Magnetic properties of some cerium-based alloys

Journal of Alloys and Compounds, 210 (1994) 339-342 JALCOM 1178

339

Magnetic properties of some cerium-based alloys S.K. D h a r , R . B a l a s u b r a m a n i u m ,

S.M. P a t t a l w a r a n d R . V i j a y a r a g h a v a n

Solid State Physics, Tam Institute of Fundamental Research, Homi Bhabha Road, Bombay 400005 (India)

(Received February 21, 1994)

Abstract We report the results for d.c. electrical resistivity (1.5-300 K), low temperature heat capacity (2-25 K) and magnetic susceptibility (5-300 K) of CeSil.sCuo.2, CeSil.sNio.2, CeSiL6Coo.4, CeSil.sNio.5 and CemGe~.4Nio.6. The first two alloys have the ct-ThSi2 type structure of the parent CeSi2 while the latter crystallize in the hexagonal AIB2 type. The influence of both the Ruderman-Kittel-Kasuya-Yoshida exchange interaction and the single-ion Kondo exchange interaction are observed in CeSi~.sCuo.2 and CeSi~.sNio.2. CeSil.6Coo.4 is non-magnetic but magnetic ordering takes place in CeSi~.sNio.5 near 3 K. The data indicate that the crystal field splitting in the alloy containing germanium may be relatively smaller.

The structural aspects relating to the replacement of Si in RESi2 (RE = L a - Gd) by 3d atoms Fe, Co and Ni have been reported in the literature [1]. It was found that the alloys RESi2_x(Fe, Co, Ni)x retained the parent ThSi2-type structure for x < 0.4. A structural transformation to the AlB2 hexagonal type was observed for the alloys with x = 0.4. The unit cell volume of the alloys in all three series for x = 0.4 decreases smoothly owing to the lanthanide contraction, indicating the trivalency of the rare earth ions. It was also reported in ref. 1 that for x > 0.4 the X-ray diffraction patterns of the alloys were complex and could not be interpreted as arising from a single-phase material. In a later work, however, Mayer and Felner observed that in PrSi2_xNix and NdSi2_xNi~ the range of AlB2-type phase formation extended up to x=0.75 [2]. The measurement of the magnetization showed that for samples with x = 0.4 both the RE and the 3d atom sublattices are paramagnetic at least down to 4.2 K [3]. Narasimahan and Steinfink found that LaSil.6Nio.4 and LaSi~.6Feo.4 are Pauli paramagnets indicating filled 3d bands [4]. Contardi et al. investigated the REGe2_xNi~ phase diagram and reported that alloys with the nominal composition REGel.4Nio.6 ( R E = L a - G d ) are single phase and have the AIB2-type structure [5]. For the cerium compound, a homogeneous material was obtained at the composition Cel.~Gel.4Nio.6. More recently, it has been reported that the replacement of Si by Cu up to about 8 at.% in CeSi2 causes ferromagnetic ordering at low temperatures [6]. Interestingly, CeSi2 is a well-known valence fluctuating compound and has a non-magnetic ground state.

In the present work we have studied the magnetization, electrical resistivity and heat capacity of CeSil.sCuo.2, CeSil.sNio.2, CeSil.sNio.5, CeSil.6Coo.4 and Cel.lGel.4Nio.6. Keeping in view the change in the magnetic behaviour of cerium ions due to a small replacement of Si by Cu in CeSi2, it was of interest to see the effect due to the. substitution of other 3d metal atoms Fe, Co and Ni. We were also interested to investigate the magnetic behaviour of the alloys crystallizing in the A1B2-type hexagonal structure. As mentioned above, in ref. 3 these alloys were found to be paramagnetic down to 4.2 K. This indicates that, for the alloys containing cerium, either the magnetic ordering temperature is lower than 4.2 K or the alloys are non-magnetic because of the possible valence-Kondo spin fluctuations of the cerium ions. Alloys of composition CeSi2_xMx ( M - F e , Co, Ni) for x=0.2 and 0.4, CeSil.sNio.5, CeSi~.sCuo.2 and CemGel.4Nio. 6 were prepared by melting the constituents taken in the proper ratios in an arc furnace under an atmosphere of argon. The alloy buttons were repeatedly melted to ensure proper homogenization and they were given the same heat treatment as reported in ref. 1, 2 and 5. The X-ray diffraction patterns of the alloys were recorded using Cu Ka radiation. We find that our results are partially at variance with the results reported in refs. 1 and 2. While CeSil.sNio.2 and CeSi~.aCuo.2 are single phase and their X-ray diffraction patterns are similar to that of CeSi2, alloys of the same stoichiometry but containing Fe and Co have a slight impurity phase. At the higher concentrations of 3d metal atoms we could obtain single-phase alloys for

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S.K. Dhar et aL / Magnetic properties of Ce-based alloys

340

only CeSil.6Coo.4, CeSil.sNio.5 and Cel.lGel.4Nio.6 and their X-ray diffraction patterns could be indexed on the basis of a hexagonal cell. In the remaining alloys an extra minor impurity phase was discernible in the diffraction pattern. The impurity phase persisted despite several heat treatments. It was also present in the as cast alloys. The lattice constants of the single-phase alloys are listed in Table 1. They have been obtained from the first two most intense lines of the X-ray diffraction spectrum of each alloy. The magnetic susceptibility as a function of temperature is plotted in Fig. 1. A Curie-Weiss behaviour is observed in all the alloys. For Cel.~Gel.4Nio.6, CeSil.sNio.5 and CeSi~.sCuo.2 the effective paramagnetic moment per cerium ion is close to the free trivalent cerium ion value and the paramagnetic Curie temperature is small. However, for CeSil.6Coo.4 the CurieWeiss fit X = c / T - Op yields/~r = 2.9/XB (Ce ion) - 1 and T A B L E 1. The lattice parameters, effective paramagnetic moment /-¢~fr, paramagnetic Curie temperature 0p and temperature-independent susceptibility Xo of various alloys Alloy

Lattice parameters

CeSiLsCu0. 2 CeSit.sNio. 2 CeSiL6Co0.4 CeSiLsNio.5 CemGeL4Ni0.6

500

i

a

c

(A)

(A)

4.183 4.242 4.055 4.048 4.178

14.011 14.642 4.226 4.291 4.224

'

I

I

'

0 CeSil.5Nio.5

400

V Cel •,Ge, .4Nio .6 + CeSi, °Cur, o ......

550

~

Z50 I200~-

I o/ ..

Ioo II 0

-34

1.5 2.46 2.63

- 26.8 -- 18 - 10.3

'

I

X0 (e.m.u. mo1-1)

'

10-3

I

..::+

5

i i , i,

• • •

•

+

+

o

++ + +o o o ° ~VVV 9vv~ o vv

++ o o

4

, , , i t t i i i,,i, ,t~ f ~ o CeSil8

Cuo2

•

Ni02

f

~vvvv

CeSi18

i~ , ,

++oO ° ,vvvv

++

+++oo ::' vvvv o°

I.

.+o,e

,

,

,

,

=- I;.

ol;" .... 0 1 z a

I.=1~ 0

2.38

ee e .'"

•

.=,"

I

0p (K)

~

.CeSi, 6Co04

-~00 " I

--..

'

450

/%fr (/tB)

0p = - 2 0 0 K. Therefore, for this alloy a temperatureindependent term Xo was added to the fitting function. The results reveal an appreciable temperature-independent contribution to the susceptibility of magnitude 10 -3 e.m.u, mo1-1 with a reduced /z~fr of 1.5ttB. The values of ~ r and Op are listed in Table 1. Isothermal magnetization at 5 K up to a maximum applied field of 5 T was measured for CeSil.sCuo.2 and CeSil.6Coo.4. A ferromagnetic response is observed in the alloy containing copper and the magnetic moment at 5 T is 0.68/~B (Ce ion) -1, which is close to the theoretical value of 0.71/Zn (Ce ion) -~ for a crystal field split doublet ground state of cerium. For CeSiL6Coo.4 the magnetization at 5 K in maximum applied field is an order of magnitude smaller. The field response of the magnetization is paramagnetic but it exhibits a slight curvature. The latter may be due to a small impurity phase which is below the detection limit of the X-ray diffraction. The heat capacity C of the alloys is plotted in Fig. 2 and 3. A magnetic transition is seen in CeSil.sCuo.2 at 8 K. The integrated entropy associated with the magnetic transition is 4 J mo1-1 K -~ which is much less than the value of R In 2 (5.76 J mol-1 K-1) for a doublet ground state. The reduced entropy and the temperature dependence of the resistivity (see below) indicate a substantial Kondo interaction of the cerium ions. Our results are in conformity with those reported by Boni et al. [7]. They find that the transition temperature Tc and the magnetic moment/xs increase rapidly with x in CeSi2_~Cux up to the phase limit of x = 0.28. For CeSil.76Cuo.24 they find Tc = 8 K,/~s = (0.62 + 0.05)/~B and a low temperature 3/value of 400 mJ mol-1 K-1. The reduced value of the entropy could also arise if

.soo

l =. lo

4 e

l

X('l'e=ta)

..50

100

150

200

250

300

T(K)

Fig. 1. Inverse magnetic susceptibility vs. temperature of CeSiLsCuo.2, CeSiL6Co0.4, CeSil.5Nio.5 and CeLiGeL4Nio.6. The isothermal magnetization at 5 K of CeSil.sCuo. 2 and CeSil.6Co0.4 is also shown.

0

, , , I , , ,

0

5

,I,

10

, , , I , , , , I , , ,

15

20

I

25

T (K) Fig. 2. The heat capacity of CeSiLsCuo.2 and CeSil.aNio.2.

S.K. Dhar et aL / Magnetic properties of Ce-based alloys 7

!

I

I

I

|

I

I

I

i

/

/

6

o CeSil. 5 Nio. 5 V Cel'l Gel'4Nio'6

5 ~

Z L)

/

4

~1 =f-I

2 1

0 0

2

4

6

8

I0

12

14

16

18

20

T(K) Fig. 3. T h e h e a t c a p a c i t y o f CeSix.sNio.5 a n d Ce1.1GeL4Nio.6.

the magnetic state of the cerium ion is sensitive to the near-neighbour environment in these pseudobinary alloys such that some cerium ions are non-magnetic as in CeSi2. In the isostructural alloy CeSil.sNio.2 a broad hump in the heat capacity is observed at 6 K with a peak heat capacity value of about 1.3 J mo1-1 K -1. Such a behaviour could arise from either a spin glass ordering or a very inhomogeneous magnetic ordering of cerium ions influenced by the possible near-neighbour environment. A magnetic transition at 3.2 K is observed in the heat capacity data of CeSi~.sNio.5. The anomaly in the heat capacity reaches a peak value of 2.9 J mol-~ K-1. The pronounced tail above Tc indicates appreciable short-range order in the paramagnetic region. At the lowest temperature data point of 2 K, C/T is still large and data at lower temperatures would be required for the estimation of magnetic entropy. In Cel.~Gel.4Nio.6 the heat capacity C increases gradually below 8 K and reaches a value of 2.5 J mol -~ K -1 near 2 K and appears to decrease below 2 K. Data at lower temperatures are required to confirm this. C/T increases from 200 mJ mol -~ K -2 at 10 K to 1.1 J mo1-1 K -2 at 2 K. It may be premature to associate this large increase in C/T with the formation of a heavy fermion state in this alloy. It has been shown in the literature that in some systems such as CeAI6.sCur.5 and CePd3B0.a where cerium ions occupy regular positions in the crystal lattice the disorder at the non-magnetic atom sites gives rise to a spin glass ordering of cerium ions at low temperatures [8]. The low temperature heat capacity of such systems exhibits broad anomalies with peak heights exceeding 1 J mol- ~ K - ~ and C/T also exceeds 1 J tool -1 K -2. However, when the measurements are

341

made down to the 100 mK range it is found that C~ T decreases rapidly by more than an order of magnitude. Extension of heat capacity measurements of Cex.]Ge].aNio.6 below 1 K would be interesting in this regard. It is interesting to note that, although CeSil.sNio.5 and Cel.lGel.4Nio.6 are isotypic, there is an appreciable difference in the heat capacity of these two materials above 10 K. At 18 K, for example, the heat capacity of CeHGel.4Nio.6 exceeds that of CeSil.sNio.5 by about 3.3 J mol-1 K-1. For isotypic compounds the Debye temperatures scale inversely as the square root of their molecular weights. The latter ratio in the present case is 1.17. Therefore, the large difference in heat capacities above 10 K probably does not have a lattice origin. The linear extrapolation of the C/T vs. T z plots from above 14 K in CeSil.sNio.5 and 11 K in Cel.lGel.4Nio.6 gives for T= 0 K C/T values of 17 mJ mol-1 K-Z and 145 mJ mo1-1 K -2 respectively. Since the lattice and other possible contributions to the heat capacity have not been subtracted, these numbers indicate that the temperature-independent electronic contribution to the heat capacity may partially account for the observed difference in the heat capacities of the two materials above 10 K. It is quite plausible that the crystal field splitting in Cel.lGel.4Nio.6 is smaller than that usually encountered in cerium compounds which is of the order of 100 K. A smaller level separation would give rise to an appreciable Schottky contribution to the heat capacity. In this regard there is a distinct change of slope in the X -1 vs. T curve of Cel.lGel.4Nio.6 at 30 K followed by yet another change of slope at 15 K and this could arise from crystal field effects. The electrical resistivity of some of the alloys in the temperature range from 1.5 to 300 K is shown in Fig. 4. Since the alloys are very brittle we could obtain workable pieces for the measurement of resistivity for only some of them. The resistivity of CeSi~.sCuo.z initially decreases with temperature below 300 K but exhibits a Kondo-like upturn at temperatures below 100 K. Below 8 K the resistivity decreases owing to the magnetic ordering of the cerium ions. The magnetic contribution to the electrical resistivity of a Kondo impurity is known to increase logarithmically below a temperature of the order of the characteristic Kondo temperature. The Kondo temperature in cerium-based alloys varies from a few to a few hundred kelvin. In CeSil.sCuo.2 the effects of both the indirect Ruderman-KittelKasuya-Yoshida exchange interaction between the cerium ions which leads to magnetic ordering at 8 K and the single-ion Kondo exchange interaction which gives rise to an upturn in the resistivity and presumably the reduced entropy associated with the transition at 8 K are observed. The electrical resistivity of CeSil.aNio.2 increases with temperature below 300 K down to 8 K

342

S.K~ Dhar et aL / Magnetic properties of Ce-based alloys

'1.21'3~

~

I ~ ' ' ' I ' ~ ' ' I ' ' ' ~ I ' ' ' ~ I ' . ~ '

0.9

tCeSil. 8 Cuo.2

value at 300 K. The resistivity of CeSil.sNio.5 exhibits the normal metallic behaviour and decreases with the decrease in temperature. A sharp drop in the resistivity is seen at the transition to the magnetically ordered state. To conclude, we have investigated the magnetic behaviour of some cerium-based pseudobinary alloys mostly derived from the parent CeSi2 by substituting 3d transition metal atoms for Si. While CeSi2 is paramagnetic, a magnetically ordered ground state, with a possible influence of near-neighbour environment, is favoured in some of the pseudobinary alloys.

0.8

References tlllllll

0.7 0

50

I00

lJllillJt

150

200

ililll

250

300

T{K) Fig. 4. The electrical resistivity vs. temperature of CeSiLaCu0.2, CeSiLsNio.2 and CeSil.sNi0.5.

indicating stronger single-ion Kondo effects compared with CeSil.aCuo.2. Below 8 K the decrease in the resistivity is apparently correlated with the broad hump seen in the heat capacity at around the same temperature. The resistivity at 1.4 K is higher than its

1 I. Mayer and M. Tassa, J. Less-Common Met., 19 (1969) 173. 2 I. Mayer and I. Felner, Z Solid State Chem., 7 (1973) 292. 3 I. Felner and M. Schieber, Solid State Commun., 13 (1973) 457. 4 K.S.V.L Narasimahan and H. Steinfink, Z Solid State Chem., 10 (1974) 137. 5 V. Contardi, R. Ferro, R. Marazza and D. Rossi, J. LessCommon Met., 51 (1977) 277. 6 M. Ishikawa, H.F. Braun and J.L. Jorda, Phys. Rev. B, 27 (1983) 3092. 7 P. Boni, G. Shirane, Y. Nakazawa, M. Ishikawa and S. Tomiyoshi, J. Phys. Soc. Jpn., 56 (1987) 3801. 8 K.A. Gschneidner, Jr., J. Tang, S.K. Dhar and A. Goldman, Physica B, 163 (1990) 507.