High-pressure Raman scattering studies of two-magnons in transition metal oxides.

Abstract

We present the results of Raman scattering experiments under high-pressures, and at low and high temperatures that probe the magnetic and lattice excitations in the antiferromagnets $\alpha$-Fe$\sb2$O$\sb3$, NiO, and FeBO$\sb3$. For $\alpha$-Fe$\sb2$O$\sb3$, we report measurements of the effects of high pressures and O-isotope substitution on the Raman spectrum. High pressure spectra were obtained from single crystals in a diamond-anvil cell. The Gruneisen parameters and isotope shifts of the first-order phonons are found to be comparable to the same quantities for the dominant 1320-cm$\sp{-1}$ line, confirming the assignment of this line as a phonon overtone. We identify the two-magnon scattering as a weak and broad feature at $\sim$1525 cm$\sp{-1}$, showing a temperature dependence that is consistent with results for two-magnon scattering in kindred materials. As expected, the magnon band does not shift with isotopic substitution, and its frequency is in good agreement with that predicted by an Ising-model calculation. We also report on the pressure dependence of Raman scattering by magnon pairs in NiO for the pressure range 0-30 GPa. This is the first high-pressure work on light scattering by magnons. The data are in excellent agreement with Bloch's empirical law describing the dependence of superexchange on volume. Using Anderson's theory, we find that the pressure dependence of the antiferromagnetic exchange constant is mainly determined by the hopping parameter of a Hubbard model involving Ni(e$\sb{\rm g}$)-O(2p) hybridization. The implications of these results to high-temperature superconductors and early Raman data are discussed. Finally, we have explored the coupling between a phonon and spin fluctuations by using high-pressure Raman scattering at low temperatures in FeBO$\sb3$. FeBO$\sb3$ is a pure-spin antiferromagnet for which the two-magnon cut-off at P = 0 occurs below the energies of internal (BO$\sb3)\sp{3-}$ vibrations. By applying high-pressures (up to 25 GPa at T = 90K), we were able to rapidly cause a shift in the two-magnon band so that it first resonates with and eventually overcomes one of the internal modes ($\nu\sb4$) of symmetry E$\sb{\rm g}$. In the crossover region, the Raman spectra show a complex lineshape reflecting coupling of the phonon to the two-spin continuum. Results agree well with Fano theory describing interference of coupled discrete-continuum scattering channels. Here, the magnon-phonon interaction relies on the modulation of the superexchange parameter J by ion displacements. Fits to the data give an unexpectedly large coupling constant.