Magnetism in U2T2X compounds

Journal of Magnetism and Magnetic Materials 140-144 (1995) 1367-1368

~ i ~ journal of " . " " magnetism and magnetic materials

ELSEVIER

Magnetism in U2T2Xcompounds L. Havela a,*, V. Sechovsk2~ a p. Svoboda a H. Nakotte b K. Proke~ a,b, F.R. de Boer b A. Seret c J.M. Winand c, j. Rebizant c, J.C. Spirlet c A. Purwanto d, R.A. Robinson d a Charles Unit,ersity, Ke Karlovu 5, 121 16 Prague 2, Czech Republic b Van der Waals-Zeeman Laboratory, Uni~,ersity of Amsterdam, Valckenierstraat 65, 1018 XEAmsterdam, The Netherlands c European Commission, Joint Research Centre, Institute for Transuranium Elements, Postfach 2340, 76125 Karlsruhe, Germany d LANL, Los Alamos, NM 87545, USA

Abstract Magnetic and other electronic properties are presented for the U2T2X compounds (T = late transition metal, X = Sn or In), that crystallize in the tetragonal U3Si2-type structure. They show the formation of 5f magnetic moments and antiferromagnetic ordering in compounds where weak 5 f - d hybridization is expected. Near the onset of magnetic ordering strongly enhanced y-values are found. Neutron-diffraction experiments on U2Pd2Sn and U2Pd2In point to a connection between the mutual coordination of U atoms and the type of magnetic anisotropy.

We have studied systematically the magnetic and other electronic properties of U2T2X compounds (T = transition metal, X = Sn or In), which crystallize in the tetragonal U3Si 2 structure [1,2]. Similar to other actinide compounds with transition metals, the character of the 5f electron states is strongly dependent on the strength of the 5 - d hybridization. Here, we review basic magnetic data obtained on polycrystalline samples and the results of lowtemperature specific-heat measurements. The magnetic structure of U2Pd2In and U2Pd2Sn was investigated by powder neutron diffraction. Antiferromagnetic (AF) ordering is indicated by the peak in the temperature dependence of the magnetic susceptibility x ( T ) and the specific heat C(T) in U2T2Sn for T = Ni, Pd, Pt, Rh. The ordering temperatures are given in the Table 1. For In compounds, the range of stability of the AF ordering is restricted to U2Ni2In and U2Pd2In. In the paramagnetic range, x ( T ) can be well accounted by the Modified Curie-Weiss law (MCW):x = C / ( T - ~ g p ) + Xo, where Xo represents a temperature-independent susceptibility. Values of the paramagnetic Curie temperature ~gp and the effective moments ]'£eff obtained from C are also summarized in Table 1. Nevertheless, we are aware that

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/Xeff-values can be severely affected by magnetic anisotropy in the paramagnetic range, which leads to values generally smaller than those obtained on a single crystal. The MCW type of behaviour was detected also for U2Pt2In and U2Co2Sn, which do not show magnetic ordering down to 1.6 K, but for which X increases to rather large values, and the magnetic behaviour can be labelled as spin fluctuator. U2Pt2In also displays a very strong y-enhancement. U2Rh2In was not yet subjected to susceptibility measurement, but the absence of any anomaly in C(T) and a certain y-enhancement found also in this case point to the presence of spin fluctuations, too. The MCW behaviour at high temperatures can be followed also in U2Ir2Sn, but the presence of a ferromagnetic impurity (UIr) does not allow to assess the low-temperature behaviour. The remaining compounds behave as more or less enhanced temperatureindependent Pauli paramagnets. Comparison of the values of the electronic specific-heat coefficient y (Table 1) shows a strong enhancement tendency from the Pauli paramagnets U2Ru2Sn and U2Co2In towards spin fluctuators (largest y = 850 m J / m o l K 2 in U2PtzIn - note that 1 mole contains 2 U atoms in this case), but high y-values are found also in antiferromagnets. Some of them display very small magnetic entropy S (in U2RhzSn and UzPtzSn S does not exceed 0.4R In 2). Neutron powder diffraction experiments at LANSCE, Los Alamos, have indicated that in UzPd21n and U2Pd2Sn the same non-collinear ordering exists with U moments

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L. Havela et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1367-1368

1368

30 ,

lying within the basal plane along the directions of the [1 1 0] type [3]. T h e m a g n e t i c and crystallographic unit cells are identical. In U 2 P d 2 S n , w h i c h is p r e s u m a b l y the

20

•

(a)

U2Co21n WP

U2Ni2In AF

7,4 K = 6

M C W : [~ = 2 0 gB

~ m3/mol

Op = -81 K 7,0 = 2 5 1 0 ~ m3/mol

y = 32 mJ/molK2

!U2Ru21n

"f= 200 mJ/mol K2 (350 mJ/mol K2 aboveTN)

U2Pd21n AF

U2Rh2In SF

TN=37K M C W : fl= 2 1 NB Op - -32 K Xo 1 9 1 0 4 m3/mol

y = 280 mJ/mol K2

7 = 393 mJ/mol K2

U2Ir21n WP?

U2Pt2ln

SF

M C W : ~t 2 4 ~tB Op = -106 K l O - 1 0 10"s m3/mol

y = 850 mJ/mol K2

(b)

U2Fe2Sn WP

U2Co2Sn SF M C W : ~t = 1.5 IzB Op = -51 K Xo = 2 3 10"s m3/mol

U2Ru2Sn WP

TN=25 K I

M C W : ~t = 2.30 laB ®p=-ll0K 7,0 = 1 8 1 0 qs m3/mol

y = 250 mJ/tool K2

7 = 172 mJ/mol K2

U2Rh2Sn A F

U2Pd2Sn A F

TN=Z4K M C W : ~ - 2 4 1 IIB @p - - 106 K X0 10.10"8 m3/mol

7 = 20 mJ/mol K2

U2Ni2Sn AF

TN=4I K M C W : [a = 2 5 0 ~B Op = -30 K XO = 1 4 10-8 m3/mol

y = 131 mJ/mol K2

, = 203 mJ/mol K2

U21r2Sn SF

U2Pt2Sn AF

M C W : ~t = 1.73 I~B Op = - 1 0 0 K ~0 = 1 6 1 0 ~ m3/mol

y = 130 mJ/molK2

TN= 15.5 K M C W : ~t = 2.03 ~tB ®p = -34 K 7.0 = 2 5.10"8 m3/mol

, = 334 mJ/mol K2 390 mJ/mol K2 above TN)

%

.

.

.

.

a)

°%°

~., %°

°*%...,.

10

2 ~<

-'<%~Co;?r ...........................~'~'k)] 50

100 150 200 250 300 350

UzPtzSn

30

b)

U2CozSn UzPdzSn

~2 20 e~

lo

TN= I4K 10

.

,, U2PtzIn

I,) ~°

Table 1 Summary of bulk properties of U2T2In (a) and U2T2Sn (b) compounds. The type of ground state is displayed in the upper right corner of each rectangle (WP - weak paramagnet, SF - spin fluctuator, AF - antiferromagnet). The MCW parameters account for the high-temperature x(T) behaviour (~eff related to 1 Uatom). In the case of WP, the x-value at 4.2 K is given instead. In the lower part, the low-temperature y-coefficient of the specific heat is given (the values are normalized to 1 f.u., 2 U-atoms). For some of the compounds, magnetic properties have not been studied yet (U 2 Ru2 In). For U 2 Ir 2 In the susceptibility data are contaminated by a considerable amount of ferromagnetic binary UIr

:.

2

UzIrzSn

><

o

0

50

100

T (K) 150

200

250

300

Fig. 1. Temperature dependence of the magnetic susceptibility of selected U2TeIn (a) and U2T2Sn (b) compounds. The dotted lines shown in some cases represent the MCW fits with parameters shown in Table 1.

c o m p o u n d with the m o s t apparent l o c a l - m o m e n t b e h a v i o u r in the 2 : 2 : 1 family, m o m e n t s o f 1.89/x B on U a t o m s (at 10 K) have been obtained by the r e f i n e m e n t procedure. In U 2 P d 2 I n the m o m e n t s are reduced to 1.40/x a. T h e stronger tendency to m a g n e t i s m in Sn c o m p o u n d s s e e m s to be a general feature c o m p a r e d to the U2T2In group. A s the U m o m e n t s are perpendicular to the nearest U - U link (which is along c), we m a y speculate about the hybridization-induced anisotropy m e c h a n i s m in the U2T2X c o m p o u n d s . Acknowledgements: This w o r k w a s supported by the ' S t i c h t i n g voor F u n d a m e n t a l O n d e r z o e k der Materie' (FOM), by the U S - C z e c h o s l o v a k Science and T e c h n o l o g y Joint F u n d u n d e r project N u m b e r 93039, and by the Grant A g e n c y o f C z e c h Republic (grant no. 2 0 2 / 9 3 / 0 1 8 4 ) . Support to A.S. and J.M.W. g i v e n in the f r a m e w o r k of the E.C. f u n d e d training p r o g r a m H u m a n Capital and Mobility is a c k n o w l e d g e d .

References [1] M.N. Peron et al., J. Alloys Comp. 201 (1993) 203. [2] F. Mirambet, P. Gravereau, B. Chevalier, L. Trut and J. Etourneau, J. Alloys Comp. 191 (1993) L1. [3] A. Purwanto et al., Phys. Rev. B 50 (1994) 6792.