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Superconducting and magnetic properties of Pb(Eu, Ce)Mo6S8
517
Journal of Magnetism and Magnetic Materials 31-34 (1983) 517-518 S U P E R C O N D U C T I N G A N D M A G N E T I C P R O P E R T I E S O F Pb(Eu, C e ) M o n S s K. O K U D A *, S. N O G U C H I t, K. S U G I Y A M A t a n d M. D A T E t * Department of Electronics, University of Osaka Prefecture, Sakai, Osaka 591, Japan : Department of Physics, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan
Electric and magnetic properties of PbL2_xEuxM06S8 and PbL2_~,CexMorS8 were investigated by the electrical resistance and magnetization measurements in the range of temperature down to 0.1 K and magnetic field up to 650 kOe. A clear enhancement of He: was found in the composition Pb0.7Eu0.sM06S8.
Chevrel phase compounds MM06S s have attracted much attention because of their peculiar superconducting and magnetic properties, such as high upper critical magnetic field He2 in M = Pb, Sn and the coexistence of magnetic ordering and superconductivity in M = rareearth elements except for Ce and Eu. In the present work, the effect of magnetic ions on the superconductivity in Pbl.2_xEu~,M06S8 and Pbl.2_xCexM06S 8 was investigated by electrical resistance and magnetization measurements, with interest in: (1) high H¢~ in a region of low x and (2) lack of superconductivity in Ce- and EuM% Ss. Samples used in the present experiments were prepared by sintering from PbS, MoS 2, Mo, EuS and CezS 3 powders. Superconducting transition temperature T~was measured by conventional four-probe dc electrical resistance method. The obtained dependence of T¢ on the concentration x in Pbl.2_xEuxMo6S8 and Pbl.2_ xCexMo6S 8 are shown in fig. 1. As the concentration x was increased, the T¢ of the former system was almost unchanged up to x = 0.5 and for x > 0.5 it dropped abruptly to zero for x = 0.9. On the other hand, the Tc of the latter system was depressed at lower concentration than that of Eu-doped system and it may be attributed to the strong exchange scattering associated with the Kondo effect [1].
151
~
Temperature dependence of H¢2 was measured by magnetoresistance method under a pulsed magnetic field up to 650 kOe in the High Magnetic Field Laboratory of Osaka University [2]. The Hc2(T ) curves for the both systems showed a positive curvature with decreasing temperature. A clear enhancement of He2 was found in the composition Pb0.7Eu0.sMo6S8 as shown in fig. 2. These results are explained as a consequence of the compensation of applied field by a negative exchange field due to the antiferromagnetic exchange interaction between conduction electron spins and local magnetic moments [3]. Magnetization measurements on the compounds CeMo6S 8 and Eul.2Mo6Ss, whose magnetic ordering temperature Tm are 2.5 and 0.3 K, respectively, were done near Tm in the fields up to 400 kOe. The magnetization process of CeMo6S s at 1.3 K showed a linear increase with field above 160 kOe as shown in fig. 3. The high field susceptibility above 200 kOe, Xhr, is !
60C
I \,
40C
~i
!
\,Pb.l2Mo6S8
(b) U -'r
x,. i (a) ,_,I(
200
'~
5
I
\
%4z
I
0 0.5 1.0 Pb X Eu,Ce Fig. 1. The dependence of T~ on the concentration x for (a) Pbt.2_xCexMorS8 and (b) Pbl.2_:,EuxMorSs.
0
I
5T ( K )
"%,"\',, I ~ ,
10
i
15
Fig. 2. Temperature dependence of He2 in Pbo.7Euo.sM06Ss and Pbl.2M06Ss.
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
518
K. Okuda et aL / Superconducting and magnetic PR of Pb(Eu, Ce)Mo 6 S 8 I
I
I
1.5
~4
f
f
Y,
O0
0.5 i
100
J
200 H(~Oe)
i
300
Fig. 3. Magnetization processes in CeMo6S 8 at 1.3 K and Eu].2Mo6S s at 0.6 K. 1.07 x 10 -5 e m u / g . The saturated magnetic moment extrapolated from the linear region to zero field is (0.6 _ 0.1)/xa/Ce and it means that the ground state of Ce 3+ is doublet. O n the other hand, the magnetization of Eul.2Mo6S s at 0.6 K was almost saturated above 200 kOe as shown in fig. 3. The saturated magnetic moment of the compound is (5.9 + 0.3)/~a/Eu, which is 84% of that of divalent Eu. This result suggests the presence of 16% Eu 3+ in the samples, because the contribution of Eu 3+ to the observed magnetic moment is negligible at low temperatures. In facts, our suggestion is supported by the results in M6ssbaner study on Sn0.75Eu0.25Mo6S s by Dunlap et al. [4]. The MiSssbauer spectra taken by them showed two lines characteristic of Eu 2 + and Eu 3 + over the temperature range from 4.2 to 300 K.
Temperature dependence of the magnetoresistance was measured for high concentrations of Eu and Ce in Pbl.2Mo6S 8. A large negative magnetoresistance was found near Tm for CeMo6S 8 and EUl.2Mo6S 8 and it suggests that the effect of spin fluctuations is large in both compounds. Of particular interest is the compound Pb9.4Eu0.sMo6S8, which has a resistance peak at 10 K and becomes superconducting at 2.0 K in zero field. However, in low fields below 3 kOe the normal state reappears at 0.8 K with decreasing temperature. This temperature dependence of the magnetoresistance is similar to that in DYl.2Mo6S s and ErL2Mo6S s [5] and it may be caused by magnetic ordering such as spin-glass ordering. This work was supported by the Grant in Aid for Scientific Research from the Ministry of Education in Japan.
References [1] M.B. Maple, L.E. DeLong, W.A. Fertig, D.C. Johnston, R.W. McCallum and R.N. Shelton, Valence Instabilities and Related Narrow-Band Phenomena, ed. R.D. Parks (Plenum, New York, 1977) p. 17. [2] M. Date, IEEE Trans. on Magn. MAG-12 (1976) 1024. [3] V. Jaccarino and M. Peter, Phys. Rev. Lett. 9 (1962) 290. [4] B.D. Dunlap, G.K. Shenoy, F.Y. Fradin, C.D. Barnet and C.W. Kimball, J. Magn. Magn. Mat. 13 (1979) 319. [5] M. lshikawa and O. Fischer, Solid State Commun. 24 (1977) 747.
Journal of Magnetism and Magnetic Materials 31-34 (1983) 517-518 S U P E R C O N D U C T I N G A N D M A G N E T I C P R O P E R T I E S O F Pb(Eu, C e ) M o n S s K. O K U D A *, S. N O G U C H I t, K. S U G I Y A M A t a n d M. D A T E t * Department of Electronics, University of Osaka Prefecture, Sakai, Osaka 591, Japan : Department of Physics, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan
Electric and magnetic properties of PbL2_xEuxM06S8 and PbL2_~,CexMorS8 were investigated by the electrical resistance and magnetization measurements in the range of temperature down to 0.1 K and magnetic field up to 650 kOe. A clear enhancement of He: was found in the composition Pb0.7Eu0.sM06S8.
Chevrel phase compounds MM06S s have attracted much attention because of their peculiar superconducting and magnetic properties, such as high upper critical magnetic field He2 in M = Pb, Sn and the coexistence of magnetic ordering and superconductivity in M = rareearth elements except for Ce and Eu. In the present work, the effect of magnetic ions on the superconductivity in Pbl.2_xEu~,M06S8 and Pbl.2_xCexM06S 8 was investigated by electrical resistance and magnetization measurements, with interest in: (1) high H¢~ in a region of low x and (2) lack of superconductivity in Ce- and EuM% Ss. Samples used in the present experiments were prepared by sintering from PbS, MoS 2, Mo, EuS and CezS 3 powders. Superconducting transition temperature T~was measured by conventional four-probe dc electrical resistance method. The obtained dependence of T¢ on the concentration x in Pbl.2_xEuxMo6S8 and Pbl.2_ xCexMo6S 8 are shown in fig. 1. As the concentration x was increased, the T¢ of the former system was almost unchanged up to x = 0.5 and for x > 0.5 it dropped abruptly to zero for x = 0.9. On the other hand, the Tc of the latter system was depressed at lower concentration than that of Eu-doped system and it may be attributed to the strong exchange scattering associated with the Kondo effect [1].
151
~
Temperature dependence of H¢2 was measured by magnetoresistance method under a pulsed magnetic field up to 650 kOe in the High Magnetic Field Laboratory of Osaka University [2]. The Hc2(T ) curves for the both systems showed a positive curvature with decreasing temperature. A clear enhancement of He2 was found in the composition Pb0.7Eu0.sMo6S8 as shown in fig. 2. These results are explained as a consequence of the compensation of applied field by a negative exchange field due to the antiferromagnetic exchange interaction between conduction electron spins and local magnetic moments [3]. Magnetization measurements on the compounds CeMo6S 8 and Eul.2Mo6Ss, whose magnetic ordering temperature Tm are 2.5 and 0.3 K, respectively, were done near Tm in the fields up to 400 kOe. The magnetization process of CeMo6S s at 1.3 K showed a linear increase with field above 160 kOe as shown in fig. 3. The high field susceptibility above 200 kOe, Xhr, is !
60C
I \,
40C
~i
!
\,Pb.l2Mo6S8
(b) U -'r
x,. i (a) ,_,I(
200
'~
5
I
\
%4z
I
0 0.5 1.0 Pb X Eu,Ce Fig. 1. The dependence of T~ on the concentration x for (a) Pbt.2_xCexMorS8 and (b) Pbl.2_:,EuxMorSs.
0
I
5T ( K )
"%,"\',, I ~ ,
10
i
15
Fig. 2. Temperature dependence of He2 in Pbo.7Euo.sM06Ss and Pbl.2M06Ss.
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
518
K. Okuda et aL / Superconducting and magnetic PR of Pb(Eu, Ce)Mo 6 S 8 I
I
I
1.5
~4
f
f
Y,
O0
0.5 i
100
J
200 H(~Oe)
i
300
Fig. 3. Magnetization processes in CeMo6S 8 at 1.3 K and Eu].2Mo6S s at 0.6 K. 1.07 x 10 -5 e m u / g . The saturated magnetic moment extrapolated from the linear region to zero field is (0.6 _ 0.1)/xa/Ce and it means that the ground state of Ce 3+ is doublet. O n the other hand, the magnetization of Eul.2Mo6S s at 0.6 K was almost saturated above 200 kOe as shown in fig. 3. The saturated magnetic moment of the compound is (5.9 + 0.3)/~a/Eu, which is 84% of that of divalent Eu. This result suggests the presence of 16% Eu 3+ in the samples, because the contribution of Eu 3+ to the observed magnetic moment is negligible at low temperatures. In facts, our suggestion is supported by the results in M6ssbaner study on Sn0.75Eu0.25Mo6S s by Dunlap et al. [4]. The MiSssbauer spectra taken by them showed two lines characteristic of Eu 2 + and Eu 3 + over the temperature range from 4.2 to 300 K.
Temperature dependence of the magnetoresistance was measured for high concentrations of Eu and Ce in Pbl.2Mo6S 8. A large negative magnetoresistance was found near Tm for CeMo6S 8 and EUl.2Mo6S 8 and it suggests that the effect of spin fluctuations is large in both compounds. Of particular interest is the compound Pb9.4Eu0.sMo6S8, which has a resistance peak at 10 K and becomes superconducting at 2.0 K in zero field. However, in low fields below 3 kOe the normal state reappears at 0.8 K with decreasing temperature. This temperature dependence of the magnetoresistance is similar to that in DYl.2Mo6S s and ErL2Mo6S s [5] and it may be caused by magnetic ordering such as spin-glass ordering. This work was supported by the Grant in Aid for Scientific Research from the Ministry of Education in Japan.
References [1] M.B. Maple, L.E. DeLong, W.A. Fertig, D.C. Johnston, R.W. McCallum and R.N. Shelton, Valence Instabilities and Related Narrow-Band Phenomena, ed. R.D. Parks (Plenum, New York, 1977) p. 17. [2] M. Date, IEEE Trans. on Magn. MAG-12 (1976) 1024. [3] V. Jaccarino and M. Peter, Phys. Rev. Lett. 9 (1962) 290. [4] B.D. Dunlap, G.K. Shenoy, F.Y. Fradin, C.D. Barnet and C.W. Kimball, J. Magn. Magn. Mat. 13 (1979) 319. [5] M. lshikawa and O. Fischer, Solid State Commun. 24 (1977) 747.