Raman Scattering by Magnetic Excitations in RbNiF3

Abstract

An investigation of Raman scattering of argon laser light (λ=5145 Å) from the hexagonal ferrimagnet RbNiF₃ revealed at T≅15°K an intense, broad, symmetrical line shifted ∼510 cm⁻¹ from the laser excitation energy. At higher temperatures as its amplitude decreased, the line broadened and its shift decreased. It was still intense at the Curie temperature (139°K), and remained observable to temperatures near 200°K. This line, assumed to arise from magnetic excitations, is best described in the ordered state as due to two magnon scattering, and at T_c and above due to scattering from a cluster. From the computed magnon modes and energies, optical absorption data, and susceptibility measurements, we determine a set of exchange constants for RbNiF₃. These constants are then used to relate computed magnetic properties with the results of NMR and other measurements. Comparison of the Raman shifts with the predictions of a molecular field model are used to obtain estimates of the short‐range order persisting above T_c.