MAGNETIC PROPERTIES OF NpAs2

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MAGNETIC PROPERTIES OF NpAs2

M. Bogé, J. Chappert, A. Blaise, J. Fournier, D. Damien, J. Rebizant, J.

Spirlet

To cite this version:

M. Bogé, J. Chappert, A. Blaise, J. Fournier, D. Damien, et al.. MAGNETIC PROPERTIES OF NpAs2. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-167-C1-168. �10.1051/jphyscol:1980145�.

�jpa-00219714�

JOURNAL DE PHYSIQUE Colloque C1, supplkment au no 1, Tome 41, janvier 1980, page C1-167

MAGNETIC PROPERTIES OF NpAs2

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M. Boge, J. Chappert, A. Blaise

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38041 GrenobZe, France.

x Ddpartement de &nie Radioactif, C.E.N./F.A.R.

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The magnetic p r o p e r t i e s o f some Neptunium p n i c t i d e s have a1 ready been i n v e s t i g a t e d , m a i n l y i n t h e s e r i e s o f c u b i c compounds NpM (M = N,P,As, Sb) [I]. Seve'ral kinds o f magnetic behaviour are observed. Complementary i n f o r m a t i o n s a r e obtained from r e s u l t s o f e l e c t r i c a l r e s i s t i v i t y , neutron d i f f r a c t i o n , s u s c e p t i b i l i t y and Miissbauer e x p e r i - ments. A few years ago, J.M. F o u r n i e r and a l . 121 examined t h e magnetic p r o p e r t i e s o f NpAs2 by sus- c e p t i b i l i t y measurements from 4,2 K t o 316 K. T h i s

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compound has a t e t r a g o n a l s t r u c t u r e ( a = 3.96 A,

0

c = 8.11 A). These authors showed t h a t below 22 K, NpAs2 i s ferromagnetic and t h a t between 22 K and 180 K t h e s u s c e p t i b i l i t y curve l / $ ( T ) shows two d i s t i n c t minima a t 54 K and 136 K, when t h e tempe- r a t u r e i s i n c r e a s i n g ( F i g u r e 1). Above 180 K t h e compound i s paramagnetic. The valence s t a t e o f t h e Neptuni um was thought 4+.

We used t h e Mdssbauer technique on 2 3 7 ~ p i n order t o i n v e s t i g a t e these magnetic p r o p e r t i e s , and t o measure d i r e c t l y t h e valence s t a t e by means o f the isomer s h i f t . Hyperfine s p e c t r a were o b t a i - ned u s i n g t h e 59,6 keV MSssbauer resonance o f 2 3 7 ~ p w i t h a source o f m e t a l l i c 241~m having an a c t i v i t y o f 30 mCi. T h i s source i s prepared i n C.E.N./Fontenay aux Roses. The absorbers (200 mg o f Np) were prepared i n Euratom Karlsruhe. These samples (source and absorbers) are encapsulated i n two soldered aluminium containers. On t h e g a m a rays p a t h each, w a l l i s 0.5 mm t h i c k . Ue have used NpAlz (which has an extended magnetic h y p e r f in e p a t t e r n ) t o ' c a l i b r a t e our spectrometer.

F i g u r e 1 : i n v e r s e o f t h e s u s c e p t i b i l i t y o f NpAsz as a f u n c t i o n o f temperature F u l l l i n e : as temperature i s increased Dashed l i n e : as temperature i s

decreased.

A t 77 K ( F i g u r e 2) t h e NpAs2 spectrum c o n s i s t s o f a s i n g l e l i n e

( d a t h r &BeC,a

T h i s isomer s h i f t value confirms t h e 4 t valence s t a t e o f Np. Thus, t h e e l e c t r o n i c c o n f i g u r a t i o n o f t h e Np i o n i s 5f3. A t 4.2 K ( F i g u r e 2) we ob- t a i n a m a g n e t i c a l l y s p l i t h y p e r f i n e spectrum. The a n a l y s i s gives a magnetic f i e l d Hhf of about

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980145

C1-168 JOURNAL DE PHYSIQUE

2900 kOe and a quadrupole i n t e r a c t i o n e 2 q ~ o f about 24 mm/sec w i t h an asymmetry parameter o f 0.12.

We are p r e s e n t l y r e c o r d i n g MGssbauer spectra a t i n t e r m e d i a t e temperatures between 4.2 K and 77 t(- i n o r d e r t o i n v e s t i g a t e t h e ;-egion around 22 K. Our p r e l i m i n a r y d a t a shows t h a t t h e spectruri does n o t change s i g n i f i c a n t l y between 4.2 K and 20 K. A t 25 K t h e spectrum i s c l e a r l y m o d i f i e d w i t h a s t r o n g unresolved background c o n t r i b u t i o n . We t h i n k t h a t t h i s i s a good i n d i c a t i o n

f o r t h e appearance o f the t r a n s i t i o n towards a new s p i n arrangement. Neutron d i f f r a c t i o n s t u d i e s 131 show t h a t NpAs2 keep i t s t e t r a g o n a l s t r u c t u r e down t o 4.2 K.

I t i s g e n e r a l l y assumed t h a t t h e magnetic h y p e r f i n e f i e l d a t t h e 2 3 7 ~ p nucleus i s propor- t i o n a l t o t h e e l e c t r o n i c moment c a r r i e d by t h e Np4' ion. Dunlap and Lander 141 have e s t a b l i s h e d t h e r e l a t i o n s h i p :

!-I = l(0.522 vB)/MOel Hhf

i n a number o f Neptunium i n t e r m e t a l l i c . Using t h i s r e l a t i o n s h i p we f i n d an e l e c t r o n i c moment o f 1.5 pg f o r t h e N ~ i o n . ~ +

Discussion

I t i s i n t e r e s t i n g t o compare t h i s value w i t h the s a t u r a t i o n magnetization o f 0.56 pB 121. The f a c t o r t h r e e c l e a r l y i n d i c a t e s t h a t below 22 K NpAs, i s n o t ferromagnetic b u t most probably f e r r i m a g n e t i c ; t h i s a l s o e x p l a i n s why no magnetic s c a t t e r i n g c o u l d be detected by neutron d i f f r a c - t i o n C3J (a ferromagnetic moment o f 1.5 uB would have e a s i l y been measured even w i t h t h e small sample used).

Between 22 K and 54 K Massbauer s p e c t r a t o - g e t h e r w i t h magnetic measurements show t h a t a complex a n t i ferromagnetic o r d e r i n g e x i s t s .

Thus,NpAs2 r e v e a l s i t s e l f as a very i n t e r e s - t i n g magnetic compound. F u r t h e r measurements are underway ( e l e c t r i c a l r e s i s t i v i t y , s p e c i f i c h e a t ) . A Miossbauer study under magnetic f i e l d i n t h e f e r r i m a g n e t i c s t a t e would be v e r y u s e f u l .

F i g u r e 2 : Mossbauer s p e c t r a o f NpAs2 between 4.2 K and 77 K.

References

I11 A.T. ALDRED and a l . ,

Ph. Review B,

3 ,

[21 J.M. FOURNIER, A. BLAISE, P. SALMON,

I n t e r n . Conf. of Magnetism, Moscow, v o l . I V , p. 65 (1973).

C31 A. BLAISE, J.M. FOURNIER, P. SALMON, unpublished.

141 B.5. DUNLAP and G.H. LANDER, Phys. Rev. L e t t .

33,