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Interaction of magnetic ions and superconductivity in YBa2Cu3O7−δ
Physica C 162-164 (1989) 965-966 North-Holland
Interaction
of Magnetic Ions and
Superconductivity
i n YBazCuzOT-Q
R. W S r d e n w e b e r , B. U l l m a n n * , K. H e i n e m a n n * a n d H. C. F r e y h a r d t * ( I n s t . f. S c h l c h t u. l o n e n t e c h n i k , t a l s t r . 3 / 5 , 34 G S t t i n g e n , FRG)
KFA, P.O. Box
1913, 517
J l l l l c h , FRG; * I n s t . f.
Metallphys., Hospi-
T h e 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 of T B a z ( C u l - z M x ) O 7 4 w i t h M f F e , N i a n d T= Y ,G d w e r e s t u d i e d . Ni a n d Fe c a u s e a l i n e a r d e c r e a s e of T© w i t h x, b u t f o r s m a l l c o n c e n t r a t i o n s o f Fe, x < 0 . 0 2 3 , Tc i s a l m o s t c o n s t a n t . I f Fe a n d Gd i s s u b s t i t u t e d s i m u l t a n c o u s l y f o r Cu a n d Y, t h e d e c r e a s e o f Tc i s l a r g e r ( - d T c / d x f f i 7 0 0 K i n s t e a d o f 5 9 i K ) . F o r Ni p l u s Gd s u b s t i t u t i o n no a d d i t i o n a l d e c r e a s e of t h e t r a n s i t i o n t e m p e r a t u r e Is o b s e r v e d . T h e s e r e s u l t s a n d r e s i s t i v e m e a s u r e m e n t s of Bcs i n d i c a t e , t h a t t h e d e c r e a s e o f T© i s a t l e a s t p a r t l y c a u s e d b y s c a t t e r i n g of s u p e r c o n d u c t i n g quasi particles at the magnetic ions substituted at different positions in the perovskite structure o f YBazCuaOv-6. 1. I n t r o d u c t i o n The e f f e c t s c a u s e d by s u b s t i t u t i n g Cu in YBasCusOv-e (YBCO) by a 3d t r a n s i t i o n m e t a l or Y by a 4f r a r e e a r t h elemen t, r e s p e c t i v e l y , are of common i n t e r e s t for t h e s t u d y of t h e mech anism of h i g h - T o s u p e r c o n d u c t i v i t y . The s t r o n g e l e c t r o n c o r r e l a t i o n s a s s o c i a t e d with l o c a l i z e d m a g n e t i c moments are n o r m a l l y i n c o m p a t i b l e w i t h t h e c o n d e n s a t i o n of e l e c t r o n s into e l e c t r o n p a i r s of the s u p e r c o n d u c t i n g s t a t e . N e v e r t h e l e s s , t h e high T¢ of a b o u t 92K for YHCO is h a r d l y a f f e c t e d by t h e s u b s t i t u t i o n of Y w i t h a r a r e - e a r t h ion h a v i n g a l a r g e l o calized moment, w h e r e a s Tc is s u b s t a n t i a l l y d e c r e a s e d by t h e i n c o r p o r a t i o n of a 3d t r a n s i t i o n metal. These o b s e r v a t i o n s s u g g e s t t h a t t h e s i t e , which is occupied by t h e m a g n e t i c s u b s t i t u t e , and t h e p h y s i c a l c h a n g e s i n duced on t h e h o s t m a t e r i a l are of Importance for t h e u n d e r s t a n d i n g of t h e e f f e c t each s u b s t i t u t i o n h a s on the superconducting properties. In t h i s p a p e r we r e p o r t t h e 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 of YBCO, where Cu is s u b s t i t u t e d by Fe or Ni and, a d d i t i o n a l l y , Y by Gd. The T© d e p e n d e n c e on Fe or Ni c o n c e n t r a t i o n x is d i s c u s s e d In t h e c o n t e x t of t h e a t o mic p o s i t i o n of t h e s u b s t i t u t e and t h e c o h e r e n c e l e n g t h o b t a i n e d from t h e u p p e r c r i t i c a l field Bcs. 2 SAMPLE PREPARATION AND CHARACTERIZATION Ceramic YBCO s a m p l e s were p r e p a r e d by s t a n d a r d powder m e t a l l u r g i c a l methods. YzOs, GdsO3, BaCOn, CuO. FesOa and NiO powders were mixed, c a l e t n a t e d , reground, a n n e a l e d and f i n a l l y s i n t e r e d a t 960"C for 4 hrs In flowing oxygen, and s u b s e q u e n t l y cooled to room t e m p e r a t u r e a t a r a t e of I K/mtn. In order to o b t a i n comp a r a b l e r e s u l t s t h e same " s t a n d a r d recipe" was a p p l i e d to all samp les. It s h o u l d be n o t i c e d t h a t each h e a t t r e a t m e n t was c a r r i e d out In flowing oxygen down to room t e m p e r a t u r e . T h is seems to be i m p o r t a n t for t h e s i t e o c c u p a n c y in F e - s u b s t i t u t e d samples / 1 / and t h e related physical properties. Chemical composition and h o m o g e n e i t y of t h e s a m p l e s were c h e c k e d by TEM, e n e r g y d i s p e r s i v e x - r a y a n a l y s i s (EDX), chemical analysis, and xIray diffraction measurements. For x<0.15 (for Fe) and x<0.05 (for NI) t h e d i s t r i b u t i o n of Fe or NI, r e s p e c t i v e l y , was homogeneous and no second p h a s e s could be d e t e c t e d w i t h i n t h e r e s o l u t i o n of t h e e x p e r i m e n t s . M i c r o s t r u c t u r e and s t r u c t u r e of t h e s a m p l e s were d e t e r m i n e d by ( h i g h - r e s o l u t i o n ) TEM and x - r a y e x p e r i m e n t s /2/. The twin s t r u c t u r e of orthorhombic YBCO Is not changed by Ni s u b s t i t u t i o n . However, Fe s u b s t i t u t ion in a t f i r s t (x<0.015) c a u s e s a pronounced d e c r e a s e
0921-4534/89/$03.50 (North-Holland)
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
of t h e t w i n s i z e d. For l a r g e r c o n c e n t r a t i o n s of Fe, x>0.015, t h e m i c r o s t r u c t u r e c o m p l e t e l y c h a n g e s a nd t w i n domains of 5nm size a re obs e rve d. Similar t r e n t s are o b s e r v e d for GdBa2Cu~O~-o (GBCO) w i t h Fe or NI, but t h e t w i n s i z e is g e n e r a l l y found to be s l i g h l y s m a l l e r t h a n for t h e YBCO c o u n t e r p a r t . In bot h c a s e s , Ni and Fe s u b s t i t u t i o n , t h e o r t h o r h o m b i c i t y is r e t a i n e d in t h e c o n c e n t r a t i o n a l regime x
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F i g . l : C r i t i c a l t e m p e r a t u r e vs. c o n c e n t r a t i o n x.
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R. W6rdenweber et al. /Interaction o f magnetic ions and superconductivity
966
t u t e d YBCO Tc d e c r e a s e s l i n e a r l y with dT©/dx=360 K s t a r t i n g a t xffiO. F e - s u b s t l t u t e d YBCO and GBCO show two l i n e a r regimes. In YBCO T¢ s t a y s c o n s t a n t for x~O,02 and d e c r e a s e s with -dTe/dx=691K for x>O.02. Whereas in GBCO i t d e c r e a s e s w i t h - d T c / d x = 3 4 0 K for x<0.02 and-dT©/dxffi700 K for x>0.02. The a d d i t i o n a l d e c r e a s e of Tc c a u s e d by Gd s u b s t i t u t i o n In F e - d o p e d YBCO r e p r e s e n t s a c o n t r a s t to t h e i n v a r i a b l e Tc v a l u e o b t a i n e d for r a r e e a r t h doping only. A d d i t i o n a l l y , t h e r e s i s t i v e t r a n s i t i o n to t h e s u p e r c o n d u c t i n g s t a t e In an e x t e r n a l m a g n e t i c field B<13 T was measured. The u pper c r i t i c a l field Bet is a g a i n d e f i n e d by t h e mid point of the t r a n s i t i o n . Because of t h e e x t r e m e l y low c u r r e n t d e n s i t y , which was a p p l i e d during the experiment, this value represents the i n t r i n s i c Bcz v a l u e of the g r a i n s and not of the i n t e r g r a n u l a r m a t e r i a l . The s l o p e s -dBcz/dT taken at a t e m p e r a t u r e of ~0.9 Tc (to close to Tc i n h o m o g e n e i t l e s p l a y a d o m i n a n t role) are g i v e n in Fig.2. Again d i f f e r e n t b e h a v i o r s of Ni- and F e - d o p e d samples can be seen. In Ni s u b s t i t u t e d YBCO -dBcz/dT d e c r e a s e s only slowly with t h e c o n c e n t r a t i o n . Whereas F e - d o p e d s a m ples show a s t r o n g d e c r e a s e a t c o n c e n t r a t i o n s c o r r e s p o n d i n g to t h e v a l u e where dTc/dx changes. The d e c r e a s e is more pronounced for G d - s u b s t i t u t e d samples.
The drastlcal change of ~j. at x=0.016 for Fe-doped samples seems to be correlated with the change of the slope dT/dx in Flg.l. The effect of the magnetic moment on the superconducting properties can be interpreted as follows. Nl is substituted into the C u - O plans, which posses a large density [~z[ of superconducting quasi particles. The magnetic moment directly Interacts with the Cooper pairs. The linear decrease agrees with the Abrlkosov Gor'kov theory /7/. Evaluation of the exchange integral J according to /7/: dT©/dx=_0.26 jz S(S+I) N(Er)/kN, yields a reasonable value Jr0.02 eV. On the other hand, Fe is substituted on the Cu(1) position, where the local denslty [~t[ is reduced. No direct interactlon takes place. However, large concentrations of Fe (x>0.016) cause an increase of li'j., i.e. J~Uz[ "smears out" along c-dlrectlon and the propablllty for direct interaction between the magnetic moment and the Cooper pairs increases. Evaluation of the exchange Integral for x>0.02 yields again a reasonable value Jf0.025eV. In this picture the additional decrease of Tc by Gd substitution in Fe-doped samples follows automatically. The increase of l e a d s to a d d i t i o n a l p a i r b r e a k i n g a t t h e m a g n e t i c mome nt of Gd. '
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Fig,2: -dBca/dT vs. c o n c e n t r a t i o n x. A comparison of our v a l u e -dBcffi/dT=2.66T/K o b t a i n e d for t h e undoped YBCO w i t h t h e l i t e r a t u r e shows a good a g r e e m e n t /3/.On t h e o t h e r hand, e x p e r i m e n t s on s i n g l e c r y s t a l s / 4 / r e v e a l two slopes: -dB~z/dT=2.3T/K and dB~t/dTm0.46T/K, where II and A r e f e r s to t h e d i r e c t i o n of t h e a p p l i e d field p a r a l l e l or p e r p e n d i c u l a r to t h e C u O p l a n e s , r e s p e c t i v e l y . The q u a n t i t a t i v e a g r e e m e n t b e t ween our r e s u l t (for xffiO) and t h e v a l u e for B p a r a l l e l to t h e Cu-O p l a n e s I n d i c a t e s , t h a t our d a t a in Fig. 2 r e p r e s e n t t h e Bcmz-values. E x t a p o l a t i o n of B~2 back to zero t e m p e r a t u r e a c c o r ding to t h e d i r t y limit l s o t r o p i c formula (with no Pauli p a r a m a g n e t i s m l i m i t i n g effects), /5/ Bc(0)=0.69 Tc dBc/dT, y i e l d s B~(O). For s t r o n g l y a n l s o t r o p l c s u p e r c o n d u c t o r s a more a p p r o p r i a t e e x t r a p o l a t i o n might be l i n e a r or e v e n h i g h e r t h a n l i n e a r . B~= f u r t h e r g i v e s an i n d i c a t i o n for ~ . The coherence l e n g t h in t h e Cu-O plane, ~o, is o b t a i n e d from Bes(O)ffi~o/2n ~o2 /7/. The r a t i o of B~t/B~tffi ~ o / ~ , y i e l d s ~ . Our B e t - m e a s u r e m e n t s i n d i c a t e , t h a t ~ is s t r o n g l y i n c r e a s e d b y F e - d o p i n g (in GBCO as well as YBCO) b u t not by N l - d o p l n g (Fig.3).
Our e x p e r i m e n t s d e m o n s t r a t e t h a t I n c o r p o r a t e d m a g n e t i c moments re duc e t h e s u p e r c o n d u c t i v i t y of YBCO. The Tc d e p e n d e n c e on t h e c o n c e n t r a t i o n of t h e m a g n e t i c ions seems to be a complex, b u t u n d e r s t a n d a b l e f u n c t i o n of t h e atomic p o s i t i o n of t h e m a g n e t i c s u b s t i t u t e , t h e exchange integral , t h e local d e n s i t y of s u p e r c o n d u c t i n g q u a s l p a r t i c l e s , t h e c ohe re nc e l e n g t h and. may be, a d d i t i o n p r o p e r t i e s , l i k e s t r u c t u r a l or c he mi c a l changes, which a re c a u s e d by t h e chemical n a t u r e of t he s u b s t i tute. References 1. Y. Oda, H. FuJlta, H. Toyoda, T. Kaneko, T. Kohare, I. Nahada, Jap.J.Appl.Phys.26, L1660 (1987). 2. G.V.S. S a s t r y , R. WSrdenweber and H.C. F r e y h a r d t , subm. to J. Appl. Phys. 3. D.O. Welch, M. Suenaga and T. Asano, Phys. Rev.B36, 2390 (1987). 4. T.K. Worthington. W.J. G a l l a g h e r and T.R. Dlnger, Phys. Rev. Lett.59, 1160 (1987). 6. N.R. Werthamer, E. Helfand and P.C. Hohenberg. Phys. Rev,147, 296 (1966). 6. W.E, Lawrence and S. Doniach, in "Proc. of LT12", Kyoto (1970), edt. E.Kanda (Keigaku, Tokyo. 1971), p.361. 7. A,A, A brl kos ov and L.P. Gor'kov, Sov. Phys. JETP_6, 1174 (1967).
Interaction
of Magnetic Ions and
Superconductivity
i n YBazCuzOT-Q
R. W S r d e n w e b e r , B. U l l m a n n * , K. H e i n e m a n n * a n d H. C. F r e y h a r d t * ( I n s t . f. S c h l c h t u. l o n e n t e c h n i k , t a l s t r . 3 / 5 , 34 G S t t i n g e n , FRG)
KFA, P.O. Box
1913, 517
J l l l l c h , FRG; * I n s t . f.
Metallphys., Hospi-
T h e 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 of T B a z ( C u l - z M x ) O 7 4 w i t h M f F e , N i a n d T= Y ,G d w e r e s t u d i e d . Ni a n d Fe c a u s e a l i n e a r d e c r e a s e of T© w i t h x, b u t f o r s m a l l c o n c e n t r a t i o n s o f Fe, x < 0 . 0 2 3 , Tc i s a l m o s t c o n s t a n t . I f Fe a n d Gd i s s u b s t i t u t e d s i m u l t a n c o u s l y f o r Cu a n d Y, t h e d e c r e a s e o f Tc i s l a r g e r ( - d T c / d x f f i 7 0 0 K i n s t e a d o f 5 9 i K ) . F o r Ni p l u s Gd s u b s t i t u t i o n no a d d i t i o n a l d e c r e a s e of t h e t r a n s i t i o n t e m p e r a t u r e Is o b s e r v e d . T h e s e r e s u l t s a n d r e s i s t i v e m e a s u r e m e n t s of Bcs i n d i c a t e , t h a t t h e d e c r e a s e o f T© i s a t l e a s t p a r t l y c a u s e d b y s c a t t e r i n g of s u p e r c o n d u c t i n g quasi particles at the magnetic ions substituted at different positions in the perovskite structure o f YBazCuaOv-6. 1. I n t r o d u c t i o n The e f f e c t s c a u s e d by s u b s t i t u t i n g Cu in YBasCusOv-e (YBCO) by a 3d t r a n s i t i o n m e t a l or Y by a 4f r a r e e a r t h elemen t, r e s p e c t i v e l y , are of common i n t e r e s t for t h e s t u d y of t h e mech anism of h i g h - T o s u p e r c o n d u c t i v i t y . The s t r o n g e l e c t r o n c o r r e l a t i o n s a s s o c i a t e d with l o c a l i z e d m a g n e t i c moments are n o r m a l l y i n c o m p a t i b l e w i t h t h e c o n d e n s a t i o n of e l e c t r o n s into e l e c t r o n p a i r s of the s u p e r c o n d u c t i n g s t a t e . N e v e r t h e l e s s , t h e high T¢ of a b o u t 92K for YHCO is h a r d l y a f f e c t e d by t h e s u b s t i t u t i o n of Y w i t h a r a r e - e a r t h ion h a v i n g a l a r g e l o calized moment, w h e r e a s Tc is s u b s t a n t i a l l y d e c r e a s e d by t h e i n c o r p o r a t i o n of a 3d t r a n s i t i o n metal. These o b s e r v a t i o n s s u g g e s t t h a t t h e s i t e , which is occupied by t h e m a g n e t i c s u b s t i t u t e , and t h e p h y s i c a l c h a n g e s i n duced on t h e h o s t m a t e r i a l are of Importance for t h e u n d e r s t a n d i n g of t h e e f f e c t each s u b s t i t u t i o n h a s on the superconducting properties. In t h i s p a p e r we r e p o r t t h e 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 of YBCO, where Cu is s u b s t i t u t e d by Fe or Ni and, a d d i t i o n a l l y , Y by Gd. The T© d e p e n d e n c e on Fe or Ni c o n c e n t r a t i o n x is d i s c u s s e d In t h e c o n t e x t of t h e a t o mic p o s i t i o n of t h e s u b s t i t u t e and t h e c o h e r e n c e l e n g t h o b t a i n e d from t h e u p p e r c r i t i c a l field Bcs. 2 SAMPLE PREPARATION AND CHARACTERIZATION Ceramic YBCO s a m p l e s were p r e p a r e d by s t a n d a r d powder m e t a l l u r g i c a l methods. YzOs, GdsO3, BaCOn, CuO. FesOa and NiO powders were mixed, c a l e t n a t e d , reground, a n n e a l e d and f i n a l l y s i n t e r e d a t 960"C for 4 hrs In flowing oxygen, and s u b s e q u e n t l y cooled to room t e m p e r a t u r e a t a r a t e of I K/mtn. In order to o b t a i n comp a r a b l e r e s u l t s t h e same " s t a n d a r d recipe" was a p p l i e d to all samp les. It s h o u l d be n o t i c e d t h a t each h e a t t r e a t m e n t was c a r r i e d out In flowing oxygen down to room t e m p e r a t u r e . T h is seems to be i m p o r t a n t for t h e s i t e o c c u p a n c y in F e - s u b s t i t u t e d samples / 1 / and t h e related physical properties. Chemical composition and h o m o g e n e i t y of t h e s a m p l e s were c h e c k e d by TEM, e n e r g y d i s p e r s i v e x - r a y a n a l y s i s (EDX), chemical analysis, and xIray diffraction measurements. For x<0.15 (for Fe) and x<0.05 (for NI) t h e d i s t r i b u t i o n of Fe or NI, r e s p e c t i v e l y , was homogeneous and no second p h a s e s could be d e t e c t e d w i t h i n t h e r e s o l u t i o n of t h e e x p e r i m e n t s . M i c r o s t r u c t u r e and s t r u c t u r e of t h e s a m p l e s were d e t e r m i n e d by ( h i g h - r e s o l u t i o n ) TEM and x - r a y e x p e r i m e n t s /2/. The twin s t r u c t u r e of orthorhombic YBCO Is not changed by Ni s u b s t i t u t i o n . However, Fe s u b s t i t u t ion in a t f i r s t (x<0.015) c a u s e s a pronounced d e c r e a s e
0921-4534/89/$03.50 (North-Holland)
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
of t h e t w i n s i z e d. For l a r g e r c o n c e n t r a t i o n s of Fe, x>0.015, t h e m i c r o s t r u c t u r e c o m p l e t e l y c h a n g e s a nd t w i n domains of 5nm size a re obs e rve d. Similar t r e n t s are o b s e r v e d for GdBa2Cu~O~-o (GBCO) w i t h Fe or NI, but t h e t w i n s i z e is g e n e r a l l y found to be s l i g h l y s m a l l e r t h a n for t h e YBCO c o u n t e r p a r t . In bot h c a s e s , Ni and Fe s u b s t i t u t i o n , t h e o r t h o r h o m b i c i t y is r e t a i n e d in t h e c o n c e n t r a t i o n a l regime x
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t u t e d YBCO Tc d e c r e a s e s l i n e a r l y with dT©/dx=360 K s t a r t i n g a t xffiO. F e - s u b s t l t u t e d YBCO and GBCO show two l i n e a r regimes. In YBCO T¢ s t a y s c o n s t a n t for x~O,02 and d e c r e a s e s with -dTe/dx=691K for x>O.02. Whereas in GBCO i t d e c r e a s e s w i t h - d T c / d x = 3 4 0 K for x<0.02 and-dT©/dxffi700 K for x>0.02. The a d d i t i o n a l d e c r e a s e of Tc c a u s e d by Gd s u b s t i t u t i o n In F e - d o p e d YBCO r e p r e s e n t s a c o n t r a s t to t h e i n v a r i a b l e Tc v a l u e o b t a i n e d for r a r e e a r t h doping only. A d d i t i o n a l l y , t h e r e s i s t i v e t r a n s i t i o n to t h e s u p e r c o n d u c t i n g s t a t e In an e x t e r n a l m a g n e t i c field B<13 T was measured. The u pper c r i t i c a l field Bet is a g a i n d e f i n e d by t h e mid point of the t r a n s i t i o n . Because of t h e e x t r e m e l y low c u r r e n t d e n s i t y , which was a p p l i e d during the experiment, this value represents the i n t r i n s i c Bcz v a l u e of the g r a i n s and not of the i n t e r g r a n u l a r m a t e r i a l . The s l o p e s -dBcz/dT taken at a t e m p e r a t u r e of ~0.9 Tc (to close to Tc i n h o m o g e n e i t l e s p l a y a d o m i n a n t role) are g i v e n in Fig.2. Again d i f f e r e n t b e h a v i o r s of Ni- and F e - d o p e d samples can be seen. In Ni s u b s t i t u t e d YBCO -dBcz/dT d e c r e a s e s only slowly with t h e c o n c e n t r a t i o n . Whereas F e - d o p e d s a m ples show a s t r o n g d e c r e a s e a t c o n c e n t r a t i o n s c o r r e s p o n d i n g to t h e v a l u e where dTc/dx changes. The d e c r e a s e is more pronounced for G d - s u b s t i t u t e d samples.
The drastlcal change of ~j. at x=0.016 for Fe-doped samples seems to be correlated with the change of the slope dT/dx in Flg.l. The effect of the magnetic moment on the superconducting properties can be interpreted as follows. Nl is substituted into the C u - O plans, which posses a large density [~z[ of superconducting quasi particles. The magnetic moment directly Interacts with the Cooper pairs. The linear decrease agrees with the Abrlkosov Gor'kov theory /7/. Evaluation of the exchange integral J according to /7/: dT©/dx=_0.26 jz S(S+I) N(Er)/kN, yields a reasonable value Jr0.02 eV. On the other hand, Fe is substituted on the Cu(1) position, where the local denslty [~t[ is reduced. No direct interactlon takes place. However, large concentrations of Fe (x>0.016) cause an increase of li'j., i.e. J~Uz[ "smears out" along c-dlrectlon and the propablllty for direct interaction between the magnetic moment and the Cooper pairs increases. Evaluation of the exchange Integral for x>0.02 yields again a reasonable value Jf0.025eV. In this picture the additional decrease of Tc by Gd substitution in Fe-doped samples follows automatically. The increase of l e a d s to a d d i t i o n a l p a i r b r e a k i n g a t t h e m a g n e t i c mome nt of Gd. '
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Fig,2: -dBca/dT vs. c o n c e n t r a t i o n x. A comparison of our v a l u e -dBcffi/dT=2.66T/K o b t a i n e d for t h e undoped YBCO w i t h t h e l i t e r a t u r e shows a good a g r e e m e n t /3/.On t h e o t h e r hand, e x p e r i m e n t s on s i n g l e c r y s t a l s / 4 / r e v e a l two slopes: -dB~z/dT=2.3T/K and dB~t/dTm0.46T/K, where II and A r e f e r s to t h e d i r e c t i o n of t h e a p p l i e d field p a r a l l e l or p e r p e n d i c u l a r to t h e C u O p l a n e s , r e s p e c t i v e l y . The q u a n t i t a t i v e a g r e e m e n t b e t ween our r e s u l t (for xffiO) and t h e v a l u e for B p a r a l l e l to t h e Cu-O p l a n e s I n d i c a t e s , t h a t our d a t a in Fig. 2 r e p r e s e n t t h e Bcmz-values. E x t a p o l a t i o n of B~2 back to zero t e m p e r a t u r e a c c o r ding to t h e d i r t y limit l s o t r o p i c formula (with no Pauli p a r a m a g n e t i s m l i m i t i n g effects), /5/ Bc(0)=0.69 Tc dBc/dT, y i e l d s B~(O). For s t r o n g l y a n l s o t r o p l c s u p e r c o n d u c t o r s a more a p p r o p r i a t e e x t r a p o l a t i o n might be l i n e a r or e v e n h i g h e r t h a n l i n e a r . B~= f u r t h e r g i v e s an i n d i c a t i o n for ~ . The coherence l e n g t h in t h e Cu-O plane, ~o, is o b t a i n e d from Bes(O)ffi~o/2n ~o2 /7/. The r a t i o of B~t/B~tffi ~ o / ~ , y i e l d s ~ . Our B e t - m e a s u r e m e n t s i n d i c a t e , t h a t ~ is s t r o n g l y i n c r e a s e d b y F e - d o p i n g (in GBCO as well as YBCO) b u t not by N l - d o p l n g (Fig.3).
Our e x p e r i m e n t s d e m o n s t r a t e t h a t I n c o r p o r a t e d m a g n e t i c moments re duc e t h e s u p e r c o n d u c t i v i t y of YBCO. The Tc d e p e n d e n c e on t h e c o n c e n t r a t i o n of t h e m a g n e t i c ions seems to be a complex, b u t u n d e r s t a n d a b l e f u n c t i o n of t h e atomic p o s i t i o n of t h e m a g n e t i c s u b s t i t u t e , t h e exchange integral , t h e local d e n s i t y of s u p e r c o n d u c t i n g q u a s l p a r t i c l e s , t h e c ohe re nc e l e n g t h and. may be, a d d i t i o n p r o p e r t i e s , l i k e s t r u c t u r a l or c he mi c a l changes, which a re c a u s e d by t h e chemical n a t u r e of t he s u b s t i tute. References 1. Y. Oda, H. FuJlta, H. Toyoda, T. Kaneko, T. Kohare, I. Nahada, Jap.J.Appl.Phys.26, L1660 (1987). 2. G.V.S. S a s t r y , R. WSrdenweber and H.C. F r e y h a r d t , subm. to J. Appl. Phys. 3. D.O. Welch, M. Suenaga and T. Asano, Phys. Rev.B36, 2390 (1987). 4. T.K. Worthington. W.J. G a l l a g h e r and T.R. Dlnger, Phys. Rev. Lett.59, 1160 (1987). 6. N.R. Werthamer, E. Helfand and P.C. Hohenberg. Phys. Rev,147, 296 (1966). 6. W.E, Lawrence and S. Doniach, in "Proc. of LT12", Kyoto (1970), edt. E.Kanda (Keigaku, Tokyo. 1971), p.361. 7. A,A, A brl kos ov and L.P. Gor'kov, Sov. Phys. JETP_6, 1174 (1967).