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Depression of Tc by nonmagnetic impurities in antiferromagnetic superconductor with Tc<TN
Physica B 259—261 (1999) 606—607
Depression of T by nonmagnetic impurities in antiferromagnetic superconductor with T (T , Z. Hossain, R. Nagarajan, S.K. Dhar, L.C. Gupta* Tata Institute of Fundamental Research, Solid State Physics Group, Honi Bhabha Road, Colaba, Mumbai 400 005, India
Abstract Theoretical considerations show that nonmagnetic impurities (which in conventional superconductors are not expected to have much influence on ¹ ) should depress ¹ in antiferromagnetic superconductors with ¹ (¹ . , DyNi B C (¹ "6.5 K, ¹ "11 K) and ErNi B C (¹ "10.5 K, ¹ "6.5 K) form an ideal pair of antiferromagnetic , , superconductors, with ¹ (¹ and ¹ '¹ , respectively, to investigate this effect experimentally which is the aim of the , , present work. Our most important finding is that in Y Dy Ni B C, dilute substitution of Y results in a decrease of \V V ¹ (*¹ &!2.5 K for x"0.9) in direct contrast to Y Er Ni B C, where substitution of Y results in an increase of \V V ¹ (*¹ �.3 K for x"0.9). Although depression of ¹ in DyNi B C by nonmagnetic Lu ions has been reported, that report did not consider the above-mentioned theoretical consideration. Our work points out the importance of nonmagnetic impurities in magnetic superconductors. 1999 Elsevier Science B.V. All rights reserved. Keywords: Quaternary borocarbides; Magnetic superconductors; Nonmagnetic impurity; DyNi B C
Coexistence of superconductivity and magnetism has been one of the fundamental problems in condensed matter physics [1]. Of particular interest is the case of magnetic superconductors in which superconductivity occurs in an already magnetically ordered lattice (superconducting transition temperature, ¹ (¹ , the mag , netic ordering temperature). In a conventional superconductor, nonmagnetic impurities are expected to have no appreciable effect on ¹ [2]. In the case of magnetic superconductors with ¹ (¹ , nonmagnetic impurities , were theoretically conjectured to suppress ¹ due to destruction of translational symmetry of antiferromagnetic lattice ([3], see several relevant articles in Ref. [1] also). However, this aspect has not been verified experimentally, possibly because, only two systems with ¹ ( ¹ were known, viz., Er Fe Si , (¹ &1 K, ¹ &2.9 K) , + [4] and Tb Mo Si (¹ &1.5 K, ¹ &19 K) [5]. And +
* Correspondence author. Tel.: #91-22-215-2971/2326; fax: #91-22-215-2110; e-mail: [email protected].
also because, they have rather low ¹ ((2 K). The dis covery of quaternary borocarbide superconductors [6—8] provides an opportunity to investigate the coexistence phenomena in detail due to the high coexistence temperature found in the magnetic superconductors of the family, RNi B C (R"Tm, Er, Ho, Dy; ¹ &11, 10.5, 8, 6 K; ¹ , &1.5, 7, 8, 11 K, respectively) [9]. Of these, , DyNi B C has ¹ (¹ [10—13] with both the transition , temperatures above 5 K which gives an ideal opportunity to test the above-mentioned conjecture ([3], see several relevant articles in Ref. [1] also). Here, we present a summary of our studies of the effect of substitution of the nonmagnetic Y on ¹ of DyNi B C (¹ (¹ ) and , ErNi B C (¹ '¹ ). , Materials Y R Ni B C (R"Dy, Er) (0(x(1), \V V were prepared by arc melting together appropriate amounts of master alloys and annealed in vacuum at 1050°C for one week. Two batches were prepared to check for consistency. ¹ was determined by resistivity (down to 1.5 K) and susceptibility (down to 4.2 K). The superconducting transitions were sharp (*¹ &0.3 K). Mid point of the transition was taken as ¹ . Bulk
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 8 0 0 - X
Z. Hossain et al. / Physica B 259—261 (1999) 606—607
607
DyNi B C [14]. These authors, however, do not con sider theoretical argument as suggested in Ref. [3], but attribute the depression of ¹ to softening of magnon spectrum. We have also carried out investigations on the system Y Tb Ni B C and find that the depression of ¹ is \V V much larger than that by nonmagnetic impurity, as expected. We also find coexistence of superconductivity and magnetism over certain concentrations. These studies will be published elsewhere.
Acknowledgements Fig. 1. ¹ (filled circle) and ¹ (filled triangle) in the system , Y Dy Ni B C as a function of x. \V V
magnetic order and its coexistence with superconductivity was confirmed by heat capacity measurements. Our results show that in Y ErxNi B C, ¹ varies \V almost linearly with x. ¹ of YNi B C being higher than that of ErNi B C, substitution of Y in ErNi B C in creases ¹ of the material. The variation of ¹ in Y Dy Ni B C with respect to x is shown in Fig. 1. It is \V V clearly seen from the figure that for dilute substitutions (x"0.90 and 0.95) of Y, ¹ decreases. Thus, the most important aspect of the result is that even though the end members (x"1) of the two systems, Y Dy Ni B C \V V and Y Er Ni B C are magnetic superconductors, di\V V lute substitution of nonmagnetic ion, Y, in ErNi B C (¹ '¹ ) results in an increase of ¹ whereas it results in , a decrease of ¹ in DyNi B C (¹ (¹ ). This clearly , establishes that nonmagnetic impurities do depress ¹ in materials where ¹ (¹ , as theoretically conjectured. , Similar effect has been reported by substitution of Lu in
We thank S.K. Paghdar and S.M. Pattalwar for their help in some of the experiments.
References [1] T. Matsubara, A. Kotani (Eds.), Superconductivity in Magnetic and Exotic Materials, Springer, Berlin, 1984. [2] P.W. Anderson, J. Phys. Chem. Solids 11 (1959) 26. [3] M.J. Nass et al., Phys. Rev. B 25 (1982) 4541. [4] S. Nagochi, K. Okuda, Physica B 194—196 (1994) 1975. [5] F.G. Aliev et al., Euro. Phys. Lett. 25 (1994) 143. [6] Chandan Mazumdar et al., Solid State Commun. 87 (1993) 413. [7] R. Nagarajan et al., Phys. Rev. Lett. 72 (1994) 274. [8] R.J. Cava et al., Nature 367 (1994) 252. [9] L.C. Gupta, Philos. Mag. B 77 (1998) 717, and references therein. [10] Z. Hossain et al., IEEE Trans. Magn. 31 (1995) 4133. [11] C.V. Tomy et al., Physica C 248 (1995) 349. [12] B.K. Cho et al., Phys. Rev. B 52 (1995) 3844. [13] Z. Hossain et al., Physica B 223 & 224 (1996) 99. [14] B.K. Cho et al., Phys. Rev. Lett. 77 (1996) 163.
Depression of T by nonmagnetic impurities in antiferromagnetic superconductor with T (T , Z. Hossain, R. Nagarajan, S.K. Dhar, L.C. Gupta* Tata Institute of Fundamental Research, Solid State Physics Group, Honi Bhabha Road, Colaba, Mumbai 400 005, India
Abstract Theoretical considerations show that nonmagnetic impurities (which in conventional superconductors are not expected to have much influence on ¹ ) should depress ¹ in antiferromagnetic superconductors with ¹ (¹ . , DyNi B C (¹ "6.5 K, ¹ "11 K) and ErNi B C (¹ "10.5 K, ¹ "6.5 K) form an ideal pair of antiferromagnetic , , superconductors, with ¹ (¹ and ¹ '¹ , respectively, to investigate this effect experimentally which is the aim of the , , present work. Our most important finding is that in Y Dy Ni B C, dilute substitution of Y results in a decrease of \V V ¹ (*¹ &!2.5 K for x"0.9) in direct contrast to Y Er Ni B C, where substitution of Y results in an increase of \V V ¹ (*¹ �.3 K for x"0.9). Although depression of ¹ in DyNi B C by nonmagnetic Lu ions has been reported, that report did not consider the above-mentioned theoretical consideration. Our work points out the importance of nonmagnetic impurities in magnetic superconductors. 1999 Elsevier Science B.V. All rights reserved. Keywords: Quaternary borocarbides; Magnetic superconductors; Nonmagnetic impurity; DyNi B C
Coexistence of superconductivity and magnetism has been one of the fundamental problems in condensed matter physics [1]. Of particular interest is the case of magnetic superconductors in which superconductivity occurs in an already magnetically ordered lattice (superconducting transition temperature, ¹ (¹ , the mag , netic ordering temperature). In a conventional superconductor, nonmagnetic impurities are expected to have no appreciable effect on ¹ [2]. In the case of magnetic superconductors with ¹ (¹ , nonmagnetic impurities , were theoretically conjectured to suppress ¹ due to destruction of translational symmetry of antiferromagnetic lattice ([3], see several relevant articles in Ref. [1] also). However, this aspect has not been verified experimentally, possibly because, only two systems with ¹ ( ¹ were known, viz., Er Fe Si , (¹ &1 K, ¹ &2.9 K) , + [4] and Tb Mo Si (¹ &1.5 K, ¹ &19 K) [5]. And +
* Correspondence author. Tel.: #91-22-215-2971/2326; fax: #91-22-215-2110; e-mail: [email protected].
also because, they have rather low ¹ ((2 K). The dis covery of quaternary borocarbide superconductors [6—8] provides an opportunity to investigate the coexistence phenomena in detail due to the high coexistence temperature found in the magnetic superconductors of the family, RNi B C (R"Tm, Er, Ho, Dy; ¹ &11, 10.5, 8, 6 K; ¹ , &1.5, 7, 8, 11 K, respectively) [9]. Of these, , DyNi B C has ¹ (¹ [10—13] with both the transition , temperatures above 5 K which gives an ideal opportunity to test the above-mentioned conjecture ([3], see several relevant articles in Ref. [1] also). Here, we present a summary of our studies of the effect of substitution of the nonmagnetic Y on ¹ of DyNi B C (¹ (¹ ) and , ErNi B C (¹ '¹ ). , Materials Y R Ni B C (R"Dy, Er) (0(x(1), \V V were prepared by arc melting together appropriate amounts of master alloys and annealed in vacuum at 1050°C for one week. Two batches were prepared to check for consistency. ¹ was determined by resistivity (down to 1.5 K) and susceptibility (down to 4.2 K). The superconducting transitions were sharp (*¹ &0.3 K). Mid point of the transition was taken as ¹ . Bulk
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 8 0 0 - X
Z. Hossain et al. / Physica B 259—261 (1999) 606—607
607
DyNi B C [14]. These authors, however, do not con sider theoretical argument as suggested in Ref. [3], but attribute the depression of ¹ to softening of magnon spectrum. We have also carried out investigations on the system Y Tb Ni B C and find that the depression of ¹ is \V V much larger than that by nonmagnetic impurity, as expected. We also find coexistence of superconductivity and magnetism over certain concentrations. These studies will be published elsewhere.
Acknowledgements Fig. 1. ¹ (filled circle) and ¹ (filled triangle) in the system , Y Dy Ni B C as a function of x. \V V
magnetic order and its coexistence with superconductivity was confirmed by heat capacity measurements. Our results show that in Y ErxNi B C, ¹ varies \V almost linearly with x. ¹ of YNi B C being higher than that of ErNi B C, substitution of Y in ErNi B C in creases ¹ of the material. The variation of ¹ in Y Dy Ni B C with respect to x is shown in Fig. 1. It is \V V clearly seen from the figure that for dilute substitutions (x"0.90 and 0.95) of Y, ¹ decreases. Thus, the most important aspect of the result is that even though the end members (x"1) of the two systems, Y Dy Ni B C \V V and Y Er Ni B C are magnetic superconductors, di\V V lute substitution of nonmagnetic ion, Y, in ErNi B C (¹ '¹ ) results in an increase of ¹ whereas it results in , a decrease of ¹ in DyNi B C (¹ (¹ ). This clearly , establishes that nonmagnetic impurities do depress ¹ in materials where ¹ (¹ , as theoretically conjectured. , Similar effect has been reported by substitution of Lu in
We thank S.K. Paghdar and S.M. Pattalwar for their help in some of the experiments.
References [1] T. Matsubara, A. Kotani (Eds.), Superconductivity in Magnetic and Exotic Materials, Springer, Berlin, 1984. [2] P.W. Anderson, J. Phys. Chem. Solids 11 (1959) 26. [3] M.J. Nass et al., Phys. Rev. B 25 (1982) 4541. [4] S. Nagochi, K. Okuda, Physica B 194—196 (1994) 1975. [5] F.G. Aliev et al., Euro. Phys. Lett. 25 (1994) 143. [6] Chandan Mazumdar et al., Solid State Commun. 87 (1993) 413. [7] R. Nagarajan et al., Phys. Rev. Lett. 72 (1994) 274. [8] R.J. Cava et al., Nature 367 (1994) 252. [9] L.C. Gupta, Philos. Mag. B 77 (1998) 717, and references therein. [10] Z. Hossain et al., IEEE Trans. Magn. 31 (1995) 4133. [11] C.V. Tomy et al., Physica C 248 (1995) 349. [12] B.K. Cho et al., Phys. Rev. B 52 (1995) 3844. [13] Z. Hossain et al., Physica B 223 & 224 (1996) 99. [14] B.K. Cho et al., Phys. Rev. Lett. 77 (1996) 163.