UC Riverside

The physics of mutual interaction of phonon quasiparticles with electronic spin degrees of freedom, leading to unusual transport phenomena of spin and heat, has been a subject of continuing interest for decades. Understanding the underlying mechanisms of spin-phonon interactions is pivotal for engineering spin and phonon transport and developing novel spintronic applications. By means of inelastic neutron scattering and first-principles calculations, spin-phonon dynamics in transition metal oxides are investigated.In the first part of the thesis, spin-phonon interactions and their effects on thermal transport were investigated in the exemplary collinear antiferromagnetic NiO. Anomalous scattering spectral intensity from acoustic phonons was identified, unveiling spin precession driven by phonon and strong spin-lattice correlations that renormalize the polarization of acoustic phonons. Time-domain thermoreflectance measurements of the thermal conductivity vs. temperature follow T$^{-1.5}$ in the antiferromagnetic phase. This temperature dependence cannot be explained by phonon-isotope and phonon-defect scattering or phonon softening. Instead, we attribute this to magnon-phonon scattering and spin-induced dynamic symmetry breaking. Our results provide approaches to identify and quantify strong spin-phonon interactions, shedding light on engineering functional spintronic and spin-caloritronic materials through these interactions. In the second part of the thesis, temperature-dependent spin and phonon dynamics in Cr$_2$O$_3$ were characterized and analyzed. We unveiled the emergence of paramagnons above the T$_N$ and at 280 K, closely below T$_N$. We demonstrated a significant softening of linear magnons upon heating in the antiferromagnetic state. Further analysis revealed that this softening primarily originated from four-magnon interactions, while thermal expansion played a subsidiary role. For phonon dynamics, while most phonon modes exhibit expected softening from 50 to 450 K, we observe significant stiffening in the longitudinal optical modes, which involve changing the distances between the nearest Cr$^{3+}$ pairs. Instead of effects from thermal expansion, phonon anharmonicity, magnetostriction, or electron-phonon interactions, the anomalous stiffening can be attributed to the renormalization of electron states due to the change of spin order. Our results point to a purely static magnetoelectric-coupling origin for the observed phonon stiffening, suggest the high-tunability of phonon energies in Cr$_2$O$_3$, and provide insights into controlling lattice dynamics in novel magnetoelectric spintronics.