Re-entrant superconductivity and magnetic ordering in the pseudoternary system (Er1−xHox)Rh4B4

S o h d State Communications, Vol. 26, pp. 141-144. ~ P e r g a m o n Press Ltd. 1978. Printed in Great Britain

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SUPERCONDUCTIVITY AND MAGNETIC ORDERING IN THE PSEUDOTERNARY SYSTEM (Erl_xHOx)R_h4B 4

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C. J o h n s t o n , W A Fert~g ~ and M B Maple ~ Institute for Pure and Apphed Physical Sciences U m v e r s ~ t y of C a h f o r r a a , S a n D~ego, L a J o l l a , C a h f o r r n a 9 2 0 9 3 and B T. Matth~as Institute for Pure and Apphed Physical Sciences ~ U m v e r s ~ t y of C a h f o r m a , San Diego, La Jolla, Cahforma 92093 and Bell Laborator:e s M u r r a y H~ll, N . J . 0 7 9 7 4 ( R e c e i v e d I I J u l y 1977 by H. S u h l ) The boundaries between the normal paramagnet~c, superconducting, and normal magnetically-ordered p h a s e s ~n t h e t e t r a g o n a l p s e u d o ternary rare earth system (Erl_xHOx)R~h4B 4 have been established b y m e a n s of a c s u s c e p t ~ b ~ h t y m e a s u r e m e n t s to temperatures as low as0 0714

.h_eereln t h e c o m p o s i t i o n d e p e n d e n c i e s of t h e superconducting critical temperatures Tcl and TcZ and the magnetic ordering temperature T M of a s e r i e s of p s e u d o t e r n a r y ( E r l _ x H O x ) R_h4B 4 c o m p o u n d s . Samples of (Er 1 _xHOx)Rh4B 4 were syntheszzed by arc-melting the high purity elem e n t s u n d e r A r , f o l l o w e d b y a n n e a h n g ~n sealed Ta tubes at IZ00 C for one week and t h e n a t 900 C f o r t h r e e w e e k s . Superconducting and magnetic critical temperatures were determined from low frequency (15-20 Hz) ac magnetic susceptibility (Xac) measurements down to temperatures a s l o w a s 0 07 E w h i c h were achieved in a He3-He 4 d~lutaon refrlge rator S h o w n i n F i g 1 a r e p l o t s of X a c v s t e m p e r a t u r e f o r a r e p r e ~ o ~ t a t i v e s e l e c t i o n of six (Erl_xHOx)R_h4B 4 sam[ties. The samples w i t h x - 0, 0. Z70, 0 81B a n d 0 8 9 0 e a c h e x hlbzt a normal to superconducting state transition at an upper critical temperature Tel followed by a loss of superconductlvaty at a lower critical temperature TcZ In c o n t r a s t , t h e c o m p o s i t i o n s x = 0 915 a n d x = 1 . 0 0 o r d e r magnetically at temperatures T M o f 6 - 7 H, and no ewdence for superconductivity at lower temperatures was observed The Xac plot for x = 0 890 is particularly interesting: Tcl and TeE nearly coincide so that the sample returns to the normal state before it becomes completely superconducting, this composition is therefore very close to the critical concen-

Recently, the compound ErRh4B 4, which becomes superconducting at a critical temperat u r e T e l = 8 . 7 I4, w a s r e p o r t e d t o e x h i b i t a r e t u r n to t h e n o r m a l s t a t e a t a l o w e r c r i t i c a l temperature T c 2 = 0 9 14. 1 T h e t r a n s i t i o n a t Tc2 cmnclded with and was apparently induced b y t h e o n s e t of l o n g - r a n g e o r d e r i n g o f t h e m a g n e t i c m o m e n t s of t h e E r 3+ ~ons w h i c h c o m p l e t e l y o c c u p y o n e s e t of c r y s t a l l o g r a p h i c sites in tbas compound. The long-range magnetic order was inferred from a pronounced lambdatype specific heat anomaly at TcZ and a decrease of the Er contr~butlon to the low frequency ac magnetic susceptlblhty below TcZ T h e c o m p o u n d E r R h 4 B 4 i s a m e m b e r of the class of primitive tetragonal ternary comp o u n d s w i t h t h e g e n e r a l f o r m u l a MR_h4B 4 Z, 3 The remaining MRh4B 4 compounds whxch have been formed are superconducting for M = Th, Y, Nd, Sin, T m a n d L u , a n d m a g n e t i c f o r M = Gd, T b , I)y a n d H o Z A s p r e w o u s l y pointed out, 2 a comparison between the two MRh4B 4 compounds formed by the neighboring r a r e e a r t h ( R E ) ~ons Ho a n d E r ~s p a r t i c u l a r l y ~ n t e r e s t x n g , s ~ n c e t h e ~ 7 14 C u r i e t e m p e r a t u r e of t h e H o c o m p o u n d xs ~n j u x t a p o s z t ~ o n wxth t h e 8 7 14 s u p e r c o n d u c t i n g t r a n s l h o n t e m p e r a t u r e T e l of t h e n e ~ g h b o r z n g E r c o m p o u n d In order to explore the reasons for the d~vergence in the b e h a v i o r of E r R h 4 B 4 a n d H o R h 4 B 4 , a n d to further identify the factors responsible for and influencing re-entrant superconductivity m the MRh4B 4 materials, we m e a s u r e d a n d r e p o r t

* R e s e a r c h m L a J o l l a , C a i s s p o n s o r e d b y t h e A i r F o r c e O f f i c e o f Sclent~f~c R e s e a r c h , Air F o r c e S y s t e m s C o m m a n d , U S A F , u n d e r A F O S R c o n t r a c t No. F 4 9 6 2 0 - 7 7 - C - 0 0 0 9 . ~ R e s e a r c h s p o n s o r e d b y t h e U. S E n e r g y R e s e a r c h a n d D e v e l o p m e n t A d m ~ m s t r a t ~ o n u n d e r contract ERDA E(04-3)-34PAZ27 141

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tratlon Xcr at which Tcl, Tc2 and T M become coincident T h e s h a p e of t h e X a c v s . temperature curves indicates that all samples with x < Xcr order magnetically somewhere b e t w e e n T c l a n d T c z , a l t h o u g h T M -~ T c 2 a t t h e c o m p o s i t i o n x = 01 a n d a t x = X c r , t h e p r e s e n t measurements do n o t d i s t i n g u i s h w h e t h e r t h i s i s a l s o the c a s e f o r the r e m a i n d e r of the s a m p l e s w~th 0 < x < X e r It I s p o s s i b l e t h a t t h e r e i s a region at intermediate compositions in which superconductxvlty and magnetic order coex,st over an appreciable temperature range,4 future heat capacity and neutron diffraction measurem e n t s w~ll h e l p to c l a r i f y t h i s p o i n t . The superconducting critical temperatures Tel and TcZ for x Xcr a r e p l o t t e d v s c o m p o s l t u o n m F~g. 2 f o r a l l samples investigated The d a t a y i e l d the phase boundaries between the normal paramagnetic, superconducting, and normal magn e t i c a l l y - o r d e r e d p h a s e s i n t h e ( E r 1 _xHOx)~h4B 4 system. T h e r e a r e s e v e r a l f e a t u r e s to b e ~ n o t e d i n F i g 2: (1) W i t h I n c r e a s i n g x, T c l is nearly linear in x up to x ~ 0.8, but bends d o w n w a r d a s x a p p r o a c h e s X c r . (Z) t h e T c Z vs. x phase boundary for x
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f r o m x > X c r , (3) t h e r e i s a s h a l l o w m l m m u r n in the Tc2 vs x phase boundary whzch occurs n e a r x = 0. 25. a n d (4) t e m p e r a t u r e h y s t e r e s i s o c c u r s i n T c 2 ( i n d i c a t e d by v e r t i c a l b a r s i n F i g 2), t h e m a g n i t u d e of \ v ] u c h i s l a r g e s t i n the w c l m t y of the m l m m u m m the Tc2 vs x phase boundary, since great care was exerc i s e d to i n s u r e t h a t at e a c h t e m p e r a t u r e t h e sample and thermometer were m thermal equih b r l u r n , t h i s h y s t e r e s i s a p p e a r s to b e a n z n t n n s z c p r o p e r t y of t h e s a m p l e s r a t h e r t h a n a n experimental artifact E x t r a p o l a t i o n o f the T e l v s x d a t a f o r x ~ 0 8 indicates that were it not for the occurr e n c e o f m a g n e t i c o r d e r n e a r x = 1 a t 6 - 7 K. H o l ~ 4 B 4 w o u l d b e c o m e s u p e r c o n d u c t i n g at a s o m e w h a t l o w e r t e m p e r a t u r e o f 5-6 I~ For 0 8 ~ x < X c r , t h e d o w n w a r d c u r v a t u r e o f the Tcl vs x phase boundary apparently reflects a stronger reductlon of T c l due to the proximlty in t e m p e r a t u r e to T M Thls effect probably orlglnates f r o m fluctuations and/or s a m p l e Inhomogenextle s w h i c h lead to addltlonal de palring of superconductlng electrons vzs the effectlve fleld f r o m m a g n e t i c orderlng of the R E Ions zn the regLrne x ~ Xcr T h e latter behavlot is quahtatlvely slrnilar to that prevlously o b s e r v e d in the supercondhctlng d11ute substi-

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points i n d i c a t e the o b s e r v e d t e m p e r a t u r e h y s t e r e s i s . t u t l o n a l a l l o y s y s t e m s L a t . x G d x 5 and (La I _xGdx)sIn, & in w h i c h the m o s t r a p l d d e p r e s s z o n of T c was o b s e r v e d f o r x v a l u e s at wh2ch the " s p i n g l a s s " m a g n e t i c o r d e r i n g t e m p e r a t u r e of the Gd ~ n p u r l t y ions b e c a m e c o m p a r a b l e to T c A n u n d e r s t a n d t n g of the o t h e r t h r e e f e a t u r e s of F i g . Z n o t e d a b o v e m u s t a w a i t f u r t h e r m e a s u r e m e n t s whxch e s t a b h s h the n a t u r e and t e m p e r a t u r e s o f m a g n e t i c o r d e r i n g i n the r e g z o n 0 < x < Xcr H o w e v e r , we note that i f TNI = T cZ t h r o u g h o u t t h i s region, t h e n the d e p r e s s 2 o n o~ TcZ f o r 0. 3 ~ x < X c r r e l a t i v e to the d a s h e d line e x t r a p o l a t i o n of T M f r o m x > x c r , and a l s o the t e m p e r a t u r e h y s t e r e s i s o f T c Z n e a r the m l n i m u m in the Tc~(X ) p h a s e b o u n d a r y m i g h t both be e x p e c t e d to o c c u r i f the d o m i n a n t m a g n e t i c x n t e r a c t i o n b e t w e e n the R E iolts is of the R K K Y ty p e . Since tlns indirect xnferace~on between R E xons is medlated by the conductlon electrons, the decrease ~n the conduct~on e l e c t r o n susceptzl~ht7 ~n t h e superconducting s t a t e r e l a t i v e to that i n the n o r m a l state would w e a k e n the R K K Y ~nteraction and t e n d to decrease T M . Further, the value of T M ~ T c Z ob~alned w h e n cooling f r o m the superconduc~ng s t a t e m i g h t t h e r e f o r e be l o w e r t h a n the v a l u e o b t a i n e d w h e n ~ncreas~ng the t e m p e r a t u r e f r o m the m a g n e t i c a l l y o r d e r e d s t a t e at v e r y l o w t e m p e r a t u r e s , giving r~se to the observed hysteresis. Of course, magnetic d~pole-d~pole xnteract~ons between the R E ~ons m u s t also be a contributing factor ~n the over-

a l l b e h a v i o r b e c a u s e the m a g n e t i c o r d e r i n g t e m p e r a t u r e s c a l c u l a t e d f r o m this i n t e r a c t i o n a l o n e a r e about 1 K. The m l m m u m in the TcZ(X ) phase boundary is Itself an interestlng f e a t u r e w h i c h m a y o r i g i n a t e f r o m a change in the d e t a i l e d m a g n e t i c s t r u c t u r e a s x i s v a r i e d (both HoRh4B 4 ~ and e v e n t u a l l y E r R h 4 B 4 7 a r e f e r r o m a g n e t i c ) , If T M > T c Z , the m l m m u r n m a y be a m a m f e s t a h o n of the H o - E r d ~ s o r d e r on the R E sublatt~ce, which would Increase the orbital crltzcal fleld and hence decrease T c Z . Additional m e a s u r e m e n t s are n e c e s s a r y to differentiate between these posslbdlbes. in c o n c l u s i o n , we h a v e d e t e r m i n e d the b o u n d a r i e s b e t w e e n the n o r m a l pararrmgnet~c, s u p e r c o n d u c t i n g , and n o r m a l m a g n e t i c a l l y ordered phases zn the ( E r 1 . x H o x } l ~ 4 B 4 pseudoternary system. The re-entrant superconductivity p r e w o u s l y r e p o r t e d f o r E r R h 4 B 4 1 w as found to persist to Xcr ~ 0 89, and only m a g netic orderlng was found for x > xcr . Therefore, superconductlvlty and ferrornagnehsm are mutually excluslve p h e n o m e n a over an appreciable temperature range for ~ = 0 1 and x > Xcr , w h e t h e r this i s a l s o the c a s e f o r intermediate compositions remalns to be determined. The present m e a s u r e m e n t s and other m e a s u r e m e n t s in progress on pseudoternary rare earth rhodium borlde systems v~ll provlde an exper~rctental basls upon whlch theorJes d e a h n g wlth the coexistence of superconductlvlty and long-range magnetic order can be tested.

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REFERENCES i Z

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FERTIG, W A , JOHNSTON, D C , DeLONG, L E , McCALLUM. R W , MAPLE, M ]3 a n d M A T T H I A S , B.T , PhyslcalRevaewLetters 38, 987 (1977) MATTHIAS, B T , CORENZWIT, E , VANDENBERG, J M and BARZ, H E , Proceedrags of the National Academy of Sciences, USA, 74, 1334 (1977) VANDENBERG, J M and MATTHIAS, B T , P r o c e e d l n g s of the Natlonal A c a d e m y of Sclences, U S A , 74, 1336 (1977) GOR'KOV, L P and RUSINOV, A I , Sovlet P h y s l c s J - E T P 19, 922 (1964) HEIN, R A , FALGE, R. Z , Jr , M A T T H I A S , B T and CORENZWIT, E , Physical R e v : e w Letters 2, 500 (1959) CROW, J E and PARKS, R D , P h y s l c s Letters 21, 378 (1966) MONCTON, D E , McWHAN, D B , ECKERT, J , SHIRANE, G and THOMLINSON, W Physical Review Letters 39, 1 1 6 4 ( 1 9 7 7 )

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