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Suppression of superconductivity by nonmagnetic impurities, structural properties and magnetic ordering in HoxLa1−xNi2B2C
Physica C 315 Ž1999. 91–98
Suppression of superconductivity by nonmagnetic impurities, structural properties and magnetic ordering in Ho x La 1yx Ni 2 B 2 C J. Freudenberger a,) , A. Kreyssig b, C. Ritter c , K. Nenkov a,1, S.-L. Drechsler a , a G. Fuchs a , K.-H. Muller , M. Loewenhaupt b, L. Schultz a ¨ a
Institut fur und Werkstofforschung Dresden, Postfach 270016, D-01171 Dresden, Germany ¨ Festkorper¨ b Institut fur ¨ Angewandte Physik, TU Dresden, D-01062 Dresden, Germany c Institut M.V. Laue, P. LangeÕin, 156 X, F-38042 Grenoble, France Received 24 December 1998; received in revised form 19 February 1999; accepted 26 February 1999
Abstract Superconducting and magnetic properties as well as the lattice structure of polycrystalline Ho x La 1yx Ni 2 B 2 C samples have been studied by susceptibility, X-ray, and neutron diffraction measurements, respectively. A miscibility gap has been found between x s 0.4 and x s 0.7 if the samples are prepared by standard arc-melting. Samples in this concentration range have been successfully prepared by melt-spinning. All samples exhibit the LuNi 2 B 2 C-type structure and the lattice parameters depend linearly on the La content. Already a low lanthanum concentration of about 10% leads to a rapid depression of Tc from 8.5 K to 2 K. Furthermore this concentration leads to a complete depression of the a-axis modulated incommensurate antiferromagnetic structure. Two other magnetic structures, one with commensurate Ž0 0 1. and one with incommensurate wave vector Ž0 0 0.915. were found to be less sensitive to the La concentration. The neutron-diffraction peak intensities connected to these structures decrease with increasing La content and disappear at x s 0.75 Žcommensurate structure. and x s 0.55 Žincommensurate structure.. A comparison with Ho x R 1yx Ni 2 B 2 C compounds ŽR s Y, Lu. shows that these two magnetic structures are influenced in different way by the size of the nonmagnetic rare-earth ion R. No size effect was observed for the magnetic ordering temperature of the commensurate antiferromagnetic structure, whereas the incommensurate spiral structure along c-axis was found to be sensitive to the difference in the size of the R and the Ho ions. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Ho x La 1yx Ni 2 B 2 C; Superconductivity; Nonmagnetic impurities
1. Introduction The discovery of superconductivity in rare earth transition metal borocarbides has stimulated much )
Corresponding author. Tel.: q49-351-4659-553; Fax: q49351-4659-537; E-mail: [email protected] 1 On leave from: Int. Lab. of High Magn. Fields, Wroclaw; ISSP-BAS, Sofia.
research on these compounds. Of particular interest is HoNi 2 B 2 C because of its fascinating interplay between superconductivity and complex magnetic structures. Three different types of antiferromagnetic structures have been observed by neutron scattering, Ži. a commensurate structure, Žii. an incommensurate spiral along the c-axis and Žiii. a further incommensurate magnetic component that develops along the a-axis w1,2x. Whereas the two incommensurate mag-
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 2 0 8 - 7
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netic components coexist only in a finite temperature range between 7 K and 4.5 K, the commensurate magnetic structure appears below about 8 K and shows a saturation behaviour at low temperatures. On the other hand, HoNi 2 B 2 C becomes superconducting at Tc , 8.5 K, then reenters the normal state in a narrow temperature interval of about 4.3 to 5.5 K to become superconducting again with further reduction of temperature w3x. Below this characteristic temperature of about 4.5 K, only the commensurate antiferromagnetic structure remains coexisting with the superconducting state. The spiral structure along the c-axis w4x as well as the component along the a-axis w5x have been considered to be responsible for the reentrant behaviour observed in HoNi 2 B 2 C. Consequently, the question arose which of these two incommensurate structures is responsible for the suppression of the superconductivity. This problem has been clarified recently by investigating polycrystalline samples of pseudo-quaternary Ho xY1yxNi 2 B 2 C compounds w6,7x. The three distinct types of antiferromagnetic order, mentioned above, have also been detected for x - 1. Fortunately, the temperature ranges where they exist are sensitive to the Ho concentration and these x-dependencies have been found to be different for the different types of magnetic order. For this particular Ho xY1yx Ni 2 B 2 C system not only the commensurate antiferromagnetic state but also the incommensurate c-axis spiral can coexist with superconductivity whereas the incommensurate a-axis component is strongly correlated to the suppression of superconductivity w6,7x. A linear decrease of the superconducting transition temperature Tc with increasing x has been observed for the Ho xY1yx Ni 2 B 2 C samples w8x. On the other hand, for Lu diluted HoNi 2 B 2 C the dependence of Tc on the concentration is more complicated w9,10x. This different behaviour has been attributed to differences in the lattice parameters of the Ho–Y and the Ho–Lu systems which result in distinct electronic structures controlling the exchange interaction between the magnetic ions as well as the depression of Tc w9x. In this context also, the investigation of the Ho xLa 1y x Ni 2 B 2 C system is of interest because La, being nonmagnetic as Y and Lu, not only has the largest ionic radius of all rare earth ions but also the final LaNi 2 B 2 C compound is nonsuperconducting. In the present paper, the influence of lanthanum on
the superconducting and magnetic properties of Ho x La 1yx Ni 2 B 2 C compounds has been studied.
2. Experimental details Polycrystalline Ho x La 1yx Ni 2 B 2 C samples were prepared by a standard arc melting technique. 11 B was used instead of the natural B to reduce the absorption for neutron scattering experiments Žthis holds for all samples mentioned in this paper.. Powders of the elements were weighted in the stoichiometric compositions with a surplus of 10 wt.% boron to compensate the high losses of boron caused by the arc melting. The powder was pressed to pellets that were melted under argon gas on a water-cooled copper plate in an arc furnace. To get homogeneous samples, they were turned over and melted again four times. After the melting procedure the solidified samples were homogenized at 11008C for 10 days. Using this method single-phase samples can be obtained only for Ho concentrations below x s 0.4 and above x s 0.7. Samples in the range of 0.4 - x - 0.7 consist of two phases with two lattice structures belonging to Ho concentrations of x s 0.4 and x s 0.7, respectively. Thus, single-phase samples can be prepared by standard arc melting only for Ho concentrations out of this miscibility gap. The rapid quenching single-roller melt-spinning technique was used to suppress this phase separation. For this purpose ingots of nominal composition were prepared by arc melting as above. Each ingot was heated at radio frequency in a quartz jet over the melting temperature and then pressed out by an overpressure. The melt was quickly solidified by getting into contact with the copper wheel. The actual sample composition is uncertain due to evaporation losses during melting. Owing to this, the nominal composition, in particular the nominal Ho content x, is given in this paper. To investigate the lattice structures Žat room temperature. as well as the phase purity of the samples, a powder X-ray diffractometer in Bragg–Brentano geometry was used. The X-ray diffraction experiments were done on crushed powders using CoK a Ž l s 0.1789 nm. radiation. The scans were taken from 2Q s 208 up to 1408 in steps of DQ s 0.0158. Finally the lattice parameters for the regarded phases
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in the samples were determined from measured diffraction data using DBWS Rietveld program. Powder neutron diffraction experiments were carried out at the multi-detector instrument D20 at the ILL, Grenoble, to investigate the magnetic as well as the nuclear structures. The scans were taken at several temperatures below and above the magnetic ordering temperature in a range of 2Q between 58 and 1608 using a graphite monochromator Ž l s 0.241 nm.. The powder samples were placed in cylindrical sample holders ŽVanadium, f s 8 mm.. The samples were first cooled down to the lowest temperature of 1.5 K and then heated in temperature steps of 0.3 K up to 15 K. Each diffraction pattern was measured for about 15 min. The superconducting properties of the samples were determined by susceptibility and resistivity measurements. The AC susceptibility x was measured in a temperature range between 4.5 K and 300 K at different DC magnetic fields using an AC field amplitude of 0.01 mT. To determine the superconducting transition for samples with lower Tc , the standard four-probe technique was utilized to measure the temperature dependence of the resistance down to 1.4 K.
3. Results and discussion
Fig. 1. X-ray diffraction patterns around the Ž112. peak for different Ho concentrations. Upper part: arc molten samples; lower part: melt spun samples.
The phase purity of the investigated Ho xLa 1y x Ni 2 B 2 C samples was checked by X-ray diffraction. No significant fraction of phase impurities was found. The X-ray structure analysis revealed the LuNi 2 B 2 C structure type w11x Žspace group I4rmmm. in the whole range of Ho concentrations. For concentrations within the miscibility gap Ž0.4 F x F 0.7. two patterns with I4rmmm space group symmetry were found. In the upper part of Fig. 1, cuts of diffraction patterns near the Ž112. peak are shown for different Ho x La 1yx Ni 2 B 2 C sample compositions prepared by arc melting. The patterns for the two Ho concentrations x s 0.4 and x s 0.6 reveal a splitting of the Ž112. peak into two peaks that can be related to two structures with different lattice parameters. The miscibility gap found in the Ho x La 1yx Ni 2 B 2 C series in the range of 0.4 F x F 0.7 may be caused by the large difference of the temperatures of peritectic phase formation for LaNi 2 B 2 C and HoNi 2 B 2 C. The
peritectic temperature T P , 13308C of LaNi 2 B 2 C w12x is much lower than that obtained for HoNi 2 B 2 C ŽT P s 15268C.. Assuming conditions of a thermodynamic equilibrium HoNi 2 B 2 C is expected to solidify separately. Only samples with compositions near that of the pure phase solidify in the given composition which can be explained by the reduced diffusion of the minor phase in the melt. In the case of rapid quenching, the diffusion in the melt during cooling through the temperature range between the two peritectic temperatures can be kept very small. Therefore, the rapid quenching melt-spinning technique was used in order to suppress the miscibility gap. In the lower part of Fig. 1, three cuts of diffraction patterns of melt-spun samples are shown around the Ž112. Bragg reflection. The pure melt-spun samples exhibit larger peak widths of the Ž112. peaks than the arc-molten material, but the positions of corresponding peaks agree. The diffraction pattern of
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the melt-spun sample for x s 0.5 reveal only one peak. No splitting of this peak could be detected by in situ high-temperature X-ray analysis carried out in Debye–Scherrer geometry. However, the broad foot of this peak indicates small amounts of compositions near the borders of the miscibility gap or mostly fine-crystalline material. Thus, the cooling rate obtained by the melt-spinning procedure still remains insufficient to produce a single-phase sample. The structure obtained for x s 0.5 by melt spinning was found to be stable during tempering below the eutectic temperature of about T , 10208C, but it starts to decay above this temperature. In Fig. 2, the lattice parameters a and c are shown as function of the Ho concentration. The standard deviations of the Rietveld analysis result in errors that are related to the peak widths. They are shown in Fig. 2 as error bars. Peaks corresponding to phases near or within the miscibility gap have dramatically enlarged widths compared with the peaks of the pure RNi 2 B 2 C ŽR s Ho, La. compounds. The lattice parameters for the two pure RNi 2 B 2 C compounds are in good agreement with those obtained by other groups w11x. The dashed lines in Fig. 2 indicate the almost linear scaling of the lattice pa-
Fig. 2. Lattice parameters a Žv . and c ŽI. of the arc-molten Ho x La 1yx Ni 2 B 2 C samples versus Ho concentration. Additionally, the lattice parameters a Ž`. and c ŽB. of the melt-spun sample with x s 0.5 are shown. The concentration range of the miscibility gap is marked gray. Each couple of lattice parameters Ž a, c . that belong to one lattice are marked with the same Greek symbol.
Fig. 3. Susceptibility of Ho x La 1yx Ni 2 B 2 C samples Ž x s 0.95, 0.97, 1. in dependence on temperature. In the inset the resistivity is shown for x s 0.9 and 0.8.
rameters with x in the range of small values of x F 0.2. The lattice parameters of the melt-spun Ho 0.5 La 0.5 Ni 2 B 2 C sample shown in Fig. 2 with inverse symbols and without error bars lie exactly on these dashed lines. The miscibility gap Ž0.4 F x F 0.7. is marked gray. Deviations from the linear scaling of the lattice parameters with x are observed for large Ho concentrations x G 0.75. In Fig. 3, the temperature dependence of the susceptibility is shown for three Ho x La 1yx Ni 2 B 2 C samples with Ho concentrations x G 0.95. In the inset, the temperature dependence of the resistivity is displayed for x s 0.9 and 0.8. The LaNi 2 B 2 C compound is nonmagnetic as well as nonsuperconducting w11x. Therefore, it is natural that superconductivity of HoNi 2 B 2 C is suppressed substituting a large amount of Ho by La. But it is surprising that only 10% lanthanum is sufficient for a suppression of Tc from 8.5 K to 2 K in Ho xLa 1y x Ni 2 B 2 C, as can be seen in Figs. 3 and 5. Reentrant behaviour is observed in weakly diluted samples. The onset temperature of the superconducting transition of the sample with a nominal composition of Ho 0.97 La 0.03 Ni 2 B 2 C shifts to a value of about 7.5 K. With decreasing temperature the susceptibility decreases, goes through a minimum at a
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temperature of about 6 K and rises again for decreasing temperature. Traces of superconductivity and reentrant behaviour are still visible in the x ŽT . curve of the sample with x s 0.95. The magnetic structure of some samples of the Ho x La 1yx Ni 2 B 2 C system has been measured by neutron diffraction. For HoNi 2 B 2 C, the well known three antiferromagnetic structures have been found: the structure with commensurate wave vector t 1 s Ž0 0 1. was observed below 9 K. Two incommensurate modulated structures with t 2 s Ž0 0 0.915. and t 3 s Ž0.585 0 0. exist between 4.8 K and 9 K. In Fig. 4, the peak intensities connected to the incommensurate c-axis modulated spiral Župper part. and to the commensurate antiferromagnetic structure along c-axis Žlower part. are shown for four different Ho concentrations. The peak intensities for both structures decrease with decreasing Ho content and are below the detection limit for x - 0.6. It should be noted that the temperature dependence of the c-axis spiral structure Žt 2 . which exists for x s 1 only in a narrow temperature range, is quite different for x F 0.9: its intensity increases with decreasing temperature down to the lowest measured temperature. Fur-
Fig. 4. Temperature dependence of the neutron-diffraction peak intensities of the incommensurate Žt 2 . and the commensurate Žt 1 . magnetic reflections of Ho x La 1yx Ni 2 B 2 C samples normalized to the Ž002. nuclear reflection for x s1, 0.9, 0.8 and 0.6.
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Fig. 5. Variation of the different ordering temperatures in dependence on the Ho concentration in Ho x La 1yx Ni 2 B 2 C Ž0.6 F x F 1.: superconducting ordering Tc , Temperature of the commensurate Tt 1 and incommensurate Tt 2 antiferromagnetic structures and the reentrant temperature Tre .
thermore, the incommensurate a-axis modulated structure observed in HoNi 2 B 2 C does not exist in the Ho x La 1yx Ni 2 B 2 C samples with x F 0.9 investigated, so far, by neutron scattering. Thus, this magnetic structure disappears already at La concentrations smaller than 10%. The values for the pure HoNi 2 B 2 C sample are taken from former measurements at the multi-detector instrument D1B at the ILL Grenoble w10x. It has not yet been checked directly whether in Ho x La 1yx Ni 2 B 2 C with sample compositions of x s 0.97 and x s 0.95 an incommensurate a-axis modulated antiferromagnetic structure exists. But this is suggested by the similarity of the susceptibility data of these samples and the data for Ho x R 1yx Ni 2 B 2 C ŽR s Y, Lu. w7,10x. The dependence of the ordering temperatures of the different magnetic structures as well as of the superconducting transition temperatures of Ho xLa 1y x Ni 2 B 2 C on the Ho concentration is shown in Fig. 5 in the range of x G 0.6. Superconductivity in Ho x La 1y x Ni 2 B 2 C is strongly suppressed by a small amount of lanthanum. The magnetic ordering temperatures of the magnetic
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structures along the c-axis show a linear decrease with decreasing holmium content. The breakdown of the a-axis modulated structure in the range 0.9 - x - 1 indicates a comparably strong influence of lanthanum on this magnetic structure as observed for the influence of La on the superconductivity in this system. The superconducting and magnetic ordering temperatures of Ho x La 1yx Ni 2 B 2 C depend on the concentration of the magnetic Ho ions and on the lattice parameters which are strongly affected by the large size of the La ions. In order to separate these two effects the influence of nonmagnetic impurities R with different size on the superconducting and magnetic properties of Ho x R 1yx Ni 2 B 2 C were compared. The experimental results for R s Y, Lu and La are shown in Figs. 6 and 7 for the magnetic and superconducting ordering temperatures, respectively. The dependence of the properties of Ho xY1yx Ni 2 B 2 C on the Ho concentration is relatively simple because Y 3q has approximately the same ionic radius as Ho 3q. In this case, an almost linear scaling of the magnetic and superconducting ordering temperatures with x has been found w6,8x. Substituting Ho by R s La leads to an expansion of the a-axis and a contraction of the c-axis whereas for R s Lu the opposite trends have been found w9x. It should also be noted that the lattice parameters of Ho x R 1yx Ni 2 B 2 C are much stronger influenced for R s La than for R s Lu. The lattice parameter a, for instance, increases by 7.8% for R s La, but decreases only by 1.6% for R s Lu, if x changes from 1 to 0. It is surprising that the very different size of the lattice parameters for the Ho x R 1yx Ni 2 B 2 C compounds with R s Y, Lu and La has almost no effect on the magnetic ordering temperature of the commensurate antiferromagnetic structure as shown in Fig. 6a. The same linear decrease of the magnetic ordering temperature with decreasing Ho content is observed between x s 1 and x s 0.8 for all these compounds. No commensurate magnetic structure was detected below x s 0.75. On the other hand, the ordering temperature of the incommensurate spiral structure along the c-axis Žt 2 . is sensitive to the difference in the size of the rare-earth ions: the slope of the Tt 2Ž x . curve in Fig. 6b increases with increasing difference between the size of the R ion and the Ho ion. However, it should also be noted that the
Fig. 6. Ordering temperatures of Ža. the c-axis modulated commensurate, Žb. the c-axis modulated incommensurate and Žc. the a-axis modulated incommensurate magnetic structure versus Ho concentration for Ho x R 1yx Ni 2 B 2 C with R s La, Y w6x and Lu w9x. The dashed line in Žc. stands for the fact that this magnetic structure could not be seen by neutron-diffraction for x F 0.9 and T G1.8 K.
influence of decreasing Ho concentration on the magnetic ordering temperature of the incommensurate spiral structure is distinctly weaker than in the case of the commensurate structure. The spiral structure was detected down to x s 0.6. Very different results were found for the a-axis modulated structure for compounds with R s Lu and La. As shown in Fig. 6c, no size effect seems to be present for the a-axis modulated incommensurate structure of Ho x R 1yx Ni 2 B 2 C for R s Lu or Y in the range of
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Fig. 7. Superconducting transition temperature versus Ho concentration for Ho x R 1yx Ni 2 B 2 C compounds with R s La, Y w8x and Lu w9x.
Ho concentrations between x s 0.75 and x s 1. On the other hand, the substitution of Ho by a small amount of La less than 10% is sufficient to suppress this magnetic structure completely. In Fig. 7, the superconducting transition temperatures of Ho x R 1yx Ni 2 B 2 C compounds are compared for R s Y, Lu and La. For R s Y, a linear scaling of Tc with the Ho concentration x was found in the whole range of x w8x. In the case of R s Lu, the value of Tc remains nearly unchanged for concentrations x G 0.7 w9x and changes linearly for smaller Ho concentrations. There is a rapid suppression of Tc by a small amount of lanthanum. The mean lattice parameters of Ho x La 1yx Ni 2 B 2 C are changed at these concentrations only by about 0.3% compared with those of HoNi 2 B 2 C. From the corresponding small increase of the in-plane Ni–Ni distance only a small change of the electronic structure of the NiŽ3d.dominated conduction band could be expected. Thus the change of the lattice parameters of the Ho x R 1yx Ni 2 B 2 C compounds alone cannot explain the decrease of Tc . Concerning the slope of Tc Ž x . there is a clear correlation with < dr < being the absolute value of the difference of the ionic radius R under consideration and the host value of Ho. With
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increasing < dr < the slope near x Q 1 changes its magnitude from positive values for almost identical ionic radii Žin the case of Y, < dr < s 0.011 nm. passing a nearly zero value Žfor Lu, < dr < s 0.046 nm. to a negative value Žfor La, < dr < s 0.177 nm.. The values of the ionic radii were taken from Ref. w11x. According to this result, Ho x R 1yx Ni 2 B 2 C with R s Sc and Th are expected to show an intermediate slope in between the values observed for R s Lu and La. According to Morosov w13,14x, under some special circumstances in antiferromagnetic superconductors the nonmagnetic impurities can strongly suppress Tc . This concept working exclusively in the antiferromagnetic ordered state is supported by the data for Lu x La 1yx Ni 2 B 2 C. In these nonmagnetic compounds Tc has been reported to decrease from 16.5 K for x s 1 to 14.8 K for x s 0.5 w15x. In this case, the large size of the La ions has only a moderate influence on Tc , although the < dr < for both systems ŽHo–La and Lu–La. are quite similar. According to Fig. 5 only the c-axis modulated structures are likely candidates in the temperature interval under consideration. Their specific influence will be discussed below in more detail. Quite interestingly, the much steeper decrease of Tc Ž x . of Ho x La 1yxNi 2 B 2 C is observed for x - 0.95 when Tc Ž x . reaches a value of about 5.9 K which marks the onset of the antiferromagnetic incommensurate a-axis modulated structure. This characteristic temperature has been determined for HoNi 2 B 2 C by specific heat measurements w16x and by neutron diffraction w17x. In addition, the destruction of superconductivity induced diamagnetism, which is well known as reentrant behaviour of the paramagnetic normal state is shown in Fig. 3. This might be ascribed due to the neighbourhood Žfluctuations. of the a-axis modulated antiferromagnetic structure, which is expected in this region. Another important factor unfavourable for superconductivity seems to be the strong enhancement of the order parameter related to the c-axis modulated structures which set in near 6 K and saturates near 3 K Žsee Fig. 4.. As a result Tc decreases steeply in a narrow concentration range 0.93 - x - 0.95 from 6 K to about 2.5 K. Outgoing from this point Tc does continue to decrease but now already with a reduced slope. This is attributed to the strongly diminished magnitude of the order parameter of the a-axis structure and the saturated magni-
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tude of the t 1 , t 2 order parameters. However in the concentration range 0.8 - x - 0.9 the t 1 structure starts to be strongly suppressed and the slope of the decrease of Tc is reduced again. The possibly remaining moderate and smooth decrease of Tc in the range of concentrations 0 - x - 0.85 could be ascribed to the alloying effect with the nonsuperconducting LaNi 2 B 2 C system in a nonmagnetic phase. Further studies at very low temperatures might sheet light on details of this behaviour. A common feature for all the investigated Ho x R 1yx Ni 2 B 2 C samples, R s Y, Lu and La, is that the slope of the decrease of Tc as a function of x seems to be the effect of disorder. Disorder was found to be responsible also for the depression of Tc in nonmagnetic YxLu 1y x Ni 2 B 2 C compounds in the range of x values around 0.5 w17x. A quite different interpretation has been proposed for the strong suppression of Tc observed for Dyx Lu 1yx Ni 2 B 2 C in the range of Dy concentrations between 0.75 and 0.85 w18x. In this case, collective magnetic excitations have been assumed to be the main origin for the strong suppression of superconductivity. In Ho x La 1yx Ni 2 B 2 C the La impurities strongly suppress Tc as the difference in the radii of the R3q ions and Ho 3q is very large. The development of the antiferromagnetic c-axis structures is well reflected in the Tc Ž x . dependence.
4. Conclusion Using the melt-spinning technique the miscibility gap appearing for arc-melted Ho x La 1yx Ni 2 B 2 C samples can be avoided. Both superconductivity and the a-axis modulated incommensurate antiferromagnetic structure are suppressed by a small amount of lanthanum in the range of Ž1 y x . between 5 and 10%. For a better understanding of this unusually strong suppression of Tc investigations of the electronic structure are essential. In nonmagnetic R x RX1yx Ni 2 B 2 C systems the large La ion acts on Tc due to the difference in the size of the R ionic radii. On the other hand in Ho x La 1yx Ni 2 B 2 C the strong suppression of Tc Ž x . can be explained by impurity effects in the antiferromagnetically ordered state.
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft within the project Sonder¨ forschungsbereich 463, ‘Seltenerd-Ubergangsmetallverbindungen: Struktur, Magnetismus und Transport’ and the SMWK Project No. LC21.09. Valuable discussions with A. Morosov are gratefully acknowledged.
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Suppression of superconductivity by nonmagnetic impurities, structural properties and magnetic ordering in Ho x La 1yx Ni 2 B 2 C J. Freudenberger a,) , A. Kreyssig b, C. Ritter c , K. Nenkov a,1, S.-L. Drechsler a , a G. Fuchs a , K.-H. Muller , M. Loewenhaupt b, L. Schultz a ¨ a
Institut fur und Werkstofforschung Dresden, Postfach 270016, D-01171 Dresden, Germany ¨ Festkorper¨ b Institut fur ¨ Angewandte Physik, TU Dresden, D-01062 Dresden, Germany c Institut M.V. Laue, P. LangeÕin, 156 X, F-38042 Grenoble, France Received 24 December 1998; received in revised form 19 February 1999; accepted 26 February 1999
Abstract Superconducting and magnetic properties as well as the lattice structure of polycrystalline Ho x La 1yx Ni 2 B 2 C samples have been studied by susceptibility, X-ray, and neutron diffraction measurements, respectively. A miscibility gap has been found between x s 0.4 and x s 0.7 if the samples are prepared by standard arc-melting. Samples in this concentration range have been successfully prepared by melt-spinning. All samples exhibit the LuNi 2 B 2 C-type structure and the lattice parameters depend linearly on the La content. Already a low lanthanum concentration of about 10% leads to a rapid depression of Tc from 8.5 K to 2 K. Furthermore this concentration leads to a complete depression of the a-axis modulated incommensurate antiferromagnetic structure. Two other magnetic structures, one with commensurate Ž0 0 1. and one with incommensurate wave vector Ž0 0 0.915. were found to be less sensitive to the La concentration. The neutron-diffraction peak intensities connected to these structures decrease with increasing La content and disappear at x s 0.75 Žcommensurate structure. and x s 0.55 Žincommensurate structure.. A comparison with Ho x R 1yx Ni 2 B 2 C compounds ŽR s Y, Lu. shows that these two magnetic structures are influenced in different way by the size of the nonmagnetic rare-earth ion R. No size effect was observed for the magnetic ordering temperature of the commensurate antiferromagnetic structure, whereas the incommensurate spiral structure along c-axis was found to be sensitive to the difference in the size of the R and the Ho ions. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Ho x La 1yx Ni 2 B 2 C; Superconductivity; Nonmagnetic impurities
1. Introduction The discovery of superconductivity in rare earth transition metal borocarbides has stimulated much )
Corresponding author. Tel.: q49-351-4659-553; Fax: q49351-4659-537; E-mail: [email protected] 1 On leave from: Int. Lab. of High Magn. Fields, Wroclaw; ISSP-BAS, Sofia.
research on these compounds. Of particular interest is HoNi 2 B 2 C because of its fascinating interplay between superconductivity and complex magnetic structures. Three different types of antiferromagnetic structures have been observed by neutron scattering, Ži. a commensurate structure, Žii. an incommensurate spiral along the c-axis and Žiii. a further incommensurate magnetic component that develops along the a-axis w1,2x. Whereas the two incommensurate mag-
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 2 0 8 - 7
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netic components coexist only in a finite temperature range between 7 K and 4.5 K, the commensurate magnetic structure appears below about 8 K and shows a saturation behaviour at low temperatures. On the other hand, HoNi 2 B 2 C becomes superconducting at Tc , 8.5 K, then reenters the normal state in a narrow temperature interval of about 4.3 to 5.5 K to become superconducting again with further reduction of temperature w3x. Below this characteristic temperature of about 4.5 K, only the commensurate antiferromagnetic structure remains coexisting with the superconducting state. The spiral structure along the c-axis w4x as well as the component along the a-axis w5x have been considered to be responsible for the reentrant behaviour observed in HoNi 2 B 2 C. Consequently, the question arose which of these two incommensurate structures is responsible for the suppression of the superconductivity. This problem has been clarified recently by investigating polycrystalline samples of pseudo-quaternary Ho xY1yxNi 2 B 2 C compounds w6,7x. The three distinct types of antiferromagnetic order, mentioned above, have also been detected for x - 1. Fortunately, the temperature ranges where they exist are sensitive to the Ho concentration and these x-dependencies have been found to be different for the different types of magnetic order. For this particular Ho xY1yx Ni 2 B 2 C system not only the commensurate antiferromagnetic state but also the incommensurate c-axis spiral can coexist with superconductivity whereas the incommensurate a-axis component is strongly correlated to the suppression of superconductivity w6,7x. A linear decrease of the superconducting transition temperature Tc with increasing x has been observed for the Ho xY1yx Ni 2 B 2 C samples w8x. On the other hand, for Lu diluted HoNi 2 B 2 C the dependence of Tc on the concentration is more complicated w9,10x. This different behaviour has been attributed to differences in the lattice parameters of the Ho–Y and the Ho–Lu systems which result in distinct electronic structures controlling the exchange interaction between the magnetic ions as well as the depression of Tc w9x. In this context also, the investigation of the Ho xLa 1y x Ni 2 B 2 C system is of interest because La, being nonmagnetic as Y and Lu, not only has the largest ionic radius of all rare earth ions but also the final LaNi 2 B 2 C compound is nonsuperconducting. In the present paper, the influence of lanthanum on
the superconducting and magnetic properties of Ho x La 1yx Ni 2 B 2 C compounds has been studied.
2. Experimental details Polycrystalline Ho x La 1yx Ni 2 B 2 C samples were prepared by a standard arc melting technique. 11 B was used instead of the natural B to reduce the absorption for neutron scattering experiments Žthis holds for all samples mentioned in this paper.. Powders of the elements were weighted in the stoichiometric compositions with a surplus of 10 wt.% boron to compensate the high losses of boron caused by the arc melting. The powder was pressed to pellets that were melted under argon gas on a water-cooled copper plate in an arc furnace. To get homogeneous samples, they were turned over and melted again four times. After the melting procedure the solidified samples were homogenized at 11008C for 10 days. Using this method single-phase samples can be obtained only for Ho concentrations below x s 0.4 and above x s 0.7. Samples in the range of 0.4 - x - 0.7 consist of two phases with two lattice structures belonging to Ho concentrations of x s 0.4 and x s 0.7, respectively. Thus, single-phase samples can be prepared by standard arc melting only for Ho concentrations out of this miscibility gap. The rapid quenching single-roller melt-spinning technique was used to suppress this phase separation. For this purpose ingots of nominal composition were prepared by arc melting as above. Each ingot was heated at radio frequency in a quartz jet over the melting temperature and then pressed out by an overpressure. The melt was quickly solidified by getting into contact with the copper wheel. The actual sample composition is uncertain due to evaporation losses during melting. Owing to this, the nominal composition, in particular the nominal Ho content x, is given in this paper. To investigate the lattice structures Žat room temperature. as well as the phase purity of the samples, a powder X-ray diffractometer in Bragg–Brentano geometry was used. The X-ray diffraction experiments were done on crushed powders using CoK a Ž l s 0.1789 nm. radiation. The scans were taken from 2Q s 208 up to 1408 in steps of DQ s 0.0158. Finally the lattice parameters for the regarded phases
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in the samples were determined from measured diffraction data using DBWS Rietveld program. Powder neutron diffraction experiments were carried out at the multi-detector instrument D20 at the ILL, Grenoble, to investigate the magnetic as well as the nuclear structures. The scans were taken at several temperatures below and above the magnetic ordering temperature in a range of 2Q between 58 and 1608 using a graphite monochromator Ž l s 0.241 nm.. The powder samples were placed in cylindrical sample holders ŽVanadium, f s 8 mm.. The samples were first cooled down to the lowest temperature of 1.5 K and then heated in temperature steps of 0.3 K up to 15 K. Each diffraction pattern was measured for about 15 min. The superconducting properties of the samples were determined by susceptibility and resistivity measurements. The AC susceptibility x was measured in a temperature range between 4.5 K and 300 K at different DC magnetic fields using an AC field amplitude of 0.01 mT. To determine the superconducting transition for samples with lower Tc , the standard four-probe technique was utilized to measure the temperature dependence of the resistance down to 1.4 K.
3. Results and discussion
Fig. 1. X-ray diffraction patterns around the Ž112. peak for different Ho concentrations. Upper part: arc molten samples; lower part: melt spun samples.
The phase purity of the investigated Ho xLa 1y x Ni 2 B 2 C samples was checked by X-ray diffraction. No significant fraction of phase impurities was found. The X-ray structure analysis revealed the LuNi 2 B 2 C structure type w11x Žspace group I4rmmm. in the whole range of Ho concentrations. For concentrations within the miscibility gap Ž0.4 F x F 0.7. two patterns with I4rmmm space group symmetry were found. In the upper part of Fig. 1, cuts of diffraction patterns near the Ž112. peak are shown for different Ho x La 1yx Ni 2 B 2 C sample compositions prepared by arc melting. The patterns for the two Ho concentrations x s 0.4 and x s 0.6 reveal a splitting of the Ž112. peak into two peaks that can be related to two structures with different lattice parameters. The miscibility gap found in the Ho x La 1yx Ni 2 B 2 C series in the range of 0.4 F x F 0.7 may be caused by the large difference of the temperatures of peritectic phase formation for LaNi 2 B 2 C and HoNi 2 B 2 C. The
peritectic temperature T P , 13308C of LaNi 2 B 2 C w12x is much lower than that obtained for HoNi 2 B 2 C ŽT P s 15268C.. Assuming conditions of a thermodynamic equilibrium HoNi 2 B 2 C is expected to solidify separately. Only samples with compositions near that of the pure phase solidify in the given composition which can be explained by the reduced diffusion of the minor phase in the melt. In the case of rapid quenching, the diffusion in the melt during cooling through the temperature range between the two peritectic temperatures can be kept very small. Therefore, the rapid quenching melt-spinning technique was used in order to suppress the miscibility gap. In the lower part of Fig. 1, three cuts of diffraction patterns of melt-spun samples are shown around the Ž112. Bragg reflection. The pure melt-spun samples exhibit larger peak widths of the Ž112. peaks than the arc-molten material, but the positions of corresponding peaks agree. The diffraction pattern of
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the melt-spun sample for x s 0.5 reveal only one peak. No splitting of this peak could be detected by in situ high-temperature X-ray analysis carried out in Debye–Scherrer geometry. However, the broad foot of this peak indicates small amounts of compositions near the borders of the miscibility gap or mostly fine-crystalline material. Thus, the cooling rate obtained by the melt-spinning procedure still remains insufficient to produce a single-phase sample. The structure obtained for x s 0.5 by melt spinning was found to be stable during tempering below the eutectic temperature of about T , 10208C, but it starts to decay above this temperature. In Fig. 2, the lattice parameters a and c are shown as function of the Ho concentration. The standard deviations of the Rietveld analysis result in errors that are related to the peak widths. They are shown in Fig. 2 as error bars. Peaks corresponding to phases near or within the miscibility gap have dramatically enlarged widths compared with the peaks of the pure RNi 2 B 2 C ŽR s Ho, La. compounds. The lattice parameters for the two pure RNi 2 B 2 C compounds are in good agreement with those obtained by other groups w11x. The dashed lines in Fig. 2 indicate the almost linear scaling of the lattice pa-
Fig. 2. Lattice parameters a Žv . and c ŽI. of the arc-molten Ho x La 1yx Ni 2 B 2 C samples versus Ho concentration. Additionally, the lattice parameters a Ž`. and c ŽB. of the melt-spun sample with x s 0.5 are shown. The concentration range of the miscibility gap is marked gray. Each couple of lattice parameters Ž a, c . that belong to one lattice are marked with the same Greek symbol.
Fig. 3. Susceptibility of Ho x La 1yx Ni 2 B 2 C samples Ž x s 0.95, 0.97, 1. in dependence on temperature. In the inset the resistivity is shown for x s 0.9 and 0.8.
rameters with x in the range of small values of x F 0.2. The lattice parameters of the melt-spun Ho 0.5 La 0.5 Ni 2 B 2 C sample shown in Fig. 2 with inverse symbols and without error bars lie exactly on these dashed lines. The miscibility gap Ž0.4 F x F 0.7. is marked gray. Deviations from the linear scaling of the lattice parameters with x are observed for large Ho concentrations x G 0.75. In Fig. 3, the temperature dependence of the susceptibility is shown for three Ho x La 1yx Ni 2 B 2 C samples with Ho concentrations x G 0.95. In the inset, the temperature dependence of the resistivity is displayed for x s 0.9 and 0.8. The LaNi 2 B 2 C compound is nonmagnetic as well as nonsuperconducting w11x. Therefore, it is natural that superconductivity of HoNi 2 B 2 C is suppressed substituting a large amount of Ho by La. But it is surprising that only 10% lanthanum is sufficient for a suppression of Tc from 8.5 K to 2 K in Ho xLa 1y x Ni 2 B 2 C, as can be seen in Figs. 3 and 5. Reentrant behaviour is observed in weakly diluted samples. The onset temperature of the superconducting transition of the sample with a nominal composition of Ho 0.97 La 0.03 Ni 2 B 2 C shifts to a value of about 7.5 K. With decreasing temperature the susceptibility decreases, goes through a minimum at a
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temperature of about 6 K and rises again for decreasing temperature. Traces of superconductivity and reentrant behaviour are still visible in the x ŽT . curve of the sample with x s 0.95. The magnetic structure of some samples of the Ho x La 1yx Ni 2 B 2 C system has been measured by neutron diffraction. For HoNi 2 B 2 C, the well known three antiferromagnetic structures have been found: the structure with commensurate wave vector t 1 s Ž0 0 1. was observed below 9 K. Two incommensurate modulated structures with t 2 s Ž0 0 0.915. and t 3 s Ž0.585 0 0. exist between 4.8 K and 9 K. In Fig. 4, the peak intensities connected to the incommensurate c-axis modulated spiral Župper part. and to the commensurate antiferromagnetic structure along c-axis Žlower part. are shown for four different Ho concentrations. The peak intensities for both structures decrease with decreasing Ho content and are below the detection limit for x - 0.6. It should be noted that the temperature dependence of the c-axis spiral structure Žt 2 . which exists for x s 1 only in a narrow temperature range, is quite different for x F 0.9: its intensity increases with decreasing temperature down to the lowest measured temperature. Fur-
Fig. 4. Temperature dependence of the neutron-diffraction peak intensities of the incommensurate Žt 2 . and the commensurate Žt 1 . magnetic reflections of Ho x La 1yx Ni 2 B 2 C samples normalized to the Ž002. nuclear reflection for x s1, 0.9, 0.8 and 0.6.
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Fig. 5. Variation of the different ordering temperatures in dependence on the Ho concentration in Ho x La 1yx Ni 2 B 2 C Ž0.6 F x F 1.: superconducting ordering Tc , Temperature of the commensurate Tt 1 and incommensurate Tt 2 antiferromagnetic structures and the reentrant temperature Tre .
thermore, the incommensurate a-axis modulated structure observed in HoNi 2 B 2 C does not exist in the Ho x La 1yx Ni 2 B 2 C samples with x F 0.9 investigated, so far, by neutron scattering. Thus, this magnetic structure disappears already at La concentrations smaller than 10%. The values for the pure HoNi 2 B 2 C sample are taken from former measurements at the multi-detector instrument D1B at the ILL Grenoble w10x. It has not yet been checked directly whether in Ho x La 1yx Ni 2 B 2 C with sample compositions of x s 0.97 and x s 0.95 an incommensurate a-axis modulated antiferromagnetic structure exists. But this is suggested by the similarity of the susceptibility data of these samples and the data for Ho x R 1yx Ni 2 B 2 C ŽR s Y, Lu. w7,10x. The dependence of the ordering temperatures of the different magnetic structures as well as of the superconducting transition temperatures of Ho xLa 1y x Ni 2 B 2 C on the Ho concentration is shown in Fig. 5 in the range of x G 0.6. Superconductivity in Ho x La 1y x Ni 2 B 2 C is strongly suppressed by a small amount of lanthanum. The magnetic ordering temperatures of the magnetic
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structures along the c-axis show a linear decrease with decreasing holmium content. The breakdown of the a-axis modulated structure in the range 0.9 - x - 1 indicates a comparably strong influence of lanthanum on this magnetic structure as observed for the influence of La on the superconductivity in this system. The superconducting and magnetic ordering temperatures of Ho x La 1yx Ni 2 B 2 C depend on the concentration of the magnetic Ho ions and on the lattice parameters which are strongly affected by the large size of the La ions. In order to separate these two effects the influence of nonmagnetic impurities R with different size on the superconducting and magnetic properties of Ho x R 1yx Ni 2 B 2 C were compared. The experimental results for R s Y, Lu and La are shown in Figs. 6 and 7 for the magnetic and superconducting ordering temperatures, respectively. The dependence of the properties of Ho xY1yx Ni 2 B 2 C on the Ho concentration is relatively simple because Y 3q has approximately the same ionic radius as Ho 3q. In this case, an almost linear scaling of the magnetic and superconducting ordering temperatures with x has been found w6,8x. Substituting Ho by R s La leads to an expansion of the a-axis and a contraction of the c-axis whereas for R s Lu the opposite trends have been found w9x. It should also be noted that the lattice parameters of Ho x R 1yx Ni 2 B 2 C are much stronger influenced for R s La than for R s Lu. The lattice parameter a, for instance, increases by 7.8% for R s La, but decreases only by 1.6% for R s Lu, if x changes from 1 to 0. It is surprising that the very different size of the lattice parameters for the Ho x R 1yx Ni 2 B 2 C compounds with R s Y, Lu and La has almost no effect on the magnetic ordering temperature of the commensurate antiferromagnetic structure as shown in Fig. 6a. The same linear decrease of the magnetic ordering temperature with decreasing Ho content is observed between x s 1 and x s 0.8 for all these compounds. No commensurate magnetic structure was detected below x s 0.75. On the other hand, the ordering temperature of the incommensurate spiral structure along the c-axis Žt 2 . is sensitive to the difference in the size of the rare-earth ions: the slope of the Tt 2Ž x . curve in Fig. 6b increases with increasing difference between the size of the R ion and the Ho ion. However, it should also be noted that the
Fig. 6. Ordering temperatures of Ža. the c-axis modulated commensurate, Žb. the c-axis modulated incommensurate and Žc. the a-axis modulated incommensurate magnetic structure versus Ho concentration for Ho x R 1yx Ni 2 B 2 C with R s La, Y w6x and Lu w9x. The dashed line in Žc. stands for the fact that this magnetic structure could not be seen by neutron-diffraction for x F 0.9 and T G1.8 K.
influence of decreasing Ho concentration on the magnetic ordering temperature of the incommensurate spiral structure is distinctly weaker than in the case of the commensurate structure. The spiral structure was detected down to x s 0.6. Very different results were found for the a-axis modulated structure for compounds with R s Lu and La. As shown in Fig. 6c, no size effect seems to be present for the a-axis modulated incommensurate structure of Ho x R 1yx Ni 2 B 2 C for R s Lu or Y in the range of
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Fig. 7. Superconducting transition temperature versus Ho concentration for Ho x R 1yx Ni 2 B 2 C compounds with R s La, Y w8x and Lu w9x.
Ho concentrations between x s 0.75 and x s 1. On the other hand, the substitution of Ho by a small amount of La less than 10% is sufficient to suppress this magnetic structure completely. In Fig. 7, the superconducting transition temperatures of Ho x R 1yx Ni 2 B 2 C compounds are compared for R s Y, Lu and La. For R s Y, a linear scaling of Tc with the Ho concentration x was found in the whole range of x w8x. In the case of R s Lu, the value of Tc remains nearly unchanged for concentrations x G 0.7 w9x and changes linearly for smaller Ho concentrations. There is a rapid suppression of Tc by a small amount of lanthanum. The mean lattice parameters of Ho x La 1yx Ni 2 B 2 C are changed at these concentrations only by about 0.3% compared with those of HoNi 2 B 2 C. From the corresponding small increase of the in-plane Ni–Ni distance only a small change of the electronic structure of the NiŽ3d.dominated conduction band could be expected. Thus the change of the lattice parameters of the Ho x R 1yx Ni 2 B 2 C compounds alone cannot explain the decrease of Tc . Concerning the slope of Tc Ž x . there is a clear correlation with < dr < being the absolute value of the difference of the ionic radius R under consideration and the host value of Ho. With
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increasing < dr < the slope near x Q 1 changes its magnitude from positive values for almost identical ionic radii Žin the case of Y, < dr < s 0.011 nm. passing a nearly zero value Žfor Lu, < dr < s 0.046 nm. to a negative value Žfor La, < dr < s 0.177 nm.. The values of the ionic radii were taken from Ref. w11x. According to this result, Ho x R 1yx Ni 2 B 2 C with R s Sc and Th are expected to show an intermediate slope in between the values observed for R s Lu and La. According to Morosov w13,14x, under some special circumstances in antiferromagnetic superconductors the nonmagnetic impurities can strongly suppress Tc . This concept working exclusively in the antiferromagnetic ordered state is supported by the data for Lu x La 1yx Ni 2 B 2 C. In these nonmagnetic compounds Tc has been reported to decrease from 16.5 K for x s 1 to 14.8 K for x s 0.5 w15x. In this case, the large size of the La ions has only a moderate influence on Tc , although the < dr < for both systems ŽHo–La and Lu–La. are quite similar. According to Fig. 5 only the c-axis modulated structures are likely candidates in the temperature interval under consideration. Their specific influence will be discussed below in more detail. Quite interestingly, the much steeper decrease of Tc Ž x . of Ho x La 1yxNi 2 B 2 C is observed for x - 0.95 when Tc Ž x . reaches a value of about 5.9 K which marks the onset of the antiferromagnetic incommensurate a-axis modulated structure. This characteristic temperature has been determined for HoNi 2 B 2 C by specific heat measurements w16x and by neutron diffraction w17x. In addition, the destruction of superconductivity induced diamagnetism, which is well known as reentrant behaviour of the paramagnetic normal state is shown in Fig. 3. This might be ascribed due to the neighbourhood Žfluctuations. of the a-axis modulated antiferromagnetic structure, which is expected in this region. Another important factor unfavourable for superconductivity seems to be the strong enhancement of the order parameter related to the c-axis modulated structures which set in near 6 K and saturates near 3 K Žsee Fig. 4.. As a result Tc decreases steeply in a narrow concentration range 0.93 - x - 0.95 from 6 K to about 2.5 K. Outgoing from this point Tc does continue to decrease but now already with a reduced slope. This is attributed to the strongly diminished magnitude of the order parameter of the a-axis structure and the saturated magni-
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tude of the t 1 , t 2 order parameters. However in the concentration range 0.8 - x - 0.9 the t 1 structure starts to be strongly suppressed and the slope of the decrease of Tc is reduced again. The possibly remaining moderate and smooth decrease of Tc in the range of concentrations 0 - x - 0.85 could be ascribed to the alloying effect with the nonsuperconducting LaNi 2 B 2 C system in a nonmagnetic phase. Further studies at very low temperatures might sheet light on details of this behaviour. A common feature for all the investigated Ho x R 1yx Ni 2 B 2 C samples, R s Y, Lu and La, is that the slope of the decrease of Tc as a function of x seems to be the effect of disorder. Disorder was found to be responsible also for the depression of Tc in nonmagnetic YxLu 1y x Ni 2 B 2 C compounds in the range of x values around 0.5 w17x. A quite different interpretation has been proposed for the strong suppression of Tc observed for Dyx Lu 1yx Ni 2 B 2 C in the range of Dy concentrations between 0.75 and 0.85 w18x. In this case, collective magnetic excitations have been assumed to be the main origin for the strong suppression of superconductivity. In Ho x La 1yx Ni 2 B 2 C the La impurities strongly suppress Tc as the difference in the radii of the R3q ions and Ho 3q is very large. The development of the antiferromagnetic c-axis structures is well reflected in the Tc Ž x . dependence.
4. Conclusion Using the melt-spinning technique the miscibility gap appearing for arc-melted Ho x La 1yx Ni 2 B 2 C samples can be avoided. Both superconductivity and the a-axis modulated incommensurate antiferromagnetic structure are suppressed by a small amount of lanthanum in the range of Ž1 y x . between 5 and 10%. For a better understanding of this unusually strong suppression of Tc investigations of the electronic structure are essential. In nonmagnetic R x RX1yx Ni 2 B 2 C systems the large La ion acts on Tc due to the difference in the size of the R ionic radii. On the other hand in Ho x La 1yx Ni 2 B 2 C the strong suppression of Tc Ž x . can be explained by impurity effects in the antiferromagnetically ordered state.
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft within the project Sonder¨ forschungsbereich 463, ‘Seltenerd-Ubergangsmetallverbindungen: Struktur, Magnetismus und Transport’ and the SMWK Project No. LC21.09. Valuable discussions with A. Morosov are gratefully acknowledged.
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