DIRECT OBSERVATION OF EXCHANGE SPLITTINGS IN RARE EARTH DIMERS BY INELASTIC NEUTRON SCATTERING

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DIRECT OBSERVATION OF EXCHANGE

SPLITTINGS IN RARE EARTH DIMERS BY

INELASTIC NEUTRON SCATTERING

A. Dönni, A. Furrer, H. Blank, A. Heidemann, H. Güdel

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Supplbment au no 12, Tome 49, dkcembre 1988

DIRECT OBSERVATION OF EXCHANGE SPLITTINGS IN RARE EARTH DIMERS BY INELASTIC NEUTRON SCATTERING

A. Donni (I), A. Furrer (I), H. Blank ('), A. Heidemann (') and H. U. Giidel (3)

( I ) Laboratorium fiir Neutronenstreuung ETHZ, 5303 Wiirenlingern, Switzerland (2) ILL, 38042 Grenoble, France

(3) Institut fiir anorganische Chemie, Freiestrasse 3, 3000 Bern S/Sulitzerland

Abstract. - The crystal structures of the insulating dimer compounds Cs3RaBrg (R = ~ b ~ + , D ~ ~ + , H O ~ + , ~ r ~ + , yb3+) were determined by neutron diffraction at 8 K and 295 K. Magnetic dimer excitations were measured by inelastic neutron scattering. Exchange splittings were thus directly determined. In the ~ b ~ + , D ~ and yb3+ dimers the interaction ~ + is of the Heisenberg type.

1. I n t r o d u c t i o n 2. R e s u l t s a n d Discussion Magnetic ordering phenomena a t low temperatures

are widespread in insulating rare earth compounds. An understanding of the basic underlying mechanisms is made difficult by their cooperative nature. The study of discrete dimers of magnetic ions offers a way around this problem. It has been widely used to in- vestigate exchange interactions in 3d transition metal systems. Inelastic neutron scattering (INS) is a very powerful technique for the study of small clusters of magnetic ions [ I , 21. Here we report INS results on the title compounds containing the ~ 2 ~ r i - dimers. Supportive neutron diffraction results are also sum- marised.

The compounds Cs3R2Brg (R = ~ b ~ + , H O ~ + , Ek3+, yb3+) d l crystallise in the space group

R.3

Z = 6. They were synthesized according t o reference [3]. Neutron diffraction experiments were carried out at the reactor Saphir in Wiirenlingen using a multi- detector. Table I summarises the unit cell dimensions for all the compounds a t 8 K and 295 K. The ther- mal expansion is anisotropic with Ac approximately 40 % larger than Aa. Distances between the dimeric ~ 2 ~ r i - units increase significantly between 8 K and 295 K, whereas the intradimer distances show a slight decrease.

INS spectra were measured of all the title com-

Table I. - Lattice constants, molar volumes ( M V ) and heat expansion of Cs3R2Brg. Space group m c , Z = 6.

DELTA a = [a (295 K) - a (8 K)] /a (8 K) R T = 8 K : a

("1

("1

("1

(4

C8 - 1514 JOURNAL DE PHYSIQUE pounds and their diluted analogues C S ~ Y I . S R O . ~ B ~ ~ to determine the crystal field (CEF) splitting!. High-

resolution experiments, carried out on the instruments IN5 and IN13 at the ILL and on IN2 at Wurenlingen, were then used to determine the additional splittings due to exchange interactions in the undiluted sam- ples. In the ~ b ~ + , Dy3+ and ~ 0 compounds the ~ ' first CEF excitations situated at 2.53 (with two com- ponents at 2.48 and 2.57), 1.87 and 0.48 meV, respec- tively, were found to split into several components with total spreads of 0.4, 0.6 and 0.2 meV, respectively, in the undiluted samples.

The CEF ground state of Dy3', ~ r ~ ' and yb3' is a Kramers doublet in this crystal environment. For D~,B~:- and ~b2Br;- we were able to directly mea- sure the exchange splitting of this state. As an illustra- tion the result for Cs3Yb2Br9 is shown in figure 1. The temperature dependence of the band at 373 f 2 peV is exactly that of a singlet to triplet excitation. The cor- responding singlet to triplet excitation in Cs3Dy2Bi-9 was found at 82 f 4 peV.

Fig. 1. - Energy spectra of neutrons scattered from poly-

1

crystalline Cs3Yb2Br9 for Q = 2.3

A-

,

Magnetic dimer excitations show a very typical Q dependence (Q is the scattering vector) of their INS intensities. For a singlet-triplet excitation and a poly- crystalline sample it is given by [4]:

where R is the separation of the rare earth ions in the dim2r, F (Q) the magnetic form factor, and the ex- pression in the bracket is an interference term reflect- ing the dimeric arrangement of the scattering centers

in the crystal. Figure 2 compares the experimental re- sults for the singlet-triplet excitationis in the D ~and ~ + yb3+ dimers with the calculated curve from equation

(1). The good agreement confirms our assignments.

Cs,Yb2Br9 Ilhll3. ILL)

o Cs,Yb2 Br, Ilh2. LNSI

-

1

Fig. 2. - Q-dependence of the intensities of t h e singlet- triplet ground-state transitions observed for R~~ dimers in Cs3R2Brg (R = yb3+, D ~ ~ + ) a t T = 1.5 K. The full line corresponds t o t h e interference term of equation (1).

The observed singlet-triplet exchange splittings can be described by the isotropic Hamiltonian

Treating the ground Kramers doubdets of the single ions as effective S1 = S2= 1/2 states, we thus obtain antiferromagnetic 25 values of - 82 and - 373 peV for D y 2 ~ r ; - and ~ b z ~ r i - , respectively. These are at least one order of magnitude bigger than the magnetic dipole interaction energies. We conclude that exchange interactions are primarily responsible for the magnetic coupling in these dimers. So far we have had no need to go beyond an isotropic Heisenberg Hamiltonian to in- terpret our data. With CEF effects rnuch smaller than spin-orbit coupling, the angular momentum remains fairly isotropic and the Hamiltonian in equation (2) is expected to be adequate. To our knowledge we here re- port the first direct measurements of energy splittings due to exchange interactions in rare earth dimers. Acknowledgment

This work was financially supported by the Swiss National Science Foundation.

[I] Giidel, H. U., Furrer, A. and Kjems, J. K., J. Magn. Magn. Mater. 54-57 (1986) 1453. [2] Furrer, A., Giidel, H. U. and Darriet, J., J. Less

Common Met. 111 (1985) 223. [3] Meyer, G., Inorg. Synth. 22 (1983) 1.

[4] Furrer, A. and Gudel, H. U., Phys. Rev. Lett. 39