Quantum anomalous Hall phase and half-metallic phase in ferromagnetic (111) bilayers of $4d$ and $5d$ transition metal perovskites

Extraordinary electronic phases can form in artificial oxide heterostructures, which will provide a fertile ground for new physics and also give rise to novel device functions. Based on a systematic first-principles density functional theory study of the magnetic and electronic properties of the (111) superlattices ${\left({AB\text{O}}_3\right)}_2/{\left({AB}^{\prime }{\mathrm{O}}_3\right)}_{10}$ of $4d$ and $5d$ transition metal perovskite ($B$ = Ru, Rh, Ag, Re, Os, Ir, Au; ${AB}^{\prime }{\mathrm{O}}_3={\mathrm{LaAlO}}_3, {\mathrm{SrTiO}}_3)$, we demonstrate that due to quantum confinement, bilayers ${\left({\text{La}B\text{O}}_3\right)}_2$ ($B$ = Ru, Re, Os) and ${\left({\text{Sr}B\text{O}}_3\right)}_2$ ($B$ = Rh, Os, Ir) are ferromagnetic with ordering temperatures up to room temperature. In particular, bilayer ${\left({\mathrm{LaOsO}}_3\right)}_2$ is an exotic spin-polarized quantum anomalous Hall insulator, while the other ferromagnetic bilayers are metallic with large Hall conductances comparable to the conductance quantum. Furthermore, bilayers ${\left({\mathrm{LaRuO}}_3\right)}_2$ and ${\left({\mathrm{SrRhO}}_3\right)}_2$ are half metallic, while the bilayer ${\left({\mathrm{SrIrO}}_3\right)}_2$ exhibits a peculiar colossal magnetic anisotropy. Our findings thus show that $4d$ and $5d$ metal perovskite (111) bilayers are a class of quasi-two-dimensional materials for exploring exotic quantum phases and also for advanced applications such as low-power nanoelectronics and oxide spintronics.

Figure 1

Atomic and magnetic structures of a ${\left({AB\text{O}}_3\right)}_2/{\left({AB}^{\prime }\mathrm{O}\right)}_{10}$ superlattice. (a) Part of the unit cell containing the ${\left({AB\text{O}}_3\right)}_2$ bilayer, (b) z-AF and (c) i-AF magnetic configuration in the buckled honeycomb lattice formed by the $B$ atoms. Arrows represent the directions of spin moments on the $B$ atoms. Two colors denote the $B$ atoms on the two different planes in the bilayer.

Figure 2

Band structures of the ${\left(\mathrm{LaRuO}\right)}_2/{\left(\mathrm{LAO}\right)}_{10}$ (a, b), ${\left(\mathrm{LaReO}\right)}_2/{\left(\mathrm{LAO}\right)}_{10}$ (c, d), and ${\left(\mathrm{LaOsO}\right)}_2/{\left(\mathrm{LAO}\right)}_{10}$ (e, f) superlattices. Zero refers to the Fermi level.

Figure 3

Band structures of the ${\left(\mathrm{SrRhO}\right)}_2/{\left(\mathrm{STO}\right)}_{10}$ (a, b), ${\left(\mathrm{SrOsO}\right)}_2/{\left(\mathrm{STO}\right)}_{10}$ (c, d), and ${\left(\mathrm{SrIrO}\right)}_2/{\left(\mathrm{STO}\right)}_{10}$ (e, f) superlattices. Zero refers to the Fermi level.

Figure 4

(a) Anomalous Hall conductivity $\left({\sigma }_{xy}^A\right)$, (b) band structure, and (c) atom- and Os $d$ orbital-decomposed densities of states (DOS) of the ${\left(\mathrm{LaOsO}\right)}_2/({\mathrm{LAO}}_{10}$ superlattice. Zero refers to the top of valence bands.

Figure 5

(a) Charge- and (b) spin-density distributions of conduction electrons in the ${\left(\mathrm{LaRuO}\right)}_2/{\left(\mathrm{LAO}\right)}_{10}$ superlattice. Both charge and spin densities are confined within the ${\left(\mathrm{LaRuO}\right)}_2$ bilayer located at the central part.