Superconducting and magnetic studies on GdBa2(Cu1−xFex)3O7+z oxide system

~Solid State Communications, Vol. 67, No. 5, pp. 521-524, 1988. ~_~ P r i n t e d t n G r e a t B r i t a i n .

SUPERCONDUCTING AND HAGNETIC STUDIES

0038-1098/88 $3.00 + .00 Pergamon Press pie

ON GdBa2(CUl_xFex)307+ z OXIDE SYSTEM

S.K. M a l i k , C.V. Tomy, D.T. A d r o j a * , S.K. Dhar and R. Nagarajan Tata I n s t i t u t e of Fundamental Research, Bombay 400 005, I n d i a and Ram Prasad and N.C. Soni Bhabha Atomic Research C e n t e r , Bombay 400 085~ I n d i a (Received on May 24, 1988 by C.N.R. Rao) The compounds CdBa~(Cu. Fe )~0~ have been prepared t o study the /-X X +Z e f f e c t of p a r t l a l replacement o~ ~u by Fe on the s t r u c t u r e , superconductivity and magnetic b e h a v i o r o f t h i s o x i d e system. A change from o r t h o r h o m b i c t o t e t r a g o n a l s t r u c t u r e t a k e s p l a c e w i t h i n c r e a s i n g x . The superconducting transition temperature, T , decreases as the Fe C . . concentration increases. A l l the compounds exhzbzt Curie-Wezss b e h a v i o r in their ~agnetic susceptibility which i s dominated by the paramagnetism o f the Cd~+ i o n . •

L

1. INTRODUCTION

[2,3,9,10]. The r a r e e a r t h i o n s have been shown t o be m a g n e t i c a l l y i s o l a t e d from the e o n d u e t i o n electrons [11,12]. In f a c t , magnetism o f the r a r e e a r t h i o n and s u p e r c o n d u c t i v i t y c o e x i s t i n many o f these compounds [ 9 , 1 0 ] . The s u b s t i t u t i o n a t the copper s i t e i s expected t o have s t r o n g e r e f f e c t s on the s u p e r c o n d u c t i n g p r o p e r t i e s since copper i s t h o u g h t t o be i n v o l v e d i n some way i n the s u p e r c o n d u c t i v i t y o f such compounds. In t h i s communieatJon we r e p o r t the r e s u l t s o f s t r u c t u ral, s u p e r c o n d u c t i n g and magnetic s t u d i e s on GdBa~(Cu. Fe )~0~ compounds. The magnetic Cd /-X X J /+Z 1on was Chosen t o l n v e s t z g a t e the i n t e r a c t i o n between the r a r e e a r t h moments and the Fe moments, i f any.

The discovery of high temperature superconductivity in YBa~Cu~O~ [I] and related comE J /-y pounds [ 2 , 3 ] has c r e a t e d a g r e a t d e a l o f a c t i v i t y t o understand the p h y s i c a l p r o p e r t i e s of such systems. The compound YBa2Cu~O7 crystallizes i n t h e o r t h o r h o m b i c s t r u c t u r e ~spa~e group Pmmm) which i s c h a r a c t e r i z e d by t w o - d i m e n s i o n a l Cu-O planes and a o n e - d i m e n s i o n a l chain l i k e Cu-O n e t w o r k . A t r a n s f o r m a t i o n from low t e m p e r a t u r e o r t h o r h o m b i e s t r u c t u r e t o high t e m p e r a t u r e t e t r a g o n a l s t r u c t u r e occurs a t about 700C i n one atmosphere of oxygen [ 4 ] . There are two s a l i e n t features associated with this structural transformation. F i r s t , the oxygen s t o i c h i o m e t r y falls below 6.5 i n the t e t r a g o n a l phase and, s e c o n d l y , a r a n d o m i z a t i o n o f the oxygen atoms occurs a t 0 ( I ) and 0(5) s i t e s which become symmetry e q u i v a l e n t i n the t e t r a g o n a l phase [ 4 ] . In g e n e r a l , under normal p r e p a r a t i o n c o n d i t i o n s , the o r t h o r h o m b i c phase occurs f o r oxygen s t o i c h i o m e t r y between 6.5 and 7.0 and i s supercond u c t i n g w h i l e t h e t e t r a g o n a l phase occurs f o r oxygen s t o i c h i o m e t r y between 6 and 6.5 and i s n o n - s u p e r c o n d u c t i n g . However, e x c e p t i o n a l samp i e s can be prepared by v a r y i n g the h e a t t r e a t ment or p r e p a r a t i o n procedures [ 5 - 7 ] . At p r e s e n t t h e r e i s an i n t e n s e debate i n t h e l i t e r a t u r e r e g a r d i n g the importance of chains versus the planes i n the occurrence o f high t e m p e r a t u r e superconductivity i n these o x i d e s . The r e c e n t r e s u l t s suggest t h a t the i n t e g r i t y o f the planes i s more i m p o r t a n t than t h a t of the chains i n o b t a i n i n g high t e m p e r a t u r e s u p e r c o n d u c t i v i t y i n these compounds [8]. It is of interest to study the effect of substituting magnetic impurities at various sites on the superconducting properties of these compounds. Earlier work has shown that replacement of Y by trivalent magnetic rare earth ions has no effect on the superconducting properties, in particular, on the Tc of these compounds

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2. EXPERIMENTAL DETAILS The samples of GdBa~(Cu. Fe )~0~ with x /-X ) /+Z = 0.0, 0.025, 0.05, 0.0~5 and 0.~ were prepared by the standard solid state reaction procedure. The oxides Gd20~ , BaC0~, Cu0 and Fe~0~ in the appropriate me~al ion " Jratios were Lt~oroughly mixed and heated at 900C overnight in a flowing oxygen atmosphere. The r e s u l t i n g m i x t u r e was pulverized, compacted and r e h e a t e d a t 900C f o r a n o t h e r 12 hours. A l l the samples were g i v e n an i n t e r m e d i a t e heat t r e a t m e n t a t 450C f o r 12 hours i n f l o w i n g oxygen b e f o r e f i n a l l y c o o l i n g them s l o w l y t o room t e m p e r a t u r e . Powder X - r a y d i f f r a c t i o n p a t t e r n s were o b t a i n e d a t room temperat u r e u s i n g Cu K~ r a d i a t i o n . R e s i s t i v i t i e s were measured using a f o u r probe do t e c h n i q u e i n the t e m p e r a t u r e range of 4.2 t o 30OK. Magnetic susceptibilities i n t h e same t e m p e r a t u r e range were o b t a i n e d using Faraday method. 3. RESULTS AND DISCUSSION Powder X - r a y d i f f r a c t i o n s t u d i e s showed t h a t a l l the compounds mentioned above are s i n g l e phase m a t e r i a l s . T y p i c a l X - r a y p a t t e r n s of some of the compounds are shown i n F i g u r e I . It i s seen t h a t a s t r u c t u r a l t r a n s f o r m a t i o n from o r t h o r h o m b i c t o t e t r a g o n a l s t r u c t u r e takes p l a c e around x=O.05 c h a r a c t e r i z e d by the decrease and

Powai, Bombay 400 076. 521

Vol. 67, No. 5

MAGNETIC STUDIES ON GdBa2(CU1_xFex)307+z OXIDE SYSTEM

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Fig. 2. Lattice parameters a, b and c in the GdBa2(CU1_xFex)307+z as a function of x.

eventual d i s a p p e a r a n c e o f t h e (200) and (020) splitting. Samples c o n t a i n i n g Fe i n e x c e s s o f 5% a r e t e t r a g o n a l a t room t e m p e r a t u r e . T h i s t y p e o f i m p u r i t y d r i v e n s t r u c t u r e change has been o b s e r ved i n YBa.(Cu. M )~0. compounds f o r M = Fe,

Co, Ca, e~c. u[~3-x1~]z.+ZThe l a t t i c e parameters obtained by a least-squares fit of the observed d values are plotted in Figure 2 and summarized in Table I along with other experimental results. The lattice parameter in the a direction increases and that in the b direction decreases before both of them become almost indistinguishable around x=0.05. There is also a small contraction in c l a t t i c e parameter as x increases. Neutron diffraction studies on YBa~(Cu. Fe )~0~ have shown [ 1 4 ] that the /. X / Z o r t ~ o r h o m ~ z c ~o { e t r a g o n a l s t r u c t u r a l transforn , a t i o n i s s i m i l a r t o t h a t which t a k e s p l a c e in YBa2Cu~O 7 as a f u n c t i o n o f t e m p e r a t u r e i n the sense { h a t Y a r a n d o m i z a t i o n o f oxygen atoms t a k e s p l a c e a t 0 ( I ) and 0 ( 5 ) s i t e s . However, i n t h e Fe s u b s t i t u t e d samples, t h e t o t a l oxygen occupancy is still close to 7 or slightly higher [14]. A similar situation i s e x p e c t e d i n t h e compounds o f CdBa~(Cu. Fe ) 0 . system a l s o .

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Th~ r ~ s t ~ v i { ~ Z o f various samples of the GdBa~(CuA Fe )~0. series is shown in Figure 3 ~ /+Z as a Z f u n c/ -t~z o n X OT ~emperature. The s u p e r c o n d u c ting transition t e m p e r a t u r e d e c r e a s e s as x i n c r e a s e s as shown i n F i g u r e 4. However, this d e c r e a s e i n T i s smooth even a c r o s s t h e o r t h o rhombic to te~ragonal structural transformation. The r e s i s t i v i t y increases with increasing Fe concentration and t h e r e i s a t e n d e n c y t o w a r d s localization for higher x. The results of magnetic susceptibility measurements on GdBa~(Cu. Fe ) ~ 0 . compounds • z . t - x . x ~ 1+z are shown in Figure 5 wnere inverse susceptibility is plotted against temperature for some of the samples. A11 the compounds e x h i b i t CurieWeiss behavior in the temperature range i n v e s t i gated. The e f f e c t i v e paramagnetic moments are ~+ close to that of free Gd ion (7.94 PB) and

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i n c r e a s e s l i g h t l y with i n c r e a s i n g Fe concentrat i o n implying the existence of a magnetic moment on Fe. The paramagnetic Curie temperatures are small and somewhat uncertain showing both positive and negative values. The results of susceptibil~y measurements are summarized in Table I. Fe Mossbauer studies have been carried out on GdBa~(CuA Fe )~0. compounds to get information abou~ ~he magne~ic orderlng of Fe moments. These studies reveal that Fe occupies at least two sites, presumably the two copper sites, and possibly an interstitial site as well. In the x=0.075 compound which was examined in detail, at 4.2K, only one quadrupole doublet •

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Vol. 67, No. 5

MAGNETIC

523

STUDIES ON GdBa2(CU1_xFex)307+ z OXIDE SYSTEM

Table I Lattice parameters, a, b and c, mid-point superconducting transition temperature, T (mid), effective magnetic moment, P ~ , and paramagne• . C tzc Curie temperature, 8p, of various compounds in C ~ 2 ( C U 1 _ x F e x ) 3 0 7 + z oxide system.

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was observed. The other doublet(s) has presumabl y s p l i t i n t o several lines due to the Fe moments ordering magnetically, r e s u l t i n g in a reduction in i n t e n s i t y of each component of the hyperfine s p l i t lines [17]. The magnetic ordering of the Fe moments could be of the spin

Fig. 5. Inverse magnetic s u s c e p t i b i l i t y versus temperature in GdBa~(Cu. Fe )~0~

glass type. Our preliminary heat capacity measurements on GdBa~(Cu. Fe )~0~ [18] with

In conclusion, we have investigated superconducting and magnetic behavior of the Fe containing GdBa~(Cu. Fe )~0~ samples. A trans. £ I/+Z formatlon from ort~or~o~bzc to tetragonal structure occurs for x ~ 0 . 0 5 . The superconducting temperature reduces smoothly with increasing iron concentration even across the structural transition. The presence of iron_moment and its ! ordering is inferred from - Fe Mossbauer measurements. The paramagnetic susceptibility is dominated by the Gd moments. Preliminary heat capacity measurements show a lowering of the Neel temperature of Gd moments in the presence of Fe moments which can be accounted for in terms of the reduction in dipolar interaction due to lattice expansion. The magnetic interaction between the Cd moments and the Fe moments appear to be weak.

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x=0.075 suggest tnat tne presence Ot iron does not enhance the magnetic ordering temperature of the Gd moments. In f a c t , the heat capacity of the Fe doped sample is lower compared to that of the undoped sample and also rises at a somewhat lower temperature i n d i c a t i n g t h a t t h e Gd magnetic ordering temperature (T N) has been lowered on Fe addition. The lowering-of T~ can be understood in terms of the expansion in the ~lattice parameter in the a direction and the r -- dependence of the T~ as would be expected for ordering based on-dipolar interactions. It also appears that the magnetic interaction between the Gd moments and the Fe moments is very weak as is the case for the interaction between the Gd moments and the conduction electron spins.

system with x=O.O, 6.02~Xo.~5Ja~Zo.l[,.

524

MAGNETIC STUDIES ON GdBa2(CU1_xFex)307+z OXIDE SYSTEM

Vol. 67, NoZ 5'

REFERENCES

[I]

[2]

[3] [4]

[5] [6]

[7] [8] [9]

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