Electronic, magnetic, and spin lattice interactions of magnonic spinels

Spinel structure ferrites are promising for low loss magnonics, and are comprised of Fe ions on ferrimagnetically aligned octahedral and tetrahedral sites. For lithium aluminum ferrites, ab initio calculations identify that the octahedral sites preferably accommodate Al and Li atoms and strain the ferrite, thereby reducing the density of states at the Fermi level, a known hallmark of low magnetic damping. Al ferrite compositions, such as Al$_{0.5}$Fe$_{2.5}$O$_4$ (AFO), show the half-metallic ferrimagnetic ground state; however, Li substituted AFO compositions, such as Li$_{0.5}$Al$_{1.0}$Fe$_{1.5}$O$_4$ (LAFO), completely remove the density of states at the Fermi level, thus transforming the material into a ferrimagnetic insulator. Depending on the substrate the in-plane magnetocrystalline anisotropy is in general preferred over out-of-plane in AFO. However, LAFO prefers to exhibit out-of-plane magnetocrystalline anisotropy, which is critical to reducing the threshold for current-induced magnetization switching. Our exchange interaction calculations identify and confirm ferrimagnetic super-exchange interactions between oppositely aligned magnetic moments of octahedral Fe and tetrahedral Fe via oxygen. Here, the ferrimagnetic super-exchange angle is 123 degree, which is different from the standard anti-ferromagnetic super-exchange of 180 degree. The octahedral Fe and octahedral Fe, on the other hand, exhibit ferromagnetic super-exchange interactions with 92 degree, which is closer to the standard ferromagnetic super-exchange angle of 90 degree. Interestingly, spin dependent phonon calculations find chiral phonons in Li$_{0.5}$Fe$_{2.5}$O$_4$ (LFO), which may ultimately produce highly stable and low damping excitations, leading to potential applications in low-loss magnonic interconnects. The inherent straining effect in LFO strongly affects both low-frequency acoustic and low- and intermediate-frequency optical modes and produces Raman modes with low magnon-phonon coupling, another hallmark of low-loss magnonics.