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Observation of superconductivity in polycrystalline DyNi2B2C
ELSEVIER
Physica B 223&224 (1996) 99-101
Observation of superconductivity in polycrystalline DyNi2B2C Z. Hossain a, L.C. Gupta a, R. Nagarajan a,., S.K. Dhar a, C. Godart b, R. Vijayaraghavan a a Tata Institute of Fundamental Research, Bombay 400 005, India b UPR-209, CNRS, 92195 Meudon Cedex, France
Abstract
One of the offshoots of the discovery of superconductivity (SC) in the Y - N i - B - C system is the coexistence of SC and magnetism in RNi2BzC (R = Er, Ho, Tm). Here we report our observation of SC ( T o ~ 6 K) and magnetic order (TN~ 11 K) in polycrystalline DyNi2B2C. This is one of a few systems where SC sets in with T~ < TN. From the known systematics of T~ and TN in borocarbide systems, we expect YbNizB2C to exhibit SC around 12 K. However, our resistivity measurements in YbNizB2C do not reveal SC down to 1.5 K.
One of the important features that emerged from the discovery of superconductivity (SC) in the Y - N i - B - C system [1,2] is the coexistence of superconductivity and magnetism in the single phase materials [3] of this rare earth based family of compounds, RNi2B2C (R = Ho, Er, Tin) I-4-6]. What sets the borocarbides apart from other known magnetic superconductors, such as RRh4B4, is that their superconducting transition temperatures, To, as well as their magnetic ordering temperatures are relatively high. In the magnetic borocarbide superconductors, TmNi2B2C, ErNi2B2C and HoNi2B2C, TN and ATe, the depression of Tc with respect to LuNi2B2C, nearly follow the well-known deGennes scaling. Accordingly, one would expect DyNi2B2C to order magnetically and superconduct around 11 K and 6 K respectively. Polycrystalline samples of DyNi2BzC [4] were earlier reported to exhibit (antiferro) magnetic ordering at about 11 K but existence of SC was not established down to 2 K [4, 7]. Recently SC has been reported in a single crystal specimen of DyNi2B2C [8] which seemed to imply as if SC depends on crystallinity of the material.
* Corresponding author.
Therefore we reinvestigated the system and found SC in polycrystalline DyNi2B2C [9]. Five independent samples were prepared by standard arc melting procedure. Details of sample preparation are given elsewhere [6]. All the samples were annealed at 1050°C for one week. Powder X-ray diffraction patterns (XRD) of these materials showed that all of them were essentially of the expected YNizB2C-type phase (Fig. 1). AC susceptibility (in a field of 1.5 G r.m.s, at 313 Hz) of the sample with best SC property (sample I) is shown in Fig. 2. The peak at ~ 11 K is due to antiferromagnetic transition. The distinct sharp reduction in signal, around 6 K, leading to diamagnetism clearly shows occurrence of SC in this material. The onset of SC is at ~6.5 K. Resistivity measurements on this sample (inset Fig. 2) clearly show a resistivity drop at ~ 11 K which correspond to the magnetic ordering and a sharp drop to zero resistance confirming the superconducting transition. The superconducting onset temperature as observed from resistivity measurement is ~6.5 K, in agreement with the AC susceptibility results. The resistance goes to zero below 4.8 K. From the results of other samples we find that the superconducting properties vary from sample to sample. The diamagnetic signal strength is considerably reduced
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved P l l S092 1 - 4 5 2 6 ( 9 6 ) 0 0 0 5 0 - 6
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Z Hossain et al. / Physica B 223&224 (1996) 99 101 I
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20 ( d e g r e e ) Fig. 1. Powder X-ray diffraction patterns of DyNi2B2C: (top) for sample I with best SC property (with Rietveld refinement); (bottom) for sample II with no SC.
15
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Fig. 2. AC susceptibilityof DyNi2B2C. Sample I shows the best diamagnetic signal. Sample 11 has no diamagnetic signal. The peak at ~ 16 K is due to the magnetic ordering of the impurity phase, possibly DyB2C2 (Tm~16K). The inset shows the resistivity result.
in three of the samples and in one (sample II) there is no diamagnetism at all down to 4.2 K (Fig. 2). We note here that in DyNi2B2C, Tc < TN. Under this circumstance, even nonmagnetic impurities are detrimental to SC because of uncompensated local fields. The sample exhibiting the best SC properties has a nearly
ideal XRD pattern of YNi2B2C-tyI~e with lattice parameter a = 3.531 A and c = 10.486 A (note the satisfactory Rietveld refinement in Fig. 1). The most intense impurity line is 2% of the most intense line of the 1221phase. We find that SC properties deteriorate as impurity fraction increases. The sample which does not exhibit diamagnetism shows the maximum impurity content in the XRD pattern ( ~ 8 % ) (Fig. 1). A plausible cause for the suppression of SC is the disorder/vacancy at the B- and C-sites [103. Such a possibility for borocarbide materials is indicated from the NMR experiments on YNi2B2C [10]. Our LAZY PULVERIX simulations indicate that it will be difficult to detect such a disorder/vacancy from the XRD pattern. For the same reason, these are also difficult to detect in EDAX/EPMA analysis. Off-stoichiometry in Dy or Ni is relatively more prominent in the XRD pattern. A very recent investigation on DyNi2B2C [I1] confirms that off-stoichiometry in initial carbon content has a detrimental effect on the SC of DyNi2B2C. We expect that nonstoichiometry of B also may have a similar effect. It has been shown that stoichiometry of all the four elements is critical in HoNi2B2C [12]. The observation of SC in DyNi2B2C may have important and far-reaching consequences. It may happen that in a particular series of compounds, SC is not observed in some AF members, whereas one may expect them to show SC (with Tc < TN) on the basis of certain systematics. One reason for not observing SC could be due to such metallurgical problems as pointed out above. Such compounds should be reinvestigated for SC. Tc of Tb2Mo3Si,~, the only other known magnetic superconductor with Tc < TN, is also sample dependent [-13]. We reported recently, magnetic properties of DyNizBzC [9]. The magnetic susceptibility of DyNi2B2C (measured in a SQUID magnetometer) at 1 kG shows a peak at ~ 11 K characteristic of antiferro magnetic order. At 5 K, though the sample is superconducting no diamagnetism is seen because, He2 < 1 kG, and also because the paramagnetic contribution to the magnetic signal at this field may override the diamagnetic signal. Recent neutron diffraction measurements of DyNizB2C clearly show that Dy moments order antiferromagnetically below TN ~ 10.5 K [14]. The isothermal magnetization of DyNi2B2C at 5 K shows a field induced metamagnetic transition at ~7.5 kG (Fig. 3). The saturation magnetic moment at 55 kG is ~ 8.9/~B which is close to the saturation magnetic moment (10#a) of the free trivalent Dy ion. This indicates that at 55 kG field, spins are forced to align ferromagnetically. Normal state magnetic susceptibility follows a Curie Weiss behaviour with an effective magnetic moment of 10.6#B (close to that of the free Dy 3+ ion) with a Op of 8.1 K.
Z. Hossain et al. / Physica B 223&224 (1996) 99-101 60
40.
/
DyNi2B2C 5K
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Fig. 3. Magnetization of DyNi2B2C. Note the field induced magnetic transition.
Heat capacity data in DyNi2B2C [9] show a large anomaly at ~ 10 K due to magnetic ordering in this compound. The magnetic entropy is calculated to be 9.88 J/mol K which is close to R In 4, indicating that the ground state of Dy 3+ ions is either a quartet or two closely spaced doublets. From the known Tc's of RNi2B2C (R = Y, Ho, Er, Tm) and from the present To of Dy we point out that the suppression of To with heavy rare earth follows deGennes scaling. We have synthesized YbNi2B2C and the lattice parameters (a = 3.479 A, c = 10.614 A) indicate Yb to be in the 3 + state. By deGennes scaling, one would expect this material also to exhibit SC around 12 K. Our resistivity data show no SC down to 1.5 K [15]. Considering the problems due to stoichiometry discussed above, it is too early to conclude the presence or absence of SC in this material. In any case, this would be an interesting material. If Yb in this material is very close to valence instability, then the suppression of T~ in YbNi2B2C could be much larger than predicted by deGennes scaling. In fact Yb impurity in LuRh4B4 gives rise to a much stronger suppression of Tc than that suggested by deGennes scaling. Our preliminary heat capacity measurements down to 1.SK on YbNi2B2C give 7 ~200 mJ/mol K 2 [15], suggesting the material to be a possible heavy fermion system. Again, from deGennes
101
scaling we expect that TbNi2B2C is likely to exhibit SC around ~ 2 K. The results of this system known so far [4, 9] did not reveal SC down to 2 K. It is likely that the same material/stoichiometric problems that are associated with DyNi2B2C and HoNi2B2C may be present in the case of TbNi2B2C too. It would therefore be interesting to prepare TbNi2B2C with variation in stoichiometry and heat treatment and also extend the studies to temperatures lower than 2 K. In conclusion, we have shown that superconductivity occurs and coexists with antiferromagnetism in polycrsytalline samples of DyNi2B2C. The superconducting transition temperature (T~ = 6 K) is less than the magnetic ordering temperature (Tin = 11 K) which is rather rare. The superconducting properties in this material are severely affected by stoichiometry, the presence of impurity phases and/or atomic disorder. The material orders antiferromagnetically and undergoes field induced metamagnetic transition in a field of ~ 7.5 kG. Our samples of YbNi2BzC do not show SC down to 1.5 K. This large suppression of Tc in YbNi2B2C could be due to valence instability of Yb. We also suggest that SC may have to be carefully reinvestigated in TbNi2BzC.
References [1] Chandan Mazumdar et al., Solid State Commun. 87 (1993) 413. [2] R. Nagarajan et al., Phys. Rev. Lett. 72 (1994) 274. [3] R.J. Cava et al., Nature 367 (1994) 252. [4] H. Eisaki et al., Phys. Rev. B 50 (1994) 647. [5] L.C. Gupta et al., in: Proc. Int. Conf. On Physical Metallurgy, Bombay, 9-11 March 1994, eds. S. Banerjee and R.V. Ramanujan (Gordon & Breach, New York, 1994) p. 494. [6] C. Godart et al., Phys, Rev. B 51 (1995) 489. [7] C.V. Tomy et al., Physica C 235-240 (1994) 2551. [8] B.K. Cho et al., Phys. Rev. B 52 (1995) R3844. See also High Tc update, 1 April 1995. [9] Z. Hossain et al., IEEE Trans. Magn. 31 (1995) 4133; see also High Tc update, 1 June 1995. [101 T. Kohara et al., Phys. Rev. B 51 (1995) 3985. [11] C.V. Tomy et al., Physica C 248 (1995) 349. [12] H. Schmidt et al., Physica C 246 (1995) 177. [13] F.G. Aliev et al., Europhys. Lett. 25 (1994) 143. [14] J.W. Lynn et al., Physica B 223&224 (1996) 66. [15] S.K. Dhar et al., Solid State Commun. (in press).
Physica B 223&224 (1996) 99-101
Observation of superconductivity in polycrystalline DyNi2B2C Z. Hossain a, L.C. Gupta a, R. Nagarajan a,., S.K. Dhar a, C. Godart b, R. Vijayaraghavan a a Tata Institute of Fundamental Research, Bombay 400 005, India b UPR-209, CNRS, 92195 Meudon Cedex, France
Abstract
One of the offshoots of the discovery of superconductivity (SC) in the Y - N i - B - C system is the coexistence of SC and magnetism in RNi2BzC (R = Er, Ho, Tm). Here we report our observation of SC ( T o ~ 6 K) and magnetic order (TN~ 11 K) in polycrystalline DyNi2B2C. This is one of a few systems where SC sets in with T~ < TN. From the known systematics of T~ and TN in borocarbide systems, we expect YbNizB2C to exhibit SC around 12 K. However, our resistivity measurements in YbNizB2C do not reveal SC down to 1.5 K.
One of the important features that emerged from the discovery of superconductivity (SC) in the Y - N i - B - C system [1,2] is the coexistence of superconductivity and magnetism in the single phase materials [3] of this rare earth based family of compounds, RNi2B2C (R = Ho, Er, Tin) I-4-6]. What sets the borocarbides apart from other known magnetic superconductors, such as RRh4B4, is that their superconducting transition temperatures, To, as well as their magnetic ordering temperatures are relatively high. In the magnetic borocarbide superconductors, TmNi2B2C, ErNi2B2C and HoNi2B2C, TN and ATe, the depression of Tc with respect to LuNi2B2C, nearly follow the well-known deGennes scaling. Accordingly, one would expect DyNi2B2C to order magnetically and superconduct around 11 K and 6 K respectively. Polycrystalline samples of DyNi2BzC [4] were earlier reported to exhibit (antiferro) magnetic ordering at about 11 K but existence of SC was not established down to 2 K [4, 7]. Recently SC has been reported in a single crystal specimen of DyNi2B2C [8] which seemed to imply as if SC depends on crystallinity of the material.
* Corresponding author.
Therefore we reinvestigated the system and found SC in polycrystalline DyNi2B2C [9]. Five independent samples were prepared by standard arc melting procedure. Details of sample preparation are given elsewhere [6]. All the samples were annealed at 1050°C for one week. Powder X-ray diffraction patterns (XRD) of these materials showed that all of them were essentially of the expected YNizB2C-type phase (Fig. 1). AC susceptibility (in a field of 1.5 G r.m.s, at 313 Hz) of the sample with best SC property (sample I) is shown in Fig. 2. The peak at ~ 11 K is due to antiferromagnetic transition. The distinct sharp reduction in signal, around 6 K, leading to diamagnetism clearly shows occurrence of SC in this material. The onset of SC is at ~6.5 K. Resistivity measurements on this sample (inset Fig. 2) clearly show a resistivity drop at ~ 11 K which correspond to the magnetic ordering and a sharp drop to zero resistance confirming the superconducting transition. The superconducting onset temperature as observed from resistivity measurement is ~6.5 K, in agreement with the AC susceptibility results. The resistance goes to zero below 4.8 K. From the results of other samples we find that the superconducting properties vary from sample to sample. The diamagnetic signal strength is considerably reduced
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved P l l S092 1 - 4 5 2 6 ( 9 6 ) 0 0 0 5 0 - 6
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Z Hossain et al. / Physica B 223&224 (1996) 99 101 I
I
I
DyNizBzC
I
I
I
r
I
1000
I
I
I
1000
II o
,
10
.... t ,IA ......
20
30
,
40
50
60
70
20 ( d e g r e e ) Fig. 1. Powder X-ray diffraction patterns of DyNi2B2C: (top) for sample I with best SC property (with Rietveld refinement); (bottom) for sample II with no SC.
15
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',,,,,,,,,,,,, ,,,,,,,,,,,,, 8
12
16
20
Fig. 2. AC susceptibilityof DyNi2B2C. Sample I shows the best diamagnetic signal. Sample 11 has no diamagnetic signal. The peak at ~ 16 K is due to the magnetic ordering of the impurity phase, possibly DyB2C2 (Tm~16K). The inset shows the resistivity result.
in three of the samples and in one (sample II) there is no diamagnetism at all down to 4.2 K (Fig. 2). We note here that in DyNi2B2C, Tc < TN. Under this circumstance, even nonmagnetic impurities are detrimental to SC because of uncompensated local fields. The sample exhibiting the best SC properties has a nearly
ideal XRD pattern of YNi2B2C-tyI~e with lattice parameter a = 3.531 A and c = 10.486 A (note the satisfactory Rietveld refinement in Fig. 1). The most intense impurity line is 2% of the most intense line of the 1221phase. We find that SC properties deteriorate as impurity fraction increases. The sample which does not exhibit diamagnetism shows the maximum impurity content in the XRD pattern ( ~ 8 % ) (Fig. 1). A plausible cause for the suppression of SC is the disorder/vacancy at the B- and C-sites [103. Such a possibility for borocarbide materials is indicated from the NMR experiments on YNi2B2C [10]. Our LAZY PULVERIX simulations indicate that it will be difficult to detect such a disorder/vacancy from the XRD pattern. For the same reason, these are also difficult to detect in EDAX/EPMA analysis. Off-stoichiometry in Dy or Ni is relatively more prominent in the XRD pattern. A very recent investigation on DyNi2B2C [I1] confirms that off-stoichiometry in initial carbon content has a detrimental effect on the SC of DyNi2B2C. We expect that nonstoichiometry of B also may have a similar effect. It has been shown that stoichiometry of all the four elements is critical in HoNi2B2C [12]. The observation of SC in DyNi2B2C may have important and far-reaching consequences. It may happen that in a particular series of compounds, SC is not observed in some AF members, whereas one may expect them to show SC (with Tc < TN) on the basis of certain systematics. One reason for not observing SC could be due to such metallurgical problems as pointed out above. Such compounds should be reinvestigated for SC. Tc of Tb2Mo3Si,~, the only other known magnetic superconductor with Tc < TN, is also sample dependent [-13]. We reported recently, magnetic properties of DyNizBzC [9]. The magnetic susceptibility of DyNi2B2C (measured in a SQUID magnetometer) at 1 kG shows a peak at ~ 11 K characteristic of antiferro magnetic order. At 5 K, though the sample is superconducting no diamagnetism is seen because, He2 < 1 kG, and also because the paramagnetic contribution to the magnetic signal at this field may override the diamagnetic signal. Recent neutron diffraction measurements of DyNizB2C clearly show that Dy moments order antiferromagnetically below TN ~ 10.5 K [14]. The isothermal magnetization of DyNi2B2C at 5 K shows a field induced metamagnetic transition at ~7.5 kG (Fig. 3). The saturation magnetic moment at 55 kG is ~ 8.9/~B which is close to the saturation magnetic moment (10#a) of the free trivalent Dy ion. This indicates that at 55 kG field, spins are forced to align ferromagnetically. Normal state magnetic susceptibility follows a Curie Weiss behaviour with an effective magnetic moment of 10.6#B (close to that of the free Dy 3+ ion) with a Op of 8.1 K.
Z. Hossain et al. / Physica B 223&224 (1996) 99-101 60
40.
/
DyNi2B2C 5K
20. o
E
0
~O~e
•
r
% -20. -40.
~o
u
-60 -6
-4
-~
0 H (Tesla)
2
4
6
Fig. 3. Magnetization of DyNi2B2C. Note the field induced magnetic transition.
Heat capacity data in DyNi2B2C [9] show a large anomaly at ~ 10 K due to magnetic ordering in this compound. The magnetic entropy is calculated to be 9.88 J/mol K which is close to R In 4, indicating that the ground state of Dy 3+ ions is either a quartet or two closely spaced doublets. From the known Tc's of RNi2B2C (R = Y, Ho, Er, Tm) and from the present To of Dy we point out that the suppression of To with heavy rare earth follows deGennes scaling. We have synthesized YbNi2B2C and the lattice parameters (a = 3.479 A, c = 10.614 A) indicate Yb to be in the 3 + state. By deGennes scaling, one would expect this material also to exhibit SC around 12 K. Our resistivity data show no SC down to 1.5 K [15]. Considering the problems due to stoichiometry discussed above, it is too early to conclude the presence or absence of SC in this material. In any case, this would be an interesting material. If Yb in this material is very close to valence instability, then the suppression of T~ in YbNi2B2C could be much larger than predicted by deGennes scaling. In fact Yb impurity in LuRh4B4 gives rise to a much stronger suppression of Tc than that suggested by deGennes scaling. Our preliminary heat capacity measurements down to 1.SK on YbNi2B2C give 7 ~200 mJ/mol K 2 [15], suggesting the material to be a possible heavy fermion system. Again, from deGennes
101
scaling we expect that TbNi2B2C is likely to exhibit SC around ~ 2 K. The results of this system known so far [4, 9] did not reveal SC down to 2 K. It is likely that the same material/stoichiometric problems that are associated with DyNi2B2C and HoNi2B2C may be present in the case of TbNi2B2C too. It would therefore be interesting to prepare TbNi2B2C with variation in stoichiometry and heat treatment and also extend the studies to temperatures lower than 2 K. In conclusion, we have shown that superconductivity occurs and coexists with antiferromagnetism in polycrsytalline samples of DyNi2B2C. The superconducting transition temperature (T~ = 6 K) is less than the magnetic ordering temperature (Tin = 11 K) which is rather rare. The superconducting properties in this material are severely affected by stoichiometry, the presence of impurity phases and/or atomic disorder. The material orders antiferromagnetically and undergoes field induced metamagnetic transition in a field of ~ 7.5 kG. Our samples of YbNi2BzC do not show SC down to 1.5 K. This large suppression of Tc in YbNi2B2C could be due to valence instability of Yb. We also suggest that SC may have to be carefully reinvestigated in TbNi2BzC.
References [1] Chandan Mazumdar et al., Solid State Commun. 87 (1993) 413. [2] R. Nagarajan et al., Phys. Rev. Lett. 72 (1994) 274. [3] R.J. Cava et al., Nature 367 (1994) 252. [4] H. Eisaki et al., Phys. Rev. B 50 (1994) 647. [5] L.C. Gupta et al., in: Proc. Int. Conf. On Physical Metallurgy, Bombay, 9-11 March 1994, eds. S. Banerjee and R.V. Ramanujan (Gordon & Breach, New York, 1994) p. 494. [6] C. Godart et al., Phys, Rev. B 51 (1995) 489. [7] C.V. Tomy et al., Physica C 235-240 (1994) 2551. [8] B.K. Cho et al., Phys. Rev. B 52 (1995) R3844. See also High Tc update, 1 April 1995. [9] Z. Hossain et al., IEEE Trans. Magn. 31 (1995) 4133; see also High Tc update, 1 June 1995. [101 T. Kohara et al., Phys. Rev. B 51 (1995) 3985. [11] C.V. Tomy et al., Physica C 248 (1995) 349. [12] H. Schmidt et al., Physica C 246 (1995) 177. [13] F.G. Aliev et al., Europhys. Lett. 25 (1994) 143. [14] J.W. Lynn et al., Physica B 223&224 (1996) 66. [15] S.K. Dhar et al., Solid State Commun. (in press).