patterns reveal the presence of SrRuO3 , a well known metallic and ferromagnetic perovskite material. Higher pressures or higher temperatures result in a higher proportion of this ruthenate. On the other hand, for pressures below 60 Kbar the RE ruthenate–cuprates are either much more impure or not obtained. Also CuRE2 O4 have been found in some samples and the double perovskite Sr2 RERuO6 [9,10] can be identified if temperatures higher than 1373 K are used. Table 1 shows the unit cell parameters and the ionic radii. Roughly linear relationships can be found between them. The unit cell volume of the RuSr2 RECu2 O8 family vs the cube of the rareearth trivalent radii for coordination number eight according to Shannon and Prewitt [11] gives an essentially linear relationship.
R. Ruiz-Bustos et al. / Physica C 382 (2002) 395–400 Table 1 Structural parameters derived from Rietveld refinement of Xray power diffraction data for RuSr2 RECu2 O8 ) ) 3 ) ) a (A c (A V (A v2 r (A Er Ho Y Dy Tb Gd Eua Sma a
3.8178(6) 3.82665(4) 3.82683(6) 3.8352(1) 3.8320(4) 3.8341(3) 3.843(2) 3.852(2)
11.521(4) 11.5341(3) 11.5303(3) 11.5354(7) 11.543(3) 11.5479(1) 11.55(1) 11.56(1)
167.93(8) 168.898(6) 168.856(7) 169.671(7) 169.51(5) 169.760(4) 170.58(3) 171.526(3)
2.82 1.79 2.89 1.05 1.61 1.51
1.004 1.015 1.019 1.027 1.040 1.053 1.066 1.079
Eu and Sm data from [3].
The appearance of this type of linear relationship is usually interpreted as due to an essentially ionic bonding. The lanthanide ions are characterized by having a common trivalent state in which the external f electrons are rather strongly attracted by the nucleus so that they do not usually participate in the bonding. Moreover, the lanthanides have the added interest of the ‘‘lanthanide contraction’’ resulting in a progressive reduction in size along the series [12]. In this connection it is not yet clear why some of the materials (in particular that of Dysprosium) depart from linearity. Concerning stoichiometry, it is usually assumed, in a first approximation, that ruthenium will be in the V oxidation state and copper as Cu(II). If this is the case, oxygen stoichiometry will be 8, i.e. RuSr2 GdCu2 O8 . Superconductivity will then be due to so-called self-doping [13]: Ru52p Sr2 Gd(Cu2þp )2 O8 . Indeed changing the Ru5þ /Ru4þ ratio and concomitantly the Cu3þ /Cu2þ could affect the superconducting properties and in particular Tc without affecting the oxygen content. Even more, the oxide chemistry of ruthenium is mostly concerned with the VIII (as in RuO4 [14]) and IV (as in RuO2 [15] or BaRuO3 [16]) oxidation states and little, if at all with oxidation state III [17]. That one will allow some oxygen under-stoichiometry ‘‘RuSr2 GdCu2 O8d ’’. Obviously if the labile oxygen was that of the Cu–O2 planes, i.e. affecting the Cu3þ / Cu2þ ratio, superconductivity will be affected as it is observed in all superconducting cuprates. It is worth pointing out that the cell parameters reported in the literature for the gadolinium
397
compound can be quite different, i.e. the a parameter may vary between 3.830 [6] and 3.841 [5], depending on the preparation conditions. This point is worth of further study.
4. Magnetic properties As representative examples the v vs T plots for the RuSr2 RECu2 O8 samples are given in Fig. 2(a), (b) and (c) for Eu, Ho and Y respectively. In all cases, the susceptibility obeys a Curie– Weiss law in the high temperature range (200–300 K). The obtained magnetic moments agree with the values calculated for the combination of the RE3þ and Ru5þ contributions; in the case of the Ho compound, the estimated value for Ru5þ is 1.57 lB after discounting Ho3þ contributions. The Eu sample was obtained as essentially pure (see below) and its magnetic behaviour (Fig. 2(a)) agrees with that reported previously for this compound [18] and, in order to compare it, with that of the other—new, but not so pure materials—it can be described as follows: The sharp increase in the susceptibility observed at around 150 K can be attributed to the Dzialoshinsky–Moriya ferromagnetic component due to the spin canting in the a–b plane of the antiferromagnetic ordering of the Ruthenium moments along the c-axis, as recently reported by Lynn et al. [19] in the case of the isostructural Gadolinium rutheno-cuprate. Although the origin of this canting is not yet well established, it is generally assumed to be due to the tilting of the [Ru–O6 ] octahedra out of the c-axis. A cusp is observed at this temperature since, below 140 K the magnetic susceptibility levels off due to the non-magnetic Eu 7 F0 ground term, so that the non-diagonal interactions of the van Vleck equation are solely responsible of the well known temperature independent paramagnetic susceptibility. It is worth mentioning that, below 35 K, the susceptibility signal, instead of going up—as usual due to the common gadolinium impurities that normally accompany europium—it goes down (Fig. 2(a)), something that we attribute to the superconducting properties of this RuSr2 RECu2 O8 sample.
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synthesis, in a decomposition process or both, according to RuSr2 RECu2 O8 () Sr2 RuREO6 þ 2CuO:
ð2Þ
The eventual presence of the very stable SrRuO3 perovskite is a rather common product of the HP synthesis; however it does not seem to affect the magnetic properties below 150 K. The v vs T plots for the Ho and Y samples appear on Fig. 2(b) and (c) respectively; the cusp, observed in the Eu case at 140 K, appears now at 148 and 141 K respectively. These are the Neel temperatures corresponding to the canted antiferromagnetic ordering of the Ruthenium sublattice in RuSr2 HoCu2 O8 and RuSr2 YCu2 O8 respectively. The estimated value of this ferromagnetic component is 0.1 lB, (Fig. 3 shows the magnetization curve corresponding to the Ho compound at 125 K, from which this value is extrapolated) in good agreement with the value recently obtained, by Lynn et al for the isostructural Gd compound by neutron diffraction [19]. Below TN v decreases and, at 50 K, a slight increase is observed reaching maxima at 31 K for Ho and 33 K for Y. These maxima can be attributed to the presence of Sr2 RuREO6 as a decomposition product (see Eq. (2) above). From X-ray diffraction (Fig. 4) we have observed that the amount of this double perovskite increases with time at room temperature conditions. All the RuSr2 RECu2 O8 HP phases seem then to be metastable at ambient
Fig. 2. DC susceptibility vs temperature measured at H ¼ 100 Oe and ZFC for (a) Eu, (b) Ho and (c) Y compounds.
The other two samples show a more complex behaviour due to the accompanying impurities that are either produced in the High Pressure
Fig. 3. Field dependence of the magnetization for the Ho sample.
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Fig. 4. X-ray diffraction patterns for the Y sample: (a) t ¼ 0, (b) t ¼ after 30 days and (c) after 90 days.
pressure and temperature. Magnetic susceptibility measurements for the Y sample, 90 days after its synthesis, show a much more developed peak due to the increasing amount of the Sr2 YRuO6 impurity (see inset in Fig. 2(c)). In order to confirm the presence of these magnetic impurities, Fig. 5 shows the v vs T plot for the pure Sr2 HoRuO6 compound; a marked increase in the value of v is observed, this peak at 33 K corresponds to the TN of this canted antiferromagnetic material. The estimated value of this canted ferromagnetic component is 0.3 lB (see the magnetization curve at 30 K on Fig. 3 corresponding to the Ho compound, from which this value is extrapolated). Even more, when the susceptibility of RuSr2 YCu2 O8 is measured both under field cooled (6 Oe, Fig. 6) and zero-field cooled conditions, before the decomposition starts, v decreases much faster and
Fig. 5. DC susceptibility measured under ZFC and FC conditions (100 Oe) for Sr2 RuHoO6 .
reaches negative values at 51 K, a most likely signature of the presence of superconductivity.
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Dr. J. Romero (UCM, technical staff) performed the SQUID measurements. R. Ruiz-Bustos thanks P. Attfield and A.C. McLaughlin for their help with the X-ray data.
References
Fig. 6. Magnetic susceptibility vs temperature for the Y compound FC (6 Oe) and ZFC measurements.
5. Conclusions We have performed several substitutions at the gadolinium site in RuSr2 GdCu2 O8 . Thanks to high pressure and high temperature conditions, we have succeeded in replacing Gd by Tb, Dy, Y, Ho and Er, all of them presenting similar structures to the gadolinium compound; as one could expect the unit cell volume reflects the lanthanide contraction in the Ru-1212 family. Magnetic susceptibility measurements reveal the existence of very interesting phenomena. Interpretation is, however, somewhat complicated due to the presence of magnetic impurities. The corresponding magnetic moments in the RuSr2 HoCu2 O8 compound are 0.1 lB for Ru and 0.3 lB for Ho. Such low values are then not incompatible with superconductivity.
Acknowledgements Financial support from Spanish research agency (CICYT, Proyect MAT98-0729) and a MEC grant given to R. Ruiz-Bustos made possible this work.
[1] C. Bernhard, J.L. Tallon, Ch. Niedermayer, Th. Blasius, A. Golnik, E. Br€ ucher, R.K. Kremer, D.R. Noakes, C.E. Stronach, E.J. Ansaldo, Phys. Rev. B 59 (1999) 14099. [2] A.C. McLaughlin, W. Zhou, J.P. Attfield, A.N. Fitch, J.L. Tallon, Phys. Rev. B 60 (1999) 7512. [3] L. Bauernfein, W. Widder, H.D. Braun, Physica C 254 (1995) 151. [4] J.L. Tallon, C. Bernhard, M.E. Bouwden, P.W. Giilberd, T.M. Soto, D.J. Pringle, IEEE Trans. Appl. Supercond. 9 (1999) 1696. [5] I. Felner, U. Asaf, S. Reich, Y. Tsabba, Physica C 311 (1999) 163. [6] O. Chmaissem, J.D. Jorgensen, H. Shaked, P. Dollar, J.L. Tallon, Phys. Rev. B 61 (9) (2000) 6401. [7] H.M. Rietveld, Acta Crystallogr. 22 (1967) 151. [8] A.C. Larson, R.B. Von Dreele, Los Alamos National Laboratory Report No. LA-UR-86-748, 1994. [9] P.C. Donohue, E.L. McCann, Mater. Res. Bull. 12 (1977) 519. [10] R. Ruiz-Bustos, J.M. Gallardo-Amores, E. Moran, J.P. . Alario-Franco, in: Mat. Res. Attfield, R. Saez-Puche, M.A Soc. Symp. Proc., vol. 659, 2001. [11] R.D. Shannon, C. Prewitt, Acta Cryst. B 26 (1970) 1046. [12] M.A. Alario-Franco, E. Moran Miguelez, R. Saez-Puche, F. Garcıa-Alvarado, U. Amador, M. Barahona, F. Fernandez, M.T. Perez-Frıas, J.L. Vicent, Mat. Res. Bull. 23 (1988) 313–321. [13] A.C. McLaughlin, J.P. Attfiel, Phys. Rev. B 60 (1999) 14605. [14] F.A. Cotton, G. Wilkinson, P.L. Gaus, Basic Inorganic Chemistry, Wiley, New York, 1995, p. 600. [15] F.A. Cotton, J.T. Mague, Inorg. Chem. 5 (1966) 317. [16] P.C. Donohue, L. Katz, R. Ward, Inorg. Chem. 4 (1965) 306. [17] N.N. Greenwood, A. Earnshaw, The chemistry of the elements, Pergamon Press, 1984, p. 1263. [18] G.V.M. Williams, S. Kramer, Phys. Rev. B 62 (2000) 4132. [19] J.W. Lynn, B. Keimer, C. Ulrich, C. Berhard, J.L. Tallon, Phys. Rev. B 61 (2000) 14964.
Lanthanide substitution by high pressure in the RuSr2GdCu2O8 magnetic superconductor R. Ruiz-Bustos, J.M. Gallardo-Amores, R. S aez-Puche, * E. Moran, M.A. Alario-Franco Facultad de Ciencias Quımicas, Laboratorio Complutense de Altas Presiones and Departamento de Quımica Inorg anica I, Universidad Complutense de Madrid, 28040 Madrid, Spain Received 6 August 2001; received in revised form 3 January 2002; accepted 16 January 2002
Abstract A systematic study of the structural and magnetic properties of the family RuSr2 RECu2 O8 (RE ¼ Er, Ho, Y, Dy, Tb, Gd and Eu) has been performed. All these materials appear to be tetragonal (P4/mmm) and the unit cell volume decreases along with the lanthanide ion dimension. Differences are observed in the magnetic behaviour of these compounds. High pressure and high temperature are needed to synthesize most of the members of this family. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.72.)h; 74.25.Ha; 74.62.)c Keywords: Ruthenates–cuprates; High pressure; Superconductors magnetic materials; Lanthanides
1. Introduction Recent reports claiming the coexistence of superconductivity and magnetic order have focused attention on the properties of the so-called ruthenate–cuprate RuSr2 GdCu2 O8 [1,2]. This material has a tetragonal ‘‘1212’’ type structure (space group: P4/mmm) similar to that of YBa2 Cu3 O7 with [Cu–O2 ] chains being replaced by [Ru–O6 ] octahedral layers. The ruthenium atoms, which seem to be in an oxidation state intermediate between IV and V, order magnetically below a Curie temperature of 130–140 K while the ma-
terial becomes superconducting below a critical temperature (Tc 15–40 K), that seems to depend on the synthesis conditions and on the sample microstructure [3–6]. At room pressure, Sm, Eu and Gd are the only RE elements that accept to enter into the structure of RuSr2 GdCu2 O8 [3]. However by high pressure and high temperature we have performed several substitutions in the gadolinium site in order to extend the knowledge of the RuGd1212 family. In this way, we have been successful in replacing Gd by Tb, Dy, Y, Ho and Er.
2. Synthesis and characterization *
Corresponding author. Tel.: +34-1-394-4338; fax: +34-1394-4352. E-mail address: [email protected] (M.A. Alario-Franco).
A ‘‘belt-type’’ apparatus was used to carry out all experiments in the 20–80 Kbar and 1073–1473 K
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 2 3 3 - 9
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pressure and temperature ranges, respectively. Samples were prepared by reacting stoichiometric RuO2 , SrCuO2 and RE2 O3 (previously calcined between 1173 and 1273 K), according to RuO2 þ SrCuO2 þ RE2 O3 () RuSr2 RECu2 O8 :
ð1Þ
SrCuO2 was previously prepared by reacting strontium carbonate and copper oxide at 1323 K for three days in air. This step aids to minimize the formation of the SrRuO3 impurity phase, which once formed appears to be very stable. Sr2 RERuO6 is a useful precursor to avoid the formation of SrRuO3 for the room pressure synthesis. Nevertheless, these phases seem to be very stable under high pressure, therefore the use of SrCuO2 as precursor is to be preferred in our case. The reactants were placed in sealed gold capsules for experiments at or below 1373 K while platinum capsules were used for the higher temperatures. Best results were obtained for experiments performed at 60 Kbar and 1373 K. All samples have been characterized by means of XRD power diffraction performed either in a high resolution STOE transmission diffractometer using CuKa1 radiation (Y, Dy and Ho substitutions) or/and a Siemens D-500 diffractometer also equipped with a CuKa source (Fig. 1).
Fig. 1. Rietveld refinement fit of the CuKa X-ray diffraction data for (i) RuSr2 DyCu2 O8 , (ii) SrRuO3 and (iii) Cu2 DyO4 impurities are identified.
The X-ray diffraction patterns were all fitted by the Rietveld method [7] using a GSAS program [8] with the starting model previously reported for the RuSr2 GdCu2 O8 , see Fig. 1. The average crystal structure of RuSr2 RECu2 O8 is tetragonal with P4/ mmm space group symmetry. Backgrounds were fitted using a linear interpolation and the peak shapes were modelled using a pseudo-Voight function. Perovskite-type SrRuO3 appears in all samples, its amount being relatively low ( K 6%) in the Y, Dy and Ho samples. However, other members of the family present variable amounts of the SrRuO3 impurity in an extent between 8% and 60%. Magnetic properties were measured with a Quantum Design MPMS-XL SQUID magnetometer. The temperature dependence of the magnetic susceptibility was measured under an applied magnetic field of H ¼ 100 Oe in the temperature range from T ¼ 2 to 300 K.
3. Results and discussion High pressure and high temperatures are then needed for the synthesis of the Er, Ho, Y, Dy, Tb, compounds, while the Gd, Eu, Sm can be synthesized at ambient pressure: it then seems that high pressure stabilizes this family of phases when small cations are present. However, all samples are biphasic and the XRD patterns reveal the presence of SrRuO3 , a well known metallic and ferromagnetic perovskite material. Higher pressures or higher temperatures result in a higher proportion of this ruthenate. On the other hand, for pressures below 60 Kbar the RE ruthenate–cuprates are either much more impure or not obtained. Also CuRE2 O4 have been found in some samples and the double perovskite Sr2 RERuO6 [9,10] can be identified if temperatures higher than 1373 K are used. Table 1 shows the unit cell parameters and the ionic radii. Roughly linear relationships can be found between them. The unit cell volume of the RuSr2 RECu2 O8 family vs the cube of the rareearth trivalent radii for coordination number eight according to Shannon and Prewitt [11] gives an essentially linear relationship.
R. Ruiz-Bustos et al. / Physica C 382 (2002) 395–400 Table 1 Structural parameters derived from Rietveld refinement of Xray power diffraction data for RuSr2 RECu2 O8 ) ) 3 ) ) a (A c (A V (A v2 r (A Er Ho Y Dy Tb Gd Eua Sma a
3.8178(6) 3.82665(4) 3.82683(6) 3.8352(1) 3.8320(4) 3.8341(3) 3.843(2) 3.852(2)
11.521(4) 11.5341(3) 11.5303(3) 11.5354(7) 11.543(3) 11.5479(1) 11.55(1) 11.56(1)
167.93(8) 168.898(6) 168.856(7) 169.671(7) 169.51(5) 169.760(4) 170.58(3) 171.526(3)
2.82 1.79 2.89 1.05 1.61 1.51
1.004 1.015 1.019 1.027 1.040 1.053 1.066 1.079
Eu and Sm data from [3].
The appearance of this type of linear relationship is usually interpreted as due to an essentially ionic bonding. The lanthanide ions are characterized by having a common trivalent state in which the external f electrons are rather strongly attracted by the nucleus so that they do not usually participate in the bonding. Moreover, the lanthanides have the added interest of the ‘‘lanthanide contraction’’ resulting in a progressive reduction in size along the series [12]. In this connection it is not yet clear why some of the materials (in particular that of Dysprosium) depart from linearity. Concerning stoichiometry, it is usually assumed, in a first approximation, that ruthenium will be in the V oxidation state and copper as Cu(II). If this is the case, oxygen stoichiometry will be 8, i.e. RuSr2 GdCu2 O8 . Superconductivity will then be due to so-called self-doping [13]: Ru52p Sr2 Gd(Cu2þp )2 O8 . Indeed changing the Ru5þ /Ru4þ ratio and concomitantly the Cu3þ /Cu2þ could affect the superconducting properties and in particular Tc without affecting the oxygen content. Even more, the oxide chemistry of ruthenium is mostly concerned with the VIII (as in RuO4 [14]) and IV (as in RuO2 [15] or BaRuO3 [16]) oxidation states and little, if at all with oxidation state III [17]. That one will allow some oxygen under-stoichiometry ‘‘RuSr2 GdCu2 O8d ’’. Obviously if the labile oxygen was that of the Cu–O2 planes, i.e. affecting the Cu3þ / Cu2þ ratio, superconductivity will be affected as it is observed in all superconducting cuprates. It is worth pointing out that the cell parameters reported in the literature for the gadolinium
397
compound can be quite different, i.e. the a parameter may vary between 3.830 [6] and 3.841 [5], depending on the preparation conditions. This point is worth of further study.
4. Magnetic properties As representative examples the v vs T plots for the RuSr2 RECu2 O8 samples are given in Fig. 2(a), (b) and (c) for Eu, Ho and Y respectively. In all cases, the susceptibility obeys a Curie– Weiss law in the high temperature range (200–300 K). The obtained magnetic moments agree with the values calculated for the combination of the RE3þ and Ru5þ contributions; in the case of the Ho compound, the estimated value for Ru5þ is 1.57 lB after discounting Ho3þ contributions. The Eu sample was obtained as essentially pure (see below) and its magnetic behaviour (Fig. 2(a)) agrees with that reported previously for this compound [18] and, in order to compare it, with that of the other—new, but not so pure materials—it can be described as follows: The sharp increase in the susceptibility observed at around 150 K can be attributed to the Dzialoshinsky–Moriya ferromagnetic component due to the spin canting in the a–b plane of the antiferromagnetic ordering of the Ruthenium moments along the c-axis, as recently reported by Lynn et al. [19] in the case of the isostructural Gadolinium rutheno-cuprate. Although the origin of this canting is not yet well established, it is generally assumed to be due to the tilting of the [Ru–O6 ] octahedra out of the c-axis. A cusp is observed at this temperature since, below 140 K the magnetic susceptibility levels off due to the non-magnetic Eu 7 F0 ground term, so that the non-diagonal interactions of the van Vleck equation are solely responsible of the well known temperature independent paramagnetic susceptibility. It is worth mentioning that, below 35 K, the susceptibility signal, instead of going up—as usual due to the common gadolinium impurities that normally accompany europium—it goes down (Fig. 2(a)), something that we attribute to the superconducting properties of this RuSr2 RECu2 O8 sample.
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synthesis, in a decomposition process or both, according to RuSr2 RECu2 O8 () Sr2 RuREO6 þ 2CuO:
ð2Þ
The eventual presence of the very stable SrRuO3 perovskite is a rather common product of the HP synthesis; however it does not seem to affect the magnetic properties below 150 K. The v vs T plots for the Ho and Y samples appear on Fig. 2(b) and (c) respectively; the cusp, observed in the Eu case at 140 K, appears now at 148 and 141 K respectively. These are the Neel temperatures corresponding to the canted antiferromagnetic ordering of the Ruthenium sublattice in RuSr2 HoCu2 O8 and RuSr2 YCu2 O8 respectively. The estimated value of this ferromagnetic component is 0.1 lB, (Fig. 3 shows the magnetization curve corresponding to the Ho compound at 125 K, from which this value is extrapolated) in good agreement with the value recently obtained, by Lynn et al for the isostructural Gd compound by neutron diffraction [19]. Below TN v decreases and, at 50 K, a slight increase is observed reaching maxima at 31 K for Ho and 33 K for Y. These maxima can be attributed to the presence of Sr2 RuREO6 as a decomposition product (see Eq. (2) above). From X-ray diffraction (Fig. 4) we have observed that the amount of this double perovskite increases with time at room temperature conditions. All the RuSr2 RECu2 O8 HP phases seem then to be metastable at ambient
Fig. 2. DC susceptibility vs temperature measured at H ¼ 100 Oe and ZFC for (a) Eu, (b) Ho and (c) Y compounds.
The other two samples show a more complex behaviour due to the accompanying impurities that are either produced in the High Pressure
Fig. 3. Field dependence of the magnetization for the Ho sample.
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399
Fig. 4. X-ray diffraction patterns for the Y sample: (a) t ¼ 0, (b) t ¼ after 30 days and (c) after 90 days.
pressure and temperature. Magnetic susceptibility measurements for the Y sample, 90 days after its synthesis, show a much more developed peak due to the increasing amount of the Sr2 YRuO6 impurity (see inset in Fig. 2(c)). In order to confirm the presence of these magnetic impurities, Fig. 5 shows the v vs T plot for the pure Sr2 HoRuO6 compound; a marked increase in the value of v is observed, this peak at 33 K corresponds to the TN of this canted antiferromagnetic material. The estimated value of this canted ferromagnetic component is 0.3 lB (see the magnetization curve at 30 K on Fig. 3 corresponding to the Ho compound, from which this value is extrapolated). Even more, when the susceptibility of RuSr2 YCu2 O8 is measured both under field cooled (6 Oe, Fig. 6) and zero-field cooled conditions, before the decomposition starts, v decreases much faster and
Fig. 5. DC susceptibility measured under ZFC and FC conditions (100 Oe) for Sr2 RuHoO6 .
reaches negative values at 51 K, a most likely signature of the presence of superconductivity.
400
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Dr. J. Romero (UCM, technical staff) performed the SQUID measurements. R. Ruiz-Bustos thanks P. Attfield and A.C. McLaughlin for their help with the X-ray data.
References
Fig. 6. Magnetic susceptibility vs temperature for the Y compound FC (6 Oe) and ZFC measurements.
5. Conclusions We have performed several substitutions at the gadolinium site in RuSr2 GdCu2 O8 . Thanks to high pressure and high temperature conditions, we have succeeded in replacing Gd by Tb, Dy, Y, Ho and Er, all of them presenting similar structures to the gadolinium compound; as one could expect the unit cell volume reflects the lanthanide contraction in the Ru-1212 family. Magnetic susceptibility measurements reveal the existence of very interesting phenomena. Interpretation is, however, somewhat complicated due to the presence of magnetic impurities. The corresponding magnetic moments in the RuSr2 HoCu2 O8 compound are 0.1 lB for Ru and 0.3 lB for Ho. Such low values are then not incompatible with superconductivity.
Acknowledgements Financial support from Spanish research agency (CICYT, Proyect MAT98-0729) and a MEC grant given to R. Ruiz-Bustos made possible this work.
[1] C. Bernhard, J.L. Tallon, Ch. Niedermayer, Th. Blasius, A. Golnik, E. Br€ ucher, R.K. Kremer, D.R. Noakes, C.E. Stronach, E.J. Ansaldo, Phys. Rev. B 59 (1999) 14099. [2] A.C. McLaughlin, W. Zhou, J.P. Attfield, A.N. Fitch, J.L. Tallon, Phys. Rev. B 60 (1999) 7512. [3] L. Bauernfein, W. Widder, H.D. Braun, Physica C 254 (1995) 151. [4] J.L. Tallon, C. Bernhard, M.E. Bouwden, P.W. Giilberd, T.M. Soto, D.J. Pringle, IEEE Trans. Appl. Supercond. 9 (1999) 1696. [5] I. Felner, U. Asaf, S. Reich, Y. Tsabba, Physica C 311 (1999) 163. [6] O. Chmaissem, J.D. Jorgensen, H. Shaked, P. Dollar, J.L. Tallon, Phys. Rev. B 61 (9) (2000) 6401. [7] H.M. Rietveld, Acta Crystallogr. 22 (1967) 151. [8] A.C. Larson, R.B. Von Dreele, Los Alamos National Laboratory Report No. LA-UR-86-748, 1994. [9] P.C. Donohue, E.L. McCann, Mater. Res. Bull. 12 (1977) 519. [10] R. Ruiz-Bustos, J.M. Gallardo-Amores, E. Moran, J.P. . Alario-Franco, in: Mat. Res. Attfield, R. Saez-Puche, M.A Soc. Symp. Proc., vol. 659, 2001. [11] R.D. Shannon, C. Prewitt, Acta Cryst. B 26 (1970) 1046. [12] M.A. Alario-Franco, E. Moran Miguelez, R. Saez-Puche, F. Garcıa-Alvarado, U. Amador, M. Barahona, F. Fernandez, M.T. Perez-Frıas, J.L. Vicent, Mat. Res. Bull. 23 (1988) 313–321. [13] A.C. McLaughlin, J.P. Attfiel, Phys. Rev. B 60 (1999) 14605. [14] F.A. Cotton, G. Wilkinson, P.L. Gaus, Basic Inorganic Chemistry, Wiley, New York, 1995, p. 600. [15] F.A. Cotton, J.T. Mague, Inorg. Chem. 5 (1966) 317. [16] P.C. Donohue, L. Katz, R. Ward, Inorg. Chem. 4 (1965) 306. [17] N.N. Greenwood, A. Earnshaw, The chemistry of the elements, Pergamon Press, 1984, p. 1263. [18] G.V.M. Williams, S. Kramer, Phys. Rev. B 62 (2000) 4132. [19] J.W. Lynn, B. Keimer, C. Ulrich, C. Berhard, J.L. Tallon, Phys. Rev. B 61 (2000) 14964.