ELSEVIER Journal of Magnetism and Magnetic Materials 140-144 (1995) 1333-1334 journal of magnetism and magnetic materials Collective magnetic excitations of R 3+ ions in grain-aligned RBa2Cu307 (R = Ho, Er) F. Fauth a,*, U. Staub a M. Guillaume a, j. Mesot a, A. Furrer a p. Dosanjh b, H. Zhou b, p. Vorderwisch c, U. Stuhr c a Laboratory for Neutron Scattering, ETH Ziirich & Paul Scherrer Institut, CH-5232 VilligenPSI, Switzerland b University of British Columbia, Department of Physics, VancouverBC, Canada Berlin Neutron Scattering Center, Hahn-Meitner-lnstitut, D-14109 Berlin, Germany Abstract The lowest ground-state crystalline-electric-field transition in grain-aligned RBa2Cu30 7 (R = Ho, Er) has been studied in detail by means of inelastic neutron scattering. The observed dispersion in energy is analyzed in terms of a generalized Heisenberg model including anisotropies in the coupling of spin operators. So far very detailed results are obtained for HoBa2Cu30 7. In order to understand the interplay of superconductiv- ity and magnetism in the high-T c superconductors RBa2Cu30 x (R = rare earth), we need information on the coupling mechanism between the R 3+ ions. Measurements of the collective magnetic excitations, which contain infor- mation on both the nature and the size of the R3+-R 3+ magnetic coupling, turn out to be the most direct way to characterize the magnetic interaction. The inelastic neutron scattering (INS) technique is particularly suited for this kind of studies which are usually performed on single crystals. Because of the absence of large and perfect crystals of RBa2Cu30 x, we have performed INS experi- ments on grain-aligned samples. The preparation and char- acterization of the samples has been described in Ref. [1]. Due to the CEF single-ion anisotropy the grains align along the z- and x-axis in HoBa2Cu307 and ErBazCU3OT, respectively. The INS experiments were performed on a high-resolution triple-axis spectrometer at the Berlin Neu- tron Scattering Center. Scans with constant scattering vec- tor Q = 27r(x/a, y/b, z/c) were performed throughout the experiments. Due to the random orientation of the HoBazCu30 7 sample in the (a, b)-plane, it was impossi- ble to distinguish the components Qx and Qy, therefore the actual scattering vector was Q = 27r(x'/a, z/c) with x' = (X 2 -I- y2)1/2 (assuming a = b). Similar considera- tions hold for ErBa2Cu30 7. A large region of reciprocal space was covered in order to examine the dispersion behaviour of the CEF excitation in detail. * Corresponding author. Fax: +41-56-99 29 39; ernail: bitnet/earn fauth@cageir5a. In HoBa2Cu30 7 the CEF transition F 3 ~ F4 of energy A = 0.5 meV [2] shows up as a well resolved inelastic line and exhibits a pronounced energy dispersion for Q perpen- dicular to the z-axis, whereas for Q involving a zero x' component the line position remains essentially constant (Fig. 1). The data obtained for HoBa2fu30 7 are analyzed in terms of a generalized Heisenberg model including anisotropies in the coupling of the spin operators. In the random phase approximation, the energy of the magnetic excitations is determined by the poles of the dynamic magnetic susceptibility [3]: X'~'~(Q, w)= (a=x, y, z), 1 -J~"(Q)x,'["(w) (1) where J~(Q) is the Fourier transform of the exchange coupling and Xo~'~(o)) the single-ion susceptibility. The intensities I~(Q) of the excitations are proportional to the residues of the corresponding poles. The energy of the first ground-state CEF transition in HoBa2Cu307 is sufficiently small compared to the energies of the other CEF transi- tions, thus we can assume an effective two-level system for which the following expressions for w'~(Q) and I°'(Q) result: wa(Q) = A~I - (2MeJ~(Q)/A) , (2) I°'(Q) =M~A/w'~(Q). (3) Here, A is the energy of the ground-state CEF transition and M,, is the corresponding transition matrix element. For the calculation of jo,,~(Q) we take into account the 0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)01426-4