Magnetic studies on the superconducting oxide ErBa2Cu3O7−y

0038-1098/88 $3 00 + 00 Pergamon Journals Ltd.

% S o l i d State Communlcatlons, Vol.65,No.7, pp.581-584, 1988. %~.[~ Prlnted in Great Brltaln.

MAGNETIC STUDIES ON THE SUPERCONDUCTING OXIDE ErBa2Cu307_y S.K. Mallk , C.V

Tomy , Ram P r a s a d @, N.C.

S o n l @, Ashok Mohan @ and C.K

Gupta @

*Tat~ Institute of Fundamental Research, Bombay 400 005, India Bhabha Atomic Research Center, Bombay 400 085, India (Received 20 September 1987 by E.F. Bertaut) Magnetic studies have been carried out on the oxide superconductor ErBa^Cu~O. (T =88 K) at various temperatures and fields From the L d /-C hysteresis ~oop recorded at 4.2K, the values of the lower critical field and magnetization critical current densities are determined In the superconducting state, the magnetlzatlon is found to become positive at some temperature depending on the applied field. Thls is thought to be due to some field penetration under the influence of which the paramagnetlsm of Er ions overtakes the diamagnetic response. From the susceptlbliity data above the superconducting state, the effective rare earth ion moment is found to be very close to its free Ion value In spite of the large moment, the rare earth ion has no discernible effect on T . c

i. Introduction

2. Experimental

The observation of superconductivity in the liquid nitrogen temperature range in multlphase Y-Ba-CU-O system [i] and the subsequent identlflcation of the superconducting phase as orthorhomblc YBa2Cu~O 7 [2-8] has created tremendous interest and excitement in this field. Thls structure consists of corrugated 0(2)-Cu(2)-O(3) sheets perpendicular to the c axls, and O(1)Cu(1)-O[l) linear chains along the b direction The oxygen stolchlometry in thls compound is close to O_ From the consideration of charge 7 neutrality, thls would require the presence of both Cu 2% and Cu 3+ Ions wlth an average copper valence of about 2 33 An orthorhomblc to tetragonal structural transformation is observed in thls compound at about 750C [9]. Shortly afterwards it was found that superconductivity in thls class of materials was not confined to only Y-contalnlng compound but could also be obtained in several REBa^Cu~O~ comZ J /pounds wlth RE as one of the trlvalent rar~ earth ion There is very little or no change in the superconducting transition temperature on rare earth substitution [10-14] in striking contrast to the situation in intermetallic compounds [15] The interaction between the magnetic rare earth ions and the conduction electrons appears to be very weak. Oxides containing Gd, Dy, Ho and Er are found to order magnetically at temperatures ( T ) of 2.24K 0 95K, 0.17K and 0 59K, respectivelym[10,14] Magnetic order and superconductivity coexist below T In these compounds We present here the results of resistivity and magnetic measurements on the oxide superconductor ErBa2Cu.O_ from which we obtain the value of lower ~ r ~ c a l field H . , the magnetic critical current density and it~itemperature dependence, effective p a r a m a g n e ~ c moment etc. Below T , the c paramagnetlsm of Er- ions overtakes the dlamag-

The oxide ErBa Cu 0 was prepared by the 2 3 7followlng procedure App~oprlate amounts of Er20_, BaCO_ and CuO powders were thoroughly mixed and heated J in a flowing oxygen atmosphere at 900C for 16 hours. The resulting mixture was reground, pelletized and heated to 900C for another 16 hours. The sample was then cooled slowly to 400C at the rate of 2C/mlnute and kept there for 4 hours after which the temperature was reduced to room temperature at the rate of 2C/mlnute all in flowing oxygen atmosphere. Thls procedure resulted in a single phase orthorhomblc material as confirmed by powder X-ray diffraction studies. The lattice parameters obtained from the observed d values are a=3 833A, b=3 883A and c=ii.651A, corresponding to a theoretical density of 7.12gm/cc Four probe dc reslstlvlty measurements were performed on a bar shaped sample in the temperature range of 4 2K to 300K The contacts were made wlth the help of conducting silver paint. Temperatures were measured either wlth a carbon glass thermometer or wlth a Pt resistance thermometer Magnetic measurements were made on the same sample (dimensions 0.25cm x 0.30cm x 0 5cm) having a weight of 0 1925gm, in the temperature range of 4.2K to 300K using a set up based on Faraday method wlth the longer axls of the sample perpendicular to the direction of the applled field. The above dimensions and weight correspond to an experimental density of 5.13gm/cc compared to the theoretical value of 7.12gm/cc for the same Thls points towards the porosity of the material. 3

Results and Discussion

Figure i. shows the results of resistivity measurements on ErBa Cu.O. . This material is 2 /metallic. Its reslstlvl~y small and decreases wlth decreasing temperatures eventually showing a large drop starting at a temperature of about 92 K. The zero resistance Is achieved at about 88 K.

netlc response at some temperature depending on the applied fields Above the superconducting transition temperature Tc, the susceptibility is close to that of Er ions 581

582

Vol. 65, No. 7

M ~ C N E T I C STUDIES ON THE S U P E R C O N D U C T I N G O X I D E E r B a 2 C u 3 0 7 _ y

1.0

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--

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0.2

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~ . - 1 ~ - - .

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50

0

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Temperoture

Fig

,

150

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,

,

,

250

,

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300

(K)

R e s i s t i v i t y versus t e m p e r a t u r e for

ErBa2Cu307_y Results of low field s u s c e p t i b i l i t y m e a s u r e ments are shown in Fig 2 For o b t a ~ n l n g curve labeled "ZFC", the sample was cooled b e l o w the superconducting transltlon temperature in zero applied field and then the m a g n e t i z a t i o n was recorded wlth increasing t e m p e r a t u r e in a field of 65Oe To obtain curve labeled "FC", the m a g n e tlzatlon was o b t a i n e d w h l l e cooling the sample below the transition t e m p e r a t u r e in the same field as in curve m a r k e d "ZFC" Using the theoretlcal d e n s i t y and n e g l e c t i n g demagnetization correction, the shielding effect is 87% of the full d l a m a g n e t l s m expected while the M e l s s n e r

_'

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fraction is 33% C o n s i d e r i n g the p o r o s i t y of the sample, and the fact that these are highly anlsotropic type II materials, the above fractions may be taken to imply oulk s u p e r c o n d u c t i v i t y A plot of the part of the h y s t e r e s i s loop recorded at 4 2K in flelds up to 8 kOe is shown in Fig 3 To obtain thls, the sample was cooled to 4.2K in zero applied field As the field is increased, a d l a m a g n e t l c szgnal appears because of the shielding effect The d i a m a g n e t i s m is linearly p r o p o r t i o n a l to the a p p l i e d field up to a field of about 600 Oe after w h i c h it d e v i a t e s from the linear behavior due to p e n e t r a t z o n of the field in the interior of the sample Thus the lower c r i t i c a l field (H ~) for p o l y c r y s t a l l i n e ErBa Cu O is about ~ 0 G at 4 2K From the magn~tl~a~n data (neglecting any d e m a g n e t l z a tlon corrections), using the crltlcal state model [16,17], the m a g n e t i c c r i ~ i c a l 2 c u r r e n t denslty is estimated to be about i0 A/cm at 4 2K in zero applied field. The t e m p e r a t u r e varlatlon of the remnant m a g n e t l z a t : o n w h i c h reflects the varlat~on of m a g n e t i c crltlcal current density as a functlon of t e m p e r a t u r e is shown in figure 4 The value of the crltlCal current d e n s l t ~ ma~ be compared wlth the value of about i0 u ~ / c m 2 in p o l y c r y s t a l l i n e YBa~Cu~O_ and about i0 A / c m in single crystal YBa^~u_~_ I-y Thus it appears that the m a g n e t i c crlt~cal ~u~rent d e n s l t y is nearly the same in p o l y c r y s t a l l i n e Y or Er compounds of thls family Magnetlc s u s c e p t i b i l i t y of the s u p e r c o n d u c tlng sample o b t a i n e d in applied fields of 2 5, 4 and 6 5 kOe is shown in Fig 5 It is noted that in low a p p l i e d fields, the s u s c e p t i b i l i t y in the superconducting state is n e g a t i v e and b e c o m e s positive at or close to the transition temperature as shown in Fig 2. However, as the applied field is increased above H _, p e n e t r a t i o n of the field in the interior of ~ e sample takes place In the p r e s e n c e of thls field, the p a r a m a g n e t l c susceptibility of the Er ions (though reduced from the normal state value) more than compensates the d i a m a g n e t i c response at some temperature and results in a p o s i t i v e value of the

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Fig

M a g n e t i c s u s c e p t l b i i i t y of E r B a 2 C u 3 0 7 _ y in a field of 50G. To obtain curve labelled "ZFC", the sample was cooled to 4 2K in zero fleld and t e m p e r a t u r e d e p e n d e n c e of the m a g n e t l z a t l o n was o b t a i n e d on w a r m i n g the sample in 65G a p p l i e d field w h l l e curve labeled "FC" was o b t a i n e d w h i l e c o o l i n g the sample in the above field

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Field (kOe)

Part of the hysteresis loop at 4 2K for ErBa Cu O The saple was initially 2 3 7cooled in z~ro a p p l i e d field

Vol.

7

65, No

MAGNETIC

STUDIES

ON

THE S U P E R C O N D U C T I N G

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=

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m o d e l of the p e n e t r a t i o n zs n e e d e d to q u a n t z t a t l vely e x p l a l n the m a g n e t i c s u s c e p t i b i l i t y data at various fields. The s u s c e p t i b i l i t y above the s u p e r c o n d u c t i n g t r a n s i t i o n t e m p e r a t u r e has been fitted to formula X=~+C/(T-8 ) (Fig. 6) w h e r e X- represents the t e m p e r a t u r e P z n d e p e n d e n t term. T ~ e o b s e r v e d value of e f f e c t l v e m o m e n t of 9 5 2 ~ per Er ion is ve~[ close to the t h e o r e t i c a l val~e 9 59 ~ = for Er-tons The n e g a t i v e p a r a m a g n e t l c Curie ~ e m p e r a t u r e 8 (-24K) reflects the p r e s e n c e of a n t z f e r r o m a g n~tlc interactions (The value of Xo is v e r y small and unreliable) Thus in spite o~ the large

1.4 ,

OXIDE E r B a 2 C u 3 0 7 _ y

200

250

300

(K)

M a g n e t i c s u s c e p t i b i l i t y of E r B a ^ C u ~ O _ in a p p l i e d flelds of 2 5, 4 andZ6 ~k6e y

m a g n e t m z a t l o n (Fig 5) The exact field p e n e t r a tion d e p e n d s on the m l c r o s t r u c t u r e of the grains and the t e m p e r a t u r e On lowering the t e m p e r a t u r e of the s a m p l e from room t e m p e r a t u r e end, the dlamagnetlc response suddenly appears at the superconductlng transition temperature, which reduces the p a r a m a g n e t l s m and t h e r e f o r e reflects as a b r o a d peak in the s u s c e p t l b l l l t y at or near T A k n o w l e d g e of the grain sizes and a s u i t a b l e c

magnetic moment, the rare earth ions do not affect T zn a d l s c e r n z b l e manner relative to that of The non m a g n e t i c Y c o n t a z n l n g compound This is in contrast to the Sltuatlon in rare earth intermetallic compounds and implies an extremely weak c o u p l i n g b e t w e e n the rare earth sp~ns and the c o n d u c t i o n e l e c t r o n spins in these oxldes Thls may be due to several factors such as low rare earth concentration, low electron density, large s u p e r c o n d u c t i n g e n e r g y gap due to high T , t w o - d l m e n s l o n a l nature of the s t r u c t u r e of these oxide systems etc In conclusion, reslstzvlty and detailed magnetic studies have been carried out on the oxlde s u p e r c o n d u c t o r ErBa Cu O It is shown 2 3 7that strong p a r a m a g n e t l s m of Ehe ions p e r s i s t s even in the s u p e r c o n d u c t i n g sta~e Values of lower crlt~cal field and m a g n e t i z a t i o n current denslty are d e t e r m i n e d Acknowledgment - The authors thank Mr D T Adro3a for help in e x p e r i m e n t a l m e a s u r e m e n t s and Prof R V 1 3 a y a r a g h a v a n for hls Interest in the work

References

[i]

[2]

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[4]

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584 [5]

MAGNETIC STUDIES ON THE SUPERCONDUCTING OXIDE ErBa2Cu3OT_y

M.A. Beno, L. Soderholm, D.W Capone II, D.G H1nks, J D. Jorgensen, J.D. Grace, I.K. Schuller, C U Segre, and K Zhang, Appl. Phys. Lett. 51, 57 (1987) [6] J.J. Capponl, C. Challlout, A.W. Hewat, P Le3ay, M Marezlo, N Nguyen, B Raveau, J.L soubeyroux, J.L. Tholence, and R. Tournler, Europhys Lett. 3, 1301 (1987) [7] W I.F Davld, W.T.A Harrlson, J.M.F. Gunn, O. Moze, A.K Soper, P. Day, J.D. Jorgensen, D G. H1nks, M.A Beno, L Soderholm, D W. Capone II, I.K Schuller, C.U Segre, K. Zhang, and J.D. Grace, Nature 327, 310 (1987). [8] F. Beech, S Mlraglla, A. Santoro, and R S. Roth, Phys Rev. B 35, 8778 (1987). [9] Ivan K. Schuller, D.G. Hlnks, M A Beno, D W Capone II, L Soderholm, J.-P. Locquet, Y. Bruynseraede, C.U. Segre and K Zhang, Solld State Commun. 63, 385 (1987); J D Jorgensen, M.A Beno, D.G Hlnks, L Soderholm, K J volln, R.L H1tterman, J D Grace, Ivan K Schuller, C U Segre, K. Zhang, and M S Kleeflsch, Phys. Rev. B (in press) [10] Z Flsk, J.D. Thompson, S.-W. Cheong, R M A1kln, J L. Smlth, and E Zlrnglebl, J. Magnetlsm and Magnetlc Materlals, 67, L139 (1987)

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[ii] D.W. Murphy, S. Sunshlne, R.B. van Dover, R.J. Cava, B Batlogg, S M Zahurak, and L.F Schneemeyer, Phys Rev Lett. 58, 1888 (1987). [12] P.H. Hot, R L. Meng, Y Q. Wang, L. GAo, Z.J. Huang, J. Bechtold, K Forster, and C W Chu, Phys. Rev. Lett. 58, 1891 (1987). [13] K.N Yang, Y. Dallchaouch, J M. Ferrelra, B.W. Lee, J.J Neumeler, M.S. Torlkachv1111, H. Zhou, and M B Maple, to appear in Solld State Communlcatlons. [14] B.D Dunlap, M Slaskl, D.G. Hlnks, L. Soderholm, M Beno, K. Zhang, C Segre, G.W Crabtree, W.K Kwok, S.K. Mallk, Ivan K Schuller, J.D Jorgensen, and Z Sungalla, Journal of Magnetlsm and Magnetlc Materlals 68, L139 (1987) [15] See, for instance, "Superconductlvlty In Ternary Compounds", vols 32 and 34 of Toplcs in Current Physlcs, edlted by O. Fischer and M B. Maple, Sprlnger, New York 1982. [16] C P. Bean, Phys Rev. Lett. 8, 250 (1962); Rev Mod Phys. 36, 31 (1964). [17] W A Fletz, M R. Beasley, J. Sllcox, and W W Webb, Phys. Rev 136, A335 (1964) [18] G W. Crabtree, J Z LIU, A. Umezawa, W.K Kwok, C H Sowers, S.K. Mallk, B.W. Veal, D J. Lam, M.B. Brodsky, and J.W Downey, Phys. Rev B 36, 4021 (1987)