Magnetization and neutron studies on single crystals of ferromagnetic UTe and USb0.8Te0.2

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Magnetization and neutron studies on single crystals of ferromagnetic UTe and USb0.8Te0.2

A. Delapalme, G. Busch, O. Vogt, G. Lander

To cite this version:

A. Delapalme, G. Busch, O. Vogt, G. Lander. Magnetization and neutron studies on single crystals of ferromagnetic UTe and USb0.8Te0.2. Journal de Physique Colloques, 1979, 40 (C4), pp.C4-74-C4-76.

�10.1051/jphyscol:1979424�. �jpa-00218820�

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JOURNAL DE PHYSIQUE Colloque C4, supplément au n° 4, Tome 40, avril 1979, page C4-74

Magnetization and neutron studies on single crystals of ferromagnetic UTe and USb

^0>8

Te

⁰

.

²

A . D e l a p a l m e , G. B u s c h (*), O . V o g t (*) a n d G. H . L a n d e r (**) DRF/DN, C.E.N.-G., 85 X, 38041 Grenoble Cedex, France

(*) Laboratorium fur Festkorperphysik, ETH, Zurich, Switzerland (**) CEN-G and Argonne National Laboratory, Argonne, 111 60439, U.S.A.

Résumé. — Les mesures d'aimantation sur monocristaux montrent que les moments magnétiques sont alignés selon la direction < 111 ), jusqu'à des champs de 90 kOe. Les moments saturés sont 1,91 ± 0,05 fj,B dans UTe et 2,58 ± 0,05 /j,B dans USb0,8Te0,2. Des mesures par neutrons polarisés sur les mêmes monocristaux donnent des valeurs pour le moment 5f ordonné de 2,25 ± 0,05 /u,B et 2,64 ± 0,05 /xB respectivement. Nous attribuons la différence à la polarisation des électrons de conduction 6d-7s.

Abstract. — The magnetization measurements on single crystals demonstrate that the magnetic moments are confined to the (111) direction, even in fields up to 90 kOe ; the saturated magnetic moments are 1.91 ± 0 . 0 5 (XB in UTe and 2.58 ± 0 . 0 5 /xB in USb08Te02. Polarized-neutron experiments on the same single crystals give values for the 5f ordered moments of 2.25 ± 0.05 ju,B and 2.64 ± 0.05 /AB respectively. We attribute the discrepancies between the magnetization and neutron moments to the polarization of the 6d-7s conduction electrons.

1. Introduction. — A n u m b e r of e x p e r i m e n t a l investigations o n poly crystalline u r a n i u m m o n o c h a l - c o g e n i d e s samples h a v e b e e n p e r f o r m e d in t h e past a n d a r e r e v i e w e d b y W e d g w o o d a n d K u z n i e t z [1].

Single-crystal studies on U S h a v e b e e n r e p o r t e d [2, 3]. T w o q u e s t i o n s p o s e d b y t h e s e studies are : a ) h o w t o explain t h e large d i s c r e p a n c y b e t w e e n t h e m a g n e t i c m o m e n t s as d e r i v e d from n e u t r o n a n d magnetization e x p e r i m e n t s a n d b) t o w h a t e x t e n t c a n t h e n e u t r o n magnetic f o r m factor b e u s e d t o derive t h e e l e c t r o n i c g r o u n d s t a t e ? W e r e p o r t h e r e t h e r e s u l t s of m a g n e t i z a t i o n a n d n e u t r o n studies o n U T e a n d on c o m p o u n d s w i t h t h e formula U S b0 8Te0 2

for w h i c h f e r r o m a g n e t i s m is o b s e r v e d .

2. Sample preparation. — Stoichiometric a m o u n t of t h e e l e m e n t s w e r e held at 600 K for 2 w e e k s t o get a p o w d e r . T h e r e a c t i o n p r o d u c t w a s g r o u n d , p r e s s e d a n d e n c a p s u l a t e d in a sealed t u n g s t e n c r u c i b l e . T h e crucible w a s k e p t for a b o u t t h r e e w e e k s at a t e m p e - r a t u r e 50 °C b e l o w t h e melting point with a t e m p e r a - t u r e gradient of 20 °C. T h e resulting recrystalli- z a t i o n p r o c e s s p r o d u c e d high-purity single c r y s t a l s , a n d parallelepipeds consisting of (100) faces ( — 1 x 1 x 1 . 5 m m ) w e r e u s e d in t h e e x p e r i m e n t s ; t h e crystals a r e f o u n d t o b e within 1 % of stoichio- m e t r y , a s suggested b y m i c r o p r o b e t e c h n i q u e .

3. Magnetization. — F i g u r e 1 s h o w s m e a s u r e - m e n t s t a k e n o n t h r e e different U T e single crystals a n d indicates clearly that t h e e a s y axis is t h e ( 1 1 1 ) direction as it is for U S [3]. T h e m o m e n t s in t h e ( 1 1 0 ) a n d ( 1 0 0 ) directions a r e given b y V 2 / 3 <r( n l > a n d v T / 3 c r< l u ) r e s p e c t i v e l y (cosine-

Fig. 1. — Magnetization versus applied field for the three princi- pal directions in UTe at 1.5 K (solid curve is the initial magnetization).

law). T h u s t h e m o m e n t s a r e confined t o t h e n e a r e s t ( 1 1 1 ) axis e v e n u p t o 90 k O e . E s t i m a t e s of t h e a n i s o t r o p y field in U S [3] r a n g e u p t o 1 000 k O e a n d this is a p p a r e n t l y t h e o r d e r of m a g n i t u d e w e h a v e t o e x p e c t in U T e a s well. T h e s a t u r a t e d m o m e n t is 1.91 ± 0 . 0 5 jaB.

Figure 2 s h o w s t h e initial magnetization m e a s u r e d on t h r e e different U S b0 8T e0 2 c r y s t a l s . Again w e get t h e m a x i m u m m o m e n t along t h e ( 1 1 1 ) axis ; t h e m o m e n t s in t h e o t h e r directions follow t h e cosine- l a w . H o w e v e r , t h e fields r e q u i r e d for s a t u r a t i o n a r e practically t h e s a m e for all t h r e e m a i n directions in s h a r p c o n t r a s t t o t h e m o d e l of simple uniaxial aniso- t r o p y .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979424

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MAGNETIZATION AND NEUTRON STUDIES ON SINGLE CRYSTALS C4-75

Fig. 2 . - The same as figure 1 for USbo sTea at 4.2 K .

4 . Structural studies and polarized neutron experiments. - By an independent determination, extinction was found small (7

--

2.5' of arcs) and b ,

--

b,, = 0.15 at 5 K . To determine

b,,

we obtained data on a stoichiometric sample of UO, ; we obtain b , = (0.846 2 0.005) lo-'' cm. On the question of stoichiometry we take the approach that all our experiments must be consistent and that the microprobe analysis and all experimental evidence lead into being within

1

% of stoichiometric. Finally we refine b,, = (0.614 2 0.010) lo-'* cm. For polariz- ed neutron experiments, our preliminary analysis of the magnetic scattering amplitudes (Fig. 3) shows that the moment on the tellurium site is zero, as expected (for a contrasting situation in uranium compounds, see [ 4 ] ) . To determine the saturated magnetic moment we have integrated the magnetization density p (r) over the atomic volume V,,,,. The integration can be performed as a function of radius R and for UTe we find it constant between

1

and 2

A

giving a value of 2.25 2 0.05 pB. The constant value of ,u ( R ) shows that the uranium moment is localized within a range of

1 A,

in good agreement with our expectation of localized 5f electrons.

5 . Summary.

-

We have established that in both UTe and USbo,Teo, the (

1 1 1

⁾is the easy axis and that the anisotropy field is very large. The origin of this anisotropy is difficult to determine. One obvious

s i n O/k

Fig. 3 . -Form factor of UTe ; form factor of USbo.sTeo have equivalent features. The arrow on the ordinate axis is the magneti- zation value.

ingredient is the electrostatic crystal-field at the uranium site, but there was a failure to observe discrete crystal-field levels with neutron spec- troscopy [ 2 ] . The magnetic moments of a number of actinide ferromagnets are given in table I with the difference

Po

- p

t.

Neutron experiments will see the localized magnetic moment only, but the magnetization experiments measure the total moment. A simple explanation is that the conduction electrons are polarized antiparallel to the localized moment. Since the conduction electrons participat- ing in the R.K.K.Y. exchange mechanism should be polarized parallel to the spin component S and for the light actinides J = L - S, this implies that the conduction electron contribution s should be antipa- rallel to g J , the localized moment. The sign of pep =

Fo

- pz is therefore consistent with these arguments. A number of qualitative remarks can be made about the conduction-electron polarization as

Table I. -

Magnetic moments and the conduction electron polarization in actinide compounds

Compound - US [61 UTe USbo.,Teo 2

USb [5]

NpA12 PUP

Magnetization

PO

(PB)

Neutron P (PB)

-

1.70 t 0.03 2.25 2 0.05 2.64 2 0.05 2.82 2 0.05

1 .so

²^0.05

0.77 2 0.07

pep= P o - P o N

(PB) Sample

Single crystal

>>

Powder Powder

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C4-76 A. DELAPALME, G. BUSCH, 0. VOGT AND G. H. LANDER

given in table I. Notice that since s increases from to USb,,Te,, suggests that the additional electron in U, Np, and

Pu

so I p,,

I

should increase ; uranium Te, as compared to Sb, goes into the conduction telluride is something of an exception here. On the band. This would imply that UTe is a 5f3 configura- other hand, the reduction in I peep

I

is going from UTe tion, as postulated for USb.

References

[I] WEDGWOOD, F. A. and KUZNIETZ, M. K., J. Phys. C 5 (1972) [4] DELAPALME, A . , LANDER, G. H. and BROWN, P. J., J. Phys.

3012, and LONGWORTH, G., WEDGWOOD, F. A. and C 11 (1978) 1441.

KUZNIETZ, M., J. Phys. C 6 (1973) 1652. [5] LANDER, G. H., MUELLER, M. H., SPARLIN, D. M. and VOGT, [2] WEDGWOOD, F. A., J. Phys. C 5 (1972) 2427 and J. Phys. C 7 O . , Phys. Rev. 14 (1976) 5035.

(1974) 3203. [6] WEDGWOOD, F. A., J. Phys. C 5 (1972) 2427.

[3] TILLWICK, D. L. and DU PLESSIS, P. de V., J. Magn. Mag.

Mat. 3 (1976) 319 ; ibid. 3 329.