Neutron and magnetization studies of USb0.9Te0.1 : observation of a multiaxial magnetic structure

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Neutron and magnetization studies of USb0.9Te0.1 : observation of a multiaxial magnetic structure

J. Rossat-Mignod, P. Burlet, O. Vogt, G. Lander

To cite this version:

J. Rossat-Mignod, P. Burlet, O. Vogt, G. Lander. Neutron and magnetization studies of USb0.9Te0.1 :

observation of a multiaxial magnetic structure. Journal de Physique Colloques, 1979, 40 (C4), pp.C4-

70-C4-71. �10.1051/jphyscol:1979422�. �jpa-00218818�

JOURNAL DE PHYSIQUE Colloque C4, supplément au n° 4, Tome 40, avril 1979, page C4-70

Neutron and magnetization studies of USb

^0>9

Te

^0>1

: observation of a multiaxial magnetic structure

J. R o s s a t - M i g n o d , P . Burlet, O . V o g t (*) a n d G. H . L a n d e r (**) Centre d'Etudes Nucleaires, 85 X, 38041 Grenoble, France

(*) Laboratorium fur Festkorperphysik, ETH, Zurich, Switzerland (**) C.E.N.G. and Argonne Nat. Lab., Argonne, Illinois, U.S.A.

Résumé. — Des mesures d'aimantation et de diffraction neutronique sur des monocristaux de USbo/Teo.i ont permis de déterminer une structure multiaxe à basse température tandis qu'un état incommensurable est observé à haute température. La transition incommensurable-commensurable est du premier ordre.

Abstract. — Magnetization and neutron diffraction studies of USb09Te0.i single crystal lead to the determina- tion of a multiaxial magnetic structure at low temperature whereas an incommensurate state is observed at high temperature. The incommensurate-commensurate transition is of first order.

U r a n i u m a n t i m o n i d e a n d u r a n i u m telluride h a v e t h e N a C l s t r u c t u r e . U S b is a t y p e I antiferromagnet consisting of f e r r o m a g n e t i c (001) p l a n e s , m a g n e t i z e d along [001] and s t a c k e d in t h e alternative s e q u e n c e

H 1 [1]. I n t h e f e r r o m a g n e t U T e t h e m o m e n t direction is < 1 1 1 } [ 2 ] , P ] . I n t h e solid solutions U S b j _xT ex t h e s y s t e m t e n d s rapidly t o d e v e l o p a f e r r o m a g n e t i c c o m p o n e n t a n d t h e magnetic structu- r e b e c o m e s c o m p l i c a t e d [4].

1. Low temperature multiaxial structure. — Ma- gnetization m e a s u r e m e n t s (Fig. 1) indicate t h a t t h e s a t u r a t i o n m o m e n t is only 0.92 /xB at T = 1.5 K in a ( 1 1 1 ) direction with a strong m a g n e t i c a n i s o t r o p y . A critical field is o b s e r v e d w h i c h c o r r e s p o n d s t o d o - m a i n m o t i o n . It is surprising t h a t it is l o w e r w h e n t h e field is applied along a h a r d axis t h a n along t h e e a s y ( 111 ) axis. A b o v e 160 K t h e m a g n e t i z a t i o n c u r v e s d o n o t exhibit r e m a n e n c e a n d t h e material n o longer h a s a f e r r o m a g n e t i c c o m p o n e n t .

N e u t r o n diffraction e x p e r i m e n t s p e r f o r m e d at t h e Siloe r e a c t o r ( C E N , G r e n o b l e ) indicate t h a t a t

T = 5 K t h e m a g n e t i c ordering is d e s c r i b e d b y

t h r e e w a v e v e c t o r s k, = [ 2 / 3 0 0 ] , k2 = [0 2 / 3 0 ] , k3 = [0 0 2 / 3 ] a n d t h e a s s o c i a t e d F o u r i e r c o m p o n e n t s mk a r e parallel t o k. T h e amplitudes of t h e s e t h r e e w a v e s a r e equal t o At = 2.33 ± 0.08 /nB. A f e r r o m a - gnetic c o m p o n e n t A0 = 0.93 ± 0 . 1 /xB is m e a s u r e d in g o o d a g r e e m e n t with magnetization r e s u l t s .

A m a g n e t i c field of 40 k O e applied along t h e [001]

axis d o e s n o t c h a n g e t h e diffraction p a t t e r n while this field is large e n o u g h t o i n d u c e a single d o m a i n s t a t e as e s t a b l i s h e d b y magnetization m e a s u r e m e n t . T h e n w e m u s t consider t h a t t h e f e r r o m a g n e t i c c o m p o n e n t exists t o g e t h e r with t h e t h r e e F o u r i e r c o m p o n e n t s mfcl, mt 2, mt 3. Magnetic Bragg p e a k intensities a r e compatible with t h e t h r e e m o m e n t c o m p o n e n t s mx, my, mz p r o p a g a t i n g with a + 4- — + + - (or H h + - + , - + + 1- + ) s e q u e n - ce s u c h a s

T h e n t h e m a g n e t i c ordering of U S b0 9T e0 1 at low t e m p e r a t u r e is c o m p o s e d of t h r e e s q u a r e w a v e s propagating t h e t h r e e c o m p o n e n t s mx, my, mz. T h u s e a c h u r a n i u m m o m e n t lies along a ( 1 1 1 ) direction a n d its v a l u e is 2 . 9 ± 0 . 1 /u.B. T h e e x a c t s t r u c - t u r e d e p e n d s in fact of t h e p h a s e s of e a c h w a v e I <f> = n r giving a large n u m b e r of m a g n e t i c d o - m a i n s w h i c h h a v e their magnetization along o n e of t h e eight ( 1 1 1 ) direction.

T h e m a g n e t i c s t r u c t u r e c o r r e s p o n d i n g t o a net m a g n e t i z a t i o n along t h e [111] ( n = 1 for mx, my, ra2) axis is d r a w n in figure 2. T h e m a g n e t i c cell ( 3 a x 3 a x 3 a ) c o n t a i n s 108 u r a n i u m ions w h i c h b e l o n g t o four A-units a n d four B-units (Fig. 2 a )

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979422

NEUTRON AND MAGNETIZATION STUDIES OF USb,,Te,, C4-71 which can be interchanged by a rotation of 120"

around the [I 111 axis. The projection of the magnetic structure in the (001) plane is given in figure 2b ; the moment component along the [001] direction is posi- tive for uranium ions in (001) planes at z = 0, a /2, 3 a /2 and 2 a and negative if z ⁼ a and 5 a /2. The detail of the A and B units is given in figures 2 c and 2d. It can be seen that each unit has a [ I l l ] symme- try axis, so the magnetic structure has a rhombohe- dral symmetry.

(a

(

b)

Y

. z = n A o z = n a X 2

of the diffraction pattern. For a field parallel to [06l]

the wave vector k, = [0 0 k] takes the commensura- te value 2/3 whereas k, and k, remain unchanged. To account for these experimental results we may first assume that in the incommensurate state the three modulations are independent giving rise to three domains. This implies a change of the anisotropy direction from ( 11 1 ) to ( 100 ) which has not been detected by magnetization experiment. If we consi- der the coupling of the three sine waves the magnetic moment of uranium atoms takes any value and direction and this is only consistent with a very low anisotropy. An alternative solution is to consider that, as at low temperatures the magnetic structure is composed by three longitudinal square waves. The incommensurability of the wave vector then would result of stacking fault in the

+ +

^-

+ +

^-

. . .

sequence occurring with a periodicity 1/8. The net magnetization is strickly zero and all uranium mo- ments are aligned along a ( l l 1 ) direction.

At this stage an exact solution of this high tempe- rature phase is not possible.

Fig. 2. - Magnetic structure of the commensurate phase ( k = 2/3) of USba 9TeoI.

2 . High temperature incommensurate state.

-

At a temperature TIC = 160 K a first order transition oc- curs between this commensurate state and an incom- mensurate state characterized by three longitudinal waves with wave vectors k, = [2/3-6 0 01, k, = [0 2 / 3 4 01, k, = [0 0 2/3-61. At this transition a discontinuity of the wave vector and of the superlat- tice peak intensities occur and large hysteresis ef- fects are observed. Above TI, the wave vectors value continuously decrease from 0.642 at TIC to 0.625 at T , = 205 K and no ferromagnetic compo- nent is measured. In contrast with the low tempera- ture behaviour a magnetic field induce some change

Ref el

[I] LANDER, G. H., MUELLER, M. H., SPARPLIN, D. M. and VOGT, O., Phys. Rev. B 14 (1976) 5035-5045. ^. [2] WEDGOOD, F. A. and KUZNIETZ, M. K., J. Phys. C 5 (1972)

3012-3020.

[3] BUSH, G., VOGT, O., DELAPALME, A. and LANDER, G. H., submitted t o J. Phys. C (1978).

[4] ALFRED, A. T. and LAM, D. J., The Actinides : Electronic structure and related Properties, edited by Freeman, A. J., and Darby, J. B. (Academic, New York) 1974, vol. 1, ch. 3.

[5] BAK, P. and MUKAMEL, D., Phys. Rev. B 13 (1976) 5086- 5093.

3. Discussion. - At T , the transition is second order in good agreement with theoretical prediction for a wave vector parallel to a fourfold axis and a longitudinal polarization [5], 161. Between the commensurate phase and the incommensurate phase the transition is first order and this can be accounted only if the commensurate state is described by three coupled waves [7, 8,

91.

This transition is very similar to that observed in TaSe, where a first order transition occurs between an incommensurate char- ge density wave (CDW) and a commensurate triple CDW [lo]. In the case of USbo.,Teo, in the Landau expansion of the free energy the commensurate- incommensurate transition may be driven by a 4th order term m, m i , like in the structural phase transi- tion of K,SeO, [ l l ] whereas the three waves are coupled at low temperature by the phasing 4th order term (m m

t2 ⁺

^m

t2

^m

i3 +

^m

t3

m t,).

The direction of uranium moments, parallel to ( 11 1 ) in USb,,Te,,, is in apparent conflict with the type I structure of USb. This conflict can be solved if we interpret the neutron diffraction results on USb in term of the multiaxial structure proposed by Kouvel and Kasper 1121. Such multiaxial magnetic structure can also be proposed for the others ura- nium monopnictides

.

rences

[6] BRAZOVSKIIS, A., DZYALOSHINSKII, I. E. and KUKHARENKO, B . G . , Sou. Phys. JETP 43 (1976) 1178-1 183.

[7] Mc MILLAN, W. L., Phys Rev. B 14 (1976) 1496-1502.

[8] VILLAIN, J. and MUKAMEL, D., t o b e published.

[9] NAKANISHI, K. and SHIBA, H., J. Phys Soc. Japan 44 (1978) 1465-1473.

[lo] MONCTON, D. E . , AXE, J. D. and DISALVO, F. J., Phys Rev.

B 16 (1977) 801-819.

[ l l ] IIZUMI, M., AXE, J. D., SHIRANE, G. and SHIMAOKA, K., Phys. Rev. B 15 (1977) 4392-441 1.

1121 KOUVEL, J. S. and KASPER, J . S., J. Phys. Chem. Solids 24 (1963) 529-536.