- Email: [email protected]
New superconducting and magnetic R-T-B-C systems (R = rare earth; T = Ni, Pd)
Journal of Magnetismand Magnetic Materials 140-144 (1995) 2053-2054
~,~ Journalof
amnadgneUsm
magnetic 4 ~ i materials
ELSEVIER
New superconducting and magnetic R - T - B - C (R = rare earth; T = Ni, Pd)
systems
L.C. Gupta a,* R. Nagarajan a Z. Hossain a Chandan Mazumdar b S.K. Dhar a C. Godart c C. Levy-Clement d, B.D. Padalia b, R. Vijayaraghavan a a Tam Institute of Fundamental Research, Bombay 400 005, India b Department of Physics, Indian Institute of Technology, Bombay 400 076, India e CNRS, UPR-209, 92195 Meudon Cedex, France d CNRS Laboratoire de Physique de Solides, 92195 Meudon Cedex, France
Abstract Our discovery of superconductivity in Y - N i - B - C system (Tc ~ 12 K) has opened up a new field in superconductivity and magnetism in intermetallics. A brief description of the discovery of these new borocarbide materials is given. We have synthesised NdNi2B2C and GdNi2B2C for the first time and our measurements show that NdNi2B2C (Tm ~ 5 K) and GdNi2B2C (Tm ~ 19 K) undergo magnetic ordering. CeNi2B2C does not exhibit magnetic ordering down to 4.2 K; the magnitude of its magnetic susceptibility is consistent with Ce ions in mixed valence state. We comment on the magnetic properties of the two superconducting materials ErNi2B2C and LuNi2B2C. Our recent pioneering work [1-4] on superconductivity (SC) in four-element borocarbide system Y - N i - B - C , Tc ~ 12 K, has generated a great deal of excitement, opening a new field of superconducting and magnetic materials. We reported a weak superconducting signal (resistance showed only a drop and did not go to zero) in Y - N i - B system [1]. Our further investigations showed that addition of 0.2 atom of carbon to YNi4B dramatically enhanced the diamagnetic shielding signal and resistance dropped to zero. We investigated several compositions YxNiyBzC0.2. The multiphase sample YNi2B3Co. 2 (we point out here that Y:Ni ratio in this material is 1 : 2, same as in YNi2B2C) was found to be a bulk superconductor, exhibiting a significant specific heat anomaly at Tc ( ~ 12 K) [2-4]. Details of the discovery of the bulk superconductivity in Y - N i B-C have been described in Ref. [5]. While we were attempting to optimise the superconducting fraction of the material by varying the proportion of C, Cava et al. [6] reported superconductivity in RNi2B2C (R = Y, Lu, Ho, Er, Tm) and Siegrist et al. [7] reported crystal structure (tetragonal, 14/mmm, derived from the well known ThCr2Si2-type) of a superconducting phase LuNi2B2C. Siegrist et al. [7] reported lattice constants of many materials of the series RNi2B2C. Interestingly, while the aparameter exhibits lanthanide contraction, the c-parameter,
* Corresponding author. Fax: +91-22-215 2110; email: [email protected].
in contrast, expands as one moves from La to Lu. The cell constants of CeNi2B2C deviate from this general pattern and are anomalous. These authors did not report NdNi2 B EC and GdNi2B2C. We report new results on these and on ErNiEBEC and CeNi2BEC. Magnetic properties of superconducting RNi2B2C systems, in which R has a moment, are remarkable in that the magnetic ordering temperatures are much higher than those in the two well investigated ternary SC systems, RRh4B 4 and RMo6S 8. Coexistence of magnetism and superconductivity, therefore, occurs over a wider range of temperature: HoNi2B2C (Tc ~ 8 K, Tm ~ 5 K), ErNi2B2C (Tc~ 10.5 K, Tm~ 7 K) and TmNiEB2C (Tc ~ 11 K, Tm ~ 4 K) [8]. Eisaki et al. [9] have reported similar results. We describe below magnetic properties, based on our work, of ErNi2B2C. Our specific heat (Cp) measurements show, besides the anomaly in Co at Tc, a large anomaly ( ~ 13 J/mol K) around 6 K, due to Er-moment ordering, which is more than ten times the anomaly due to SC [8]. Our recent neutron diffraction measurements not only corroborate magnetic ordering of this material at 7 K [10], they also indicate that Ni atoms have an induced magnetic moment below Tm ~ 7 K in the superconducting state. The possibility of nickel atoms carrying a small magnetic moment even when the R-atom does not have a magnetic moment is indicated by our IxSR results on LuNi2B2C [11]. These results also suggest that Ni moments order below ~ 1.8 K. Moment on Ni atoms in superconducting materials is a new phenomenon. We believe it will be a subject of
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)01592-9
L.C. Gupta et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 2053-2054
2054 1200t
NdNi2BzC
aoo
~o
2o
~o
4o
so
so 2 0
ro
Fig. 1. Powder X-ray diffraction pattern of NdNi2B2C, indicating that the sample is single phase and forms in the LuNi2B2C structure. Similar pattern is obtained for GdNi2B2C.
extensive investigations in the near future. We have been able to synthesise single phase NdNi2B2C and GdNi2B2C. Our powder X-ray diffraction measurements (Fig. 1) show that these materials also crystallise in the LuNi2B2C-structure. The lattice constants are: a = 3.69 A, 3.58/~; c = 10.10, 10.38 A for Nd- and Gd-compounds respectively. Both materials do not exhibit SC down to 4.2 K; however they order magnetically, NdNi2B2C at ~ 5 K and GdNi2B2C at ~ 19 K. Temperature dependence of magnetic susceptibility x(T) of NdNi2B2C is typical of an antiferromagnet. Further studies are needed to understand the nature of magnetic ordering of GdNi2B2C (Fig. 2). Paramagnetic x(T) of both materials follows a Curie-Weiss behaviour with /xeff = 3.6/x B and 8.1/xB; Op = - 2 . 3 and + 1 0 K respectively. We synthesised CeNi2B2C also. Our measurements show that the susceptibility of CeNi2B2C is much weaker than if the Ce ions were in trivalent state, x(T) does not follow a Curie-Weiss behaviour. We do not observe any magnetic transition in this material down to 4.2 K. In view of the anomaly in the lattice constants, our results suggest that Ce ions are in mixed valence state. Further, our preliminary Lin-edge studies show a bimodal edge structure confirming mixed valent state of Ce ions in this compound. These results are at variance with those of Kadowaki et al. [12] who reported a magnetic transition in 0.8
:!
u
0.4 ~,
M 0.3 0.2
× ×
O.l 0.0
-
×× 50
× 100 150 200 Temperature OK)
-
× 250
300
Fig. 2. Magnetic susceptibility of GdNi2B2C as a function of temperature. Inset: expanded plot below 40 K.
this material at ~ 7 K with rather small magnetic moment on Ce ions ( ~ 0 . 2 / z B / C e ion). We suspect that these authors see an antiferromagnetic ordering of an impurity phase. We point out that CeB2C 2 [13] undergoes a magnetic ordering at ~ 7 K and that the phase RB2C 2 is a commonly observed impurity phase in RNi2B2C [14]. Though we discuss magnetism in borocarbides, we point out that replacing Ni by Pd produces a superconducting phase in Y - P d - B - C system with Tc ~ 23 K which is the highest for a bulk intermetallic superconductor [15]. Our investigations of YPdaBCo. 2 show that there are at least two SC phases, with Tc ~ 22 K and ~ 10 K [16]. In conclusion, our discovery of superconductivity in quaternary Y - N i - B - C has shown a path of realising new superconducting and magnetic intermetallics. They exhibit remarkable properties. Magnetic ordering temperatures are rather high as compared to those known in ternaries RRhaB 4 and RM06S 8. NdNi2B2C and GdNi2B2C order magnetically whereas CeNi2B2C is a mixed valence system and does not exhibit any magnetic ordering and superconductivity down to 4.2 K. More detailed investigations, such as on the nature of the magnetic structure, would bring out newer aspects of the interplay of SC and magnetism. Moment on Ni atoms in ErNi2B2C and in LuNi2B2C should have an implication with respect to the mechanism of superconductivity in these materials. An intense activity in this field is expected. One may witness more quaternary materials, possibly, even without B and C but with elements such as Si, Ge, N and P and attempts may be made for more complex systems. Acknowledgment: Part of this work was done under the Indo-French Project 509-1, IFCPAR, New Delhi, India. References
[1] C. Mazumdar et al., Sol. State. Comm. 87 (1993) 413. [2] C. Mazumdar et al., Physica B 194-196 (1994) 1985 (results with and without carbon). [3] C. Mazumdar et al., Int. Conf. SCES 93, San Diego, U.S.A., Aug. 16-19, 1993. (Poster presentation of results with and without carbon). [4] R. Nagarajan et al., Phys. Rev. Lett. 72 (1994) 274. [5] L.C. Gupta et al., Physica C 235-240 (1994) 150. [6] R.J. Cava et al., Nature 367 (1994) 252. [7] T. Siegrist et al., Nature 367 (1994) 254. [8] L.C. Gupta et al., Proc. Int. Conf. on Adv. in Physical Metallurgy, Bombay, March 9-11, 1994. [9] H. Eisaki et al., Physica C 235-240 (1994) 2539. [10] S.K. Sinha et al., Phys. Rev. B, Rapid Comm. 51 (1) (1995), in press. [11] D.W. Cooke et al., Preprint. [12] K. Kadowaki et al., Int. Conf. SCES'94, Amsterdam 15-18 Aug. 1994, Physica B 206-207 (1995) to appear. [13] K. Hirota, M. Ishikawa, Physica B 186&188 (1993) 478. [14] C. Godart et al., Phys. Rev. B 50 (1) (1994), in press. [15] R.J. Cava et al., Nature 3678 (1994) 146. [15] Z. Hossain et al., Sol. State. Comm. 92 (1994) 341.
~,~ Journalof
amnadgneUsm
magnetic 4 ~ i materials
ELSEVIER
New superconducting and magnetic R - T - B - C (R = rare earth; T = Ni, Pd)
systems
L.C. Gupta a,* R. Nagarajan a Z. Hossain a Chandan Mazumdar b S.K. Dhar a C. Godart c C. Levy-Clement d, B.D. Padalia b, R. Vijayaraghavan a a Tam Institute of Fundamental Research, Bombay 400 005, India b Department of Physics, Indian Institute of Technology, Bombay 400 076, India e CNRS, UPR-209, 92195 Meudon Cedex, France d CNRS Laboratoire de Physique de Solides, 92195 Meudon Cedex, France
Abstract Our discovery of superconductivity in Y - N i - B - C system (Tc ~ 12 K) has opened up a new field in superconductivity and magnetism in intermetallics. A brief description of the discovery of these new borocarbide materials is given. We have synthesised NdNi2B2C and GdNi2B2C for the first time and our measurements show that NdNi2B2C (Tm ~ 5 K) and GdNi2B2C (Tm ~ 19 K) undergo magnetic ordering. CeNi2B2C does not exhibit magnetic ordering down to 4.2 K; the magnitude of its magnetic susceptibility is consistent with Ce ions in mixed valence state. We comment on the magnetic properties of the two superconducting materials ErNi2B2C and LuNi2B2C. Our recent pioneering work [1-4] on superconductivity (SC) in four-element borocarbide system Y - N i - B - C , Tc ~ 12 K, has generated a great deal of excitement, opening a new field of superconducting and magnetic materials. We reported a weak superconducting signal (resistance showed only a drop and did not go to zero) in Y - N i - B system [1]. Our further investigations showed that addition of 0.2 atom of carbon to YNi4B dramatically enhanced the diamagnetic shielding signal and resistance dropped to zero. We investigated several compositions YxNiyBzC0.2. The multiphase sample YNi2B3Co. 2 (we point out here that Y:Ni ratio in this material is 1 : 2, same as in YNi2B2C) was found to be a bulk superconductor, exhibiting a significant specific heat anomaly at Tc ( ~ 12 K) [2-4]. Details of the discovery of the bulk superconductivity in Y - N i B-C have been described in Ref. [5]. While we were attempting to optimise the superconducting fraction of the material by varying the proportion of C, Cava et al. [6] reported superconductivity in RNi2B2C (R = Y, Lu, Ho, Er, Tm) and Siegrist et al. [7] reported crystal structure (tetragonal, 14/mmm, derived from the well known ThCr2Si2-type) of a superconducting phase LuNi2B2C. Siegrist et al. [7] reported lattice constants of many materials of the series RNi2B2C. Interestingly, while the aparameter exhibits lanthanide contraction, the c-parameter,
* Corresponding author. Fax: +91-22-215 2110; email: [email protected].
in contrast, expands as one moves from La to Lu. The cell constants of CeNi2B2C deviate from this general pattern and are anomalous. These authors did not report NdNi2 B EC and GdNi2B2C. We report new results on these and on ErNiEBEC and CeNi2BEC. Magnetic properties of superconducting RNi2B2C systems, in which R has a moment, are remarkable in that the magnetic ordering temperatures are much higher than those in the two well investigated ternary SC systems, RRh4B 4 and RMo6S 8. Coexistence of magnetism and superconductivity, therefore, occurs over a wider range of temperature: HoNi2B2C (Tc ~ 8 K, Tm ~ 5 K), ErNi2B2C (Tc~ 10.5 K, Tm~ 7 K) and TmNiEB2C (Tc ~ 11 K, Tm ~ 4 K) [8]. Eisaki et al. [9] have reported similar results. We describe below magnetic properties, based on our work, of ErNi2B2C. Our specific heat (Cp) measurements show, besides the anomaly in Co at Tc, a large anomaly ( ~ 13 J/mol K) around 6 K, due to Er-moment ordering, which is more than ten times the anomaly due to SC [8]. Our recent neutron diffraction measurements not only corroborate magnetic ordering of this material at 7 K [10], they also indicate that Ni atoms have an induced magnetic moment below Tm ~ 7 K in the superconducting state. The possibility of nickel atoms carrying a small magnetic moment even when the R-atom does not have a magnetic moment is indicated by our IxSR results on LuNi2B2C [11]. These results also suggest that Ni moments order below ~ 1.8 K. Moment on Ni atoms in superconducting materials is a new phenomenon. We believe it will be a subject of
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)01592-9
L.C. Gupta et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 2053-2054
2054 1200t
NdNi2BzC
aoo
~o
2o
~o
4o
so
so 2 0
ro
Fig. 1. Powder X-ray diffraction pattern of NdNi2B2C, indicating that the sample is single phase and forms in the LuNi2B2C structure. Similar pattern is obtained for GdNi2B2C.
extensive investigations in the near future. We have been able to synthesise single phase NdNi2B2C and GdNi2B2C. Our powder X-ray diffraction measurements (Fig. 1) show that these materials also crystallise in the LuNi2B2C-structure. The lattice constants are: a = 3.69 A, 3.58/~; c = 10.10, 10.38 A for Nd- and Gd-compounds respectively. Both materials do not exhibit SC down to 4.2 K; however they order magnetically, NdNi2B2C at ~ 5 K and GdNi2B2C at ~ 19 K. Temperature dependence of magnetic susceptibility x(T) of NdNi2B2C is typical of an antiferromagnet. Further studies are needed to understand the nature of magnetic ordering of GdNi2B2C (Fig. 2). Paramagnetic x(T) of both materials follows a Curie-Weiss behaviour with /xeff = 3.6/x B and 8.1/xB; Op = - 2 . 3 and + 1 0 K respectively. We synthesised CeNi2B2C also. Our measurements show that the susceptibility of CeNi2B2C is much weaker than if the Ce ions were in trivalent state, x(T) does not follow a Curie-Weiss behaviour. We do not observe any magnetic transition in this material down to 4.2 K. In view of the anomaly in the lattice constants, our results suggest that Ce ions are in mixed valence state. Further, our preliminary Lin-edge studies show a bimodal edge structure confirming mixed valent state of Ce ions in this compound. These results are at variance with those of Kadowaki et al. [12] who reported a magnetic transition in 0.8
:!
u
0.4 ~,
M 0.3 0.2
× ×
O.l 0.0
-
×× 50
× 100 150 200 Temperature OK)
-
× 250
300
Fig. 2. Magnetic susceptibility of GdNi2B2C as a function of temperature. Inset: expanded plot below 40 K.
this material at ~ 7 K with rather small magnetic moment on Ce ions ( ~ 0 . 2 / z B / C e ion). We suspect that these authors see an antiferromagnetic ordering of an impurity phase. We point out that CeB2C 2 [13] undergoes a magnetic ordering at ~ 7 K and that the phase RB2C 2 is a commonly observed impurity phase in RNi2B2C [14]. Though we discuss magnetism in borocarbides, we point out that replacing Ni by Pd produces a superconducting phase in Y - P d - B - C system with Tc ~ 23 K which is the highest for a bulk intermetallic superconductor [15]. Our investigations of YPdaBCo. 2 show that there are at least two SC phases, with Tc ~ 22 K and ~ 10 K [16]. In conclusion, our discovery of superconductivity in quaternary Y - N i - B - C has shown a path of realising new superconducting and magnetic intermetallics. They exhibit remarkable properties. Magnetic ordering temperatures are rather high as compared to those known in ternaries RRhaB 4 and RM06S 8. NdNi2B2C and GdNi2B2C order magnetically whereas CeNi2B2C is a mixed valence system and does not exhibit any magnetic ordering and superconductivity down to 4.2 K. More detailed investigations, such as on the nature of the magnetic structure, would bring out newer aspects of the interplay of SC and magnetism. Moment on Ni atoms in ErNi2B2C and in LuNi2B2C should have an implication with respect to the mechanism of superconductivity in these materials. An intense activity in this field is expected. One may witness more quaternary materials, possibly, even without B and C but with elements such as Si, Ge, N and P and attempts may be made for more complex systems. Acknowledgment: Part of this work was done under the Indo-French Project 509-1, IFCPAR, New Delhi, India. References
[1] C. Mazumdar et al., Sol. State. Comm. 87 (1993) 413. [2] C. Mazumdar et al., Physica B 194-196 (1994) 1985 (results with and without carbon). [3] C. Mazumdar et al., Int. Conf. SCES 93, San Diego, U.S.A., Aug. 16-19, 1993. (Poster presentation of results with and without carbon). [4] R. Nagarajan et al., Phys. Rev. Lett. 72 (1994) 274. [5] L.C. Gupta et al., Physica C 235-240 (1994) 150. [6] R.J. Cava et al., Nature 367 (1994) 252. [7] T. Siegrist et al., Nature 367 (1994) 254. [8] L.C. Gupta et al., Proc. Int. Conf. on Adv. in Physical Metallurgy, Bombay, March 9-11, 1994. [9] H. Eisaki et al., Physica C 235-240 (1994) 2539. [10] S.K. Sinha et al., Phys. Rev. B, Rapid Comm. 51 (1) (1995), in press. [11] D.W. Cooke et al., Preprint. [12] K. Kadowaki et al., Int. Conf. SCES'94, Amsterdam 15-18 Aug. 1994, Physica B 206-207 (1995) to appear. [13] K. Hirota, M. Ishikawa, Physica B 186&188 (1993) 478. [14] C. Godart et al., Phys. Rev. B 50 (1) (1994), in press. [15] R.J. Cava et al., Nature 3678 (1994) 146. [15] Z. Hossain et al., Sol. State. Comm. 92 (1994) 341.