Physica 102B (1980) 192-195 O North-Holland Publishing Company

NEUTRON DIFFRACTION DETERMINATION OF MAGNETIC ORDERING IN THE UI_~Th~As SYSTEM: EVIDENCE FOR MODULATED MAGNETIC PHASES

P. FISCHER, J. S C H E F E R lnstitut fiir Reaktortechnik, Eidg. Technische Hochschule Ziirich, CH-5303 Wiirenlingen, Switzerland

O. V O G T Laboratorium fiir FestkSrperphysik, Eidg. Technische Hochschule, CH-8093 Ziirich, Switzerland

Magnetic structures and ordered magnetic moments of UI ,ThxAs (x-< 0.30) with NaCI structure were determined by means of neutron diffraction on single crystal samples. In addition to configurations I (+ - ) , IA(+ + - - ) with magnetic moments oriented along the (100) directions and ferromagnetic planes perpendicular to these axes, further antifer- romagnetic phases corresponding to longitudinal (100) modulations exist. Their periods depend on composition and vary for x -< 0.20 continuously with temperature.

1. Introduction

Similar to CeBi, former neutron investigations [1-3] of UAs with NaC1 structure established a first-order transition at approximately half of the N6el temperature, TN, from a type-I to a type-IA magnetic phase. At the same time an abrupt change in the ordered magnetic moment occurs. In high magnetic fields a modulated ferrimag- netic configuration is observed [3].

Substitution of U in UAs by non-magnetic Th expands the lattice [4] and reduces exchange interactions. Bulk magnetic measurements per- formed on Ul_xThxAs (x -< 0.10 [5]) show drastic changes of the magnetic phase diagram. Tran- sition to ferromagnetism and valence change from U 3÷ to U 4÷ might be anticipated similar to Ul_xThxSb [6].

2. Neutron diffraction investigation of Ut_xThxAs

In order to determine on a microscopic scale the long-range magnetic order of Ul_xThxAs, single-crystal samples were investigated by means of neutron diffraction in the concentration range x - 0.30 as a function of temperature. The measurements were performed at the reactor Saphir on a two*axis spectrometer equipped with a tilting counter. Neutrons of wavelength A = 1.051/~ and vertical [001] orientation of the

samples were used. Scale and temperature fac- tors were determined from nuclear intensities, assuming nominal compositions and neglecting absorption and extinction corrections (R = 0.03- 0.07). Magnetic Bragg reflections of possible domains were detected by means of linear scans q20, 2q0 and 20q in reciprocal space ( - 0 . 2 - q -< 1.2).

The corresponding results are summarized in table I. Fig. 1 shows illustrative magnetic phase transitions for the case of x = 0.05. The constant intensity of the weak nuclear 111 peak proves antiferromagnetism. The absence of magnetic reflections {q00} suggests longitudinal modula- tions along the {100) directions. As shown in fig. 2, the modulation depends on composition and varies for x - 0 . 2 0 continuously with tem- perature, whereas a constant q value is observed for x = 0.30 at all temperatures below TN. No higher harmonics were detected. From 0 :20 measurements of the magnetic reflections q20, 2q0 and 20q the ordered magnetic moment /x was derived using the neutron magnetic form factor of USb [7]. In the case of q = 1, cor- rections concerning A/2 contamination were made. The dependences on temperature and concentration are shown in fig. 3. The phase transitions are of second order at TN and presumably, except for x = 0.20, of first order at

192

P. Fischer et al./Magnetic ordering in U~ xTh~As 193

0

E 0

E

=

E

0

II

8 - • = I .

H ~

Z =

g~

r~

~-z~.

~_~ ,-~ ~ I I oo

+ + ~ ,

i + + VI

Vl ~e

~ " Vl

Vl o~ Vl

~ ' V l ~ " ~ ' Vl

VI " Vl

~Vl [~

v, v~

<- vl

VJ

Vl

~ Vl v , - 4

~ v ' ~ o -~

o

194 P. Fischer et al./Magnetic ordering in UI_~Th~As

U0.95Th0.05 As

Yn "10-3 TA o n t i f e r r o m g .

o 2,112,0 o o - - - - - - - - - ~

o

"8"7111 _m . ~ ' - " - " " ~ - ~

"6

I T I m o d u l a - I p a r a - t ed mg. Q

!

n 2 1 0 \ b

%

! B - - - I I . . . . . . - | . . . . . . I ._ . I

| •

I

I ,I~ . . . . \ °

A " A ~ 2 q 0

2"

- 4

-2 420A I2 D- "tip

D

'0 ' 6 ' r' i , i I 5 ' n , 1 0 ( K )

Fig. 1. Temperature dependence of integrated intensities of selected Bragg reflections of U0~sTh0.05As (linear scans with AQ = 0.005/~-~ except for 0:20-scan with A20 = 0.13 ° in the case of reflection 111).

q U l - x T h x A S Ul_xThxAS

o.° , : ,x.oo I

0.55

TN 0.10

t

- r N

I - I I | = T 80 100 120 CK)

0 . 2 0

Fig. 2. q values corresponding to modulated phases of U~_xThxAs (reflection 2q0). Tt = temperature of transition to commensurate state.

=k v =k

lower temperatures (small concentration gradients possibly enlarge the transition regions). Fig. 3 suggests a monotonous increase to satura- tion of the ordered magnetic moment with decreasing temperature, in contrast to UAs [1- 3]. This holds for x = 0.05-0.20, if one assumes a sinusoidal modulation at high temperatures. On the other hand, a large Fourier amplitude Aq =

2.7/zB results at x =0.30, which probably is caused by the commensurate magnetic super- structure indicated in table I. It consists of a type-IA configuration modified by paramagnetic layers due to Th. This magnetic structure implies maximum intensity at ql = 8/17 and considerably

. . . . . _

x . . . . . . . . . . . . . . . . . . . .

. T N

CK)

Fig. 3. Temperature dependence of ordered magnetic moment /~ of U in U~-xThxAs (©: x =0.05; I~: x = 0.10; <~: x =0.20; A: x =0.30). The lower section of the figure in- dicates the magnetic phase diagram of this system. M denotes modulated phases.

P. Fischer et al./Magnetic ordering in Ul-xThxAs 195

weaker higher harmonics (largest at q = 10/17 with about 10% of the ql intensity). Seemingly, the peak-to-background ratio of the present neutron study was insufficient to detect such weak higher harmonics.

3. Conclusions

The present neutron diffraction investigation of U~-xThxAs confirms satisfactorily the mag- netic phase diagram determined by bulk mag- netic measurements [5] and establishes the microscopic nature of these phases. Long-range antiferromagnetic order appears to exist for x -< 0.30. With increasing substitution of U by Th the Nrel temperature decreases almost linearly, in- dicating reduced exchange interactions. Up to x =0.30 the saturation moment of uranium remains approximately constant. The lower value of x = 0.10 is presumably caused by extinction associated with the approximately two times larger sample volume, compared to the other specimens. Commensurate phases with q = 0.5 and 1 are of type IA and I, similar to UAs. Compared to x = 0 the structure of type IA becomes more stable than the configuration I at x > 0. Similar to UAs in high magnetic fields [3], modulated antiferromagnetic phases dominate in Ut-xThxAs at zero field for 0 < x -< 0.20 at higher temperatures and exist at x = 0.30 in the whole temperature range below TN. These modulations are characterized by orientation of both the

propagation vector q and magnetic moments t~ along the (100) directions. The periods of modu- lation change primarily with composition (sys- tematic decrease of q with increasing Th content) and depend at x-<0.20 also continuously on temperature. Such longitudinal modulations ap- pear to be characteristic of diluted U monopnic- tides and presumably are caused by anisotropic exchange interactions associated with conduction electrons [8].

Acknowledgements

We thank Prof. W. H~ilg for support of the present investigation and both the workshop division of IRT and K. Mattenberger, ETHZ for aid in the preparation of the experiments.

References

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[2] G.H. Lander, M.H. Mueller and J.F. Reddy, Phys. Rev. B6 (1972) 1880.

[3] G.P. Felcher, G.H. Lander, P. de V. du Plessis and O. Vogt, Solid State Commun. 32 (1979) 1181.

[4] R. Ferro, Atomic Energy Review, Special Issue no. 5 (IAEA, Vienna, 1975) p. 143.

[5] O. Vogt and H. Bartholin, J. Magn. Magn. Mater. 15-18, I (1980) 531.

[6] B.R. Cooper and O. Vogt, J. de Phys. 40 (1979) C4-66. [7] G.H. Lander, M.H. Mueller, D.M. Sparlin and O. Vogt,

Phys. Rev. B14 (1976) 5035. [8] R. Siemann and B.R. Cooper, J. Magn. Magn. Mater.

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