Magnetic and valence transitions in CePd2−xMnxSi2: LIII edge study

PHYSICAi ELSEVIER

Physica B 199&200 (1994) 512-513

Magnetic and valence transitions in CePd2_ MnxSi2 : L m edge study C. Godart, a'~'* H. Flandorfer b, P. Rogi b a CNRS, UPR 209, PI. A. Briand, 92195 Meudon, France b b~stitutfiir Physikalische Chemic, Wien-A 1090, Austria CNRS, LURF-Universitb de Paris Sud, 91405 Orsay, France

Abstract We report on solid solution CePd2_ ~Mn~Si2 which shows an interesting transition of the Ce-ion from magnetism in a Kondo regime to an intermediate valence state coupled with a strongly magnetic 3d sublattice.

i. Introduction CePd2--xMn.,Si2 [1-1 is a concentrated Ce sublattice transforming from Kondo compensated 4f magnetism towards an heavy-fermion state and to 3d magnetism. Due to the lack of information on the Ce valence state in this system, it is difficult to distinguish among the dominant mechanisms driving such a transition. That persuaded us to deduce the valence state of Ce-ion from Lm absorption edge measurements.

2. Experimental CePdz_,Mn.~Siz crystallizes in the tetragonal structure of ThCr2Si2 (I4/mmm) for the full range of x, except for some faint lines in the range x = (1.0-1.5) which correspord to lower symmetry (P space group). From t ~e linear variation of the a parameter versus x (see Fig 1), which corresponds to the shortest Ce Ce distance in the cell, we would assume that the valence *Corresponding author.

v linearly increases from 3 + in CePd2Si2 t o --, 4 + in CeMn2Si2. However, neither the c parameter f~oi the unit cell volume (generally assumed to be representative of the valence changes) linearly vary with x; and they decrease as unexpectedly from the valence point of view. Moreover, if the Ce valence continuously increases from 3 + to ,-- 4 + , the magnetic moment carried by th~ Ce-ion should decrease and the 4f magnetism should decrease. In fact, as reported in Ref. [1], the total moment (carried by Ce and Mnl which is the measurable quantity roughly continuously increases in a nonlinear fashion. In parallel, an initial increase of the N6el temperature TN is observed (Fig. 2). Lm absorption edge measurements have been perfo~'med on CePdz_,Mn,Si2 (x = 0.2, 0.75, 1.00, 1.25, 1.40, !.60, !.80 and 2.00! at 300 and !0 K. Figure 3 shows that the Ce valence does not change up to x-,-0.5, then slightly increases up ,to x---!.25 and strongly for x > !.25. For r = 2.0, the valce obtained is in agreement with the saturated value generally observed by Lm absorption technique in the so-called "tetravalent" Ce compounds. These results agree with those obtained from the examination of the XANES part of the spectra

0921-4526..94/$07.00 (3 1994 Elsevier Science B.V. All rights reservcd SSDI 0921-4526(93)E0244-B

C. Godort et aL / Ph.vsica B 199&200 (1994) 512-513

Co Pd2-X Mnx Si2

4.5-

4.5T.......

3.s 4 3. ~' 4

513

Ce Pd2-x Mnx Si21

/,

T3.3

-g"

/

~"~ "~170 .

~ 4,1

2

1

~

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0 ~"

unit

[ I

c e l l volume

ICe Pd2-x Mnx St2 •D

t

Ill

~0 zso

s

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in

m 4 3~ A I

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3 A

20 x [Mal [ = t

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=~ VOI .... Vol(lit.)

and

3.1 0.5 t

.....,, 2 , o

4 3.5

j

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x [Unl & C , c(lit.)

Fig. 1. Lattice parameter CePd2 - xMn,Si2.

4.S

'~

~

:o'%, m a a(lit.)

'.

]

Fig. 2. Total paramagnetic moment and N6el temperature in CePd2 - ~MnxSi2. ( ~ 20 to ,-- 60 eV after the edge) showing for x > !, the appearance of a superstructure which is known to be due to a replicate of the second b u m p in the edge of intermediate valence compounds. Measurements at 10 K confirm this general behaviour. Interestingly, they also show that the difference between the valence at 300 and 10 K increases in the range x = 1.25 to 1.80; a behaviour not often observed in Ce-based compounds. Another important point is that the appearance of heavy-fermi,3n characteristics in the specific heat measurements, reported in Ref. [1] for x _> 1 also corresponds to the c o n c e o " a range in which the valence departs from 3 + .

Mu tot.

~

V..,3OOK

"

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l I

Fig. 3. Values of the valence at 300 and 10 K and total magnetic moment in CePdz-~MnxSiz (0.00 < x < 2.00).

3. Discussion For the low M n concentration, as the Ce valence does not change, we expect the total magnetic moment to be due to the sum of a constant value of 2.5pa for Ce 3 + and a contribution which linearly increases with the concentration of Mn. This is what is observed in Fig. 3 when x < 0.5. At Mn concentrations in the range 0.5 < x _< 1.5, as the valence of Ce increases, the magneuc moment of Ce decreases from its constant value of 2.5/~B for Ce3+; consequently, the total moment departs from the line labelled Mu(Ce + Mn) in the figure. This deviation increases with increasing x. Assuming a linear change of the Mn moment with the Mn concentration, we can calculate the m o m e n t of Ce versus x. This moment decreases rapidly to less than 0.5/zn when the measured valence of Ce reaches -,- 3.04. For concentrations higher than x ~ 1.5, the contribution of Ce to the magnetic moment is negligible and the curve for the total moment reaches the curve for the Mn moment, labelled Mu(Mn) in Fig. 3.

References [1] B. Rupp, P. Rogl, N. Pillmayr, G. Hilscher, G. Schaudy and 1. Felner, Phys. Rev. B 41 119901 9315.