Magnetic and superconducting behaviour of Y36Co28(“Y4Co3”)

Physica 109 & IlOB (1982)2041-2044 North-Holland Publishing Company

2041

MAGNETIC AND SUPERCONDUCTING BEHAVIOUR OF Y36Cozs("Y4Cofl')

B.V.B. S A R K I S S I A N , A.K. G R O V E R * and B.R. C O L E S Blackett Laboratory, Imperial College. London, UK

Results are reported of the properties of the phase in the Y-Co system which shows hoth superconductivity and strong magnetic correlations. A structure is suggested which is consistent with the latter and possibly responsible for the former.

1. Introduction A n i n t e r m e t a l l i c c o m p o u n d in the Y - C o system with a c o m p o s i t i o n close to Y4C03 has for s o m e t i m e a t t r a c t e d i n t e r e s t b e c a u s e its m a g n e t i c p r o p e r t i e s a r e much s t r o n g e r t h a n w o u l d h a v e b e e n e x p e c t e d f r o m the g e n e r a l t r e n d s in the system [1]. Of t h e c o m p o u n d s r e p o r t e d - Y 2 C 0 1 7 , YCos, Y2Co7, YC03, YC02, Y C o , Y4C03, Y3C02, Y~Cos, Y 3 C o - o n l y t h o s e c o n t a i n i n g m o r e C o than YCo2 h a d p r e v i o u s l y shown l o n g - r a n g e m a g n e t i c o r d e r , a l t h o u g h a d e g r e e of m a g n e t i c c h a r a c t e r for the C o d e v e l o p s w h e n s o m e r e p l a c e m e n t of Y by G d is m a d e in YCo2. It was t h e r e f o r e the m o r e surprising that this "Y~Co3" c o m p o u n d was f o u n d [2] to b e c o m e s u p e r c o n ducting at a r o u n d 2 K , a l t h o u g h n o n e of t h e o t h e r p h a s e s in t h e system d o e s so, with t h e p o s s i b l e e x c e p t i o n of Y3Co b e l o w 0.3 K [3]. W e n o w r e p o r t s o m e results of i n v e s t i g a t i o n s a i m e d at clarifying t h e c o m p o s i t i o n , s t r u c t u r e a n d p r o p e r t i e s of the p h a s e showing this r e m a r k a b l e behaviour. T h e results of a d e t a i l e d i n v e s t i g a t i o n of the p h a s e d i a g r a m in t h e r e l e v a n t region will b e r e p o r t e d e l s e w h e r e . It has b e e n e s t a b l i s h e d that b e t w e e n 38 a n d 50 a t % C o the only p h a s e showing significant m a g n e t i c p r o p e r t i e s o r s u p e r c o n ductivity exists o v e r a n a r r o w c o m p o s i t i o n r a n g e * On leave from the Tata Institute of Fundamental Research, Bombay. India. 0378-4363/82/0000-0000/$02.75 © 1982 N o r t h - H o l l a n d

at Y36Co2s, forms p e r i t e c t o i d a l l y at - 7 2 0 ° C a n d d o e s not include t h e c o m p o s i t i o n YnCo 3. T h e d e t a i l e d p o s i t i o n of the f o u r C o a t o m s we b e l i e v e to be r e s p o n s i b l e for t h e m a g n e t i c p r o p e r t i e s (the o t h e r 24 are e n c l o s e d in t r i a n g u l a r p r i s m s of Y a t o m s ) h a v e yet to b e e s t a b l i s h e d , but t h e y lie on t h e c-axis of t h e unit cell (fig. 1) a s c r i b e d to "Y4C03" by L e m a i r e et al. [4].

2.35

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Fig. 1. Atomic arrangements suggested for the Y36Co28 phase, assuming the positions established [4] for Y and Co atoms inside the unit cell and (i) c-axis sites proposed by Lemaire et al. [4] as half occupied at random in "Y4Co3" (unannealed); (ii) c-axis sites required if the four c-axis Co atoms in Y36Co28 are equally spaced; (iii) c-axis sites proposed for Co atoms in Y36Co28 to maximize occupation of largest interstices and yield one magnetic Co atom per Y36Co28unit cell.

2042

B.V.B. Sarkissian et al. I Magnetic and superconducting behavi¢mr of Y~6(%:,

2. Magnetic properties A f t e r a n n e a l i n g for 3 weeks at ~ 5 3 0 ° C the a.c. susceptibility

of

the

Y36Co2~ phase

is fairly

r e p r o d u c i b l e . Its b e h a v i o u r is shown in fig. 2, where it can be seen that it rises r a t h e r rapidly as the t e m p e r a t u r e falls b e l o w 6 K, a n d then at a rather slower rate at lower t e m p e r a t u r e s . T h e reciprocal susceptibility has n o e x t e n d e d r a n g e of c o n f o r m i t y to a C u r i e - W e i s s r e l a t i o n s h i p , as p o i n t e d out previously [1], b u t the data for 1/X

/ /

b e t w e e n 8 a n d 20 K suggest a 0p value of 6 K. In samples that are either some way off stoic h i o m e t r y , or i n a d e q u a t e l y a n n e a l e d , or crushed, the rise in g at lower t e m p e r a t u r e s is m u c h

,/

slower or even c o m p l e t e l y lost, d x / d T b e c o m i n g positive. T h e m a g n e t i z a t i o n (above a n d b e l o w the s u p e r c o n d u c t i n g t r a n s i t i o n t e m p e r a t u r e ) has

I

k. . . . .

b e e n s t u d i e d as a f u n c t i o n of field a n d ternFig. 3. Influencc of superimposed parallel d.c. magnetic fields up to 7 kOe on the a.c. A"signals of a sample at composition Y~Co~sla. The insets show the corresponding behaviour on an expanded scale for small values of Ha~. p e r a t u r e . T h e high field m a g n e t i z a t i o n curves at 4 . 2 K r e s e m b l e those previously r e p o r t e d for Y4Co3 [l] a n d the g e n e r a l character for the fieldinduced

normal

state

is u n c h a n g e d at

lower

t e m p e r a t u r e s . T h e m a x i m u m m a g n e t i z a t i o n corr e s p o n d s to a b o u t 0.85/,t, per Y3~,Co2~ f o r m u l a unit. M(H) curves over m o r e restricted ranges of H a b o v e T~ (fig. 2, inset) reveal the p r e s e n c e of a /./I

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small b u t definite hysteretic c o m p o n e n t of magnetization. This d i s a p p e a r s gradually with increasing t e m p e r a t u r e a n d has the general character of that in either weak f e r r o m a g n e t s or spin-glasses like C u - 2 0 % Mn which have strong f e r r o m a g n e t i c correlations. T h e r e d u c t i o n of the a.c, susceptibility by a s u p e r i m p o s e d field (fig. 3) also r e s e m b l e s that in such materials.

x.Lj/ Fig. 2. A.c. susceptibility of a sample with a nomial composition of Y36Co2811.(a) Annealed at 530°C for 21 days, and (b) after further heat treatment at 70(I°Cfor 1 day. The insets show the magnetic hysteresis loops in magnetic and superconducting regions.

3. Superconducting properties The superconducting transition temperature has b e e n studied by both a,c. susceptibility and

B. V.B. Sarkissian et al. I Magnetic and superconducting behaviour of Y36Co2s

resistivity measurements. It is much more sensitive than the magnetic properties to small variations in the quality of the material. Thus samples at the composition Y36C027 prepared with commercial yttrium (Rare Earth Products a few hundred p p m of magnetic rare earth atoms) showed superconductivity only below 1.4K, whereas specimens at the same composition prepared from high grade (Ames Research Laboratory) yttrium had T - 2.0 K. A further i m p r o v e m e n t could be obtained in alloys prepared close to composition Y36Co28. The best Tc so far observed was 2.8K. It is interesting to note that annealing such material at a higher t e m p e r a t u r e (700 C) or crushing it to a coarse powder lowered Tc to about 2.2 K (fig. 2). This suggests strongly that disordering of some Co sites was produced by such treatment. Intentional additions of gadolinium also produced a rapid depression of Tc ( - 4 m K per ppm Gd) and it seems clear that the superconductivity is in a band of d-electrons mainly associated with the Y sites and more strongly suppressed by m o m e n t s on those sites than by the magnetic character of some of the Co atoms. Substitution of Fe for Co has a much weaker effect on Tc; an alloy with 200 p p m Fe has T~ = 2.15 K. Studies of the effect on X.... of superimposed fields below Tc were made on specimens cooled to the particular t e m p e r a t u r e in zero field (fig. 3). Just below Tc the main part of the diamagnetic response is rapidly suppressed (inset B, fig. 3) but at lower temperatures fields of some kilo-oersteds are required, and Xa.c.(H) curves are not reversible. The d.c. magnetization curves (fig. 2 inset) are those of a strongly irreversible type II superconductor whose paramagnetic response has strong non-linearity in M(H). The field at which significant flux penetration begins agrees with that indicated by Xa.c.(H) and is smaller (at a given temperature) for poorer quality specimens.

4. Magneto-resistance Data for the magnetoresistance up to 60 k O e (fig. 4) throw light on both the superconductive

2043

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Fig. 4(a). Magneto-resistivity, Ap/p, of Y36Co27.9 at different temperatures in fields applied parallel to the measuring current. Curves below 3.3 K show an initial rise due to the suppression of the effects of superconducting filaments. (b). Variation of resistance with field in the superconducting region. The arrows give a rough estimate of upper critical field values. The inset shows the different effects in magnetic and superconducting regions of fields parallel and perpendicular to the measuring current. This inset also shows the magnetoresistivity (dotted line) in the structurally related nonmagnetic Y8Co5 alloy.

and the magnetic character. At low temperatures fields of more than 10 k O e are required to destroy superconductivity completely, and in low fields some superconductive filaments persist above 3 K and may be responsible for the small thermal hysteresis in X.... visible in fig. 1. The normal state magneto-resistance has two

2044

B. V.B. Sarkissian et al. / Magnetic and superconducting behaviour of Y~.('o>

components: a negative one associated with the alignment of moment-bearing Co atoms, and an anomalously large positive term visible in high fields. The small value of the normal (Kohler) magneto-resistance in the structurally related non-magnetic YsCos alloy suggests that this large term may arise from field-induced changes in the band structure associated with nearly magnetic Co atoms. These could be examined by studies of high-field modification of the normal state specific heat.

5. Discussion We believe that the unusual magnetic properties and the contrast with the absence of magnetism in neighbouring phases arise from the occupation by some of the c-axis Co atoms of sites where they are not forced into close proximity (2.44,~ for YCo> 2 . 3 7 A for Y~Co: and 2.35,~ for the 24 non-magnetic Co atoms in Y3aCo2~) with one another by enclosing polyhedra of Y atoms. (It must be emphasized that electron transfer from electropositive Y as well as small C o - C o distances prevents magnetism in YCo2, Y~Co> etc. XCo2 phases with Y, Ti, Zr, U, etc. have thereby lost the on-site Hund's rule coupling that is still present in the XFe: phases.) The Y>Coes phase only exists when there are four c-axis Co atoms for every three cells of the structure proposed by Lemaire et al. [41. Complete c-axis randomness is argued against by the limited composition range and the fairly good residual resistivity. Fig. l(ii) shows the only possible arrangement of these four atoms at equal spacings; none of them is then able to occupy the centres of the large X interstices; fig. l(iii) shows a possible arrangement that permits half of them to occupy such sites with the consequence that one of them is rather well isolated and likely to be magnetic; the intervening c-axis atoms would (as in YCo2) be nearly magnetic and provide a strongly one-dimensional coupling, the charac-

teristics of which would be a strong development of ferromagnetic correlations without a true divergence in the susceptibility and suppression of the magnetic response in large fields. A onedimensional character for the band structure of some Co atoms would provide some support for the suggested origin of the high field magnetoresistance. Faulting in the c-axis sequence could easily provide either spin glass character or pinning of induced three-dimensional magnetization. The origin of the superconductivity is a much more speculative matter, and a much nlore remarkable one in the presence of strong magnetic correlations which must surely be in a band rather decoupled from that in which the superconductive gap forms, Special phonon modes might be associated with the peculiar c-axis structure proposed. Specific heat measurements (Luzuriaga and Park; Cheng, Creuzet, Garoche, Campbell and Gratz, private communications) show characteristic superconductive behavior and are in accord with the suggestion of strong but not long-range magnetic correlations above T~. These are being studied by neutron scattering.

Acknowledgement We are greatly indebted to Professor K. (;schncidner for the provision of high purity yttrium.

References [11 F~. (;ratz, H.R. Kirchnlavcr. V S e c h o v s k y and E.P W o h l f a r l h , J. Magn. Magn. Mater. 2l (loiS(i) Dll. 121 A. K o l o d z i c i c / v k . ILV FI. Sarkissian and B.R. ( ' , H c s I Phys. F. ll) (19NO) 1.333 J3l F.I[. G e b a l l c . ILl'. Matthia~,. \ B ('ompt~m and 1 ( ' o r c n z w i t . Phys. [),c~. 1~7 ( I~J~'U,) .,\ 11 c) [41 R. l . c m a i r c . J Sch~