The specific heat and magnetic ordering at low temperature in superconducting Sm1.85−xGdxCe0.15CuO4−δ

Phystca C 235-240 (1994) 1781-1782 North-tlolland

PHYSICA

The Specific Heat and Magnetic Ordering at Low Temperature in Superconducting Sml.ss.xGdxCeo.lsCuO4.~ Jianlin Luo t , Yupeng Wang I , Hui Zhou I , Xiaobo Fan t , Yongwei Song t , Qize Ran t , Chin Lin 2, Yufen Zhang 2, D u o Jin l

tCryogenic Laboratory, Chinese Academy of Sciences, P.O.Box 2711, Beijing 100080. 2Department of Physics, Peking University, Beijing 100871. The specific heat between 1.2K and 25K for the so-called n-type superconducting system Sm ! ss.~Gdx. Ce0.tsCuO4. ~ (x=0.1,0.3,0.5,0.75)has been measured and reported. Sharp peaks due to antfferromagnetic ordering o f rare earth magnetic moments have been observed. The ordering temperature T M as well as superconducting transition temperature Tc decreases with increasing x,whde the extra entropy corresponding to the peak increases at the same time. The superconducttvtty disappear when x exceeds 0.75,and the Tc(x) behavior has been found to be well fitted by an Abnkosov-Gor'kov expression. Since the discovery of electron-doped Iq superconducting system Lnz.xCexCuO4_#(Ln = Pr, Nd, Sm, 0.14 < x < 0.18) in 1989, some distinctive properties have been found m such a system. Hall effect measurements I~'21 indicated that electrons, ra:her than holes, in the CuO 2 planes contnbute to the superconductivity as the charge carriers. Specific heat Cp of Ln I ssCe0 lsCuO4.~1351(Ln=Nd, Sm, Pr) was measured and a large magnettc anomaly was found for Ln = Nd and Sm at a temperature T M below 10K. The T M and Tc seem to coexist. What is the relationship between superconductivity and magneticordering of Ln tons? Do they mfluence each other? Here we present the mveshgatton of specific heat of Sm I ss.xGdxCe0 15CUOa.3(X = 0.1,0.3, 0.5, 0.75) in the temperature range 1.2-25K. A sharp peak m specific heat has been observed below 10K for each sample, which origmates from antiferromagnettc ordering of rare earth ions. The Neei temperature T M decreases with mcreasing x, whde the magnehc entropy increases at the same .... The, ~. . . . . ,-c,nJ~wtino !ernperature Tc becomes lower when more Sm ions are substituted by Gd, and the superconductivity d~sappears tor x>_0.75. The specific heat Cp of Sm t 85.~GdxCe015CuOa.~(x=0.1,0.3,0.5,0.75) versus temperature T is shown m fig 1. For each sample, with reducmg temperaature, Cp decreases at first. When T ts lower than 10K, the value of Cp increases raptdly and a sharp peak is observed around 4K. We

attribute this peak to the antiferromagnetic ordering of rare earth magnehc moments tn the sample , just like what happens for Sm3+ moments in Sm2CuO 4 and for Gd3+m Gd2CuO4[31. In previous work 161 the magnetizahon M as a lunchon of temperature between 1 . 2 - 30K for Sm I Ss.xGdxCe0 lsCuO4.~ was reported. A peak at around 4K with nature of antfferromagnettc 20 10 0

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T(K) Fig l. Spec,.fic heat vs. temperature

ordering was observed for each sample. The results of specific heat agree with magnetlzatlon measurements. The specific heat above 15K is well

0921-4534/94/$07 00 © 1994 - Elsevier Science B V All rights reserved S~DI 0921-4534(94)01457-4

J Luo ez al /Phvstca C 235-240 (1994) 1781-1782

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X Fig2. Superconducting transition temperature Tc and heel temperature TM vs. x. Solid hne is the A-G fit, dotted line is only for the guide o f eyes. fitted by an expression from phonon contribution for each sample. The influence of Gd doping on superconductivity is dlustrated m fig 2. The solid one is the data taken from Ref.[4]. It can be seen that the superconducting transition temperature Tc is depressed during substitution of Gd 3+ for Sm 3+. when x is less than 0.5,Tc decrease almost hnearly, and then Tc drops to zero more quickly. An AbnkosovGor'kov expnesston (solid curve in fig.2)ns found to be a good fit to all the data points. This indicates that the pair breaking mechanism due to Gd ~4 magnetic moments n,ay be responsible for the suppression ot superconductwtty. The TM changes slightly during Gd dopmg. A straight line can be drawn through the data point. It is plausible to assume that the couphng between Sm 3+ and Gd ~+ is weaker than that between only Sm 3+ ions because of energy difference between the two tons. This assumption also explams that the T M in doped system Sm I 85.~Gd~Ce0 xsCuO4.~ becomes lower tban that in its parent-compounds Sm2CuO4. 6 and GdzCuO4. 6. The change of magnettlc entropy S with x (fig.3) gives more mtbrmatlon about the magnetic ordering. S mcrease~ with Gd doping, and the dependence of S on x can be expressed as. S =(1 85-x)Rln2+xRln8 (1)

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X Fig3. Magnetic entropy as a funaion o f Gd content x, value o f = is from Ref.151. The magnetic entropy here is obtained after subtracting non-magnetic background. The second term m the right side of equation (1) is obviously the value of Gd 3+ moments with 4f 7 configuration. and the first term is that of Sm 3+. We can obtain the spin quantum number of Sm 3+ to be 1/2. This mdncates the ground state of Sm 3+ ions is doublet m the crystal-field, though Sm 3+ moments are with 4t6 configuratnon. To summarize, we have measured for the first time the specific heat of SmGdCeCuO system, and have found the effect of Gd doping on Tc to be A-G type. The two different rare earth magnetic moments couple antlferromagnetically to each other, and the system undergoes only one AF transition rather than two. This work is supported by National Center for Research and Development on Superconductivity and State Comrmsslon on Science and Technology. REFERENCES. [1] Y.Tokura et al, Nature 337(1989)345; [2] H.Takagl et al, Phys.Rev.Lett. 62(1989) 1197; [3] S Ghamaty et ai, Phystca C 160(1989) 217; [4] M S Maple et al, Phystca C 162(1989) 296; [5j T H~lubar et al, J. of Magnetism and Magnetlc Materlalb 104-107(1992)479; [6] Chin Lm et al, Solid state comu.84(1992)729