Magnetic field induced superconductors, u0.8Sn0.2Mo6S7−YSeY

Physica 148B (1987) 130-132 North-Holland, Amsterdam

MAGNETIC

FIELD

INDUCED

SUPERCONDUCTORS,

Euo.sSno.2Mo6S7_rSe Y

S. K A W A M A T A , N. K O B A Y A S H I , M. I K E B E and Y. M U T O Institute for Materials Research, Tohoku University, Katahira, Sendal 980, Japan

Received 1 August 1987 The specific heat of the magnetic field induced superconductors, Eu.~Sn 0 2M06S7 rSer has been measured. The observed Schottky anomaly shows that the Eu ions carry the magnetic moment, and that the anisotropy effect of the crystal field to Eu ions is not so small. Two mechanisms for the magnetic field induced superconductivity (MFIS), Jaccarino-Peter effect and the effect of spin fluctuation, are discussed. While we have found directly the Jaccarino-Peter effect through the hysteresis in the resistance vs. magnetic field curve as reported in LT18, it is confirmed that the effect of the spin fluctuation for the MFIS is reduced by the sizeable anisotropy of the Eu moments.

1. I n t r o d u c t i o n

In 1978, M a e k a w a and Tachiki [1] suggested the possibility of superconductivity caused by the application of a magnetic field. T h e y considered two mechanisms: the J a c c a r i n o - P e t e r effect [2] and the effect of the spin fluctuation. Following their prediction, Isino et al. [3] discovered the magnetic field induced superconductivity ( M F I S ) in a Chevrel phase c o m p o u n d , Eu0.8Sn0.2Uo6S 7 in 1980. In this paper, the experimental results on the specific heat of E u 0 8 S n 0 2 M o 6 S v _ r S e v (0 =< Y =< 0.5) are presented. T h e analysis of the specific heat reveals that the E u ions in this system carry a magnetic m o m e n t which is responsible for the m e c h a n i s m s of the M F I S , and also suggests that the effect of the spin fluctuation is weak. T h e direct evidence for the negative exchange field which is the basic assumption for the J a c c a r i n o - P e t e r effect, is to be r e p o r t e d by us elsewhere [4]. T h e M F I S in this system is caused by the J a c c a r i n o - P e t e r effect.

Y = 0. A similar result is o b t a i n e d in the sample with Y = 0.2. T h e increase of the specific heat o b s e r v e d in zero field below 4 K is considered to be the Schottky specific heat due to the crystalline field splitting of E u ions. T h e e n h a n c e m e n t of the specific heat caused by the application of the field is due to the increase of the Z e e m a n splitting. T h e p e a k of the S c h o t t k y specific heat in zero field seems to a p p e a r below 1.5 K. So above 6 K, the Schottky contribution has an asymptotic f o r m p r o p o r t i o n a l to T 2. T h e measured specific heat in zero field is fitted by the I

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T h e sample p r e p a r a t i o n is described elsewhere [4]. T h e specific heat on the samples with Y = 0 and 0.2 was m e a s u r e d by m e a n s of the thermal relaxation m e t h o d a b o v e 1.5 K and below 6 T . Fig. 1 shows the specific heat of the sample with 0378-4363/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation

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Fig. 1. Specific heat C vs. temperature T for Eu~Sn 0 2Mo687. The solid line indicates the fitting curve by the equation of C = T T + ~ST 2 + 8 T -2.

13l

S. K a w a m a t a et al. / M a g n e t i c f i e l d i n d u c e d s u p e r c o n d u c t o r s

equation C = y T + / 3 T 3 + 6 T 2 above 6 K, where y T is the electronic contribution,/3 T 3 the lattice contribution and 6 T -2 the Schottky contribution. Using the fitting values of y and/3, the electronic and lattice contributions, C o = 3 , T + /3T 3, are subtracted from the measured specific heat C. We regard the remainder C - C O represented in fig. 2 as the Schottky contribution due to the crystalline field splitting of Eu ions. In fig. 2, solid lines show the calculated Schottky specific heat due to E u 2+ ions ( J - - 7 / 2 ) on the basis of the crystal field Hamiltonian having only 0 an uniaxial field parameter, B 2 = --0.09 K. These calculated results roughly reproduce our experimental results. G o o d agreement between the measurements and the calculations in higher fields confirms that the value of J of Eu z+ is 7/2 and also that the estimated value C o is reliable. The deviation of the experimental results from the calculated ones in lower fields may come from the magnetic short range order of Eu moments, or from the higher order term in the crystalline field Hamiltonian. A b o v e 1.5 K, the contribution due to nuclear quadrupole moments is much smaller. The small parameter B2° is consistent with the 8S7/~ state of divalent Eu z+ ions. Eu 2+ ions in these compounds have the magnetic moment from which the mechanisms of I

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Fig. 3. U p p e r critical field He2 vs. temperature T. Solid lines show the calculated//c2 by the multiple pair-breaking theory. A m o n g four parameters, the experimental values of T c are used. T h e Maki p a r a m e t e r a is 5.3, the saturated value of //1, H~ 'ax is - 4 0 T, and the spin-orbit scattering parameters A~o are from 9.5 to 18.0. The inset shows the low field region of He2 of the sample with Y = 0.2.

the MFIS originates. In zero field this crystalline field Hamiltonian (B ° = - 0 . 0 9 K) gives the value of 1 . 6 K for the level separation between the lowest Kramers' doublet of Jz = - + 7 / 2 and the first excited doublet of Jz = + 5 / 2 . One possible mechanism for the MFIS is the J a c c a r i n o - P e t e r effect which is the compensation mechanism by the negative exchange field H~ due to Eu moments acting on the conduction electron spins. We confirmed directly the negative H j through the hysteretic behavior in the resistance vs. magnetic field curve [4]. Fig. 3 shows the upper critical field He2 of these compounds determined as the field where the extrapolated resistance vs. field curve intersects the zero resistance line. In the sample with Y = 0.2, the MFIS is observed. Solid lines in fig. 3 show the calculated curves by the multiple pair-breaking theory [5] based on the J a c c a r i n o - P e t e r ef-

132

s. Kawamata et al. / Magnetic field induced superconductors

fect, which reproduce the observed Hc2(T) in accordance with the previous report by Meul et al. [6]. It seems to be well established that the main origin for the MFIS in these compounds is the Jaccarino-Peter compensation mechanism. As pointed out by Maekawa et al. [1], another possible mechanism for MFIS is the effect of the spin fluctuation. The spin fluctuation has a destructive effect on the superconductivity, which is suppressed by the application of the field. From the analysis of the Schottky specific heat, the value for the level separation between the lowest doublet and the first excited one is 1.6 K. Although the value of 1.6 K is smaller than that of other rare-earth ions, it is reasonably large in comparison with the antiferromagnetic transition temperature T N of 0 . 3 K . The value of T N is inferred from the minimum of H~2(T ) in the low field region indicated in the inset of fig. 3. Th~.s anisotropy of Eu moments may considerably suppress the spin fluctuation prior to the application of the field. We suggest that the main reason which makes the effect of the spin fluctuation less important is the non-negligible anisotropy of Eu moments. In conclusion, Eu ions in these compounds have the magnetic moment. The anisotropy of Eu moments can suppress the effect of the spin fluctuation on the superconductivity. For the MFIS of these compounds, the effect of the spin

fluctuation is weak. On the other hand, our previous report confirmed the negative H~ from which the Jaccarino-Peter effect originates. Therefore, the MFIS of these compounds is caused by the Jaccarino-Peter effect.

Acknowledgments We are very grateful to Professor M. Tachiki, Professor S. Maekawa and Dr. H. Iwasaki for helpful discussions. This work was supported by the Grant-in-Aid for General Scientific Research (61460034) from the Ministry of Education, Japan.

References [1] S. Maekawa and M. Tachiki, Phys. Rev. B 18 (1978) 4688. [2] V. Jaccarino and M. Peter, Phys. Rev. Lett. 9 (1962) 290. [3] M. Isino, N. Kobayashi and Y. Muto, in: Ternary Superconductors, G.K. Shenoy et al., eds. (North-Holland, Amsterdam, 1981), p. 95. [4] S. Kawamata, N. Kobayashi, M. Ikebe and Y. Muto, Proc. of LT18, to be published. [5] 9- Fischer, Hel. Phys. Acta 45 (1972) 331. [6] H.W. Meul, C. Rossel, M. Decroux, ~. Fischer, O. Remenyi and A. Briggs, Physica 126B (1984) 44.