ULTRALOW INTERACTION TEMPERATURES AND CRYSTAL FIELD EFFECTS FOR Gd IMPURITIES IN YPd3 AND CePd3

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ULTRALOW INTERACTION TEMPERATURES AND CRYSTAL FIELD EFFECTS FOR Gd IMPURITIES IN

YPd3 AND CePd3

A. Mota, R. Hoyt, D. Wohlleben

To cite this version:

A. Mota, R. Hoyt, D. Wohlleben. ULTRALOW INTERACTION TEMPERATURES AND CRYSTAL

FIELD EFFECTS FOR Gd IMPURITIES IN YPd3 AND CePd3. Journal de Physique Colloques,

1978, 39 (C6), pp.C6-884-C6-885. �10.1051/jphyscol:19786394�. �jpa-00217863�

JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-884

ULTRALOW INTERACTION TEMPERATURES AND CRYSTAL FIELD EFFECTS FOR Gd IMPURITIES IN YPd

AND CePd

t

A . C . Mota, R . F . Hoyt and D.K. Wohlleben

J J . Physikalisehes Inst-Ctut, Univere-itat zu Koln, S KSln 41, Germany

Résumé.- La susceptibilité magnétique des impuretés de Gd montre que le comportement des ions est indépendant jusqu'à 10 mK si la concentration est inférieure à 0.15 % dans YPd3 et inférieure à 0,5%

dans CePd . On peut expliquer ces résultats dans un modèle de champ cristallin.

Abstract.- The magnetic susceptibility of Gd impurities shows single ion behaviour down to 10 mK at concentrations below 0.15 a/o in YPd and below 0.5 a/o in CePd . The data, can be fitted with crys-

tal field theory. 3 3

The search for single ion behaviour of magnetic impurities in metals has led to experiments with ex- tremely dilute alloys at temperatures in the milli- kelvin range. Most of this work /l/ has been concer- ned with 3d impurities, and it is now established that impurity-impurity interactions mask single ion effects in this temperature range at concentra- tions as low as a few ppm.

Interactions between 4 f impurities in metals are generally one, or in exceptional cases, more than two orders of magnitude smaller. For instance, from the interaction temperature of GdPd3 (T = 7K) HI one estimates 7 mK per 1000 ppm of Gd in the diamagnetic metal YPd,. Moreover, for the same con- centration of Gd in CePd , this low interaction temperature is expected to be reduced even further because of the disruption of the long range RKKY interactions caused by the valence fluctuations in this host.

We have made measurements of the low field (50 mgauss) susceptibility of such alloys form 6 K down to 10 mK. Figure 1 shows Curie behaviour for three Gd concentrations in YPd from 3 K down to 150 mK. Below that temperature we observe a smooth reduction of the susceptibility from the Curie law, which is, within the scatter, independent of con- centration for the 500 and 200 ppm alloys. The tem- perature of about 25 mK at which the 5000 ppm al- loy deviates from the low concentration data, indi- cates ordering of the expected magnitude.

t Work supported by Deutsche Forschungsgemeinschaft SFB 125

1 1 1 I 1 r

300 -

. YRjjGOOOppmGd) o. / . Y Pd3(50O ppm Gd) ,-* / 100 " « YPd3(20OppmGd) ' * ' /

i- jf -

o

J- JF / Curie Law

3 - ' " ^ /

1 I i 1 1 i 1 i_

0.01 0.03 0.1 0.3 1.0 T[K]

Fig. 1 : Reciprocal low field susceptibility of Gd impurities in YPd

Figure 2 shows similar data for the host CePd,. For the low concentration alloys, deviations of x from the Curie law become noticeable near 60 mK and again are independent of concentration down to 10 mK. From the 5.5 % and the 2 % alloys, one obtains an interaction constant of 2.5 mK per

1000 ppm, that is, a factor of three smaller than in YPd3, and a factor of thousand smaller than for Mn in Cu.

Figure 3 shows the low temperature, low concentration data on a linear temperature scale.

The data can all be fitted with Curie-Weiss laws, with the following Curie-Weiss temperatures :

8 = - 5.7 ± 0.5 mK for 1500 and 4400 ppm of Gd in CePd , 6 = - 7.8 + 0.5 mK for 200 ppm of Gd in CePd3 and 6 = - 2 0 ± 0 . 5 m k for 200 and 500 ppm of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786394

Gd in YPd3. However, EPR experiments on these sys- tems indicate crystal field effects bith b4

20 gauss, b4>>b6, and the

doublet being the ground

7

state for Gd in both YPd3 and CePdg

Stimula- ted by this finding, we have fitted our low field

x data with crystal field theory and found good agreement for the parameters indicated in figure

CePd3(1S00 ppm

Gd) Ce

d Ce Pd3[5.5 Gd)

-

- -

- -

Fig.

Reciprocal low field susceptibility of Gd impurities in CePd3.

In particular , we can fit the data in CePd3 with the same scheme and parameters as found by EPR (b4

+

gauss). However, the same data can also be fitted with b,,

-

In YPd3 on the other hand, the data can only be fitted with negative b,+, namely -

It appears that the crystal field fits are as good as the Curie-Weiss fits. The Stark split- tings obtained by these fits are in the range usu- ally observed for metals by EPR, i.e. a factor of three to ten smaller than in insulators

We wish to point out that crystal field ef- fects of similar size as the one we observe here on 4f7 ions in metals should be expected for the

ions

and ~

e in metals as well. Previous ~ + data on CuMn, =Mn, %&I, etc., as far as they could be analyzed with single ion behaviour, might be interpreted similarly on the basis of crystal field effects, rather than the Kondo effect. Clear- ly EPR data on such systems are desirable.

ACKNOWLEDGEMENT.- We thank B. Kaiser for preparing the alloys and A. Schneider and G. Vanhecke for technical assistance.

References

/ I /

Hirschkoff, E.C., Symko, O.G. and Wheatley, J.C.

J. Low Temp. Physics 5

Doran, J.C. and Symko, O.G., Sol St Comm

and AIP Conf. Proc. 18

Gardner, W.E., Penfold, J., Smith, T.F. and

Harris, I.R., J. Phys. F

133

/ 3 /

Elschner, B. et al, private communication. The

crystal field notation is that used by

A.

Abragam and N. Bleaney, Electron Paramagnetic Resonance of Transition Ions (Clarendon, Oxford,

141

de Klerk, D., Handbuch der Physik, Vbl

Fig. 3

Curie-Weiss behaviour and crystal field

fits for the low field susceptibility of those

alloys of Gd in CePdg and YPd3 whose interaction

temperatures are below