TWO MAGNON RESONANT RAMAN SCATTERING IN TRANSITION METAL OXIDES

International audienceResonant Raman scattering by two magnons is reported for the 'Ar-+ 4T2 absorption band of o-FeaOs but is absent for the corresponding band of FeBOs. This is related to the absence of inversion symmetry in the former and its presence in the latter compound. The absorption edge of NiO at m3.6 eV shows strong resonance for phonons but not for magnon processes. From this fact if is conjectured that the edge is due to O(2p)-+ Ni(3d) transitions. In this letter we present results on the resonant scattering of light by two magnons in the antiferro-magnetic transition metal compounds cw-Fe,O,, FeB03 and NiO. The optical properties of FeB03 have been well studied [ 11. There is a broad absorption band centered at 2.05 eV due to transitions from the 6Ars(" S) to the 4Tzs(4G) manifolds [ 11. These transitions occur at 1.98 eV in cw-Fe,O, [2]. The two-magnon spectra of both compounds are also well known [3,4]. We have measured the intensity of the two-magnon scattering as a function of laser frequency. The results for o-Fez-O3 are shown in fig. 1. The corresponding results for FeB03 are not shown because the two-magnon Raman scattering cross section is nearly a constant in the same optical region. The cross section of the two-magnon line can be obtained following Loudon's derivation [5]. The corresponding scattering tensor has the form /(E,-Cd-iq)(E, + s2-0-irl) , (1) where $i and $j are the ground state orbitals of ions i and j, $J " and $,, are the corresponding excited state orbitals with energies E, and E,,. The laser energy is denoted by w, the two-magnon energy by 51, erk,l are the components of the electric dipole operator. If one confines the summation to nearly degenerate intermediate states (E, NE,,) and, moreover, if the exchange matrix element is assumed isotropic and independent of the intermediate state, it is possible to transform 8