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Internal surface superconductivity: The relationship between resistive critical field and bulk upper critical field
Volume 23, number 1
PHYSICS LETTERS
background~ and peaks at 33.5 mV, then d e c r e a s e s r e a c h i n g a m i n i m u m at 37.5 mY; the signal then i n c r e a s e s and a second peak (lower than the f i r s t ) i s r e a c h e d at 43 mV. The s t r u c t u r e i s then r e peated in r e v e r s e o r d e r at 48, 52.5 and 63 mV. In conclusion, we can definitely state that t h e r e i s a change in the density of s t a t e s of P b in the voltage r a n g e , above and below the F e r m i level, of 25 to 65 mV, and this change i s p r e s e n t when P b i s in the n o r m a l or s u p e r c o n d u c t i n g state.
3October 1966
We wish to thank Dr. R. B. Flippen for use of h i s magnet, M r . J. Townley for his help in p r e p a r a t i o n of the s a m p l e s , and Mr. D. Coffin in helping with the m e a s u r e m e n t s .
References 1. J.M.Rowell et al., Phys. Rev. Letters 9 (1962) 59.
* * * * *
INTERNAL SURFACE SUPERCONDUCTIVITY: THE RELATIONSHIP BETWEEN RESISTIVE CRITICAL FIELD AND BULK UPPER CRITICAL FIELD K. M. RALLS Inorganic Materials Research Division, Lawrence Radiation Laboratory, Department of Mineral Technology, College of Engineering, University of California, Berkeley, California Received 8 August 1966
The importance of internal surface superconductivity with respect to resistive measurements is pointed out. Critical current density versus transversemagnetic field data for a well-annealed 75Nb-25Zr alloy, are presented and discussed. During the last five y e a r s a l a r g e amount of r e s i s t i v e c r i t i c a l field (t/r) data has been c o m piled for v a r i o u s high-field s o l i d - s o l u t i o n a l l o y s . A n i s o t r o p y in H r has been noted [1] or i m p l i e d [ c.f. 2,3]. T h i s a n i s o t r o p y , which is i n c o n s i s t e n t with s i m p l e t h e o r e t i c a l i n t e r p r e t a t i o n s , has r e ceived little attention until r e c e n t l y [4]. V a r i a t i o n of H r with : m i c r o s t r u c t u r a l m o d i f i c a t i o n s has been known for some time also [5], yet its s i g n i ficance has not been widely recognized, p a r t l y b e c a u s e of s c e p t i c i s m c o n c e r n i n g p r e s e n t knowledge of the effects of t h e r m a l t r e a t m e n t per se a s opposed to the effects of compositional c h a n ges r e s u l t i n g f r o m t h e r m a l t r e a t m e n t s . It is i m p o r t a n t to point our that the H r - a n i s o t r o p y and the H r - m i c r o s t r u c t u r e s e n s i t i v i t y a r e probably r e l a t e d to each other through a mutual dependence upon i n t e r n a l surface stabilized s u p e r c o n d u c t i v i t y above the bulk upper c r i t i c a l field Hc2. R e s i s t i v e c r i t i c a l field i s somewhat dependent upon the s e a r c h - c u r r e n t density (J) used and the e n t i r e r e s i s t i v e t r a n s i t i o n of b r o a d although in p a r t these may be due to s m a l l composition v a r i a t i o n s which i n v a r i a b l y occur in alloy ingots. Such composition v a r i a t i o n s e x t r e m e l y difficult to e l i -
m i n a t e e n t i r e l y b e c a u s e diffusion does not occur r e a d i l y at e a s i l y a c c e s s i b l e t e m p e r a t u r e s . Even in heavily deformed a l l o y s which a r e r e l a t i v e l y homogeneous there i s H r dependence upon J and a broad r e s i s t i v e t r a n s i t i o n , indicating that these effects a r e also due to the n a t u r e of the c o l d - w o r ked state. When, however, a s e v e r e l y coldworked alloy i s r e c r y s t a l l i z e d , diffusion is enhanced and good compositional u n i f o r m i t y r e s u l t s . F u r t h e r m o r e , the r e s i s t i v e t r a n s i t i o n i s s h a r p e ned c o n s i d e r a b l y and the dependence of H r on J i s v i r t u a l l y e l i m i n a t e d for a range of s e a r c h current densities. It has been shown that there i s a l i m i t i n g onset H r for heavily cold-worked alloys [3], and it a p p e a r s that for w e l l - a n n e a l e d a l l o y s there is a w e l l - d e f i n e d H r which i s probably the bulk upper c r i t i c a l field Hc2. The l a t t e r has been d e m o n s t r a t e d through a c o m p a r i s o n of c r i t i c a l c u r r e n t density (Jc) v e r s u s t r a n s v e r s e magnetic field, r e s i s t i v e t r a n s i t i o n and m a g n e t i z a t i o n data for pure n i o b i u m [6]. Since the magnitude of J affects the d e t e r m i n a t i o n of Hr, the choice of a single a r b i t r a r y J will not allow the d e t e r m i n a t i o n of Hc2 for all m a t e r i a l s . N e v e r t h e l e s s , a complete 29
Volume 23, number 1
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60 70 H (kG) F i g . 1. C r i t i c a l c u r r e n t d e n s i t y a s a f u n c t i o n of a p p l i e d
transverse magnetic field at 4.2°K for various voltage criteria with a 4 cm gauge length (solid lines) and for constant fractional restorted resistance (broken lines): 75Nb-25Zr, 0.0254 cm diameter, annealed in high vacuum at 1450°C for 15 min. J c v e r s u s H c u r v e a l o n g with s u i t a b l e r e s i s t i v e t r a n s i t i o n c u r v e s d o e s p e r m i t one to obtain H c 2 since Jc falls abruptly (almost discontinuously) at H c 2 f o r w e l l - a n n e a l e d m a t e r i a l s (fig. 1). C o n v e r s e l y , the r e s i s t i v e t r a n s i t i o n o c c u r s a b r u p t l y if a s u i t a b l e v a l u e of J i s u s e d . T o o high a v a l u e of J will g i v e s i m p l y a c r i t i c a l c u r r e n t d e n s i t y p o i n t (for a r e s i s t i v e o n s e t c r i t e r i o n ) , although a c o m p l e t e r e s i s t i v e t r a n s i t i o n w i l l give an a p p r o x i m a t e v a l u e f o r H c 2 . T o o low a v a l u e of J will give information about free-surface superconduct i v i t y , i . e . , a t a i l on the J c v e r s u s H c u r v e which m a y e x t e n d a s f a r a s Hc3. It i s f o r t u i t o u s that v a l u e s of J u s e f u l in obtainin~ Hc2 a r e in the r a n g e 0.1 to a b o v e 100 A / c m z w h e r e the o n s e t H r and the r e s i s t i v e t r a n s i t i o n c u r v e s a r e a l m o s t i n d e p e n d e n t of v a r i a t i o n s in J o r m o r e than an o r d e r of m a g n i t u d e f o r w e l l - a n n e a l e d m a t e r i a l s . S t r o n g e v i d e n c e that i n t e r n a l s u r f a c e s u p e r c o n d u c t i v i t y i s r e s p o n s i b l e f o r apparent bulk s u p e r c o n d u c t i v i t y (as d e t e r m i n e d r e s i s t i v e l y ) up to m a g n e t i c f i e l d s a b o v e Hc2 i s p r o v i d e d by c o m p a r i s o n s of o n s e t H r f o r a wide v a r i e t y of s o l i d s o l u t i o n a l l o y s in both d e f o r m e d and a n n e a l e d 30
Fig. 2a. Ultrasonic transition for 75Nb-25Zr [8]; 2b. Microwave transition for cold-worked 75Nb-25Zr [4]. 2c. Resistive transition for cold-worked 75Nb-25Zr [4]. 2d. Resistive transition for annealed 75Nb-25Zr: H r indicated for 1% and 50% (Hr = Hc2) resistance r e s tored (all at 4.2OK). c o n d i t i o n s [5,7]. I n v a r i a b l y , the r a t i o Hr ( c o l d w o r k e d ) / H r (annealed) is within the l i m i t s 1.1 to 1.2 f o r t r a n s v e r s e f i e l d s . It i s s i g n i f i c a n t that f o r 7 5 N b - 2 5 Z r the o n s e t of the t r a n s i t i o n at 4 . 2 ° K a s d e t e r m i n e d by an u l t r a s o n i c t e c h n i q u e [8] (fig. 2a) and by a m i c r o w a v e t e c h n i q u e [4] (fig. 2b) o c c u r s r a t h e r n e a r the H r v a l u e s of a n n e a l e d 7 5 N b - 2 5 Z r (fig. 2d), but s i g n i f i c a n t l y below the o n s e t H r v a l u e f o r c o l d - w o r k e d 7 5 N b - 2 5 Z r (fig. 2c) which was i n t e r p r e t e d a s H c 2 in r e f . 4. The w i d t h s of the t r a n s i t i o n s d e t e r m i n e d by the f o r m e r two m e t h o d s a r e s o m e w h a t p u z z l i n g . T h e y m a y be r e l a t e d to c o m p o s i t i o n f l u c t u a t i o n s o r to i n t e r n a l s u r f a c e s u p e r c o n d u c t i v i t y e v e n though no a n i s o t r o p y with f i e l d w a s o b s e r v e d . S i m i l a r e x p e r i m e n t s should be p e r f o r m e d on r e c r y s t a l l i z e d , h o m o g e n e o u s a l l o y s . In s u c h a c a s e it i s to be e x p e c t e d that the f i e l d - o r i e n t a t i o n a n i s o t r o p y of H r will d i s a p p e a r b e c a u s e any p o s s i b l e i n t e r n a l s u r f a c e c o n t r i b u t i o n s to s u p e r c o n d u c t i v e b ~ h a v i o u r w i l l be g r e a t l y m i n i m i z e d and f r e e - s u r f a c e s u p e r c o n d u c t i v i t y should be e a s i l y r e c o g n i z a b l e (or e a s i l y inhibited) f o r the J//H c a s e . Good m i c r o s s t r u c t u r a l c o n t r o l and a n a l y s i s a r e n e c e s s a r y f o r a c l e a r i n t e r p r e t a t i o n of the v a r i o u s p o s s i b l e transition fields.
Volume 23, n u m b e r 1
PHYSICS
I a m i n d e b t e d to P r o f . R . M . R o s e a n d D r . R . G . Boyd for many stimulating discussions. The experimental results reported here were obtained while the author was at the Massachusetts Instit u t e of T e c h n o l o g y . T h e s u p p o r t t h r o u g h N a t i o n a l S c i e n c e F o u n d a t i o n G r a n t G P 2863 a n d t h e u s e of t h e M . I . T . N a t i o n a l M a g n e t L a b o r a t o r y f a c i l i t i e s a t t h a t t i m e , a n d t h e c u r r e n t s u p p o r t of t h e United States Atomic Energy Commission through t h e I n o r g a n i c M a t e r i a l s R e s e a r c h D i v i s i o n of t h e Lawrence Radiation Laboratory are gratefully acknowledged.
References 1. P . R . A r o n and H . C . H i t c h c o c k , J. Appl. Phys. 33 (1963) 2242.
CURIE
TEMPERATURE
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3 O c t o b e r 1966
2. S . T . S e k u l a , R . W . B o o m and C . J . B e r g e r o n , Appl. Phys. L e t t e r s 2 (1963) 102. 3. K. M. Ralls, A. L. Donlevy, R. M. Rose and J. Wulff, in Metallurgy of advanced electronic m a t e r i a l s , ed. G . E . B r o c k , (Interscience P u b l i s h e r s , New York, 1963) p. 35. 4. S. J. Williamson and J. K. Furdyna, Phys. L e t t e r s 21 (1966) 376. 5. J.Wong, Superconductors, eds. M . T a n e n b a u m and W . V . W r i g h t , (Interscience P u b l i s h e r s , New York, 1962) p.83; H. B. Shukovsky, K.M. Ralls and R. M. Rose, T r a n s . Met. Soc., AIME 233 (1965) 1825. 6 . E . S . R o s e n b l t u n , S . H . A u t l e r and K.H.Gooen, Rev. Mod. Phys., 36 (1964) 77. 7. D.A. Colling, K.M. Ralls and J.Wulff, T r a n s . Met. Soc., AIME 236 (August 1966); K. M. Ralls unpublished r e s d l t s . 8. L. J. Neuringer and Y.Shapira, Sol. State Commun. 2 (1964) 349.
MEASUREMENTS
OF
GaxFe3_xO
4 CRYSTALS
H e l e n G A M A R I - S E A L E a n d S. K A R A V E L A S Nuclear Research Center "Democritus" Aghia Paraskevi, Attihis, Athens, Greece Received 18 August 1966
The Curie t e m p e r a t u r e s of Ga 0 5Fe2 504 and Ga 0 7Fe 2 304 have been found to be equal to 413 °:e 3°C and 347 ° * 3Oc r e s p e c t i v e l y by neuti:on diffraction technique:
T h e C u r i e t e m p e r a t u r e s of two g a l l i u m s u b s t i tuted ferrite crystals have been measured by neutron diffraction techniques. For each crystal the i n t e n s i t y of t h e n e u t r o n b e a m d i f f r a c t e d b y t h e III crystal plane was measured at different temperat u r e s . Due t o t h e h i g h c o n t r i b u t i o n of t h e m a g n e t i c s c a t t e r i n g a s c o m p a r e d to t h e n u c l e a r , t h e a c c u r a c y of d e t e r m i n a t i o n of t h e C u r i e t e m p e r a t u r e d e p e n d s m o s t l y on c r y s t a l h o m o g e n i t y a n d t e m p e r a t u r e u n i f o r m i t y . F o r t h e two c r y s t a l s w i t h x = 0.5 a n d 0.7 t h e r e s u l t s a r e T c = 413 ° + 3°C a n d T c = 3 4 7 ° ± 3 ° C r e s p e c t i v e l y . In t h e figure the above Curie points are plotted together with data for pure magnetite and for a sample w i t h x = 1.1 [ 1 , 2 ] . T h e C u r i e p o i n t i s s e e n to d e p e n d l i n e a r l y on g a l l i u m c o n t e n t f r o m x = 0 t o x = 1.1. T h i s s u g g e s t s t h a t g a l l i u m i s d i s t r i b u t e d between tetrahedral and octahedral sites at x= 0.5 a n d 0.7 s i n c e s u c h d i s t r i b u t i o n i s f o u n d f o r x = 1.1 [2]. N e u t r o n d i f f r a c t i o n s t r u c t u r e s t u d i e s a r e b e i n g c o n t i n u e d to i n v e s t i g a t e t h i s h y p o t h e s i s .
800 0 0
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0.0
I
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0.2 0.4 0.6 0.8 1.0 1.2 P A R A M E T E R X FOR G A L L I U M
Fig. 1. Curie point v e r u s Ga content.
References 1. A.W.McReynolds and R . R i s t e r , Phys. Rev. 95 (1954) 1161. 2. A . O l e s , Compt. Rend. 260 (1965) 6075.
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background~ and peaks at 33.5 mV, then d e c r e a s e s r e a c h i n g a m i n i m u m at 37.5 mY; the signal then i n c r e a s e s and a second peak (lower than the f i r s t ) i s r e a c h e d at 43 mV. The s t r u c t u r e i s then r e peated in r e v e r s e o r d e r at 48, 52.5 and 63 mV. In conclusion, we can definitely state that t h e r e i s a change in the density of s t a t e s of P b in the voltage r a n g e , above and below the F e r m i level, of 25 to 65 mV, and this change i s p r e s e n t when P b i s in the n o r m a l or s u p e r c o n d u c t i n g state.
3October 1966
We wish to thank Dr. R. B. Flippen for use of h i s magnet, M r . J. Townley for his help in p r e p a r a t i o n of the s a m p l e s , and Mr. D. Coffin in helping with the m e a s u r e m e n t s .
References 1. J.M.Rowell et al., Phys. Rev. Letters 9 (1962) 59.
* * * * *
INTERNAL SURFACE SUPERCONDUCTIVITY: THE RELATIONSHIP BETWEEN RESISTIVE CRITICAL FIELD AND BULK UPPER CRITICAL FIELD K. M. RALLS Inorganic Materials Research Division, Lawrence Radiation Laboratory, Department of Mineral Technology, College of Engineering, University of California, Berkeley, California Received 8 August 1966
The importance of internal surface superconductivity with respect to resistive measurements is pointed out. Critical current density versus transversemagnetic field data for a well-annealed 75Nb-25Zr alloy, are presented and discussed. During the last five y e a r s a l a r g e amount of r e s i s t i v e c r i t i c a l field (t/r) data has been c o m piled for v a r i o u s high-field s o l i d - s o l u t i o n a l l o y s . A n i s o t r o p y in H r has been noted [1] or i m p l i e d [ c.f. 2,3]. T h i s a n i s o t r o p y , which is i n c o n s i s t e n t with s i m p l e t h e o r e t i c a l i n t e r p r e t a t i o n s , has r e ceived little attention until r e c e n t l y [4]. V a r i a t i o n of H r with : m i c r o s t r u c t u r a l m o d i f i c a t i o n s has been known for some time also [5], yet its s i g n i ficance has not been widely recognized, p a r t l y b e c a u s e of s c e p t i c i s m c o n c e r n i n g p r e s e n t knowledge of the effects of t h e r m a l t r e a t m e n t per se a s opposed to the effects of compositional c h a n ges r e s u l t i n g f r o m t h e r m a l t r e a t m e n t s . It is i m p o r t a n t to point our that the H r - a n i s o t r o p y and the H r - m i c r o s t r u c t u r e s e n s i t i v i t y a r e probably r e l a t e d to each other through a mutual dependence upon i n t e r n a l surface stabilized s u p e r c o n d u c t i v i t y above the bulk upper c r i t i c a l field Hc2. R e s i s t i v e c r i t i c a l field i s somewhat dependent upon the s e a r c h - c u r r e n t density (J) used and the e n t i r e r e s i s t i v e t r a n s i t i o n of b r o a d although in p a r t these may be due to s m a l l composition v a r i a t i o n s which i n v a r i a b l y occur in alloy ingots. Such composition v a r i a t i o n s e x t r e m e l y difficult to e l i -
m i n a t e e n t i r e l y b e c a u s e diffusion does not occur r e a d i l y at e a s i l y a c c e s s i b l e t e m p e r a t u r e s . Even in heavily deformed a l l o y s which a r e r e l a t i v e l y homogeneous there i s H r dependence upon J and a broad r e s i s t i v e t r a n s i t i o n , indicating that these effects a r e also due to the n a t u r e of the c o l d - w o r ked state. When, however, a s e v e r e l y coldworked alloy i s r e c r y s t a l l i z e d , diffusion is enhanced and good compositional u n i f o r m i t y r e s u l t s . F u r t h e r m o r e , the r e s i s t i v e t r a n s i t i o n i s s h a r p e ned c o n s i d e r a b l y and the dependence of H r on J i s v i r t u a l l y e l i m i n a t e d for a range of s e a r c h current densities. It has been shown that there i s a l i m i t i n g onset H r for heavily cold-worked alloys [3], and it a p p e a r s that for w e l l - a n n e a l e d a l l o y s there is a w e l l - d e f i n e d H r which i s probably the bulk upper c r i t i c a l field Hc2. The l a t t e r has been d e m o n s t r a t e d through a c o m p a r i s o n of c r i t i c a l c u r r e n t density (Jc) v e r s u s t r a n s v e r s e magnetic field, r e s i s t i v e t r a n s i t i o n and m a g n e t i z a t i o n data for pure n i o b i u m [6]. Since the magnitude of J affects the d e t e r m i n a t i o n of Hr, the choice of a single a r b i t r a r y J will not allow the d e t e r m i n a t i o n of Hc2 for all m a t e r i a l s . N e v e r t h e l e s s , a complete 29
Volume 23, number 1
PHYSICS
LETTERS
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60 70 H (kG) F i g . 1. C r i t i c a l c u r r e n t d e n s i t y a s a f u n c t i o n of a p p l i e d
transverse magnetic field at 4.2°K for various voltage criteria with a 4 cm gauge length (solid lines) and for constant fractional restorted resistance (broken lines): 75Nb-25Zr, 0.0254 cm diameter, annealed in high vacuum at 1450°C for 15 min. J c v e r s u s H c u r v e a l o n g with s u i t a b l e r e s i s t i v e t r a n s i t i o n c u r v e s d o e s p e r m i t one to obtain H c 2 since Jc falls abruptly (almost discontinuously) at H c 2 f o r w e l l - a n n e a l e d m a t e r i a l s (fig. 1). C o n v e r s e l y , the r e s i s t i v e t r a n s i t i o n o c c u r s a b r u p t l y if a s u i t a b l e v a l u e of J i s u s e d . T o o high a v a l u e of J will g i v e s i m p l y a c r i t i c a l c u r r e n t d e n s i t y p o i n t (for a r e s i s t i v e o n s e t c r i t e r i o n ) , although a c o m p l e t e r e s i s t i v e t r a n s i t i o n w i l l give an a p p r o x i m a t e v a l u e f o r H c 2 . T o o low a v a l u e of J will give information about free-surface superconduct i v i t y , i . e . , a t a i l on the J c v e r s u s H c u r v e which m a y e x t e n d a s f a r a s Hc3. It i s f o r t u i t o u s that v a l u e s of J u s e f u l in obtainin~ Hc2 a r e in the r a n g e 0.1 to a b o v e 100 A / c m z w h e r e the o n s e t H r and the r e s i s t i v e t r a n s i t i o n c u r v e s a r e a l m o s t i n d e p e n d e n t of v a r i a t i o n s in J o r m o r e than an o r d e r of m a g n i t u d e f o r w e l l - a n n e a l e d m a t e r i a l s . S t r o n g e v i d e n c e that i n t e r n a l s u r f a c e s u p e r c o n d u c t i v i t y i s r e s p o n s i b l e f o r apparent bulk s u p e r c o n d u c t i v i t y (as d e t e r m i n e d r e s i s t i v e l y ) up to m a g n e t i c f i e l d s a b o v e Hc2 i s p r o v i d e d by c o m p a r i s o n s of o n s e t H r f o r a wide v a r i e t y of s o l i d s o l u t i o n a l l o y s in both d e f o r m e d and a n n e a l e d 30
Fig. 2a. Ultrasonic transition for 75Nb-25Zr [8]; 2b. Microwave transition for cold-worked 75Nb-25Zr [4]. 2c. Resistive transition for cold-worked 75Nb-25Zr [4]. 2d. Resistive transition for annealed 75Nb-25Zr: H r indicated for 1% and 50% (Hr = Hc2) resistance r e s tored (all at 4.2OK). c o n d i t i o n s [5,7]. I n v a r i a b l y , the r a t i o Hr ( c o l d w o r k e d ) / H r (annealed) is within the l i m i t s 1.1 to 1.2 f o r t r a n s v e r s e f i e l d s . It i s s i g n i f i c a n t that f o r 7 5 N b - 2 5 Z r the o n s e t of the t r a n s i t i o n at 4 . 2 ° K a s d e t e r m i n e d by an u l t r a s o n i c t e c h n i q u e [8] (fig. 2a) and by a m i c r o w a v e t e c h n i q u e [4] (fig. 2b) o c c u r s r a t h e r n e a r the H r v a l u e s of a n n e a l e d 7 5 N b - 2 5 Z r (fig. 2d), but s i g n i f i c a n t l y below the o n s e t H r v a l u e f o r c o l d - w o r k e d 7 5 N b - 2 5 Z r (fig. 2c) which was i n t e r p r e t e d a s H c 2 in r e f . 4. The w i d t h s of the t r a n s i t i o n s d e t e r m i n e d by the f o r m e r two m e t h o d s a r e s o m e w h a t p u z z l i n g . T h e y m a y be r e l a t e d to c o m p o s i t i o n f l u c t u a t i o n s o r to i n t e r n a l s u r f a c e s u p e r c o n d u c t i v i t y e v e n though no a n i s o t r o p y with f i e l d w a s o b s e r v e d . S i m i l a r e x p e r i m e n t s should be p e r f o r m e d on r e c r y s t a l l i z e d , h o m o g e n e o u s a l l o y s . In s u c h a c a s e it i s to be e x p e c t e d that the f i e l d - o r i e n t a t i o n a n i s o t r o p y of H r will d i s a p p e a r b e c a u s e any p o s s i b l e i n t e r n a l s u r f a c e c o n t r i b u t i o n s to s u p e r c o n d u c t i v e b ~ h a v i o u r w i l l be g r e a t l y m i n i m i z e d and f r e e - s u r f a c e s u p e r c o n d u c t i v i t y should be e a s i l y r e c o g n i z a b l e (or e a s i l y inhibited) f o r the J//H c a s e . Good m i c r o s s t r u c t u r a l c o n t r o l and a n a l y s i s a r e n e c e s s a r y f o r a c l e a r i n t e r p r e t a t i o n of the v a r i o u s p o s s i b l e transition fields.
Volume 23, n u m b e r 1
PHYSICS
I a m i n d e b t e d to P r o f . R . M . R o s e a n d D r . R . G . Boyd for many stimulating discussions. The experimental results reported here were obtained while the author was at the Massachusetts Instit u t e of T e c h n o l o g y . T h e s u p p o r t t h r o u g h N a t i o n a l S c i e n c e F o u n d a t i o n G r a n t G P 2863 a n d t h e u s e of t h e M . I . T . N a t i o n a l M a g n e t L a b o r a t o r y f a c i l i t i e s a t t h a t t i m e , a n d t h e c u r r e n t s u p p o r t of t h e United States Atomic Energy Commission through t h e I n o r g a n i c M a t e r i a l s R e s e a r c h D i v i s i o n of t h e Lawrence Radiation Laboratory are gratefully acknowledged.
References 1. P . R . A r o n and H . C . H i t c h c o c k , J. Appl. Phys. 33 (1963) 2242.
CURIE
TEMPERATURE
LETTERS
3 O c t o b e r 1966
2. S . T . S e k u l a , R . W . B o o m and C . J . B e r g e r o n , Appl. Phys. L e t t e r s 2 (1963) 102. 3. K. M. Ralls, A. L. Donlevy, R. M. Rose and J. Wulff, in Metallurgy of advanced electronic m a t e r i a l s , ed. G . E . B r o c k , (Interscience P u b l i s h e r s , New York, 1963) p. 35. 4. S. J. Williamson and J. K. Furdyna, Phys. L e t t e r s 21 (1966) 376. 5. J.Wong, Superconductors, eds. M . T a n e n b a u m and W . V . W r i g h t , (Interscience P u b l i s h e r s , New York, 1962) p.83; H. B. Shukovsky, K.M. Ralls and R. M. Rose, T r a n s . Met. Soc., AIME 233 (1965) 1825. 6 . E . S . R o s e n b l t u n , S . H . A u t l e r and K.H.Gooen, Rev. Mod. Phys., 36 (1964) 77. 7. D.A. Colling, K.M. Ralls and J.Wulff, T r a n s . Met. Soc., AIME 236 (August 1966); K. M. Ralls unpublished r e s d l t s . 8. L. J. Neuringer and Y.Shapira, Sol. State Commun. 2 (1964) 349.
MEASUREMENTS
OF
GaxFe3_xO
4 CRYSTALS
H e l e n G A M A R I - S E A L E a n d S. K A R A V E L A S Nuclear Research Center "Democritus" Aghia Paraskevi, Attihis, Athens, Greece Received 18 August 1966
The Curie t e m p e r a t u r e s of Ga 0 5Fe2 504 and Ga 0 7Fe 2 304 have been found to be equal to 413 °:e 3°C and 347 ° * 3Oc r e s p e c t i v e l y by neuti:on diffraction technique:
T h e C u r i e t e m p e r a t u r e s of two g a l l i u m s u b s t i tuted ferrite crystals have been measured by neutron diffraction techniques. For each crystal the i n t e n s i t y of t h e n e u t r o n b e a m d i f f r a c t e d b y t h e III crystal plane was measured at different temperat u r e s . Due t o t h e h i g h c o n t r i b u t i o n of t h e m a g n e t i c s c a t t e r i n g a s c o m p a r e d to t h e n u c l e a r , t h e a c c u r a c y of d e t e r m i n a t i o n of t h e C u r i e t e m p e r a t u r e d e p e n d s m o s t l y on c r y s t a l h o m o g e n i t y a n d t e m p e r a t u r e u n i f o r m i t y . F o r t h e two c r y s t a l s w i t h x = 0.5 a n d 0.7 t h e r e s u l t s a r e T c = 413 ° + 3°C a n d T c = 3 4 7 ° ± 3 ° C r e s p e c t i v e l y . In t h e figure the above Curie points are plotted together with data for pure magnetite and for a sample w i t h x = 1.1 [ 1 , 2 ] . T h e C u r i e p o i n t i s s e e n to d e p e n d l i n e a r l y on g a l l i u m c o n t e n t f r o m x = 0 t o x = 1.1. T h i s s u g g e s t s t h a t g a l l i u m i s d i s t r i b u t e d between tetrahedral and octahedral sites at x= 0.5 a n d 0.7 s i n c e s u c h d i s t r i b u t i o n i s f o u n d f o r x = 1.1 [2]. N e u t r o n d i f f r a c t i o n s t r u c t u r e s t u d i e s a r e b e i n g c o n t i n u e d to i n v e s t i g a t e t h i s h y p o t h e s i s .
800 0 0
Boo
400
:[
200 !
0.0
I
I
I
I
I
0.2 0.4 0.6 0.8 1.0 1.2 P A R A M E T E R X FOR G A L L I U M
Fig. 1. Curie point v e r u s Ga content.
References 1. A.W.McReynolds and R . R i s t e r , Phys. Rev. 95 (1954) 1161. 2. A . O l e s , Compt. Rend. 260 (1965) 6075.
31