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CRYSTAL-FIELD EFFECTS IN REBa2Cu3O7
V. Nekvasil, J. Stehno, J. Ebek, L. Havela, V. Sechovský, P. Svoboda
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au n o 12, Tome 49, decembre 1988
CRYSTAL-FIELD EFFECTS IN REBa2Cu307
V. Nekvasil, J. Stehno, J. Sebek, L. Havela1, V. sechovskfland P. svobodal
Institute of Physics, Czechosl. Acad. Sci., Na Slovance 2 CS-180 40 Praha 8, Czechoslovak~a
Abstract. - Superposition model of the crystal field is used to predict splittings of the lowest multiplets and ground-state magnetic properties of rare earth (RE) ions in the high-Tc superconductor REBa~Cu307. The results are compared with experimental data available in the literature.
1. I n t r o d u c t i o n
1
To interpret the magnetization data in the high -Tc superconductor REBazCu307 (REBaCuO; RE-rare earth) we have used in [I, 21 the superposition model (SM) of the crystal field (CF). The model allowed us t o transfer the 4th- and 6th-order CF parameters of the orthorhombic symmetry RE Hamiltonian from garnets t o REBaCuO. The early experimental data obtained by the Mossbauer (e.g. [3, 4]), calorimet- ric [5], neutronographic [6] and magnetization [I, 41 measurements yielded information about a few lowest- lying levels only and could not serve as a good test for the SM approach. As we show below, such test provided recent inelastic neutron scattering measure- ment of the C F spectra of ~ r[7], ~~ +d [8], and ~ +~ 0 ~ ' [9]. In this work we present the SM prediction for the CF spectra of the lowest multiplets of eleven RE ions in REBaCuO, discuss their magnetic properties, and compare the results with available experimental data. A special attention is paid to the role of the 2nd-order CF parameters, for which the SM gives no estimate. 2. Crystal-field splitting
The RE spectra were calculated using the standard CF Hamiltonian with the SM parameters given in [2] (i.e. Bzo = Bzz = 0), considering the J-mixing. It is seen in figure 1 that the model calculations compare favourably with experimental spectra in Ho- [9] and Er- [7] compounds (data from [7] reinterpreted in [S]). The symetry labelling of the experimental levels fol- lows from the calculation due t o Furrer et al. [9] who used the C F parameters obtained by the least squares treatment of their inelastic neutron data. These best- fit parameters are rather similar t o the SM ones. In this stage we avoid the discussion of the inelastic neu- tron data in the Nd-compound [8], which give the split- ting of the ground multiplet of 327 cm-'. If it were true, the REBaCuO would be the only known oxide where the ground-multiplet splitting of ~ dis smaller ~ + than in ~ r [lo]. ~ + experiment theory by Furrer et al. Er (4115,2) experiment theory by Walter et al.
-
Fig. 1. - The crystal-field levels in REBaCuO (RE-Ho, Er).
The C F spectra of five Kramers ions (ce3+, ~ d ~ + , sm3+, D ~ ~ + , yb3+) and three non-Kramers ions (pr3+, ~ b ~ + , ~ m ~ + ) are shown in figures 2 and 3, re- spectively. Let us note that the ce3+ and Tb3+ prob- ably do not form the REBaCuO-type structure [Ill. The RE spectra, except the Tm3+, are not particu- larly sensitive t o small variations of the higher-order C F parameters. Neither 2nd-order parameter appears to change the main features of the spectra, the over-all splitting and grouping of the levels, in most cases. The levels shown in figures 2, 3 are thus believed t o provide
'~harles University, Ke Karlovu 5, CS-121 1G Prague 2, Czechoslovakia.
C8 - 2178 JOURNAL DE PHYSIQUE
a useful first approximation to the RE spectra in both, superconducting and normal phases of REBaCuO.
Fig. 2. - The calculated CF levels of Kramers ions in REBaCuO (RE-Ce, Nd, Sm, Dy, Yb).
Fig. 3. - The calculated CF levels of non-Kramers ions in REB (RE-Pr, Tb, Tm).
3. M a g n e t i c p r o p e r t i e s
Contrary t o the over-all splitting, the ground-state properties of RE ions can be very sensitive t o the 2nd-order parameters for which we have no estimate.
Therefore, investigating the possible magnetic proper- ties of REBaCuO we have let t o vary in our calculation the BzO, Bz, in the range -400 t o 400 cm-l. The sum- mary of the results follows.
ce3', ~ d ~ + , yb3+: ground-state g-tensors are never very anisotropic. The typical values of the saturated moments of the doublets (in powder) are, 0.5, 1.2, and 1.7 p, per ion, respectively. The lat- ter value agrees with the Mossbauer data [3]. sm3', D ~ ~ + ,
EX-^+:
ground-state g-tensors vary with B20, B22. There are large regions in the B20, B22 plane where the g-tensors of the sm3+ and D ~ ~ + have the Ising-like character: either of the Dzh-symmetry axes can be the Ising axis in DyBaCuO; in SmBaCuO it can be the c-axis only. No choice of the Bzo, B22 allowed us t o interpret the Mossbauer data (Mc>Ma, Mb, Ma=
Mb) [4] and specific heat data (1st excited level a t ez 40 cm-l) [6] in Dy compound simultane- ously. pr3+: the well isolated ground-state quasitriplet ( r l ,r2,
r4). Ho3+: the ground-state is always the quasidoublet (r2, r 3 ) i.e., the easy axis is the a-axis. ~ b ~ + : t h e ground state is either quasidoublet ( r l , r z ) or (I'l, rq) i.e., the easy axis is the c- or a-axis, respectively. ~ m ~ + : the ground state is the isolated singlet r3.[l] Sebek, J. et al., Phys. Status Solidi A 103 (1987) K119.
[2] Nekvasil, V., Solid State Commun. 65 (1988) 1103.
[3] Hodges, J. A. et al., Solid State Commun. 64
(1987) 1209.
[4] Hodges, J. A., Imbert, P., Martmon da Cunha, Hamman J., Vincent, E., Sanchez, J. P., Physica
C (preprint)
.
[5] Dunlap, B. D. et al., J . Magn. Magn. Muter. 68 (1987) L139.
[6] Goldman, A. I. et al., Phys. Rev. B 36 (1987) 7234.
171 Walter, U. et al., Phys. Rev. B 36 (1987) 8899. [8] Walter, U., Holfand-~oritz, E., Severing, A.,
Erle, A., Schmidt, A.,'Zirngiebl, E. (preprint). [9] Furrer, A., Bruesch, P., Unternaehrer, P., Physica
C 153 (1988) (preprint).
[lo] Morrison, C. A., Leavitt, R. P., Handbook on the Physics and Chemistry of Rare Earths, Eds. K. A. Gschneider, L. Eyring (North Holland, Amster- dam) Vol. 5 (1982), p. 461.
