Interplay between defect transport and cation spin frustration in corundum-structured oxides

Despite the fundamental importance of mass transport, there is still a lack of knowledge regarding the mechanisms in many materials. In particular, how the magnetic spin structure interacts with migrating defects is unclear. Here, using density functional theory to examine cation interstitial transport in antiferromagnetic corundum-structured oxides, we reveal an interplay between the migration of the interstitial and the magnetic structure of the oxide. The magnetic spin configuration impacts the migration energy of the migrating defect but, critically, the defect modifies the spin configuration. Thus, the two aspects change in concert to modify one another. This has profound implications for mass transport in magnetic materials.

Figure 1

Cation interstitial migration path and corresponding energy barriers. The top panel demonstrates the pathway for interstitial migration along the $c$ direction as found via TAD simulations. The ground state structure of the interstitial is shown in (a), while the midpoint of the path is highlighted by (c). Small red and blue balls represent oxygen and cations, respectively. The green circle indicates the location of the cation interstitial atom for clarity. The bottom panel shows the minimum energy pathways for the migration trajectories, calculated by the CI-NEB method, in each of the three corundum-structured materials. Note that, in these calculations, ${\mathrm{Al}}_2{\mathrm{O}}_3$ is treated as nonmagnetic while ${\mathrm{Cr}}_2{\mathrm{O}}_3$ and ${\mathrm{Fe}}_2{\mathrm{O}}_3$ are treated as FM, so all spins are always aligned.

Figure 2

Effect of spin-flip interactions on the ground state energy of ${\mathrm{Fe}}_2{\mathrm{O}}_3$ with an AFM configuration where the cation interstitial, indicated by the green circle, has the same spin as the host. Small red balls are oxygen atoms. Gold and purple balls represent spin-up and spin-down cations, respectively. The values in each schematic panel (in which the oxygen ions are not shown for clarity) report the energy of the respective structure compared to the unflipped structure shown on the left.

Figure 3

Effect of adiabatic and nonadiabatic spin-relaxation dynamics on the interstitial migration barrier in ${\mathrm{Fe}}_2{\mathrm{O}}_3$. Ions with spins that are misaligned relative to the matrix or are excess spins (i.e., the interstitial cation itself) are highlighted by the green circles. As in Fig. 2, the oxygen ions are not shown for clarity. Two minimum energy paths are shown, representative of the interstitial migrating through an FM and an AFM magnetic structure, respectively. The point encircled in the minimum energy path landscape is only an estimate, due to difficulties in converging the calculation.