Study of upper critical field in superconducting borocarbides RNi2B2C (R  Y, Ho, Er, Tm)

mtUlll ELSEVIER

Physica B 223&224 (1996) 102 104

Study of upper critical field in superconducting borocarbides RNizBzC (R=Y, Ho, Er, Tm) Chandan Mazumdar"' 1, Z. Hossain b, S. Radha b, A.K. Nigam b'*, R. Nagarajan b, L.C. Gupta b, C. Godart ~, B.D. Padalia ~, G. Chandra b, R. Vijayaraghavan b ~'Department e?l Physics, Italian btstitute o/ Technology. Bombay 400 076. hlclia I'Tuta Institute Of Fundamental Research, Bombc~v 400 005, India ~UPR-209, C.N.R.S.. 92195 Meudon Cede<'< France

Abstract We have determined upper critical field, H~2(T), from resistivity measurements in the borocarbide superconductors, RNi2BzC (R =Y, Ho, Er, Tm). The Hc2(T) of these materials show a positive curvature which is unlike of a conventional type-II superconductor. Features corresponding to coexistence of superconductivity and magnetism are observed in HoNi2B2C and ErNizB2C. F r o m the resistivity measurements, we observe a double reentrant behaviour in our sample of HoNi2BzC.

Discovery of superconductivity (SC) in quaternary multiphase Y Ni B C at ~13.5 K [1,2] and the observation of SC in single phase materials RNi2B2C (R =Y, Lu, Tin, Er, Ho) [3] have catalysed the interest in SC and magnetism in intermetallics. Coexistence of SC and magnetism has been observed in some members of the series RNi2BeC (R =Tin, Er, Ho) [4-6]. These materials are unique, having highest superconducting transition temperature (To) and magnetic ordering temperature (Tin} amongst the materials exhibiting coexistence. Here we report upper critical field (H~2) studies of these materials through resistivity (p) measurements. Details of the synthesis and characterization of RNi2B2C (R =Y, rare earth) are described elsewhere [5 10]. Except in the case of YNi2B2C, in all the other cases, as cast samples were used. Longitudinal magnetoresistance (MR) were measured in magnetic fields up * Corresponding author. Present address: Tata Institute of Fundamental Research. Bombay 400 005, India.

to 45 kG and over the temperature range 4.4~20 K by conventional 4 probe method in a home-built automated set-up. Resistance of the materials were measured both as a function of magnetic field, H (at different fixed temperatures, T, both below and above To) (R H curves) and as a function of T (at different constant externally applied field) (R T curves). Due to uneven shape of the HoNizBzC sample, absolute resistivity of this material, which is not essential for the purpose of the present study, could not be determined. p T curves at different fields for YNi2B2C are shown in Fig. l(a). This material has Tc~..... t~ = 15.7K and T~o~ = 15.1 K. As H is increased, T~ decreases and transition width, A T~, increases, as expected for a type-If SC. Above Tc,,o increases with T. The MR ( = JR(H) -R(O)]/R(O)) at H = 45 kG and at 16 K is 6.5% while that at 20 K is 6.1%. We believe that this positive MR originates from Lorentz contribution. R H curves at various temperatures for YNi2B2C are shown in Fig. l(b). The isotherm at 4.4 K does not show any finite resistance upto H = 45 kG indicating

0921-4526/96/$15.00 I' 1996 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 1 9 6 ) 0 0 0 5 I -8

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Fig. 1. Resistivity of RNi2B2C as a function of magnetic field and temperature. Hc2(T) curves are shown in the insels. HCz (4.4K) to be greater than 45 kG. The width of transition increases with decreasing T, as observed in most of the type-lI SC. Inset of Fig. l(b) shows H~z(T) versus T obtained from these measurements. The onset of resistivity drop was taken as H¢2. The interesting aspect of the H~2(T) curve is that it shows a positive curvature near T~, while in conventional SC, slope of Hc2(T) curve does not change significantly below Tc over a wide range of T. Hcz(T) also appears to be sample dependent [11]. Recently, it has been shown that presence of paramagnetic impurities

[12] and intergranularity effects [13] might affect behaviour of Hc2(T). We have obtained Hc2(0 ) using the relation He(0) = - 0 . 6 9 T c ( d H c z / d T ) L. For this calculation we have taken the average slope of Hc2(T) in the temperature range 7.5 14 K and obtained H~2(0) as 66 kG. This value falls within the range of Hc2(0) reported in literature [13]. R - T curves at different H for HoNizB2C are shown in Fig. l(d). The interesting double reentrant behaviour of the material at H = 0 G can be seen in the inset of Fig. l(d); the resistance drops around 8 K, shows a

104

C Mazumdar et aL / Physica B 223&224 (1996) 102 104

m i n i m u m at 6.5 K (but does not go to zero), reaches a maximum around 5 K, then again decreases and finally reaches zero around 3.5 K. This reentrant behaviour is due to the interplay of superconductivity and magnetic ordering. We had observed a specific heat anomaly due to magnetic order in HoNizB2C [9]. Neutron scattering experiments [14] have shown that the double reentrance occurs as just below T c l ( ~ S K ) , the Ho moments undergo a magnetic transition with a small ferromagnetic component along the c-axis. This component keeps on growing and maximum suppression of SC occurs at Tc2( ~ 5 K). At a slightly lower temperature (To3) this ferromagnetic component disappears suddenly leaving only the antiferromagnetic order and the material regains SC. As H is increased, Tc~.... t) decreases. However, the t e m p e r a t u r e s , Tmi n and T . . . . at which the resistance first reaches minimum and maximum respectively, do not change appreciably with H. This means that a s Tmi n and Tma x originate as a consequence of magnetic ordering, they are not critically influenced by low fields ( ~ 2 kG). The double re-entrant-type behaviour is not seen above ~ 2 k G ; H~2 around the temperature of interest is < 2 kG. This is in agreement with other reports I-4, 15]. R - H curves taken at 3 selective temperatures, at the minimum (6 K); at the maximum (5 K); and below the maximum (4.4 K) are shown in Fig. l(c). The double reentrant behaviour is seen here as a cross-over of the curves for 5 and 6 K around 1.7kG (Fig. l(c)). The remarkable feature of a prominent maximum and m i n i m u m in the H¢2(T) curve (inset Fig. l(c)) reflects evolution of magnetism in this material with respect to T as discussed above. p - T curves for ErNi2B2C at different values of H are shown in Fig. l(f). The material has T~to)= 10 K. At H = 7 kG, T~lo) < 5 K. In this case too, the MR above T~ is positive. A notable feature is that the p - T curve for this material at 10 kG shows an anomaly around 5 K, suggesting a tendency of the material to possibly exhibit a double reentrant-type behaviour, as observed in the case of HoNi2B2C. This needs to be studied in detail; neutron diffraction studies of ErNizB2C in the presence of magnetic field and a comparison with those of HoNi2B2C should be quite informative on this aspect. The change of slope in Hc2(T) at ~ 7 K indicates the magnetic ordering temperature. p - T and p-H curves for TmNi2B2C are shown in Figs. l(g) and (h). These are quite similar to those ob-

served in the case of ErNi2B2C. He2(T) is shown in the inset of Fig. l(g). The main feature to be noticed is that Hc2(T) exhibits a small positive curvature similar to that of YNizB2C. Since the magnetic ordering temperature ( T m ~ 1.5 K ) is lower than the limit of our measurement, influence of magnetic ordering on Hc2 is not observed. In summary, our studies of YNizBzC, HoNi2B2C, ErNizB2C and TmNizBzC, show that these are type-II SC. Hc2's determined from the p - T curve and p - H curve have reasonable agreement. The values of Hc2(T) agree with those reported by Eisaki et al. [4]. Most important result is the occurrence of a prominent maximum and m i n i m u m in HCz(T) and double reentrant behaviour in the R - T curve of HoNizB2C, which indicates coexistence of SC and magnetism with ferromagnetic correlations before the material settles to antiferromagnetic order. Hc2(T) of YNizB2C and TmNizBgC appear to have a different behaviour compared to that in conventional superconductors. Above To, the magnetoresistance of all these materials is small but positive, indicating a dominant Lorentz contribution.

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., Physica C 235-240 (1994) 150 and references therein. [-6] C. Godart et al., Phys, Rev. B 51 (1995) 489. [-7] L.C. Gupta et al., J. Magn. Magn. Mater., 140-144 (1995) 2053. [8] R. Nagarajan et al., Physica B 206 207 (1995) 548. [9] R. Nagarajan et al., in: Proc. 4th Int. Conf and Exhibition: World Congress on Superconductivity, Vol. 1, eds. K. Krishen and C. Burnham (NASA Conf. Publ. 3290, 1994) p. 283. [10] R. Nagarajan et al., J. Alloys Compounds 225 (1995) 571. [11] R. Prozorov et al., Physica C 233 (1994) 363 and references therein. [12] S.B. Roy et al., Physica C 256 (1996) 90. [13] T.V. Chandrashekhar Rao et al., Physica C 249 (1995) 271 and references therein. [14] T.E. Grigereit et al., Phys. Rev. Lett. 73 (1994) 2756. [15] P.C. Canfield et al., Physica C 230 (1994) 397.