Investigation of the Spin-Phonon Coupling in Antiferromagnetic Nickel Oxide: Material Properties and Spintronic Applications
This dissertation reports the investigation into temperature dependence of magnon and phonon energies, and the strength of spin – phonon coupling in antiferromagnetic nickel oxide (NiO) - an important material for spintronic devices. A combination of Brillouin–Mandelstam and Raman spectroscopy techniques were used to elucidate the evolution of the phonon and magnon spectral signatures from the Brillouin zone center (GHz frequency range) to the second-order peaks from the Brillouin zone boundary (THz frequency range). The temperature dependent behavior of the magnon and phonon bands in the NiO spectrum indicates the presence of the antiferromagnetic order fluctuation or a persistent antiferromagnetic state, at temperatures substantially above the Neel temperature (TN = 523 K). Tuning the intensity of the excitation laser provided a method for disentangling the magnon signatures from those of the acoustic phonons in scattering spectra of NiO without the need for a magnetic field. The ultraviolet Raman spectroscopy was utilized in order to determine the spin-phonon coupling coefficients. The analysis of the second-order phonon scattering and ultraviolet laser excitation (λ=325 nm) was essential for overcoming the problem of the optical selection rules and dominance of the two-magnon band in the visible Raman spectrum of NiO. The results of this study helped to establish that the spins of Ni atoms interact more strongly with the longitudinal optical (LO) phonons than the transverse optical (TO) phonons and produce an opposite effect on the phonon energies. The experimentally determined characteristics of the spin-phonon coupling were found to be in agreement with the trends calculated by density functional theory. The results of this dissertation research can be used for the interpretation of the inelastic-light scattering spectrum of NiO and other antiferromagnetic materials. They also shed light on the nature of the spin-phonon coupling in antiferromagnetic insulators, which is important for developing spintronic devices.