CRYSTAL FIELD AND ELECTRONIC RELAXATION EFFECTS IN Rb2NaYbF6

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CRYSTAL FIELD AND ELECTRONIC RELAXATION EFFECTS IN Rb2NaYbF6

B. Dunlap, G. Davidson, M. Eibschütz, H. Guggenheim, R. Sherwood

To cite this version:

B. Dunlap, G. Davidson, M. Eibschütz, H. Guggenheim, R. Sherwood. CRYSTAL FIELD AND

ELECTRONIC RELAXATION EFFECTS IN Rb2NaYbF6. Journal de Physique Colloques, 1974, 35

(C6), pp.C6-429-C6-431. �10.1051/jphyscol:1974685�. �jpa-00215842�

JOURNAL DE PHYSIQUE Colloque

C6,

35,

1974,

C6-429

CRYSTAL FIELD

AND ELECTRONIC RELAXATION EFFECTS IN Rb2NaYbF6 (*) B. D. DUNLAP and G. R. DAVIDSON

Argonne National Laboratory, Argonne, Illinois 60439, U. S. A.

and

M. EIBSCHUTZ, H. J. GUGGENHEIM and R. C . SHERWOOD Bell Laboratories, Murray Hill, New Jersey 07974, U. S. A.

ResumB. -

Des spectres Mossbauer sur 170Yb et des mesures de susceptibilite magnetiques ont kt6 obtenus pour le composB cubique RbzNaYbFs. DYapr&s les mesures de susceptibilitk magne- tique,

est l'etat fondamental et le niveau rs se trouve

A basse tem- perature les spectres Mossbauer montrent un doublet asymetrique caractbistique de 1'6tat fondamental

A 4,2

on obtient une valeur de la constante hyperfine de l'ion libre pour 17OYb,

x

eV, et un temps de relaxation electronique

x

s.

Entre 4,2 et

zest independant de la temperature. En presence de champs magnktiques externes, les spectres passent de la description de Breit-Rabi celle du champ effectif et la frequence de relaxa- tion Blectronique diminue rapidement.

Abstract.

- Mossbauer spectra of 170Yb and magnetic susceptibility measurements have been obtained for the cubic compound RbzNaYbF6. Susceptibility measurements give a

ground state with rs ^{lying at}

At low temperatures, the Mossbauer spectra show an asymmetric doublet characteristic of the

ground state. At

one obtains a value for the free-ion hyperfine constant of 170Yb,

+

eV and an electronic relaxation time z

x 10-10 s. Between 4.2 and

is temperature independent. In the presence of external magnetic fields, the spectra show

change from Breit-Rabi to the effective field case, coupled with a rapid decrease in the electronic relaxation rate.

Compounds of cubic symmetry are particularly attractive for use in crystal field studies because the low number of unknown parameters greatly simplifies their determination. Recently there has been interest in compounds having the general formula Cs,NaRC16 (R

lanthanide or actinide). These materials are of interest because they form in the

struc- ture with the trivalent R3+ ions being octahedrally coordinated by the six chlorine atoms [I]. In addition, it is known that this coordination remains undistorted even to low temperatures [2,3]. Here we report measu- rements on the compound Rb2NaYbF6, which has the same structure. In all these materials, electronic relaxa- tion times tend to be rather long and magnetic ordering does not occur because the rare-earth ions are well separated by non-magnetic constituents. Since these materials are nonconductors, the crystal field interac- tions are relatively large, often producing a well-isolat- ed crystal field ground state. These materials thus provide the possibility of studying hyperfine interac-

(*)

Work at Argonne National Laboratory performed under

the

auspices of the

Atomic Energy Commission.

tions, crystal field effects and paramagnetic relaxation phenomena in a very simple and well-defined case.

We have obtained hyperfine spectra (using the 84 keV resonance in I7OYb) and magnetic susceptibility measurements for Rb2NaYbF6 as a function of tempe- rature and external field. The effects of cubic crystalline fields on the Yb3+ ion and the commensurate effects on the hyperfine spectra have been discussed several times previously [4]. The eightfold degeneracy of the J

712 ground state is broken into two Kramers doublets ( r 6 and r 7 ) and one quartet (r,). The eigen- functions for the isolated levels are completely known

In octahedral coordination, a point charge calculation suggests that r6 should be the ground state

Magnetic susceptibility measurements for the compound are shown in figure 1. No magnetic ordering is seen at the lowest temperature obtained (1.5 K). The strong deviations from a simple Curie law are charac- teristic crystal field effects for Yb3'. The solid line is a least squares fit to the data, which indicates a r6 ground state with the r, level at 670 If:

K. The T7 level will lie roughly twice as high but cannot be determined with

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974685

C6-430 B. D. DUNLAP, G. R. DAVIDSON, M. EIBSCHUTZ, H. J. GUGGENHEIM AND R. C. SHERWOOD

0 I I I I I

I

0 5 0 100 150 2 0 0 2 5 0 3 0 0 TEMPERATURE ( O K

FIG. 1 . - Inverse magnetic susceptibility versus temperature for RbzNaYbF6.

any accuracy from this data. The Curie law corres- ponding to the isolated T6 level is also shown on the figure, and one sees that this is obeyed at low tempera- tures. It should be noted, however, that deviations from this limiting slope begin at temperatures quite low compared to the T6-T, splitting, due to a rather large van Vleck induced contribution to the susceptibility.

For the Mossbauer experiments at temperatures T d 4.2 K and applied fields H < 35 kOe, we need consider only the T6 ground state. For this case, with a 0 + 2 nuclear transition, the hyperfine spectrum will consist of two lines at positions

and

312

with intensities of 3 and 2, respectively [4]. Here

is the hyperfine constant for the T6 level and is related to the free ion hyperfine constant by

- 713

The resulting asymmetric pattern, seen in the

0 spec- trum of figui e 2, clearly identifies the T , ground state.

For the T, ground state,

+ 3

so the asym- metry would be reversed. A T, ground state would produce a seven line spectrum in the slow relaxation limit. In the observed spectrum, the lines are somewhat broadened, due to relaxation effects. The solid line shows the results of a least squares fit to the data using an appropriate relaxation theory (61. This analysis gives

3.04 + 0.08) x eV or

AFI =

(1.30

0.03) x e V .

This agrees very well with the value of 1.29 x eV one calculates for

using relativistic <

>

values [7]. However it should be noted that no attempts have been made here to include covalency effects which generally tend to reduce

From the fit shown in figure 2 we obtain an elec- tronic relaxation time of

(9.8

0.05) x lo-'' s at 4.2 K with H

0. A similar run at 1.6 K gave

(9.3

0.07) x 10-lo s. The temperature inde- pendence indicates that spin-spin coupling provides the primary relaxation mechanism. The relaxation time is somewhat shorter than that obtained [8] in

[ " " " " ' I

- 4 0 - 2 0 0 2 0 4 0

VELOCITY (rnrn/s)

FIG. 2. - Hyperfine spectra for 170Yb in Rb2NaYbs at 4.2 K in various external magnetic fields.

Cs,NaYbCI6 due to the smaller lattice constant here

10.67 A for Cs,NaYbC16 and 8.82 21 for Rb,NaYbF,).

In the presence of an external field, two effects are seen. The first is that the electron-nucleus system undergoes a Breit-Rabi type transition. With H

0, the hyperfine spectrum is determined by the spherically symmetric hyperfine interaction

I.S. However, for

FIG. 3. - Dependence of spin-spin relaxation time on external field. The solid line is given by eq. (1).

CRYSTAL FIELD AND ELECTRONIC RELAXATION EFFECTS IN RbzNaYbFs C6-431

large fields the spectrum is determined by an effective field interaction due to the axially symmetric Zeeman interaction gp,

H. This means that the asymmetric pattern described above goes over to a more traditional magnetic splitting (polarized in the external field), giving the spectrum shown in figure 2 for H

30 kOe.

In the transition region, both hyperfine and Zeeman interactions are important and the spectra are complex.

More details concerning this region are given by Shenoy et

elsewhere in this proceedings [8].

The second major effect of the external field is a strong reduction in the spin-spin relaxation rate. In general terms this may be understood as follows [9, 101.

We consider an individual spin to be acted on by some distribution of internal fields P(Hi). In order for an energy conserving flip of a single spin to occur, it is necessary that the external field cancel the internal

field. Therefore, the relaxation rate in field H will depend on the probability P(H) that H

Hi. This will have some distribution that is a maximum at H

and falls to a small value at a few times the mean value of Hi. Since internal fields are generally on the order of hundreds of gauss

there will be a rapid decrease in the relaxation rate in small external fields. Thus we see a relaxation time z

0.98 ns at H

but

2.9 ns for H

2 kOe (Fig.

As a strictly empi- rical result we find

where A

2.4 ns and H i s in kOe. However, uncertain- ties in the actual linewidth to be used in the fitting procedure make the detailed field dependence of the relaxation time difficult to examine for this system.

References

[I] M o ~ s s , L. R., SIEGAL, M., STENGER, L. and EDELSTEIN, N., Inorg. Chem. 9 (1970) 1771.

[2] SCHWARTZ, R. W. and SCHATZ, P. N., Phys. Rev. B 8 (1973) 3229.

[3] KARRAKER, D. G., J. Chern. Phys. 55 (1971) 1084.

[4] For example, see NOWIK, I., DUNLAP, B. D. and KAL- v~us, G. M., Phys. Rev. B 6 (1972) 1048.

[5] LEA, K. R., LEASK, J. M. and WOLF, W. P., J. Phys. Chem.

Solids 23 (1962) 1381.

[6] JIMENEZ, F., GONZALEZ, IMBERT, P. and HARTMANN- BOUTRON, F., Phys. Rev. B 9 (1974) 95.

[7] For example, see DUNLAP, B. D., in Mossbauev Effect Methodology, Edited b y I . J. Gruverman, Vol. VII (Plenum Press, New York, 1972), pp. 123-145 and refe- rences therein.

[8] SHENOY, G. K., ASCH, L., FRIEDT, J. M. and DUNLAP, B. D., this conference.

[9] KRONIG, R. and BOUWKAMP, Physica 6 (1938) 521.

[lo] CASPERS, W. J., Theory of Spin Relaxation (Interscience, New York) 1964.