Magnetic neutron scattering on intermetallic uranium compounds

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Magnetic neutron scattering on intermetallic uranium compounds

M. Loewenhaupt, S. Horn, F. Steglich, E. Holland-Moritz, G. Lander

To cite this version:

M. Loewenhaupt, S. Horn, F. Steglich, E. Holland-Moritz, G. Lander. Magnetic neutron scattering on intermetallic uranium compounds. Journal de Physique Colloques, 1979, 40 (C4), pp.C4-142-C4-144.

�10.1051/jphyscol:1979446�. �jpa-00218842�

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JOURNAL DE PHYSIQUE Colloque C4, supplément au n" 4, Tome 40, avril 1979, page C4-142

Magnetic neutron scattering on intermetallic uranium compounds (*)

M . L o e w e n h a u p t (f) , S. H o r n (**), F . Steglich (**), E . H o l l a n d - M o r i t z (**) a n d G . H . L a n d e r (f t) (*) Institut fur Festkorperforschung der KFA Jiilich, West Germany

(**) II. Physikalisches Institut der Universitat zu Koln, West Germany (tt) Argonne National Laboratory, U.S.A.

Résumé. — Des mesures de diffusion inélastique de neutrons ont été effectuées de 5 K à 300 K sur UA12 et USn3, ainsi que sur les composés LaAl2, LaSn3 et ThSn3 à titre de référence non magnétique. Les spectres ont été obtenus par la méthode du temps de vol. Les expériences avec des neutrons lents donnent une ligne magnétique large quasi élastique. En utilisant des neutrons thermiques, aucune contribution inélastique à la diffusion magnétique n'a pu être mise en évidence pour UA12 à basse température. Pour ce composé la largeur résiduelle de la raie quasi élastique est r/2{T —»0) = 25 meV. A plus haute température, f / 2 augmente légèrement et reste toujours plus grand que fcB T.-

Abstract. — We report on inelastic neutron scattering experiments performed in the temperature range 5 K to 300 K on UA12, USn3 and the nonmagnetic reference compounds LaAl2, LaSn3 and ThSn3. Spectra were obtained by the time of flight (TOF) method. Experiments with cold neutrons revealed a broad magnetic quasielastic line. Using thermal neutrons, no inelastic contributions to the magnetic scattering could be resolved for UA12 at low temperatures. For this compound the residual width of the quasielastic line is r/2(T —» 0) — 25 meV. At higher temperatures, r/2 slightly increases and remains always larger than kB T.

T h e cubic L a v e s p h a s e UA12 is almost ferroma- gnetic a n d exhibits a n a r r o w 5f b a n d a t t h e F e r m i level. L o w t e m p e r a t u r e anomalies in v a r i o u s physi- cal quantities clearly d e m o n s t r a t e t h e e x i s t e n c e of p a r a m a g n o n s [ 1 , 2]. P r e v i o u s inelastic n e u t r o n scattering m e a s u r e m e n t s on UA12 (performed o n t h e diffuse scattering a p p a r a t u s D 7 a t t h e H F R G r e n o - ble using n e u t r o n s of incident e n e r g y 3.53 m e V ) r e v e a l e d a b r o a d magnetic (quasielastic) line [3]. Its half-width ( H W H M ) r/2 w a s found t o i n c r e a s e c o n s i d e r a b l y with t e m p e r a t u r e , although r/2 w a s g r e a t e r t h a n kB T in t h e w h o l e t e m p e r a t u r e r a n g e u p t o 300 K . E x t r a p o l a t i o n for T —> 0 yielded a residual line w i d t h r/2 = 22 m e V . Since this is of t h e s a m e o r d e r as F / 2 for typical r a r e e a r t h c o m p o u n d s showing t h e i n t e r m e d i a t e v a l e n c e (IV) p h e n o m e n o n (e.g. C e P d3 [4]) a local a p p r o a c h t o t h e n a r r o w b a n d in UA12 w a s e m p h a s i z e d . O n t h e basis of this ap- p r o a c h , r/2 r e s u l t s f r o m interconfigurational mixing b e t w e e n adjacent v a l e n c e states of ura- n i u m . T h e n , t h e i n c r e a s e of r/2 for increasing t e m p e r a t u r e is v e r y likely d u e t o inelastic scattering f r o m e x c i t e d crystal field ( C F ) levels having splitting energies > 30 m e V [3]. T h e first clearcut e v i d e n c e of an inelastic C F line in n e u t r o n s p e c t r a of u r a n i u m - c o m p o u n d s w a s r e c e n t l y o b s e r v e d for U P d3 [5].

I n this c o m m u n i c a t i o n t h e e x i s t e n c e of C F excitations in UA12 is c h e c k e d . N e w r e s u l t s a r e p r e s e n t e d , w h i c h w e r e obtained a t t h e diffuse scattering appa-

r a t u s I N 4 at t h e H F R G r e n o b l e w i t h n e u t r o n s of high incident e n e r g y , i.e. 51.4 m e V . T h e s e results will b e c o m p a r e d t o t h e p r e v i o u s o n e s [3] a n d also t o t h o s e for U S n3.

T h e energy r e s o l u t i o n of t h e I N 4 w a s AE/E = 0.05 .

A b s o l u t e intensities w e r e obtained using a v a n a d i u m s t a n d a r d . W e investigated a UA12 p o l y c r y s t a l (120 g) a n d a L a A l2 r e f e r e n c e s y s t e m a t f o u r different t e m p e r a t u r e s ( 5 , 30, 120, a n d 250 K ) . T h e m e a s u r e d T O F - s p e c t r a w e r e c o r r e c t e d for b a c k g r o u n d a n d a b s o r p t i o n effects a n d plotted o n an e n e r g y transfer s c a l e .

Fig. 1. —Energy loss spectra of UA12 and LaAl2 at T = 5K.

Solid line represents fit for UA12, dashed line for LaAl2. Dashed dotted line shows quasielastic magnetic contribution (2 6 : angle between initial and final momentum of neutrons).

(*) Part of this work was supported by SFB 125 Aachen/Julich/Koln.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979446

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MAGNETIC NEUTRON SCA'ITERING ON JNTERMETALLIC URANIUM COMPOUNDS C4-143

Figure 1 shows the energy loss spectra of UAl, and LaAl, at 5 K. In addition to the elastic line (from incoherent nuclear scattering), for both compounds two inelastic phonon lines are resolved. For UAl,, there is considerable intensity (dash dotted) in addition to the phonon lines. The UAl, spectra at higher temperatures look quite similar to that of figure 1 (except for increased phonon intensity).

The additional intensity mentioned above is spread over a wide energy range without significant structure. Its dependence on momentum transfer Q is consistent with a 5f form factor ; showing this scattering to be of magnetic origin.

In disagreement with the expectations originating from our previous experiments using cold neutrons [ 3 ] , no evidence for inelastic scattering, i.e. CF excitations in UAl,, is found. Therefore, our spectra could be fitted well by (')

~ ' ( 0 , 0 , T ) is the static susceptibility as measured by [ 6 ] . P(&) is a spectral function representative of a quasielastic line, centred at fro = 0 and assumed to be of a Lorentzian shape with half width r / 2 . The other symbols have their usual meanings (cf.

Ref. [3]). For the present fits, the intensity was calculated from X' (which seems to be justified according to [3]), so that r / 2 was the only adjusta- ble parameter.

Figure 2 shows r / 2 vs. T obtained this way for the measurements with cold (D 7) and thermal neutrons (IN 4). The line widths found under different

o polycrystal (INL)

0 polycrystal(

Fig. 2. - Temperature dependence of half-line width of quasi- elastic line in UAl, from energy gain spectra measured at the D 7 spectrometer and from energy loss spectra measured at the IN 4 spectrometer. Intensity from static susceptibility measure- ments.

(') In figures 1 and 3

S(Q,

^o),i.e. S(Q, o) folded with the instrumental resolution function, is plotted.

experimental conditions and with different samples show a quite uniform behaviour. A slight decrease is observed for r / 2 as the temperature decreases, namely from 33 meV at 300 K to 26 meV at 80 K, below which r/2 tends to level off. This residual line width corresponds to 280 K and is close to the Curie-Weiss temperature (310 K) obtained from re- cent static susceptibility measurements [ 2 ] . Our experiments with thermal neutrons confirm the pre- vious results [ 3 ] , namely a quasielastic line width showing considerable temperature dependence, al- though it is greater than k , T. Since we found no indication of a CF transition, we think a band des- cription of the magnetic properties of UAl, to be in favor of a local approach suggested in [ 3 ] . Then the increase in r / 2 with increasing temperature might indicate a contribution by some diffusive modes to quasielastic neutron scattering ; these contributions originating in the paramagnon excitations of UA1,.

We note that there was no evidence of such excitations in the energy loss spectra obtained on the D 7 apparatus at low temperatures ( T

<

80 K). Appa- rently, the observation of paramagnons is inhibited by lifetime broadening when working with cold neutrons.

In figure 3 , the energy gain spectra measured with the D 7 apparatus at 300 K are shown for USn, and the two isostructural compounds ThSn, and LaSn,.

For the latter compounds a distinct structure due to inelastic phonon scattering shows up below 18 meV.

In addition to this phonon scattering, a broad quasielastic line is visible for USn,. We have to admit that at present we cannot separate these two contributions quantitatively. However, it appears that magnetic neutron scattering of USn, is also dominated by a quasielastic line, whose width seems to be even larger than that for UAI,.

Fig. 3. - Energy gain spectra of USn,, LaSn3 and ThSn, at 300 K obtained using the D 7 instrument.

6 5 -

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L

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E

.

$ 3 - n E

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6 3

-

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Acknowledgments.

-

We want to thank W. Just, J. B. Suck and H. Scheuer for their help during the experiments and P. Fulde for a helpful correspon- dence.

o U S ~ , 2 0 = 2 0 ° ThSn,

- 0 LoSn, T=300 K

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M. LOEWENHAUPT, S. HORN, F. STEGLICH, E. HOLLAND-MORITZ AND G. H. LANDER

References

[I] BRODSKY, M. B. and TRAINOR, R. J., Physica 91B (1977) 271. [5] SHAMIR, N., MELAMUD, M., SHAKED, H. and WEGER, M., [2] ARMBRUSTER, H., FRANZ, W., SCHLABITZ, W. and STEGLICH, Physica 94B (1978) 225.

F . , J. Physique Colloq. 40 (1979) C4-150. [6] ARKO, A. J., FRADIN, F. Y. and BRODSKY, M. B . , Phys. Rev.

[3] HORN, S . , LOEWENHAUPT, M., STEGLICH, F. and JUST, W., J. B 8 (1973) 4104.

Magn. Magn. Mat. 9 (1978) 54.

[4] HOLLAND-MORITZ, E., LOEWENHAUPT, M., SCHMATZ, W. and WOHLLEBEN, D. K., Phys. Rev. Lett. 38 (1977) 983.