Ferromagnetism in the intermetallic phase Ni3Al

Volume 24A, number 7

PHYSICS LETTERS

FERROMAGNETISM

IN THE

27 M ~

INTERMETALLIC

PHASE

1967

Ni3AI

F . R . DE BOER, J. BIESTERBOS and C . J . SCHINKEL Nahturkundi~ L a b o r a t o r i u m d e r U n i v e r s i t e i t van A m s t e r d a m The Netherlands

Received 13 February 1967

It is found that the ordered intermetallic Ni3A1 phase behaves ferromgo~netically at temperatures below 75°K. Disordering by intense cold-working causes the ferromagnetic properties to disappear. T h e s t r u c t u r a l p r o p e r t i e s of the i n t e r m e t a l H c p h a s e Ni3A1 have been s t u d i e d t h o r o u g h l y [1-5], the r e s u l t s of t h e s e s t u d i e s a g r e e i n g f a i r l y well. T h e r e i s , h o w e v e r , much confusion about the m a g n e t i c p r o p e r t i e s of t h i s p h a s e . In an a t t e m p t to c l a r i f y t h i s s i t u a t i o n , we have s t u d i e d the m a g n e t i c p r o p e r t i e s of the Nt3A1 p h a s e in m o r e detail. W e i g h e d a m o u n t s of s p e c t r o g r a p h i c p u r e n i c k el and a l u m i n u m (Johnson, M a t t h e y and Co) w e r e a r c - m e l t e d t o g e t h e r in an a r g o n a t m o s p h e r e , d u r i n g which p r o c e d u r e no a p p r e c i a b l e l o s s in weight o c c u r r e d (< 0.15 wt. %). T h e i n g o t s (each ca. 15 g r a m ) w e r e a n n e a l e d d u r i n g 7 h o u r s at 700°C in vacuo. X - r a y i n v e s t i g a t i o n s showed the s p e c i m e n s to be s i n g l e p h a s e and to have a C u 3 A u - t y p e of o r d e r . The c o m p o s i t i o n s a s d e t e r m i n e d b y c h e m i c a l a n a l y s t s a r e in a g r e e m e n t with t h e c a l c ~ ! ~ e d c o m p o s i t i o n s . In r i g a the i n v e r s e s u s c e p t i b i l i t y X -1 a s a function of the t e m p e r a t u r e T t s given f o r a m a s s i v e 0.1 g r a m s a m p l e of NI75A125. A s s u m i n g a C u r i e - W e i s s law to hold f o r t h i s compound the s l o p e of the × - l - T c u r v e in the i n t e r v a l 220-600°K g i v e s the effective m a g n e t i c m o m e n t p e r N t - a t o m to be 1.0 ~B with 0 = 102°K *. T h e m e a s u r e d s u s c e p t i b l l l t t e s have not b e e n c o r r e c t e d f o r t e m p e r a t u r e lndepende]~t c o n t r i b u t i o n s . A s s u m i n g a v a l u e of + 0.40 × 10 - v e . m . u . , f o r t h i s c o n t r i b u t i o n , the p a r a m a g n e t i c m o m e n t p e r N t - a t o m and 0 b e c o m e 0.97 /t B and 130°K, r e s p e c t i v e l y . The m a g n e t i z a t i o n ~ a s a function of the a p p l i e d m a g n e t i c f i e l d H at liquid h e l i u m t e m p e r a t u r e s (fig.2) shows that even in a f i e l d of 170 kOe s a t u r a t i o n i s not r e a c h e d , w h i l e l o w e r i n g of the * We are very much indebted to Drs. F. van Bruggen for the high temperature measurements.

u

0

i 0

i 100

i

a 200

t~amre

a

0

{IK)

Fig.1. Resistivity and reciprocal magnetic susceptibility versus temperature of the stoichiometric Ni3A1 compound.

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| 100

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magnetic ~

! 200 (kOe)

.

I 300

Fig.2. M - H c u r v e s of N I . ~ comp~tmle c~ various compositions at liqui~ helium temperatures. 355

Volume 24A, number 7

P HYSI CS L E T T E R S

I0./, kOe - 4.6 kOe ~10 E . 7 5 @ , Ni

5

100

200

30O

temperature (*K)

Fig.3. Magnetization versus temperature of Ni75A125 Ni 73.5A126.5 and Ni78.5A121.5. t e m p e r a t u r e f r o m 5OK down to 2.5OK s e e m s to have no effect upon the m a g n e t i z a t i o n c u r v e . * F i g . 3 d i s p l a y s the m a g n e t i z a t i o n in 4.6 r e s p . 10.4 kOe a s a function of t e m p e r a t u r e . V a r i a t i o n of the Ni content within the o n e p h a s e r e g i o n (73.0 - 76.5 at % Ni) c a u s e s a s t r o n g v a r i a t i o n of the m a g n e t i z a t i o n at low t e m p e r a t u r e s (figs. 2 and 3). The effective p a r a m a g n e t i c m o m e n t p e r N i - a t o m and the p a r a m a g n e t i c C u r i e - t e m p e r a t u r e depend l e s s s t r o n g l y upon c o m p o s i t i o n : f o r 73.5 at % Ni 0.97 ~B and 84°K, and f o r 75 at % Ni 1.02 ~B and 102OK. A neutron d i f f r a c t i o n d i a g r a m obtained at r o o m t e m p e r a t u r e f r o m a s a m p l e with c o m p o sition NiT,A125 showed c o m p l e t e o r d e r with A1 at (0,0,0) ~nd Ni at (0,½,½), (½,0,½) and (½,½,0). Neutron diagrams from this sample at T= 4.2°K with zero applied field and with a field of 17 kOe applied perpendicular to the scattering vector did not show the presence of magnetic long range order. The minimum observable value for ordered

m o m e n t s on the N i - s i t e s , a s s u m i n g that t h e s e m o m o m e n t s a r e f e r r o m a g n e t i c a l l y o r d e r e d , was e s t i m a t e d a s 0.3 ~B [6]. A t e n t a t i v e m e a s u r e m e n t of the e l e c t r i c a l r e s i s t i v i t y p a s a function of the t e m p e r a t u r e i s * Mr. J. F. Fast at Philips Research Laboratories kindly performed the magnetization measurements at liquid helium temperature in low fields. 356

27 March 1967

given in fig.1 [7]. The l a r g e change of slope at 215°K obviously c o r r e s p o n d s with the much s m a l l e r a n o m a l y in the × - l - T c u r v e , while the r a p i d i n c r e a s e of m a g n e t i z a t i o n with d e c r e a s i n g t e m p e r a t u r e round 75OK i s h a r d l y r e f l e c t e d in the p - T curve. A m o s t s t r i k i n g f e a t u r e of the Ni3A1 compound i s the s t r o n g effect of i n t e n s e c o l d - w o r k i n g upon the m a g n e t i z a t i o n . It w a s o b s e r v e d that in the X - r a y d i a g r a m of an a s - c r u s h e d p o w d e r (75 at % Ni, g r a i n s i z e ca. 75 m i c r o n ) the s u p e r l a t t i c e r e f l e c t i o n s had d i s a p p e a r e d . The m a g n e t i z a t i o n s in 10 kOe at 5°K, 77OK and 300°K w e r e r e d u c e d to 7%, 13% and 74%, r e s p e c t i v e l y , of the o r i g i n a l v a l u e s . A f t e r annealing the p o w d e r at 700°C f o r 7 h o u r s in vaeuo the m a g n e t i z a t i o n at 5°K r e a c h e d 65% of the o r i g i n a l value. It i s v e r y l i k e l y that t h i s phenomenon h a s led v a r i o u s a u t h o r s to the conclusion about the Ni3A1 p h a s e being n o n - m a g n e t i c . One can r a i s e the question w h e t h e r the m a g n e t i c p r o p e r t i e s w e r e to be a s c r i b e d to a p r e c i p i t a tion of s m a l l f e r r o m a g n e t i c ~Ni-A1 p a r t i c l e s (solid solution of A1 in Ni). We p r e p a r e d t h e r e f o r e a s a m p l e of the c o m p o s i t i o n Ni78 5 A l ~ . . in the s a m e way a s we p r e p a r e d the o t h e r s a m p l e s . A s to be expected the X - r a y d i a g r a m showed t h i s s a m p l e to c o n s i s t of two p h a s e s , the Ni3A1 p h a s e with a s m a l l amount of a Ni-A1. The m a g n e t i z a t i o n - t e m p e r a t u r e c u r v e (fig. 3) is s i m i l a r to those of the s i n g l e - p h a s e compounds, a p a r t f r o m a f e r r o m a g n e t i c contribution of 3.5 e . m . u , w i t h T c ~ 300°K F r o m t h i s v a l u e and the known [8] m a g n e t i c p r o p e r t i e s of the aNi-A1 p h a s e we conclude that the p r e c i p i t a t e has about the c o m p o s i t i o n N i g 0 A l l 0 , in good a g r e e m e n t with what m u s t be expected f r o m the p h a s e - d i a g r a m of the Ni-A1 s y s t e m . F o r s a k e of c o m p l e t e n e s s we have c o l d - w o r k e d the Ni78.5A121.5. One can s e e in fig.3 that the m a g n e t i z a t i o n of the p r e c i p i t a t e i s not o r v e r y s l i g h t l y influenced, w h e r e a s the contribution of the Ni3A1phase is strongly reduced. At p r e s e n t we a r e not a b l e to give a c o m p l e t e i n t e r p r e t a t i o n of the e x p e r i m e n t a l r e s u l t s . We hope h o w e v e r , to gain m o r e u n d e r s t a n d i n g of the p r o p e r t i e s of the Ni3A1 P h a s e by e x a m i n i n g i n d i v i d u a l a t o m i c p r o p e r t i e s , e.g. with n u c l e a r m a g n e t i c r e s onance [9] and e l e c t r o n spin r e s o n a n c e . We a r e v e r y much indebted to P r o f . G. W. R a t h e nau f o r i n t e r e s t i n g us in t h i s p r o b l e m and c o n t i n uously s t i m u l a t i n g the i n v e s t i g a t i o n s . The a s s i s t ance of S. P r o o s t in s o m e of the e x p e r i m e n t s and the v a l u a b l e d i s c u s s i o n s with P r o f . A. R. M i e d e m a a r e g r a t e f u l l y mentioned.

Volume24A, number 7

PHYSICS LETTERS

References 1. A.J.Bradley and A.Taylor, Proc.Roy.Soc. (London) A159 (1937) 56. 2. H.Groeber and V.Vauk, Z.Metallk. 41 (1950) 283. 3. A.Taylor and R.W.Floyd, J. Inst. Metals 81 (19521953) 25. 4. C.L. Corey and B. Lisowski, Techn. Report Wayne State Univ. no.l, June 1966.

27 March 1967

5. R.W.Guard and J.A.Westbrook, Trans.AIME 215 (1959) 871. 6. B. van Laar and B. O. Loopstra, private communication. 7. J.Koppen, private communication. 8. J.Crangle and M.J.C.Martin, Phil.Mag. 4(1959) 1006. 9. H.W.De Wijn, F.R.De Boer and C.J.Schinkel, to be published.

ANOMALOUS ELECTROMAGNETIC MICROWAVE ABSORPTION OF SUPERCONDUCTING A L L O Y S IN A S T A T I C M A G N E T I C FIELD * K . H . BENNEMANN

Institute for the Study of Metals, University of Chicago, Chicago, Ill.

Received 21 February 1967

The microwave surface-resistance anomaly of impure superconducting A1 in a static magnetic field is qualitatively explained. It is also discussed how surface-superconductivity affects the surface-resistance.

The a n o m a l o u s d e c r e a s e in the r a t i o of the s u p e r c o n d u c t i n g to n o r m a l state s u r f a c e r e s i s t a n c e shown in fig.1 and o b s e r v e d by Budz i n s k i and Garfunkel [1] for 0.2% s i l v e r - d o p e d A1 in a static m a g n e t i c field H , p a r a l l e l to the m e t a l s u r f a c e i s explained a s follows. Due to the I

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Fig.1. * This research was supported by NASA and the ONR.

v a r i a t i o n of the e n e r g y gap in the s u r f a c e - s h e a t h of t h i c k n e s s of the o r d e r of the coherence length and the f r e q u e n c y dependence of the skin depth, the a v e r a g e superconductihg e n e r g y gap of the a b s o r p t i v e s u r f a c e r e g i o n , which is a m e a s u r e for the s u r f a c e - r e s i s t a n c e , depends on the e l e c t r o m a g n e t i c - f i e l d frequency. At low t e m p e r a t u r e s the skin depth changes slowly with i n c r e a s i n g f r e q u e n c i e s , if these a r e s m a l l e r than the e n e r g y gap, b u r the skin depth i n c r e a s e s s i g n i f i c a n t l y for f r e q u e n c i e s of the o r d e r of the e n e r g y gap, and d e c r e a s e s again for f r e q u e n c i e s somewhat l a r g e r than the e n e r g y gap [2]. The steep onset of the s u r f a c e - r e s i s t a n c e o c c u r s when the e l e c t r o m a g n e t i c - f i e l d f r e q u e n c y is equal to the a v e r a g e e n e r g y gap of the r e s i s t i v e s u r f a c e region which t h i c k n e s s i s of the o r d e r of the skin depth. With f u r t h e r i n c r e a s i n g m i c r o w a v e f r e q u e n c i e s the skin depth i n c r e a s e s and then also the a v e r age s u p e r c o n d u c t i n g e n e r g y gap, if the skin depth i s s m a l l e r than the coherence length. T h u s , the s u r f a c e - r e s i s t a n c e s t a r t s to d e c r e a s e when the a v e r a g e e n e r g y gap i n c r e a s e s m o r e r a p i d l y than the photon energy. The s u r f a c e - r e s i s t a n c e i n c r e a s e s again when the photon e n e r g i e s i n c r e a s e f a s t e r than the a v e r a g e e n e r g y gap. The s u r f a c e 357